Legacy Phosphorus Mobilization by Silicon Amendments: From Laboratory Mechanisms to Agronomic Effectiveness in Winter Wheat

Legacy Phosphorus Mobilization by Silicon Amendments: From Laboratory Mechanisms to Agronomic Effectiveness in Winter Wheat | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Legacy Phosphorus Mobilization by Silicon Amendments: From Laboratory Mechanisms to Agronomic Effectiveness in Winter Wheat Amy Shober, Zhixuan Qin, Sapana Pokhrel, Angelia L. Seyfferth, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8972522/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 6 You are reading this latest preprint version Abstract Background and Aims Historical manure applications on the Delmarva Peninsula have created legacy soil phosphorus (P) accumulation, yet winter wheat ( Triticum aestivum L.) often faces early-season P deficiency due to fixation in acidic soils. This study evaluated whether silicon (Si) amendments can mobilize legacy P across three scales: laboratory chemical desorption, in-field pot plant uptake, and on-farm agronomic yield. Methods A tiered approach used three legacy P soils. A 154-day incubation screened the desorption potential of silicic acid, Ca-Mg silicate slag, and switchgrass char. An in-field pot study isolated Si effects from liming effects using pH-balanced applications of silicic acid, silica gel, and slag. Finally, an on-farm trial compared Ca-Mg silicate slag against standard lime and starter P practices. Results In the incubation, silicic acid and slag increased water-extractable P (WEP), while switchgrass char reduced it. In the pot study, soluble Si increased soil WEP and tissue Si concentrations, but did not enhance biomass or P uptake, likely due to Ca-P precipitation and low ambient stress. Conversely, the field trial showed that while slag did not significantly alter soil P availability, it significantly increased wheat yield compared to standard management, driven by high slag solubility and Si-mediated plant health benefits. Conclusion Silicon amendments can chemically mobilize legacy P, but the mechanism is constrained by soil buffering, Ca-interference, and plant homeostasis. Silicate slag improves yield through stress resilience rather than P-mobilization. Due to variable heavy metal content, caution is advised for its use in forage systems. Legacy phosphorus Silicon fertilization Phosphorus mobilization Winter wheat Calcium-magnesium silicate slag Lime alternatives Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 INTRODUCTION In regions characterized by intensive animal agriculture, such as the Delmarva Peninsula in the Mid-Atlantic United States, the long-term application of manure to agricultural land has fundamentally altered soil phosphorus (P) dynamics (Kleinman et al. 2011 ; McDowell et al. 2002 ). Historically, poultry litter was applied on Delmarva to meet crop nitrogen (N) requirements, resulting in P application rates that far exceed crop removal (Sharpley et al. 2004 ). This imbalance has led to the accumulation of anthropogenic P (i.e., "legacy P") in soils, with soil test P concentrations often exceeding the agronomic critical level (Shober et al. 2025 ; Sims et al. 2000 ). While soil P accumulation poses a significant risk for non-point source pollution and eutrophication of sensitive water bodies like the Chesapeake Bay (Kleinman et al. 2011 ), a paradoxical challenge remains for producers: despite high soil test P, small grain crops such as winter wheat ( Triticum aestivum L.) frequently exhibit early-season P deficiency (Grant et al. 2001 ). This agronomic inefficiency is largely driven by soil chemistry and climate. Winter wheat is planted in late autumn when soil temperatures are declining, which reduces the diffusion rate of phosphate ions (Grant et al. 2001 ). Furthermore, many of these soils are naturally acidic Ultisols where P is strongly fixed by iron (Fe) and aluminum (Al) oxides (Sample et al. 1980 ). To overcome these limitations, farmers typically apply inorganic starter P fertilizers (Grant et al. 2001 ). While effective for yield, this practice further enriches the soil P pool, exacerbating the long-term environmental risk. Therefore, there is an urgent need for best management practices (BMPs) that can mobilize existing legacy P pools to meet early season crop P demand without additional P inputs. Silicon (Si) fertilization has emerged as a potential strategy to bridge this gap. Although not considered essential for higher plants, Si is beneficial, particularly for monocots like wheat, which accumulate Si in tissues to combat biotic and abiotic stress (Epstein 1994 ; Tubaña and Heckman 2015 ). Chemically, the silicate anion (SiO 4 4− ) and phosphate anion (PO 4 3− ) share similar adsorption affinities for soil mineral surfaces (Koski-Vähälä et al. 2001 ). The "competitive exchange" hypothesis suggests that increasing the concentration of soluble Si in the soil solution can displace sorbed P from Fe and Al oxides via ligand exchange, thereby increasing P bioavailability (Koski-Vähälä et al. 2001 ; Lee et al. 2004 ). However, the transition of this mechanism from the laboratory to the field has yielded inconsistent results. While some studies report enhanced P desorption and uptake following Si application (Lee et al. 2004 ; Owino-Gerroh and Gascho 2005), others report no effect or even reduced P uptake (Gao et al. 2005 ; Ma and Takahashi 1990 ). These discrepancies likely stem from the confounding effects of soil pH and the chemical composition of Si sources. Specifically, the introduction of Ca from slags may immobilize desorbed P as Ca-phosphate precipitates, potentially masking the chemical benefits of Si. Common Si amendments, such as Ca silicate (CaSiO 3 ) minerals or slags and calcium-magnesium (Ca-Mg) silicate slags, act as liming agents. The resulting pH increase can mobilize P by reducing Al toxicity and often results in a rapid dissolution of Si; however, the rapid release of soluble Si is followed by an abrupt decline in porewater Si that can result in precipitation of Si rather than plant uptake (Teasley et al. 2017 ). Plus, the introduction of Ca with these sources can also immobilize P through the formation of Ca phosphate (Ca-P) precipitates (Haynes 1982; Sample et al. 1980 ). Alternative Si amendments, such as amorphous silica gel or Si-rich plant residues (e.g., rice husk, straw) can serve as a non-liming source of soluble Si to soils; however, the availability of this Si to plants is governed by dissolution kinetics, which is affected by the source and its processing. For example, fresh Si-rich crop residues act as a slow-release, sustained source of Si that matches plant demand across the growing season. In contrast, combustion of these residues at high temperatures to form biochar can result in the formation of crystalline Si minerals (e.g., cristobalite), which effectively limits plant availability despite high total Si concentrations (Linam et al. 2023 ; Teasley et al. 2017 ). Beyond its potential interactions with soil P, Si is known to enhance cereal crop yields by alleviating both biotic and abiotic stresses. In winter wheat specifically, Si deposition in the cuticle and epidermal cells provides structural rigidity that reduces lodging and susceptibility to fungal pathogens like powdery mildew ( Blumeria graminis ; Provance-Bowley et al. 2010 ). Furthermore, Si fertilization has been shown to improve water use efficiency and biomass production under deficit irrigation or drought stress conditions in grasses (Eneji et al. 2008 ). Industrial byproducts used as Si fertilizers, such as steel or blast furnace slags, also act as multi-nutrient amendments; unlike pure silicic acid, these materials supply significant quantities of Ca and Mg, while neutralizing soil acidity (Tozsin and Oztas 2023 ). The agronomic value of these slags may therefore result from a synergistic effect of improved Si nutrition, pH correction, and the addition of secondary macronutrients. This study employs a multi-scale approach, from laboratory to field, to disentangle the chemical potential of Si-driven P mobilization from the physiological and agronomic constraints that govern its efficacy in winter wheat production. We used three distinct “legacy P” impacted soils to evaluate Si efficacy across three phases: (1) a laboratory incubation to screen the chemical desorption potential of various Si sources (silicic acid, Ca-Mg silicate slag, and biomass char) without plant interference; (2) an in-field pot study designed to decouple the "Si effect" from the "liming effect" by maintaining stable conditions across treatments; and (3) an on-farm field trial to validate the agronomic viability of locally available Si amendments under realistic management conditions. We hypothesized that soluble Si sources would chemically displace sorbed P, increasing water-extractable P (WEP), and that this mobilization would translate to reduced reliance on starter P fertilizer for winter wheat. MATERIALS AND METHODS Site Description and Soil Selection Soil samples were collected from three agricultural fields with a history of poultry litter application located in Sussex County, Delaware, USA. These soils represented major soils in the region: (1) a Fort Mott series (Loamy, siliceous, semiactive, mesic Arenic Hapludults); (2) an Ingleside series (Coarse-loamy, siliceous, semiactive, mesic Typic Hapludults); and (3) a Mullica-Berryland complex consisting of Mullica (Coarse-loamy, siliceous, semiactive, acid, mesic Typic Humaquepts) and Berryland (Sandy, siliceous, mesic Typic Alaquods) series. Bulk soil was collected from the Ap horizon (0–15 cm) of each field using shovels. Approximately 0.21 m 3 of soil was excavated per site, air-dried, crushed to break clods, and passed through a 2-mm sieve. Soil samples were collected from the homogenized bulk soil prior to the incubation and pot experiments to establish baseline physical and chemical properties, and to determine lime requirements (Table 1 ). To accommodate the experimental timeline, soils for the pot study were collected first; soils for the incubation were collected from the same locations eight months later. The outdoor pot study was located at the University of Delaware Newark Farm in Newark, Delaware, USA (39° 40' 5.91'' N, 75° 44' 45.83 W). For the field validation trial, a separate site was established at the University of Delaware Carvel Research and Education Center in Georgetown, Delaware, USA (38° 38' 7.13'' N, 75 27' 42.87 W). The soil at the field site was mapped as a Pepperbox loamy sand (Loamy, mixed, semiactive, mesic Aquic Arenic Hapludults). Phase I: Laboratory Incubation A 154-day laboratory incubation was conducted to evaluate the kinetics of P desorption by Si amendments. The study used a completely randomized design (CRD) with three replicates. Treatments included an unamended control, and three Si sources applied at five rates (0, 0.25, 0.5, 1, and 2 Mg/ha). The Si sources were: (1) Ca-Mg silicate slag (AgrowSil; Harsco Minerals, Sarver, PA); (2) switchgrass char generated from the combustion of locally grown switchgrass ( Panicum virgatum ) at the University of Maryland Wye Research and Education Center (Queenstown, Maryland, USA); and (3) laboratory grade silicic acid (SiO 2 •1.2 H 2 O; Sigma-Aldrich Co., St. Louis, MO, USA). The elemental composition of the switchgrass char and Ca-Mg silicate slag was determined via X-ray Fluorescence (XRF; Supermini200, Rigaku Americas, Inc., The Woodlands, TX, USA). Prior to the initiation of the experiment, a subsample of the air-dried, sieved soil was analyzed to determine initial physical and chemical properties. Air-dried soil (230 g) was weighed into 240-mL polyethylene snap cups with holes drilled in the top to allow gas exchange; amendments were thoroughly mixed with the soil. Deionized water was added to bring the soil to 80% field capacity, as estimated in the lab using the method of Tan ( 1996 ). Cups were snapped shut and incubated under ambient lab conditions. Soil moisture was maintained gravimetrically by weighing cups weekly and adding deionized water as needed. Destructive soil sampling was conducted at 0, 2, 7, 14, 28, 56, 90, and 154 days after treatment application. Samples were air-dried and sieved (2 mm) prior to chemical analysis. Soil samples collected at all time points were analyzed for WEP and acetic acid extractable Si (AA-Si) to track release kinetics. In addition, samples collected at days 56 and 154 were analyzed pH, Mehlich-3 (M3) P, M3-Ca, and degree of P saturation based on M3 (DPS M3 ) to evaluate treatment effects on soil acidity and stable P pools. Phase II: In-Field Pot Study A pot study was conducted to evaluate plant uptake and yield response under pH-controlled conditions at an outdoor pot-in-pot facility at the University of Delaware Newark Farm. Switchgrass char was excluded from this phase due to low reactivity observed in the laboratory incubation. The experiment used a randomized complete block design (RCBD) with four replicates. To isolate the Si effect from the liming effect, all treatments (except the Ca-Mg silicate slag) received reagent-grade CaCO 3 to raise soil pH to a target of 6.5 to match the predicted pH of the slag treatment. The six treatments included: (1) a no Si control receiving only CaCO 3 ; (2) Ca-Mg silicate slag applied at a rate to meet the soil liming requirement, providing Si at approximately 0.41, 0.41, and 0.68 Mg/ha for the Fort Mott, Ingleside, and Mullica-Berryland soils, respectively; (3) a "Low" silicic acid rate matched to the Si loading of the slag with CaCO 3 ; (4) a "Medium" silicic acid rate at two times the Si loading of the slag with CaCO 3 ; (5) a "High" silicic acid rate (2 Mg/ha) with CaCO 3 ; and (6) silica gel (2 Mg/ha; Sigma-Aldrich Co., St. Louis, MO, USA) with CaCO 3 . Calcium carbonate (lime) rates (adjusted for 100 Ca carbonate equivalence were approximately 830 mg/kg for the Fort Mott and Ingleside soils and 1383 mg/kg for the Mullica-Berryland soil). Plastic pots (12,452 cm 3 ) were filled with 16.5 kg of air-dried soil. Treatments (Si source and CaCO 3 ) were incorporated into the entire soil volume on 16 Nov 2015. Nitrogen, potassium (K) or manganese (Mn) fertilizers were also applied on 16 Nov 2015 based on University of Delaware recommendations for winter wheat to ensure non-limiting conditions (Shober et al. 2020 ). Nitrogen was applied as ammonium sulfate to all soils at an N rate of 34.3 kg/ha. Potassium was applied as potassium chloride to Fort Mott and Ingleside soils at a K rate of 37.5 and 22.8 kg/ha, respectively. Manganese fertilizer was applied as manganese sulfate at a Mn rate of 22.8 kg/ha to the Mullica-Berryland soil only. Each pot was then thoroughly wetted and left fallow for a week to allow equilibrium of applied fertilizers and lime. Winter wheat was planted to achieve a seeding rate of 400 seeds m − 2 (equal to 25 seeds per pot) in a greenhouse (to achieve rapid and uniform germination) on 24 Nov 2015. Once established (1 Dec 2015), pots were thinned to 12 plants per pot and moved to an outdoor, in-field pot-in-pot facility to simulate field temperature and photoperiod conditions. Ammonium sulfate was applied to all pots at an N rate of 66.9 and 34.3 kg/ha on 1 Apr 2016 and 2 May 2016, respectively to support late season growth. Plants were grown to physiological maturity (Feekes 11.4) and all aboveground biomass was harvested (24 Jun 2016). Harvested plant material was oven dried at 65°C, and weighed. Wheat heads were hand-threshed to separate grain. Grain yield was weighed directly, and straw biomass was calculated as the difference between the total aboveground biomass and the grain weight. Grain and straw subsamples were ground to pass a 1-mm sieve prior to analysis for Si and P. Initial operational soil P pools were characterized using a modified Hedley fractionation (Sui et al. 1999) to determine the distribution of P between soluble (H 2 O-extractable), labile (NaHCO 3 -extractable), Fe/Al-bound (NaOH-extractable), and Ca-bound (HCl-extractable) fractions. Soil samples were collected to monitor nutrient dynamics throughout the growing season starting at 4 days after treatments were incorporated (20 Nov 2015). Subsequent sampling was conducted at 40, 80, 98, 129, 161, 189, and 239 days after amendments were applied using a 10-mL syringe to extract a composite core from each pot until harvest. All soil samples were analyzed for WEP, water extractable Si (WESi), and AA-Si to track P and Si release kinetics. Comprehensive chemical characterization (pH, M3-P, M3-Ca, and DPS M3 ) was performed on a subset of samples collected during early vegetative growth (Day 40) and post-harvest (Day 239). Phase III: Field Validation Trial An on-farm trial was conducted at the University of Delaware Carvel Research and Education Center to validate the efficacy of the Ca-Mg silicate slag under commercial field conditions. The experimental design was a RCBD with four replicates and plot dimensions of 4.6-m × 9.1-m. Treatments included an unamended control, a standard agricultural lime treatment (3.36 Mg/ha calcitic lime), an agricultural lime with starter P treatment (3.36 Mg/ha calcitic lime + 25 kg/ha P as triple superphosphate), and a Ca-Mg silicate slag treatment applied at 3.36 Mg/ha to meet the liming requirement to achieve pH 6.5 (providing approximately 0.40 Mg/ha Si). Phosphorus was applied as triple superphosphate at a P rate of 25 kg/ha (lime with starter P treatment only) and potassium sulfate was applied at a K rate of 102 kg/ha prior to planting to ensure non-limiting conditions. Lime and Ca-Mg silicate slag treatments were surface applied on 2 Nov 2016. All materials were incorporated via tillage. Winter wheat (SS8340, Southern States Cooperative, Richmond, Virginia, USA) was planted the following day using a grain drill at a seeding rate of 123 kg/ha. Urea ammonium sulfate was applied at an N rate of 36.4 kg/ha at planting (3 Nov 2016). Early biomass samples were collected at green up (6 Mar 2017), dried at 60℃, and biomass was recorded. Ammonium sulfate was applied at an N rate of 54.9 kg/ha and thifensulfuron-methyl (Harmony SG) was applied at 63 g product ha − 1 on 6 Mar 2017. Grain was harvested at maturity (approximately 15 Jun 2017) using a small plot combine (Massey Ferguson 8XP; Kincaid Equipment Manufacturing, Haven, Kansas, USA); yields were adjusted to 13.5% moisture content. A grain subsample was collected from each pot, dried at 65°C to constant weight, and analyzed for P and Si. Soil samples (0–15 cm) were collected at three distinct agronomic stages: pre-plant (16 Oct 2016), dormancy (21 Dec 2017), green-up (6 Mar 2017), and post-harvest (19 Jul 2017). Samples from all four time points were analyzed for pH, WEP, M3-P, and M3-Ca. Laboratory Analysis Soil analysis Initial soil characterization and experimental samples were analyzed using standard regional procedures. Particle size analysis was performed using the hydrometer method (Bouyoucos 1962 ), and organic matter was determined by loss on ignition (Schulte and Hoskins 2011 ). Soil pH was measured in a 1:1 (w:v) soil:water suspension; Adams-Evans buffer pH was also measured to estimate lime recommendations (Sims and Eckert 2011 ). Soils were extracted with M3 solution (1:10 ratio of soil to 0.2 M CH 3 COOH + 0.025 M NH 4 NO 3 + 0.015 M NH 4 F + 0.13 M HNO 3 + 0.001 M EDTA; 5-min reaction time; Wolf and Beegle 2011 ), and concentrations of P, K, Ca, Mg, Mn, Fe, Al, and Si in extracts were analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES; Thermo iCAP 7600, Thermo Fisher Scientific, Waltham, Massachusetts, USA). The DPS M3 was calculated as the molar ratio of M3-P to the sum of M3-Al and M3-Fe, expressed as a percentage (Maguire and Sims 2002 ). Water extractable P and WESi (pot study only) were determined using a 1:10 soil:deionized water ratio shaken for 1 hour, filtered (0.45-µm; Self-Davis et al. 2009 ) and analyzed by spectrophotometer using the molybdate blue methods of Murphy and Riley ( 1962 ) for P and Hallmark et al. ( 1982 ) for Si. Acetic acid-extractable Si was determined using a 0.5 M acetic acid extraction (Korndörfer et al. 2001 ), where 10 g of soil was shaken with 100 mL of acetic acid for 1 hour, filtered, and analyzed by ICP-OES. Incubation soils receiving Ca-Mg silicate slag and silicic acid at total Si rate of 2 Mg/ha sampled at 150 d were also evaluated for CaCl 2 -extractable Si (0.01 M CaCl 2 ; 1h shaking; Korndörfer et al. 1999 ) because these treatments are known to inflate the AA-Si concentrations (Nonaka and Takahashi 1990 ; Sauer et al. 2006; Snyder 2001 ; Wu et al. 2020 ). Pot study soil samples collected at harvest (Day 239) were also analyzed for P sorption characteristics to evaluate changes in P sorption capacity of soils following Si addition. Briefly, 2 g of air-dried soils were equilibrated with 30 mL of P solution (as KH 2 PO 4 dissolved in 0.01 M CaCl 2 , with CaCl 2 being used to maintain a constant ionic environment) at P concentrations of 0, 0.1, 1, 5, 10, 35, and 50 mg/L. Samples were shaken for 24 h, centrifuged for 10 min, and filtered through 0.45-µm filters and the extracts were analyzed for P using ICP-OES. Phosphorus sorption data were fitted to the linearized Langmuir equation (Bolster and Hornberger 2007 ) to estimate soil maximum sorption capacity of P (S max ): $$\frac{C}{q}=\frac{1}{(k\times{S}_{max})}+\frac{C}{{S}_{max}}$$ where C is the equilibrium concentration (mg/L), q is the mass adsorbed (mg/kg), S max is the maximum sorption capacity (mg/kg), and k is the binding affinity (L/mg). Plant tissue analysis Wheat grain and straw samples (pot study only) were dried at 60°C to constant weight and the mass was recorded; subsamples were ground to pass a 1-mm sieve. Sub-samples of straw and grain berries were analyzed separately for Si and P after microwave-assisted nitric acid digestion following the method described in Seyfferth et al. ( 2016 ). Briefly, a 0.2 g subsample was digested using a modified nitric acid (HNO 3 ) and hydrogen peroxide (H 2 O 2 ) method with digests analyzed via ICP-OES for P and Si. For straw samples, the nitric acid digestions resulted in the formation of an acid-insoluble Si gel pellet, which was dissolved in 2 M NaOH; solutions were analyzed for Si using a spectrophotometer with the molybdenum blue method (Hallmark et al. 1982 ). Statistical Analysis All statistical analyses were performed using SAS software (Version 9.4; SAS Institute, Cary, NC). Data were analyzed separately for each soil series and experimental phase to evaluate treatment effects under different soil conditions. The Kenward-Roger adjustment was applied to calculate denominator degrees of freedom to handle the unbalanced nature of including controls within the factorial design. Normality and homoscedasticity of residuals were verified for all models using the Residual Panel graphical diagnostics. Outliers identified via studentized residuals (> 3× standard deviation) were removed prior to analysis, as the magnitude of these values suggested a likely experimental artifact, such as soil contamination during extraction, weighing errors, or incomplete filtration, rather than a true treatment effect. Retaining extreme values violated the assumptions of normality and homogeneity of variance required for the ANOVA and regression analysis. Removing extreme outliers stabilized the variance and ensured the validity of the statistical comparisons. Mixed Model Analysis for Soil Chemical Analysis, Plant Yields, and Plant Tissue Analysis Soil chemical properties were analyzed using a mixed model approach. For the incubation study, treatments consisted of a 3 × 4 factorial combination of Si sources and rates, plus a non-amended control (13 total treatments). For the pot study, six treatments were evaluated, comprising five Si-amended groups (Ca-Mg silicate slag, silica gel, and three rates of silicic acid), and a non-Si amended control; the Si gel and silicic acid treatments also received CaCO 3 to match the lime rates of the Ca-Mg silicate slag. In all models, treatment, day (or date), and their interaction were considered fixed effects. To account for repeated measures on the same experimental units over time, a REPEATED statement was used with the subject defined as replicate(treatment). For the incubation study, AA-Si data were log-transformed to satisfy model assumptions. The covariance structure varied as appropriate to address the correlation between sampling dates and ensure model stability. A First-order autoregressive [AR(1)] covariance structure was used for time-series variables (WEP, WESi, AA-Si) to provide mathematical convergence while accounting for temporal autocorrelation. Compound symmetry (CS) was used for snapshot variables measured at limited intervals (pH, M3-P, M3-Ca, DPS M3 ). When significant ( p < 0.05) treatment × day interactions were observed for soil data, treatment effects were partitioned by day using the SLICE statement. Mean separations for both plant and soil data were performed using the Tukey-Kramer honestly significant difference (HSD) test. Letter groupings for mean separations at each sampling date (soil) or at harvest (plants) were generated using the PDMIX800 macro. Planned orthogonal contrasts were used to further tease apart main effects within the unbalanced design. For the incubation study, we constructed contrasts to compare Si sources overall (silicic acid vs. alternative sources), control vs. Si amended (average of all Si-amended treatments against non-amended controls), specific Si source comparisons at the 2 Mg/ha total Si rate, and individual Si sources rate response (0.25 vs. 2 Mg/ha). For the pot study, planned orthogonal contrasts included comparisons of control vs. all Si-amended, the source effect (silicic acid vs. Ca-Mg silicate slag), and the rate effect (low vs. high). Isotherm derived parameters (S max and k ) from the pot study, as well as wheat grain yield and tissue nutrient concentrations (P and Si; pot and field studies), were analyzed using a mixed-model ANOVA (PROC MIXED). Treatment was treated as a fixed effect and replicate (block) was included as a random effect; however, where the random block variance was estimated as zero, the model automatically defaulted to a completely randomized design (CRD). Degrees of freedom were calculated using the Kenward-Roger adjustment. Mean separations were performed using the Tukey-Kramer adjustment (ɑ = 0.05) to maintain a conservative experiment-wise error rate. Regression Analysis Linear regression analysis (PROC REG) was used to quantify the relationship between Si (AA-Si or WESi) availability and WEP for the incubation and pot studies, with analyses performed independently for each soil series. To isolate the specific mechanism of competitive anion exchange, targeted regressions were conducted using the Si source groupings (e.g., silicic acid + control, Ca-Mg silicate slag + control, etc.). The coefficients of determination (R 2 ) and slope significance were compared between these two fractions to determine whether the instantaneous solution-phase Si (WESi) or the total available pool (AA-Si) served as a more robust predictor of P availability. RESULTS Initial Soil Properties and Amendment Characterization Soils used in the incubation and pot studies were characterized as loamy sands with acidic pH values ranging from 5.0 to 5.7 (Table 1 ). All three soils exhibited "excessive" M3-P concentrations, ranging from 171–684 mg/kg, which is three to ten times the Delaware agronomic critical value of 50 mg/kg (Shober et al. 2025 ). Consequently, the DPS M3 was high (27–110%), exceeding the environmental threshold of 15% established for Delaware soils (Sims et al. 2002 ). The Mullica-Berryland soil was distinct due to its higher organic matter content (42–55 g/kg) compared to that of the Fort Mott (17–20 g/kg) and Ingleside (20–21 g/kg) soils, classifying it regionally as a "black soil" with higher buffering capacity. The Pepperbox loamy sand used in the field trial had a slightly lower, yet still excessive, M3-P concentration and a DPS M3 above the 15% environmental threshold (Table 1 ). Sequential fractionation analysis of initial pot study soils indicated similar total extractable-P concentrations among all three soils (134–188 mg/kg) with more than 50% of P extracted by NaOH, followed by NaHCO 3 -P, HCl-P, and H 2 O-P (Table S1 ). Table 1 Selected initial physical and chemical properties of the soils used in the laboratory incubation, in-field pot study, and field validation trial. Extractable nutrients were determined via Mehlich-3 (M3) extraction. Degree of P saturation (DPS M3 ) was calculated as the molar ratio of M3-P / (M3-Al +M3 Fe). Phase Soil Series pH Buffer pH M3-P (mg/kg) M3-Ca (mg/kg) M3-Al (mg/kg) M3-Fe (mg/kg) DPS M3 ​ (%) Incubation Fort Mott 6.6 7.89 189 225 714 137 21.1 Ingleside 6.2 7.95 361 584 650 123 44.3 Mullica-Berryland 5.7 7.33 684 697 1487 180 37.9 Pot Study Fort Mott 5.0 7.73 171 520 508 82 27.2 Ingleside 5.0 7.72 349 540 580 104 48.3 Mullica-Berryland 5.1 7.38 492 751 1037 115 39.3 Field Trial Pepperbox 4.8 7.62 112 447 772 91 12.1 Lime requirements for the pot and field studies were determined based on the initial pH 1:1 and Adams-Evans buffer pH (Table 1 ). To isolate the effect of Si from the liming effect of the slag, calcitic lime rates were calculated to achieve a target pH of 6.5, matching the neutralization potential of the Ca-Mg silicate slag treatment (Shober et al. 2025 ). Total elemental analysis (XRF) confirmed distinct chemical profiles among the Ca-Mg silicate slag and switchgrass char amendments (Table 2 ). Both the Ca-Mg silicate slag and switchgrass char contained similar concentrations of Si. The Ca-Mg silicate slag was dominated by Ca and Mg, consistent with its utility as a liming agent, and contained appreciable Fe, Al, and Mn compared to other sources. The slag material also contained appreciable concentrations of trace elements including chromium (Cr), nickel (Ni), and molybdenum (Mo). In contrast, the switchgrass char contained lower concentrations of Ca and negligible Fe, Al, and trace elements compared to the slag. The switchgrass char did contain higher concentrations of macronutrients P, K, and S than the slag. The silicic acid and silica gel sources were purchased as high-purity synthetic amorphous silica with negligible metal cation content. Table 2 Total elemental composition of Ca-Mg silicate slag and switchgrass char as determined by X-ray Fluorescence (XRF). Silicic acid and silica gel were technical grade compounds purchased from Sigma-Aldrich; values are stoichiometric estimates based on formula weight and an assumed purity of 99%. Element (g/kg) Ca-Mg Silicate Slag Switchgrass Char Silicic Acid (H 4 SiO 2 ) Silica Gel (SiO 2 •1.2H 2 O) Si 120.55 118.74 292 467 Ca 267.36 51.72 < 0.10 < 0.10 Mg 97.47 19.22 < 0.10 < 0.10 P 0.58 25.49 < 0.10 < 0.10 K 2.34 11.06 < 0.10 < 0.10 S 1.84 8.89 < 0.10 < 0.10 Al 69.46 3.41 < 0.10 < 0.10 Fe 32.32 18.04 < 0.10 < 0.10 Cr 13.67 < 0.10 < 0.10 < 0.10 Mn 9.16 1.19 < 0.10 < 0.10 Ti 3.60 < 0.10 < 0.10 < 0.10 Nb 0.26 8.38 < 0.10 < 0.10 Zn 1.05 < 0.10 < 0.10 < 0.10 Cl < 0.10 0.48 < 0.10 < 0.10 Zr 0.30 < 0.10 < 0.10 < 0.10 Sr 0.26 0.09 < 0.10 < 0.10 Ni 0.17 < 0.10 < 0.10 < 0.10 Mo 0.11 < 0.10 < 0.10 < 0.10 Cu < 0.10 0.07 < 0.10 < 0.10 Br < 0.10 0.06 < 0.10 < 0.10 Phase I: Laboratory Incubation Water Extractable Phosphorus Mobilization A significant treatment × date interaction was reported for soil WEP across all three soils ( p < 0.001). Phosphorus desorption dynamics were strongly influenced by the solubility and chemical composition of the Si source, total Si rate, and soil chemistry. The application of switchgrass char consistently led to a significant reduction in soil WEP concentrations compared with unamended control across all these soils (Fig. 1 ; Table S2). In the coarse-textured Fort Mott soil, WEP concentrations were highly dependent on both Si source and rate, silicic acid provided the greatest WEP mobilization at lower rates. However, the Ca-Mg silicate slag resulted in the highest soil WEP concentrations when applied at the 2 Mg/ha total Si rate (7.98 mg/kg) by the end of the incubation, significantly exceeding the silicic acid at the same rate ( p < 0.0001). In contrast, the silicic acid significantly outperformed the Ca-Mg silicate slag in the Ingleside and Mullica-Berryland soils. In fact, the Ca-Mg silicate slag often resulted in a decrease in soil WEP compared to the control (as was seen with switchgrass char), especially when applied at the higher total Si rates. The treatment × date interaction was significant for soil M3-P concentrations in the Ingleside soils ( p = 0.012) but not for the Fort Mott ( p = 0.28) or Mullica-Berryland ( p = 0.08) soils (Table S3); however, the treatment main effect was significant for these soils ( p = 0.007 and 0.009 for Fort Mott and Mullica-Berryland, respectively). By the end of the study, there were no significant differences in M3-P concentrations for any treatment in the Ingleside soils. Soil M3-P concentrations in the Fort Mott soil at Day 154 were highest for the control soil (238 mg/kg). For the Mullica-Berryland soil at Day 154, the soils receiving the silicic acid or switchgrass char at the 2 Mg/ha Si rate had significantly higher M3-P concentrations (> 845 mg/kg) than the control soils (752 mg/kg). All soil M3-P concentrations remained solidly above the agronomic optimum (50 mg/kg) throughout the study. Soil WEP and Silicon Relationships A significant treatment × date interaction was observed for AA-Si across all three soil series ( p < 0.001). Consequently, the effects of Si amendments are presented by sampling date (Fig. 2 ). Silicic acid application resulted in a rapid increase in AA-Si, which remained elevated throughout the 154-day incubation for all three soils. Application of silicic acid significantly increased AA-Si concentrations relative to the control across all three soils (Fig. 2 ). In the sandy Fort Mott and Ingleside soils, silicic acid (2 Mg/ha total Si) maintained AA-Si levels between 12.8 and 17.2 mg/kg between days 56 and 154. In contrast, Si release was more attenuated in the high-organic matter Mullica-Berryland soil, where AA-Si concentrations peaked at 11.7 mg/kg at the same application rate (Fig. 2 ). Switchgrass char also increased AA-Si concentrations relative to the control, but soil-specific differences were evident. At the 2 Mg/ha Si rate, char application significantly increased AA-Si in all three soils. However, at the 1.0 Mg/ha Si rate, significant increases were observed only in the Fort Mott and Ingleside soils, whereas AA-Si levels in the Mullica-Berryland soil remained statistically similar to the control. Regression analysis confirmed a significant positive linear relationship between AA-Si and WEP in soils treated with pure silicic acid ( p < 0.0001; Fig. 3 ). This relationship was strongest in the Fort Mott soils. In contrast, the relationship between AA-Si and WEP amended with Ca-Mg silicate slag was statistically significant, but nearly flat for the Fort Mott soil ( p > 0.0001) and negative for the Ingleside ( p = 0.0028) and Mullica-Berryland ( p < 0.0001) soils. Similarly, the relationship between AA-Si and WEP with switchgrass char application was not significant for the Fort Mott soils ( p = 0.32) and strongly negative for the Ingleside and Mullica-Berryland soils ( p > 0.001). In contrast, the relationship between soluble Si and WEP in soils treated with Ca-Mg silicate slag was inconsistent (Fig. 3 ). This reduction coincided with significant increases in soil pH and M3-Ca (Table S5). Impact of Amendments on Soil pH and Mehlich 3-Ca A significant treatment × date interaction was reported for soil pH with all three soils ( p 1 Mg/ha rate resulted in significantly higher soil pH than for the control for all three soils; soil pH was 7.27, 7.4, and 5.73, respectively for Fort Mott, Ingleside, and Mullica-Berryland soils at the 2 Mg/ha Si rate, 6.7, 6.8, and 5.4 for the 1 Mg/ha Si rate, and 5.97, 5.90, and 4.73 for the control. Application of the Ca-Mg silicate slag at the 0.5 Mg/ha Si rate also increased soil pH (6.33) compared to control (5.97) for the Fort Mott soil. A significant treatment × date interaction was also reported for M3-Ca with all three soils ( p < 0.03; Table S3). By day 154, the M3-Ca concentrations were statistically higher than the control for all three soils when the Ca-Mg silicate slag was applied at rates of 0.5 Mg/ha total Si or higher; application of the Ca-Mg slag at the 0.25 Mg/ha Si rate also resulted in higher M3-Ca relative to control for the Fort Mott and Mullica-Berryland soils. Application of the switchgrass char at the 2 Mg/ha Si rate also increased soil M3-Ca relative to the control in the Ingleside and Mullica-Berryland soils (933 and 800 vs. 767 and 633 mg/kg, respectively) Phase II: In-field Pot Study Soil Phosphorus Dynamics and Sorption Capacity (S max ) The treatment × date interaction was significant for soil WEP across all three soils ( p < 0.007; Table S4). The effect on P solubility was heavily dependent on Si source (Fig. 4 ). In general, the highest rate of Si application (2 Mg/ha Si) was required to increase soil WEP compared to the control. Application of silicic acid at the high rate resulted in significantly higher soil WEP concentrations than the CaCO 3 control throughout the growing season in Fort Mott and Mullica soils (Fig. 4 ). In the Ingleside soil, silicic acid at the high Si rate increased WEP significantly at days 4 and 40, but differences were not significant compared to the control at later dates. The Ca-Mg silicate slag resulted in an immediate spike in WEP (Day 4) in Fort Mott and Ingleside soils ( p < 0.05), increasing WEP by approximately 30% compared to the control, but this effect was transient and did not persist through the growing season (Fig. 4 ). Silica gel had variable influence on soil WEP compared to the control, with similar response as silicic acid at the high (2 Mg/ha) rate in the Mullica-Berryland soil ( p < 0.0001). In contrast, the effects of Si gel were better than the Ca-Mg silicate treatment, but less effective than the high rate silicic acid when applied to Ingleside soils; Si gel did not increase soil WEP in the Fort Mott soil. Analysis of P sorption isotherms at harvest revealed that Si amendments physically altered the soil’s capacity to bind P, but results were soil specific. In the Fort Mott soil, silicic acid (2 Mg/ha Si rate) significantly reduced the S max ​ to 51 mg/kg, compared to 72 mg/kg in the control ( p < 0.0001; Table 3 ). A similar significant reduction in S max was observed in the Ingleside soil (31 mg/kg vs. 87 mg/kg in the control). However, in the high-organic matter Mullica soil, no significant treatment effects on S max ​ were observed ( p = 0.77; Table 3 ), consistent with the lack of WEP response in the incubation phase. Unlike WEP, soil M3-P and DPS M3 were largely unresponsive to the Si treatments (Table S5). Table 3 Effect of silicon treatments on phosphorus sorption maxima (S max ) and binding energy ( k ) determined by Langmuir isotherms in the in-field pot study. Values represent means (SD) for n = 3. Means within a column followed by the same letter within the same soil series are not significantly different (Tukey’s HSD, α = 0.05). Soil Series Treatment Total Si Rate (Mg/ha) S max ​ (mg/kg) k (L mg − 1 ) Fort Mott Control 0.00 72.0 (2.7) b 0.358 (0.020) a Ca-Mg Silicate Slag 0.41 84.4 (2.7) a 0.236 (0.015) b Silicic Acid (Low) 0.41 87.1 (0.5) a 0.175 (0.004) c Silicic Acid (Med) 0.82 63.3 (2.2) bc 0.271 (0.018) b Silicic Acid (High) 2.00 50.9 (2.3) c 0.313 (0.007) ab Silica Gel 2.00 68.4 (4.6) b 0.178 (0.020) c Ingleside Control 0.00 86.8 (3.5) a 0.080 (0.001) c Ca-Mg Silicate Slag 0.41 43.1 (0.4) bc 0.256 (0.016) a Silicic Acid (Low) 0.41 31.2 (0.9) c 0.286 (0.035) a Silicic Acid (Med) 0.82 40.2 (0.8) bc 0.215 (0.009) ab Silicic Acid (High) 2.00 61.1 (18.6) b 0.103 (0.034) bc Silica Gel 2.00 33.1 (2.5) c 0.206 (0.004) ab Mullica- Berryland Control 0.00 283.7 (7.9) a 0.136 (0.004) ab Ca-Mg Silicate Slag 0.68 271.8 (4.9) a 0.152 (0.004) a Silicic Acid (Low) 0.68 289.7 (3.4) a 0.123 (0.003) ab Silicic Acid (Med) 1.36 282.6 (13.8) a 0.127 (0.018) ab Silicic Acid (High) 2.00 280.4 (4.7) a 0.109 (0.003) b Silica Gel 2.00 332.3 (54.4) a 0.099 (0.018) b Soil Silicon Solubility Treatment effects on extractable silicon varied by extraction method (Fig. 5 ). As reported for the soil incubation, AA-Si concentrations for the Ca-Mg silicate slag amended soils were inflated, consistently 2 to 3 times higher than AA-Si concentrations with silicic acid application (data not shown). The WESi treatment × date interaction was significant ( p = 0.0046 and 0.015, respectively) for the Ingleside and Mullica-Berryland soils, but not significant for the Fort Mott soil ( p = 0.053; Fig. 5 ). Overall, both the Ca-Mg silicate slag and the silicic acid significantly increased WESi relative to the control ( p < 0.0001), but the magnitude depended on the source and rate (Fig. 5 ). The Ca-Mg silicate slag significantly increased WESi in all three soils ( p 2× the control levels), the Ca-Mg silicate slag was also an effective Si source, maintaining significantly elevated WESi levels comparable to the agronomic rates of silicic acid. Soil pH and Mehlich-3 Ca Soil pH and Mehlich-3 Ca (M3-Ca) concentrations responded to treatment application, with variations by soil type (Table S5). Soil pH management efficacy varied by soil type and time. In the Fort Mott soil, pH values were statistically equivalent among treatments at Day 40 ( p = 0.66), but significantly diverged by harvest (Day 209; p < 0.0001), with the slag treatment maintaining higher alkalinity than the CaCO 3 controls. In the Fort Mott soil, all Si-amended treatments resulted in soil pH values that exceeded the 6.5 target pH by the final harvest (Table S5). The Ca-Mg silicate slag treatment resulted in a soil pH of 7.2, which was significantly above the CaCO 3 control (6.5). Conversely, in the Ingleside soil, while differences were evident at Day 40 ( p < 0.001), soil pH converged by harvest, resulting in no significant treatment differences ( p = 0.07). In the Mullica-Berryland soil, significant differences persisted at both sampling dates ( p < 0.001), with the slag treatment consistently resulting in higher pH values than the CaCO 3 control. In the Fort Mott and Mullica soils, the treatment × date interaction for M3-Ca was not significant ( p = 0.30 and 0.58, respectively; Table S5), while the main effect of treatment was significant ( p = 0.037 and 0.001, respectively). In these soils, slag and CaCO 3 applications resulted in higher M3-Ca concentrations averaged across sampling dates compared to the CaCO 3 only control. In the Ingleside soil, M3-Ca concentrations also differed by treatment ( p = 0.013), with no significant interaction with time ( p = 0.22). Crop Yield and Nutrient Uptake Despite significant increases in soil P solubility (as evidenced by soil WEP) and reductions in sorption capacity (S max ) in silicic acid treatments, there were no significant differences in winter wheat grain yield or straw biomass between Si treatments and the CaCO 3 control across the three soil types ( p > 0.05; Table 4 ). The only exception was a significant increase in straw biomass for the silica gel treatment in the Mullica-Berryland soil compared to the CaCO 3 control. Similarly, tissue P concentrations in grain and straw were not significantly affected by Si treatment (Table 4 ). Total P removal (grain + straw) did not differ between the CaCO 3 control and Si treatments ( p > 0.05). However, plant Si uptake was highly responsive; wheat straw Si concentrations were significantly higher in all Si-amended treatments compared to the control. While the high-rate silicic acid treatment resulted in the highest straw Si concentrations, it can be noted that plants grown in pots receiving the Ca-Mg silicate slag treatment had straw Si concentrations that were 3–5 fold higher than the lime only control (Table 4 ). Table 4 Wheat grain yield, straw biomass, and tissue nutrient concentrations from the outdoor pot study. Means (standard deviation) for n = 3 within a column followed by the same letter are not significantly different (Tukey’s HSD, α = 0.05). Soil Series Treatment Total Si Rate (Mg/ha) Grain Yield (g/pot) Straw Biomass (g/pot) Grain P (mg/kg) Grain Si (mg/kg) Straw P (mg/kg) Straw Si (mg/kg) a Fort Mott Control 0.00 26.50 (1.48) 117.9 (4.3) ab 1421 (647) 16 (8) b 684 (135) ab 2741 (392) c Slag 0.41 23.17 (2.34) 120.6 (3.1) a 2210 (263) 101 (37) a 656 (92) b 8378 (756) b Silicic Acid (Low) 0.41 21.47 (3.59) 109.2 (2.8) b 2272 (85) 68 (52) ab 793 (73) ab 8344 (481) b Silicic Acid (Med) 0.82 22.64 (2.59) 116.0 (2.5) ab 1567 (692) 28 (14) b 990 (73) a 18259 (2570) a Silicic Acid (High) 2.00 19.51 (5.21) 115.3 (3.4) ab 2402 (228) 41 (10) ab 889 (180) ab 11151 (1275) b Silica Gel 2.00 21.61 (1.91) 116.8 (2.7) ab 2250 (71) 46 (11) ab 845 (118) 15205 (603) a Ingleside Control 0.00 22.08 (2.82) 114.4 (3.2) 2652 (394) 20 (14) 1195 (54) 1466 (344) d Slag 0.41 20.08 (1.26) 117.8 (4.1) 2475 (191) 35 (7) 1247 (67) 7143 (757) cd Silicic Acid (Low) 0.41 20.09 (3.13) 114.6 (3.5) 2735 (60) 29 (2) 1373 (355) 6501 (1104) cd Silicic Acid (Med) 0.82 23.51 (3.24) 122.8 (3.8) 2342 (428) 36 (14) 1624 (571) 16634 (4575) a Silicic Acid (High) 2.00 25.80 (3.08) 120.7 (4.5) 2294 (110) 36 (5) 1213 (233) 9560 (2998) bc Silica Gel 2.00 22.75 (0.27) 122.6 (3.9) 2385 (269) 41 (1) 1173 (212) 15361 (2188) ab Mullica- Berryland Control 0.00 24.75 (3.62) 117.1 (2.9) b 2469 (295) 19 (3) b 1150 (352) 1231 (115) Slag 0.68 24.47 (3.49) 122.7 (3.6) ab 3367 (1361) 57 (17) a 1035 (132) 9626 (150) Silicic Acid (Low) 0.68 27.17 (3.46) 126.1 (1.2) ab 2652 (245) 27 (2) b 1010 (158) 5525 (120) Silicic Acid (Med) 1.36 23.76 (0.39) 127.5 (1.3) ab 2429 (733) 38 (15) ab 1037 (44) 12292 (160) Silicic Acid (High) 2.00 22.70 (0.95) 121.3 (1.0) ab 2362 (197) 32 (4) ab 1060 (143) 11303 (190) Silica Gel 2.00 28.94 (6.27) 131.3 (1.4) a 2296 (109) 35 (8) ab 1080 (187) 9516 (170) a Statistical analysis (Tukey’s HSD) was not performed for straw Si in the Mullica-Berryland soil series due to insufficient sample size (n < 6) resulting from missing values. Phase III: On-Farm Field Trial Field Soil and Crop Response In the field trial on Pepperbox loamy sand, no treatment × date interaction ( p = 0.39) on soil pH was observed. Soils receiving the Ca-Mg silicate slag, calcitic lime only, and lime + starter P treatments had a significantly higher pH (6.13, 6.13, and 6.07, respectively) than the control soils (pH = 5.83; p = 0.0003; Table S6). Despite differences in soil pH, there was no significant treatment effect on soil P dynamics. The treatment × date interaction was highly non-significant ( p = 0.936), indicating the treatments behaved consistently (and indistinguishably) throughout the season. Mean soil WEP concentrations did not differ significantly between the Ca-Mg silicate slag treatment (4.34 mg/kg), standard calcitic lime (4.62 mg/kg), lime with starter P (5.27 mg/kg), or control (5.32 mg/kg) treatments across the study ( p > 0.066; Table S6). Similarly, treatments had no effect on M3-P ( p = 0.40) or DPS M3 ( p = 0.34) among treatments at either sampling date (Table S6). Mehlich-3 Ca concentrations showed a numerical increase in the slag-amended plots (reaching 733 mg/kg at harvest vs. 439 mg/kg in the control), resulting in a significant pairwise difference by the end of the season ( p = 0.036), although the overall treatment main effect for the trial was not significant ( p = 0.12; Table S6). Despite similar soil pH and M3-P concentrations by the end of the study, we noted a significant treatment effect on wheat yield ( p = 0.0049), where the slag-amended soils had significantly higher yields (8.25 Mg/ha) than all other treatments (mean = 7.35 Mg/ha). No significant differences in early-season vegetative biomass ( p = 0.64) were observed among treatments (Table S7). Furthermore, while tissue Si concentrations at the early green-up stage were significantly higher in the Ca-Mg silicate slag treatment (1.85 g/kg) compared to the calcitic lime treatment (1.72 g/kg), this difference was not significant at harvest ( p = 0.71). There were no significant treatment effects on grain P or Si concentrations ( p = 0.62) DISCUSSION Competitive Desorption vs. Precipitation as the Mechanisms of P Mobilization Our results confirm that Si amendments can mobilize legacy P in acidic agricultural soils, but the magnitude and direction of this effect are strictly governed by the chemical composition of the amendment. The strong positive linear relationship between AA-extractable Si and WEP in incubation soils treated with pure silicic acid (R 2 = 0.60 for Fort Mott; Fig. 3 ) supports the "competitive exchange" hypothesis, where silicate anions (H 3 SiO 4− ) displace phosphate (H 2 PO 4 − ) from specific sorption sites on Fe and Al oxides via ligand exchange (Koski-Vähälä et al. 2001 ; Lee et al. 2004 ). Yet, the discrepancy between AA-Si and WESi results in the pot study explains why the Ca-Mg silicate slag failed to mobilize P. While AA-Si indicated a large pool of 'available' Si in the slag treatment (approximately 44–54 mg/kg higher than control), the WESi data, which more accurately reflects the soil solution equilibrium, showed no significant increase over the control (Fig. 4 ). In contrast, the AA-Si concentrations in silicic acid-amended soils were only 3–4 times higher than the WESi concentrations. These results confirm that the Si applied in the Ca-Mg slag amendment was unreacted in the soil. Without a sustained elevation in dissolved H 3 SiO 4 − , the competitive exchange with H 2 PO 4 − could not occur. This validates that AA-Si overestimates the agronomically active Si pool in slag-amended soils, acting as a methodological artifact rather than a predictor of P mobilization (Wu et al. 2020 ). Critically, the P sorption isotherm data (S max ) from our pot study provides physical validation of this mechanism in a plant-soil system. The significant reduction in S max observed in Fort Mott and Ingleside soils treated with silicic acid (Table 3 ) indicates that Si anions successfully occupied binding sites that would otherwise be available for P sorption. By physically blocking these sites, soluble Si shifted the equilibrium toward the solution phase. This aligns with findings by Schaller et al. ( 2019 ), who observed that increasing Si availability mobilized P from Fe minerals in Arctic soils. Our results also corroborate the model of competitive adsorption described by Obihara and Russell ( 1972 ) and Hiemstra et al. ( 2007 ). However, the efficacy of the Ca-Mg silicate slag source was compromised by a "Ca penalty." Unlike pure silicic acid, the slag lime alternative contained significant quantities of Ca and Mg in addition to Si (Table 2 ). The significant increase in M3-Ca observed in the incubation (Table S3) supports the formation of stable Ca-P precipitates. In the Mullica and Ingleside soils, where slag reduced WEP, the high Ca loading likely shifted the equilibrium toward the formation of Ca-P minerals or the formation of Ca-P-Fe ternary complexes that outweighed the desorption pressure of the silicate anion (Haynes 1982; Penn et al. 2011 ). Ma and Takahashi (1991) similarly reported that while sodium silicate increased pH and Si, it did not always increase P availability in P-deficient soils due to complex soil chemical interactions. Our data suggests that in "legacy P" soils, the addition of Ca-rich silicate amendments may inadvertently stabilize soil P pools rather than mobilize them, effectively acting solely as a liming agent rather than a P-mobilizer. Phosphorus Sorption by Charred Biomass The consistent reduction in WEP following switchgrass char application indicates that thermal processing of biomass can negate the benefits of phytogenic Si. Despite adding total Si to the system, the char acted as a net P sink. This is likely due to the increased surface area and porosity generated during combustion, a phenomenon well-documented in biochar literature where surface functional groups and porous structures sorb phosphate (Chen et al. 2011 ; Park et al. 2015 ). This finding is critical for defining BMPs; while returning crop residues to soil is a primary pathway for Si recycling (Puppe et al. 2021 ; Vandevenne et al. 2012 ), our results suggest that incorporating charred residues may exacerbate P fixation in the short term compared to the use of un-charred phytolith-rich residues that release Si more readily (Seyfferth et al. 2016 ). Methodological Artifacts in Soil Silicon Testing Our study highlights a significant methodological limitation in assessing plant-available Si in amended soils. The AA-Si overestimated Si availability in slag-amended soils by up to 24-fold compared to the CaCl 2 extraction in the incubation study; a result that was also noted by Wu et al. ( 2020 ) for soils amended with Ca silicate materials. Similarly, the WESi concentrations for slag-amended soils in the pot study were typically 10-fold lower than AA-Si concentrations in the same soils (Fig. 5 ). Acetic acid is aggressive enough to dissolve non-labile Ca silicates in the slag that are not available to plants under ambient soil pH conditions (Nonaka and Takahashi 1990 ; Snyder 2001 ). Conversely, the 0.01 M CaCl 2 extraction provided values that were consistent between source types and aligned with thermodynamic solubility limits. This confirms that for agronomic consulting, particularly where slag amendments are used, dilute salt extractions provide a more accurate index of the active monosilicic acid pool than acid-based extractants (Miles et al. 2014; Wu et al. 2020 ). Regulation of P Uptake Due to Physiochemical Disconnect The transition from the laboratory to the pot study revealed a consistent decoupling of soil solution chemistry (WEP) from plant physiology (yield and P uptake). Silicic acid applied at the 2 Mg/ha total Si rate increased WEP by up to 70% and reduced soil sorption capacity (S max ) in the pot study, yet this did not translate to increased biomass or total P uptake in winter wheat. We propose that this lack of response is not a failure of the amendment, but a physiological regulation mechanism described by Ma and Takahashi ( 1990 ). Ma and Takahashi ( 1990 ) observed that while Si promotes P uptake in low-P conditions, it can suppress excessive P uptake in high-P environments to prevent P toxicity and maintain favorable intracellular inorganic P levels. The soils in our study were agronomically "excessive" in P (M3-P > 100 mg/kg), well above the agronomic critical level for winter wheat (Shober et al. 2025 ). Therefore, the wheat plants likely engaged homeostatic mechanisms to limit P influx, rendering the Si-mobilized P redundant for biomass production (Neu et al. 2017 ; Schaller et al. 2012 ). Although the pot study was conducted outdoors under ambient weather conditions, the soil environment was highly optimized. Fertilizers were uniformly incorporated to ensure non-limiting conditions, and the soils were thoroughly homogenized. Silicon is well-documented to enhance crop yield primarily by alleviating severe abiotic and biotic stresses, such as drought, nutrient imbalances, or fungal pathogens (Eneji et al. 2008 ; Provance-Bowley et al. 2010 ). The optimal moisture conditions during the 2015–2016 growing season, coupled with nutrient management in the pot study likely masked the potential stress-mitigation benefits of the applied Si. Additionally, the thorough mixing of liming agents in the confined volume of the pots may have exacerbated chemical interactions between Ca and P, further limiting agronomic responses compared to a heterogeneous field environment. Agronomic Implications While the Si-driven P mobilization mechanism is chemically valid, harnessing it for yield gain in legacy P soils is challenging. The transition from mechanistic pot studies to field validation trials reveals that soil buffering and environmental variability often diminish the temporary P solubilization effects observed in the lab. In our pot study, although Si amendments nearly doubled straw Si concentrations, this accumulation did not translate into higher grain yield under the buffered, low-stress conditions of the nested pot design. Consequently, Ca-Mg silicate slag should be viewed primarily as an effective liming agent and a source of plant-available Si for structural fortification (Epstein 1999; Provance-Bowley et al. 2010 ), rather than a reliable tool for "mining" legacy soil P. While slag provided a significant yield advantage in the field (likely by preserving yield potential against regional biotic stressors like powdery mildew), our data suggests a plant-health response rather than a P-mobilization response. For producers, this implies that while Si amendments enhance tissue Si and resistance to lodging (Rodrigues and Datnoff 2005 ), they do not justify the elimination of starter P fertilizers. Furthermore, the physical scale and chemical reactivity of amendments dictate their efficacy. In the pot study, lab homogenization with highly reactive CaCO 3 created a "Ca penalty," where sudden influxes of Ca likely neutralized localized P desorption through rapid Ca-P precipitation. Conversely, the coarse heterogeneity of the field trial allowed slag to neutralize soil acidity and deliver Si more effectively without widespread Ca-P precipitation. Similarly, the increased P sorption following incorporation of switchgrass char indicates that crop residues are unlikely to be a viable amendment for P solubilization. Finally, while silicic acid and silica gel showed the most promise for P mobilization in the lab, they remain cost-prohibitive for field-scale application. Moreover, evidence from our pot study suggests that high concentrations of silicic acid may induce physiological responses in small grains that negate the benefits of increased soil P solubility. As such, there are currently no viable amendments available to growers that simultaneously maximize soluble Si and enhance legacy soil P availability. Future research should focus on intermediate Si sources, such as amorphous silica-rich crop residues (Seyfferth et al. 2016 ), which may offer a middle ground between the high solubility of silicic acid and the heavy Ca-loading of industrial slags. Environmental and Safety Considerations Despite its agronomic efficacy as a value-added liming agent in grain production systems, the use of industrial byproducts like Ca-Mg silicate slag in production agriculture is not without potential environmental concern. Chemical characterizations of the Ca-Mg silicate slag used in this study (Table 2 ) and related mineral byproducts indicate highly variable concentrations of trace metals, including Cr, Ni, and Mo (Qin 2017 ; Torlon 2014 ). The liming effect of the slag can result in immobilization of cationic heavy metals, thereby reducing their phytoavailability and translocation to grain (Deus et al. 2020 ), anionic elements such as Mo can become increasingly plant-available at higher soil pH. Given the measurable Mo content in the evaluated slag (≈ 107 mg kg⁻¹), there is a distinct risk of Mo accumulation in plant tissues. As such, land managers should use caution when applying slag-based lime alternatives to forage or pasture crops where high Mo forage can induce molybdenosis (Cu deficiency) in grazing ruminants. Consequently, the use of steel slag lime alternatives is better suited to grain production systems where soils need pH adjustment and residues (straw) are incorporated into the soil or removed after grain harvest. CONCLUSION While Si can chemically mobilize legacy soil P, this mechanism does not translate into a viable agronomic strategy for winter wheat. Biological homeostasis, environmental variability, and the "Ca penalty" of common agricultural amendments effectively neutralize P-desorption at the field scale. Therefore, Si should not be promoted as a tool to mine legacy P or replace starter fertilizers. Instead, the agronomic value of Ca-Mg silicate slag lies in its performance as a highly soluble, multi-nutrient liming agent that supplies plant-available Si to preserve yield under environmental stress. However, using these industrial byproducts requires rigorous batch testing due to variable trace metal content. Specifically, the risk of Mo-induced toxicity restricts their safe application strictly to grain production systems, precluding their use in pastures or forage. Future research on Si-driven P mobilization should pivot toward Ca-free, intermediate Si sources (e.g., silica-rich plant residues) to sustainably deliver soluble Si without the confounding effects of Ca interference or heavy metal contamination. Abbreviations AA-Si, acetic acid-extractable silicon; BMP, best management practice; DPS M3 , degree of phosphorus saturation; M3, Mehlich-3; WEP, water extractable phosphorus. STATEMENTS & DECLARATIONS Acknowledgements: The authors thank Mr. Shawn Tingle for assistance with soil collection and Ms. Karen Gartley and the University of Delaware soil testing program staff for assistance with analysis. The authors also wish to thank Dr. Nicole Fiorellino from University of Maryland – College Park for her insightful review of our manuscript draft. Funding: This work was supported by a USDA Sustainable Agriculture Research and Education (SARE) graduate student grant (GNE15-111). Competing Interests: The authors have no relevant financial or non-financial interests to disclose. Author Contributions: A. Shober, Z. Qin, and A. Seyfferth contributed to the study conception and design. Material preparation and data collection were performed by Z. Qin and L. Mosesso. Data analysis was performed by Z. Qin, A. Shober, and S. Pokhrel. The first draft was written by Z. Qin and A. Shober. All authors commented on previous versions and approved the final manuscript. Data Availability: The datasets generated during the current study are available from the corresponding author on reasonable request. References Bolster CH, Hornberger GM (2007) On the use of linearized Langmuir equations. Soil Sci Soc Am J 71:1796–1806. https://doi.org/10.2136/sssaj2006.0304 Bouyoucos GJ (1962) Hydrometer method improved for making particle size analyses of soils 1. Agron J 54:464-465. https://doi.org/10.2134/agronj1962.00021962005400050028x Chen X, Chen G, Chen L et al (2011) Adsorption of copper and zinc by biochars produced from pyrolysis of hardwood and corn straw in aqueous solution. Bioresour Technol 102:8877–8884. https://doi.org/10.1016/j.biortech.2011.06.078 Deus ACF, Büll LT, Guppy CN, Santos SdMC, Moreira LLQ (2020) Effects of lime and steel slag application on soil fertility and soybean yield under a no till-system. 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Soil Sci Soc Am J 84:618–626. https://doi.org/10.1002/saj2.20013 Supplementary Files SIPaperSupplementalInformation.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major revisions 14 Apr, 2026 Reviewers agreed at journal 10 Mar, 2026 Reviewers invited by journal 09 Mar, 2026 Editor invited by journal 02 Mar, 2026 Editor assigned by journal 02 Mar, 2026 First submitted to journal 25 Feb, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8972522","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":603246765,"identity":"dfcb49e2-4373-4889-b9b4-10f9d2f7236b","order_by":0,"name":"Amy Shober","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAwklEQVRIie3LsQqCUBTG8SOtVmsQ6StcudDiy5xwcNESWhwcnFpb7S18hBOBLjdc7xSF4OwTSJe2hri6Ndz/cDjD9wMwmf4yG2bqOgBoEXz+kYQrAtPILh9Nluf7tU3SR1jWARGkvrKaVnIf8EIc41J0SCBCPQFpb9fzE8aljBhZp5ueuI1QZMCQyUNP1jCCMIoUyRGZjICsfATxZMR5UaF3ER0jrEKuJU4jvDbJ0F3UwevZZ/5GS77DaXOTyWQy/eoN9dZEemazFLYAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-5490-6284","institution":"University of Delaware College of Agriculture and Natural Resources","correspondingAuthor":true,"prefix":"","firstName":"Amy","middleName":"","lastName":"Shober","suffix":""},{"id":603246766,"identity":"236d5600-3b14-41f1-a093-320381c7e752","order_by":1,"name":"Zhixuan Qin","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Zhixuan","middleName":"","lastName":"Qin","suffix":""},{"id":603246767,"identity":"e680071a-6f37-4483-aecf-a63c5b4031f1","order_by":2,"name":"Sapana Pokhrel","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Sapana","middleName":"","lastName":"Pokhrel","suffix":""},{"id":603246768,"identity":"e0be7f3d-9618-4960-b47d-fa3e2dabe200","order_by":3,"name":"Angelia L. 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Treatments shown here include Ca-Mg silicate slag, switchgrass char, and silicic acid at the 2 Mg/ha Si rates and the unamended control. Data represent means (n=3) for each soil. Error bars indicate standard error of the mean Note that y-axis scales differ among soil types to clearly visualize treatment effects within each soil.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8972522/v1/27d9f1b42877283f8184057d.png"},{"id":104486641,"identity":"28ed33e0-fa59-414d-a19b-a5726a77e6de","added_by":"auto","created_at":"2026-03-12 10:37:28","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":105678,"visible":true,"origin":"","legend":"\u003cp\u003eTemporal kinetics of acetic acid-extractable silicon (AA-Si) release in three legacy P impacted soils (Fort Mott, Ingleside, and Mullica-Berryland) during a 154-day laboratory incubation. Treatments shown here include Ca-Mg silicate slag, switchgrass char, and silicic acid at the 2.0 Mg/ha Si rates and the unamended control. Data represent means (n=3) for each soil series. Error bars indicate standard error of the mean. The split y-axis scale used in all panels to visualize the high soluble Si concentrations in the slag treatment relative to other sources. The break in the y-axis represents the range from 35–140 mg/kg\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8972522/v1/f8bd9df9c0b655d684c96b01.png"},{"id":104486643,"identity":"54fa99ad-a66b-438b-ba80-355ad552835a","added_by":"auto","created_at":"2026-03-12 10:37:28","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":249913,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between soluble silicon (acetic acid-extractable; AA-Si) and water-extractable phosphorus (WEP) in soils treated with (a-c) silicic acid (H\u003csub\u003e4\u003c/sub\u003eSiO\u003csub\u003e4\u003c/sub\u003e), (d-f) Ca-Mg silicate slag, or (g-i) switchgrass char during the laboratory incubation. Data points represent individual sample replicates across all sampling dates and application rates. Regression lines and correlation coefficients (R\u003csup\u003e2\u003c/sup\u003e) indicate a positive linear relationship in three legacy P impacted soil series (Fort Mott: \u003cem\u003ep\u003c/em\u003e \u0026lt; 0.001; Ingleside: \u003cem\u003ep\u003c/em\u003e \u0026lt; 0.01; Mullica: \u003cem\u003ep\u003c/em\u003e \u0026lt; 0.05), supporting the mechanism of competitive desorption of phosphate by silicate anions in the absence of calcium interference\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8972522/v1/9b85b83f2928d2a68b7755a5.png"},{"id":104486642,"identity":"1099cafb-4b5f-476b-b325-4827d092643a","added_by":"auto","created_at":"2026-03-12 10:37:28","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":122169,"visible":true,"origin":"","legend":"\u003cp\u003eTemporal kinetics of soil water-extractable phosphorus (WEP) mobilization in three legacy P impacted soils during a 239-day outdoor pot study. Panels represent (a) Fort Mott, (b) Ingleside, and (c) Mullica-Berryland soil series. Treatments included (1) a no Si control receiving only CaCO\u003csub\u003e3\u003c/sub\u003e to achieve pH 6.5; (2) Ca-Mg silicate slag applied at a rate to meet the soil liming requirement, providing Si at approximately 0.41, 0.41, and 0.68 Mg/ha for the Fort Mott, Ingleside, and Mullica-Berryland soils, respectively; (3) a \"Low\" silicic acid rate matched to the Si loading of the slag with CaCO\u003csub\u003e3\u003c/sub\u003e; (4) a \"Medium\" silicic acid rate at two times the Si loading of the slag with CaCO\u003csub\u003e3\u003c/sub\u003e; (5) a \"High\" silicic acid rate (2 Mg/ha Si) with CaCO\u003csub\u003e3\u003c/sub\u003e; and (6) silica gel (2 Mg/ha Si) with CaCO\u003csub\u003e3\u003c/sub\u003e. Data represent means (n=3) for each soil. Error bars indicate standard error of the mean\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8972522/v1/94375560b9ccad1d36730193.png"},{"id":104781270,"identity":"df6ded98-7543-4251-99ba-19406e233627","added_by":"auto","created_at":"2026-03-17 07:55:16","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":212421,"visible":true,"origin":"","legend":"\u003cp\u003eTemporal dynamics of available silicon fractions in the pot study. Panels (a–c) represent acetic acid-extractable silicon AA-Si), while panels (d–f) represent water-extractable silicon (WESi) across the Fort Mott, Ingleside, and Mullica-Berryland soil series, respectively. Note the high AA-Si concentrations for slag treatments contrasted with their negligible WESi concentrations, indicating limited mineral dissolution compared to silicic acid sources. Treatments included (1) a no Si control receiving only CaCO\u003csub\u003e3\u003c/sub\u003e to achieve pH 6.5; (2) Ca-Mg silicate slag applied at a rate to meet the soil liming requirement, providing Si at approximately 0.41, 0.41, and 0.68 Mg/ha for the Fort Mott, Ingleside, and Mullica-Berryland soils, respectively; (3) a \"Low\" silicic acid rate matched to the Si loading of the slag with CaCO\u003csub\u003e3\u003c/sub\u003e; (4) a \"Medium\" silicic acid rate at two times the Si loading of the slag with CaCO\u003csub\u003e3\u003c/sub\u003e; (5) a \"High\" silicic acid rate (2 Mg/ha Si) with CaCO\u003csub\u003e3\u003c/sub\u003e; and (6) silica gel (2 Mg/ha Si) with CaCO\u003csub\u003e3\u003c/sub\u003e. Data represent means (n=3) for each soil. Error bars indicate standard error of the mean\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8972522/v1/4cd5c0d5130d9ec06c70cf69.png"},{"id":106092889,"identity":"c74939c0-41c8-437e-882e-0dc7b20f14b1","added_by":"auto","created_at":"2026-04-03 11:29:29","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2230574,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8972522/v1/18e2c599-9965-40ae-b92d-66bbff74e7c8.pdf"},{"id":104486646,"identity":"67955aac-07ac-4c4a-8cd9-5a29ecf23448","added_by":"auto","created_at":"2026-03-12 10:37:28","extension":"docx","order_by":10,"title":"","display":"","copyAsset":false,"role":"supplement","size":1732065,"visible":true,"origin":"","legend":"","description":"","filename":"SIPaperSupplementalInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-8972522/v1/8e0d80e4eb48e9893cdd697b.docx"}],"financialInterests":"","formattedTitle":"Legacy Phosphorus Mobilization by Silicon Amendments: From Laboratory Mechanisms to Agronomic Effectiveness in Winter Wheat","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eIn regions characterized by intensive animal agriculture, such as the Delmarva Peninsula in the Mid-Atlantic United States, the long-term application of manure to agricultural land has fundamentally altered soil phosphorus (P) dynamics (Kleinman et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; McDowell et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). Historically, poultry litter was applied on Delmarva to meet crop nitrogen (N) requirements, resulting in P application rates that far exceed crop removal (Sharpley et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). This imbalance has led to the accumulation of anthropogenic P (i.e., \"legacy P\") in soils, with soil test P concentrations often exceeding the agronomic critical level (Shober et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Sims et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). While soil P accumulation poses a significant risk for non-point source pollution and eutrophication of sensitive water bodies like the Chesapeake Bay (Kleinman et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), a paradoxical challenge remains for producers: despite high soil test P, small grain crops such as winter wheat (\u003cem\u003eTriticum aestivum\u003c/em\u003e L.) frequently exhibit early-season P deficiency (Grant et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2001\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThis agronomic inefficiency is largely driven by soil chemistry and climate. Winter wheat is planted in late autumn when soil temperatures are declining, which reduces the diffusion rate of phosphate ions (Grant et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). Furthermore, many of these soils are naturally acidic Ultisols where P is strongly fixed by iron (Fe) and aluminum (Al) oxides (Sample et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e1980\u003c/span\u003e). To overcome these limitations, farmers typically apply inorganic starter P fertilizers (Grant et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). While effective for yield, this practice further enriches the soil P pool, exacerbating the long-term environmental risk. Therefore, there is an urgent need for best management practices (BMPs) that can mobilize existing legacy P pools to meet early season crop P demand without additional P inputs.\u003c/p\u003e \u003cp\u003eSilicon (Si) fertilization has emerged as a potential strategy to bridge this gap. Although not considered essential for higher plants, Si is beneficial, particularly for monocots like wheat, which accumulate Si in tissues to combat biotic and abiotic stress (Epstein \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Tuba\u0026ntilde;a and Heckman \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Chemically, the silicate anion (SiO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e4\u0026minus;\u003c/sup\u003e) and phosphate anion (PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e) share similar adsorption affinities for soil mineral surfaces (Koski-V\u0026auml;h\u0026auml;l\u0026auml; et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). The \"competitive exchange\" hypothesis suggests that increasing the concentration of soluble Si in the soil solution can displace sorbed P from Fe and Al oxides via ligand exchange, thereby increasing P bioavailability (Koski-V\u0026auml;h\u0026auml;l\u0026auml; et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Lee et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2004\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eHowever, the transition of this mechanism from the laboratory to the field has yielded inconsistent results. While some studies report enhanced P desorption and uptake following Si application (Lee et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Owino-Gerroh and Gascho 2005), others report no effect or even reduced P uptake (Gao et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Ma and Takahashi \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1990\u003c/span\u003e). These discrepancies likely stem from the confounding effects of soil pH and the chemical composition of Si sources. Specifically, the introduction of Ca from slags may immobilize desorbed P as Ca-phosphate precipitates, potentially masking the chemical benefits of Si. Common Si amendments, such as Ca silicate (CaSiO\u003csub\u003e3\u003c/sub\u003e) minerals or slags and calcium-magnesium (Ca-Mg) silicate slags, act as liming agents. The resulting pH increase can mobilize P by reducing Al toxicity and often results in a rapid dissolution of Si; however, the rapid release of soluble Si is followed by an abrupt decline in porewater Si that can result in precipitation of Si rather than plant uptake (Teasley et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Plus, the introduction of Ca with these sources can also immobilize P through the formation of Ca phosphate (Ca-P) precipitates (Haynes 1982; Sample et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e1980\u003c/span\u003e). Alternative Si amendments, such as amorphous silica gel or Si-rich plant residues (e.g., rice husk, straw) can serve as a non-liming source of soluble Si to soils; however, the availability of this Si to plants is governed by dissolution kinetics, which is affected by the source and its processing. For example, fresh Si-rich crop residues act as a slow-release, sustained source of Si that matches plant demand across the growing season. In contrast, combustion of these residues at high temperatures to form biochar can result in the formation of crystalline Si minerals (e.g., cristobalite), which effectively limits plant availability despite high total Si concentrations (Linam et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Teasley et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2017\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eBeyond its potential interactions with soil P, Si is known to enhance cereal crop yields by alleviating both biotic and abiotic stresses. In winter wheat specifically, Si deposition in the cuticle and epidermal cells provides structural rigidity that reduces lodging and susceptibility to fungal pathogens like powdery mildew (\u003cem\u003eBlumeria graminis\u003c/em\u003e; Provance-Bowley et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). Furthermore, Si fertilization has been shown to improve water use efficiency and biomass production under deficit irrigation or drought stress conditions in grasses (Eneji et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Industrial byproducts used as Si fertilizers, such as steel or blast furnace slags, also act as multi-nutrient amendments; unlike pure silicic acid, these materials supply significant quantities of Ca and Mg, while neutralizing soil acidity (Tozsin and Oztas \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The agronomic value of these slags may therefore result from a synergistic effect of improved Si nutrition, pH correction, and the addition of secondary macronutrients.\u003c/p\u003e \u003cp\u003eThis study employs a multi-scale approach, from laboratory to field, to disentangle the chemical potential of Si-driven P mobilization from the physiological and agronomic constraints that govern its efficacy in winter wheat production. We used three distinct \u0026ldquo;legacy P\u0026rdquo; impacted soils to evaluate Si efficacy across three phases: (1) a laboratory incubation to screen the chemical desorption potential of various Si sources (silicic acid, Ca-Mg silicate slag, and biomass char) without plant interference; (2) an in-field pot study designed to decouple the \"Si effect\" from the \"liming effect\" by maintaining stable conditions across treatments; and (3) an on-farm field trial to validate the agronomic viability of locally available Si amendments under realistic management conditions. We hypothesized that soluble Si sources would chemically displace sorbed P, increasing water-extractable P (WEP), and that this mobilization would translate to reduced reliance on starter P fertilizer for winter wheat.\u003c/p\u003e"},{"header":"MATERIALS AND METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSite Description and Soil Selection\u003c/h2\u003e \u003cp\u003eSoil samples were collected from three agricultural fields with a history of poultry litter application located in Sussex County, Delaware, USA. These soils represented major soils in the region: (1) a Fort Mott series (Loamy, siliceous, semiactive, mesic Arenic Hapludults); (2) an Ingleside series (Coarse-loamy, siliceous, semiactive, mesic Typic Hapludults); and (3) a Mullica-Berryland complex consisting of Mullica (Coarse-loamy, siliceous, semiactive, acid, mesic Typic Humaquepts) and Berryland (Sandy, siliceous, mesic Typic Alaquods) series.\u003c/p\u003e \u003cp\u003eBulk soil was collected from the Ap horizon (0\u0026ndash;15 cm) of each field using shovels. Approximately 0.21 m\u003csup\u003e3\u003c/sup\u003e of soil was excavated per site, air-dried, crushed to break clods, and passed through a 2-mm sieve. Soil samples were collected from the homogenized bulk soil prior to the incubation and pot experiments to establish baseline physical and chemical properties, and to determine lime requirements (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). To accommodate the experimental timeline, soils for the pot study were collected first; soils for the incubation were collected from the same locations eight months later.\u003c/p\u003e \u003cp\u003eThe outdoor pot study was located at the University of Delaware Newark Farm in Newark, Delaware, USA (39\u0026deg; 40' 5.91'' N, 75\u0026deg; 44' 45.83 W). For the field validation trial, a separate site was established at the University of Delaware Carvel Research and Education Center in Georgetown, Delaware, USA (38\u0026deg; 38' 7.13'' N, 75 27' 42.87 W). The soil at the field site was mapped as a Pepperbox loamy sand (Loamy, mixed, semiactive, mesic Aquic Arenic Hapludults).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003ePhase I: Laboratory Incubation\u003c/h3\u003e\n\u003cp\u003eA 154-day laboratory incubation was conducted to evaluate the kinetics of P desorption by Si amendments. The study used a completely randomized design (CRD) with three replicates. Treatments included an unamended control, and three Si sources applied at five rates (0, 0.25, 0.5, 1, and 2 Mg/ha). The Si sources were: (1) Ca-Mg silicate slag (AgrowSil; Harsco Minerals, Sarver, PA); (2) switchgrass char generated from the combustion of locally grown switchgrass (\u003cem\u003ePanicum virgatum\u003c/em\u003e) at the University of Maryland Wye Research and Education Center (Queenstown, Maryland, USA); and (3) laboratory grade silicic acid (SiO\u003csub\u003e2\u003c/sub\u003e\u0026bull;1.2 H\u003csub\u003e2\u003c/sub\u003eO; Sigma-Aldrich Co., St. Louis, MO, USA). The elemental composition of the switchgrass char and Ca-Mg silicate slag was determined via X-ray Fluorescence (XRF; Supermini200, Rigaku Americas, Inc., The Woodlands, TX, USA).\u003c/p\u003e \u003cp\u003ePrior to the initiation of the experiment, a subsample of the air-dried, sieved soil was analyzed to determine initial physical and chemical properties. Air-dried soil (230 g) was weighed into 240-mL polyethylene snap cups with holes drilled in the top to allow gas exchange; amendments were thoroughly mixed with the soil. Deionized water was added to bring the soil to 80% field capacity, as estimated in the lab using the method of Tan (\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e1996\u003c/span\u003e). Cups were snapped shut and incubated under ambient lab conditions. Soil moisture was maintained gravimetrically by weighing cups weekly and adding deionized water as needed.\u003c/p\u003e \u003cp\u003eDestructive soil sampling was conducted at 0, 2, 7, 14, 28, 56, 90, and 154 days after treatment application. Samples were air-dried and sieved (2 mm) prior to chemical analysis. Soil samples collected at all time points were analyzed for WEP and acetic acid extractable Si (AA-Si) to track release kinetics. In addition, samples collected at days 56 and 154 were analyzed pH, Mehlich-3 (M3) P, M3-Ca, and degree of P saturation based on M3 (DPS\u003csub\u003eM3\u003c/sub\u003e) to evaluate treatment effects on soil acidity and stable P pools.\u003c/p\u003e\n\u003ch3\u003ePhase II: In-Field Pot Study\u003c/h3\u003e\n\u003cp\u003eA pot study was conducted to evaluate plant uptake and yield response under pH-controlled conditions at an outdoor pot-in-pot facility at the University of Delaware Newark Farm. Switchgrass char was excluded from this phase due to low reactivity observed in the laboratory incubation. The experiment used a randomized complete block design (RCBD) with four replicates. To isolate the Si effect from the liming effect, all treatments (except the Ca-Mg silicate slag) received reagent-grade CaCO\u003csub\u003e3\u003c/sub\u003e to raise soil pH to a target of 6.5 to match the predicted pH of the slag treatment.\u003c/p\u003e \u003cp\u003eThe six treatments included: (1) a no Si control receiving only CaCO\u003csub\u003e3\u003c/sub\u003e; (2) Ca-Mg silicate slag applied at a rate to meet the soil liming requirement, providing Si at approximately 0.41, 0.41, and 0.68 Mg/ha for the Fort Mott, Ingleside, and Mullica-Berryland soils, respectively; (3) a \"Low\" silicic acid rate matched to the Si loading of the slag with CaCO\u003csub\u003e3\u003c/sub\u003e; (4) a \"Medium\" silicic acid rate at two times the Si loading of the slag with CaCO\u003csub\u003e3\u003c/sub\u003e; (5) a \"High\" silicic acid rate (2 Mg/ha) with CaCO\u003csub\u003e3\u003c/sub\u003e; and (6) silica gel (2 Mg/ha; Sigma-Aldrich Co., St. Louis, MO, USA) with CaCO\u003csub\u003e3\u003c/sub\u003e. Calcium carbonate (lime) rates (adjusted for 100 Ca carbonate equivalence were approximately 830 mg/kg for the Fort Mott and Ingleside soils and 1383 mg/kg for the Mullica-Berryland soil).\u003c/p\u003e \u003cp\u003ePlastic pots (12,452 cm\u003csup\u003e3\u003c/sup\u003e) were filled with 16.5 kg of air-dried soil. Treatments (Si source and CaCO\u003csub\u003e3\u003c/sub\u003e) were incorporated into the entire soil volume on 16 Nov 2015. Nitrogen, potassium (K) or manganese (Mn) fertilizers were also applied on 16 Nov 2015 based on University of Delaware recommendations for winter wheat to ensure non-limiting conditions (Shober et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Nitrogen was applied as ammonium sulfate to all soils at an N rate of 34.3 kg/ha. Potassium was applied as potassium chloride to Fort Mott and Ingleside soils at a K rate of 37.5 and 22.8 kg/ha, respectively. Manganese fertilizer was applied as manganese sulfate at a Mn rate of 22.8 kg/ha to the Mullica-Berryland soil only. Each pot was then thoroughly wetted and left fallow for a week to allow equilibrium of applied fertilizers and lime. Winter wheat was planted to achieve a seeding rate of 400 seeds m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (equal to 25 seeds per pot) in a greenhouse (to achieve rapid and uniform germination) on 24 Nov 2015. Once established (1 Dec 2015), pots were thinned to 12 plants per pot and moved to an outdoor, in-field pot-in-pot facility to simulate field temperature and photoperiod conditions. Ammonium sulfate was applied to all pots at an N rate of 66.9 and 34.3 kg/ha on 1 Apr 2016 and 2 May 2016, respectively to support late season growth. Plants were grown to physiological maturity (Feekes 11.4) and all aboveground biomass was harvested (24 Jun 2016). Harvested plant material was oven dried at 65\u0026deg;C, and weighed. Wheat heads were hand-threshed to separate grain. Grain yield was weighed directly, and straw biomass was calculated as the difference between the total aboveground biomass and the grain weight. Grain and straw subsamples were ground to pass a 1-mm sieve prior to analysis for Si and P.\u003c/p\u003e \u003cp\u003eInitial operational soil P pools were characterized using a modified Hedley fractionation (Sui et al. 1999) to determine the distribution of P between soluble (H\u003csub\u003e2\u003c/sub\u003eO-extractable), labile (NaHCO\u003csub\u003e3\u003c/sub\u003e-extractable), Fe/Al-bound (NaOH-extractable), and Ca-bound (HCl-extractable) fractions. Soil samples were collected to monitor nutrient dynamics throughout the growing season starting at 4 days after treatments were incorporated (20 Nov 2015). Subsequent sampling was conducted at 40, 80, 98, 129, 161, 189, and 239 days after amendments were applied using a 10-mL syringe to extract a composite core from each pot until harvest. All soil samples were analyzed for WEP, water extractable Si (WESi), and AA-Si to track P and Si release kinetics. Comprehensive chemical characterization (pH, M3-P, M3-Ca, and DPS\u003csub\u003eM3\u003c/sub\u003e) was performed on a subset of samples collected during early vegetative growth (Day 40) and post-harvest (Day 239).\u003c/p\u003e\n\u003ch3\u003ePhase III: Field Validation Trial\u003c/h3\u003e\n\u003cp\u003eAn on-farm trial was conducted at the University of Delaware Carvel Research and Education Center to validate the efficacy of the Ca-Mg silicate slag under commercial field conditions. The experimental design was a RCBD with four replicates and plot dimensions of 4.6-m \u0026times; 9.1-m.\u003c/p\u003e \u003cp\u003eTreatments included an unamended control, a standard agricultural lime treatment (3.36 Mg/ha calcitic lime), an agricultural lime with starter P treatment (3.36 Mg/ha calcitic lime\u0026thinsp;+\u0026thinsp;25 kg/ha P as triple superphosphate), and a Ca-Mg silicate slag treatment applied at 3.36 Mg/ha to meet the liming requirement to achieve pH 6.5 (providing approximately 0.40 Mg/ha Si).\u003c/p\u003e \u003cp\u003ePhosphorus was applied as triple superphosphate at a P rate of 25 kg/ha (lime with starter P treatment only) and potassium sulfate was applied at a K rate of 102 kg/ha prior to planting to ensure non-limiting conditions. Lime and Ca-Mg silicate slag treatments were surface applied on 2 Nov 2016. All materials were incorporated via tillage. Winter wheat (SS8340, Southern States Cooperative, Richmond, Virginia, USA) was planted the following day using a grain drill at a seeding rate of 123 kg/ha. Urea ammonium sulfate was applied at an N rate of 36.4 kg/ha at planting (3 Nov 2016).\u003c/p\u003e \u003cp\u003eEarly biomass samples were collected at green up (6 Mar 2017), dried at 60℃, and biomass was recorded. Ammonium sulfate was applied at an N rate of 54.9 kg/ha and thifensulfuron-methyl (Harmony SG) was applied at 63 g product ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e on 6 Mar 2017. Grain was harvested at maturity (approximately 15 Jun 2017) using a small plot combine (Massey Ferguson 8XP; Kincaid Equipment Manufacturing, Haven, Kansas, USA); yields were adjusted to 13.5% moisture content. A grain subsample was collected from each pot, dried at 65\u0026deg;C to constant weight, and analyzed for P and Si.\u003c/p\u003e \u003cp\u003eSoil samples (0\u0026ndash;15 cm) were collected at three distinct agronomic stages: pre-plant (16 Oct 2016), dormancy (21 Dec 2017), green-up (6 Mar 2017), and post-harvest (19 Jul 2017). Samples from all four time points were analyzed for pH, WEP, M3-P, and M3-Ca.\u003c/p\u003e\n\u003ch3\u003eLaboratory Analysis\u003c/h3\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eSoil analysis\u003c/h2\u003e \u003cp\u003eInitial soil characterization and experimental samples were analyzed using standard regional procedures. Particle size analysis was performed using the hydrometer method (Bouyoucos \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e1962\u003c/span\u003e), and organic matter was determined by loss on ignition (Schulte and Hoskins \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Soil pH was measured in a 1:1 (w:v) soil:water suspension; Adams-Evans buffer pH was also measured to estimate lime recommendations (Sims and Eckert \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Soils were extracted with M3 solution (1:10 ratio of soil to 0.2 M CH\u003csub\u003e3\u003c/sub\u003eCOOH\u0026thinsp;+\u0026thinsp;0.025 M NH\u003csub\u003e4\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;0.015 M NH\u003csub\u003e4\u003c/sub\u003eF\u0026thinsp;+\u0026thinsp;0.13 M HNO\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;0.001 M EDTA; 5-min reaction time; Wolf and Beegle \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), and concentrations of P, K, Ca, Mg, Mn, Fe, Al, and Si in extracts were analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES; Thermo iCAP 7600, Thermo Fisher Scientific, Waltham, Massachusetts, USA). The DPS\u003csub\u003eM3\u003c/sub\u003e was calculated as the molar ratio of M3-P to the sum of M3-Al and M3-Fe, expressed as a percentage (Maguire and Sims \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2002\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eWater extractable P and WESi (pot study only) were determined using a 1:10 soil:deionized water ratio shaken for 1 hour, filtered (0.45-\u0026micro;m; Self-Davis et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2009\u003c/span\u003e) and analyzed by spectrophotometer using the molybdate blue methods of Murphy and Riley (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1962\u003c/span\u003e) for P and Hallmark et al. (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1982\u003c/span\u003e) for Si. Acetic acid-extractable Si was determined using a 0.5 M acetic acid extraction (Kornd\u0026ouml;rfer et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2001\u003c/span\u003e), where 10 g of soil was shaken with 100 mL of acetic acid for 1 hour, filtered, and analyzed by ICP-OES. Incubation soils receiving Ca-Mg silicate slag and silicic acid at total Si rate of 2 Mg/ha sampled at 150 d were also evaluated for CaCl\u003csub\u003e2\u003c/sub\u003e-extractable Si (0.01 M CaCl\u003csub\u003e2\u003c/sub\u003e; 1h shaking; Kornd\u0026ouml;rfer et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1999\u003c/span\u003e) because these treatments are known to inflate the AA-Si concentrations (Nonaka and Takahashi \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Sauer et al. 2006; Snyder \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Wu et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePot study soil samples collected at harvest (Day 239) were also analyzed for P sorption characteristics to evaluate changes in P sorption capacity of soils following Si addition. Briefly, 2 g of air-dried soils were equilibrated with 30 mL of P solution (as KH\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e dissolved in 0.01 M CaCl\u003csub\u003e2\u003c/sub\u003e, with CaCl\u003csub\u003e2\u003c/sub\u003e being used to maintain a constant ionic environment) at P concentrations of 0, 0.1, 1, 5, 10, 35, and 50 mg/L. Samples were shaken for 24 h, centrifuged for 10 min, and filtered through 0.45-\u0026micro;m filters and the extracts were analyzed for P using ICP-OES. Phosphorus sorption data were fitted to the linearized Langmuir equation (Bolster and Hornberger \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2007\u003c/span\u003e) to estimate soil maximum sorption capacity of P (S\u003csub\u003emax\u003c/sub\u003e):\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\frac{C}{q}=\\frac{1}{(k\\times{S}_{max})}+\\frac{C}{{S}_{max}}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eC\u003c/em\u003e is the equilibrium concentration (mg/L), \u003cem\u003eq\u003c/em\u003e is the mass adsorbed (mg/kg), S\u003csub\u003emax\u003c/sub\u003e is the maximum sorption capacity (mg/kg), and \u003cem\u003ek\u003c/em\u003e is the binding affinity (L/mg).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003ePlant tissue analysis\u003c/h3\u003e\n\u003cp\u003eWheat grain and straw samples (pot study only) were dried at 60\u0026deg;C to constant weight and the mass was recorded; subsamples were ground to pass a 1-mm sieve. Sub-samples of straw and grain berries were analyzed separately for Si and P after microwave-assisted nitric acid digestion following the method described in Seyfferth et al. (\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Briefly, a 0.2 g subsample was digested using a modified nitric acid (HNO\u003csub\u003e3\u003c/sub\u003e) and hydrogen peroxide (H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) method with digests analyzed via ICP-OES for P and Si. For straw samples, the nitric acid digestions resulted in the formation of an acid-insoluble Si gel pellet, which was dissolved in 2 M NaOH; solutions were analyzed for Si using a spectrophotometer with the molybdenum blue method (Hallmark et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1982\u003c/span\u003e).\u003c/p\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eStatistical Analysis\u003c/h2\u003e \u003cp\u003eAll statistical analyses were performed using SAS software (Version 9.4; SAS Institute, Cary, NC). Data were analyzed separately for each soil series and experimental phase to evaluate treatment effects under different soil conditions. The Kenward-Roger adjustment was applied to calculate denominator degrees of freedom to handle the unbalanced nature of including controls within the factorial design. Normality and homoscedasticity of residuals were verified for all models using the Residual Panel graphical diagnostics. Outliers identified via studentized residuals (\u0026gt;\u0026thinsp;3\u0026times; standard deviation) were removed prior to analysis, as the magnitude of these values suggested a likely experimental artifact, such as soil contamination during extraction, weighing errors, or incomplete filtration, rather than a true treatment effect. Retaining extreme values violated the assumptions of normality and homogeneity of variance required for the ANOVA and regression analysis. Removing extreme outliers stabilized the variance and ensured the validity of the statistical comparisons.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eMixed Model Analysis for Soil Chemical Analysis, Plant Yields, and Plant Tissue Analysis\u003c/h2\u003e \u003cp\u003eSoil chemical properties were analyzed using a mixed model approach. For the incubation study, treatments consisted of a 3 \u0026times; 4 factorial combination of Si sources and rates, plus a non-amended control (13 total treatments). For the pot study, six treatments were evaluated, comprising five Si-amended groups (Ca-Mg silicate slag, silica gel, and three rates of silicic acid), and a non-Si amended control; the Si gel and silicic acid treatments also received CaCO\u003csub\u003e3\u003c/sub\u003e to match the lime rates of the Ca-Mg silicate slag. In all models, treatment, day (or date), and their interaction were considered fixed effects. To account for repeated measures on the same experimental units over time, a REPEATED statement was used with the subject defined as replicate(treatment). For the incubation study, AA-Si data were log-transformed to satisfy model assumptions.\u003c/p\u003e \u003cp\u003eThe covariance structure varied as appropriate to address the correlation between sampling dates and ensure model stability. A First-order autoregressive [AR(1)] covariance structure was used for time-series variables (WEP, WESi, AA-Si) to provide mathematical convergence while accounting for temporal autocorrelation. Compound symmetry (CS) was used for snapshot variables measured at limited intervals (pH, M3-P, M3-Ca, DPS\u003csub\u003eM3\u003c/sub\u003e).\u003c/p\u003e \u003cp\u003eWhen significant (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) treatment \u0026times; day interactions were observed for soil data, treatment effects were partitioned by day using the SLICE statement. Mean separations for both plant and soil data were performed using the Tukey-Kramer honestly significant difference (HSD) test. Letter groupings for mean separations at each sampling date (soil) or at harvest (plants) were generated using the PDMIX800 macro.\u003c/p\u003e \u003cp\u003ePlanned orthogonal contrasts were used to further tease apart main effects within the unbalanced design. For the incubation study, we constructed contrasts to compare Si sources overall (silicic acid vs. alternative sources), control vs. Si amended (average of all Si-amended treatments against non-amended controls), specific Si source comparisons at the 2 Mg/ha total Si rate, and individual Si sources rate response (0.25 vs. 2 Mg/ha). For the pot study, planned orthogonal contrasts included comparisons of control vs. all Si-amended, the source effect (silicic acid vs. Ca-Mg silicate slag), and the rate effect (low vs. high).\u003c/p\u003e \u003cp\u003eIsotherm derived parameters (S\u003csub\u003emax\u003c/sub\u003e and \u003cem\u003ek\u003c/em\u003e) from the pot study, as well as wheat grain yield and tissue nutrient concentrations (P and Si; pot and field studies), were analyzed using a mixed-model ANOVA (PROC MIXED). Treatment was treated as a fixed effect and replicate (block) was included as a random effect; however, where the random block variance was estimated as zero, the model automatically defaulted to a completely randomized design (CRD). Degrees of freedom were calculated using the Kenward-Roger adjustment. Mean separations were performed using the Tukey-Kramer adjustment (ɑ = 0.05) to maintain a conservative experiment-wise error rate.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eRegression Analysis\u003c/h2\u003e \u003cp\u003eLinear regression analysis (PROC REG) was used to quantify the relationship between Si (AA-Si or WESi) availability and WEP for the incubation and pot studies, with analyses performed independently for each soil series. To isolate the specific mechanism of competitive anion exchange, targeted regressions were conducted using the Si source groupings (e.g., silicic acid\u0026thinsp;+\u0026thinsp;control, Ca-Mg silicate slag\u0026thinsp;+\u0026thinsp;control, etc.). The coefficients of determination (R\u003csup\u003e2\u003c/sup\u003e) and slope significance were compared between these two fractions to determine whether the instantaneous solution-phase Si (WESi) or the total available pool (AA-Si) served as a more robust predictor of P availability.\u003c/p\u003e \u003c/div\u003e"},{"header":"RESULTS","content":"\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eInitial Soil Properties and Amendment Characterization\u003c/h2\u003e \u003cp\u003eSoils used in the incubation and pot studies were characterized as loamy sands with acidic pH values ranging from 5.0 to 5.7 (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). All three soils exhibited \"excessive\" M3-P concentrations, ranging from 171\u0026ndash;684 mg/kg, which is three to ten times the Delaware agronomic critical value of 50 mg/kg (Shober et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Consequently, the DPS\u003csub\u003eM3\u003c/sub\u003e was high (27\u0026ndash;110%), exceeding the environmental threshold of 15% established for Delaware soils (Sims et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). The Mullica-Berryland soil was distinct due to its higher organic matter content (42\u0026ndash;55 g/kg) compared to that of the Fort Mott (17\u0026ndash;20 g/kg) and Ingleside (20\u0026ndash;21 g/kg) soils, classifying it regionally as a \"black soil\" with higher buffering capacity. The Pepperbox loamy sand used in the field trial had a slightly lower, yet still excessive, M3-P concentration and a DPS\u003csub\u003eM3\u003c/sub\u003e above the 15% environmental threshold (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Sequential fractionation analysis of initial pot study soils indicated similar total extractable-P concentrations among all three soils (134\u0026ndash;188 mg/kg) with more than 50% of P extracted by NaOH, followed by NaHCO\u003csub\u003e3\u003c/sub\u003e-P, HCl-P, and H\u003csub\u003e2\u003c/sub\u003eO-P (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSelected initial physical and chemical properties of the soils used in the laboratory incubation, in-field pot study, and field validation trial. Extractable nutrients were determined via Mehlich-3 (M3) extraction. Degree of P saturation (DPS\u003csub\u003eM3\u003c/sub\u003e) was calculated as the molar ratio of M3-P / (M3-Al +M3 Fe).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePhase\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSoil Series\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003epH\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBuffer pH\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eM3-P (mg/kg)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eM3-Ca (mg/kg)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eM3-Al (mg/kg)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eM3-Fe (mg/kg)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eDPS\u003csub\u003eM3\u003c/sub\u003e​ (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIncubation\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFort Mott\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e189\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e225\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e714\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e137\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e21.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eIngleside\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e361\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e584\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e650\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e123\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e44.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMullica-Berryland\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e684\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e697\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1487\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e180\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e37.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePot Study\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFort Mott\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e171\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e520\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e508\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e27.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eIngleside\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e349\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e540\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e580\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e104\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e48.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMullica-Berryland\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e492\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e751\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1037\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e115\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e39.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eField Trial\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePepperbox\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e112\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e447\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e772\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e91\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e12.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eLime requirements for the pot and field studies were determined based on the initial pH\u003csub\u003e1:1\u003c/sub\u003e and Adams-Evans buffer pH (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). To isolate the effect of Si from the liming effect of the slag, calcitic lime rates were calculated to achieve a target pH of 6.5, matching the neutralization potential of the Ca-Mg silicate slag treatment (Shober et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eTotal elemental analysis (XRF) confirmed distinct chemical profiles among the Ca-Mg silicate slag and switchgrass char amendments (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Both the Ca-Mg silicate slag and switchgrass char contained similar concentrations of Si. The Ca-Mg silicate slag was dominated by Ca and Mg, consistent with its utility as a liming agent, and contained appreciable Fe, Al, and Mn compared to other sources. The slag material also contained appreciable concentrations of trace elements including chromium (Cr), nickel (Ni), and molybdenum (Mo). In contrast, the switchgrass char contained lower concentrations of Ca and negligible Fe, Al, and trace elements compared to the slag. The switchgrass char did contain higher concentrations of macronutrients P, K, and S than the slag. The silicic acid and silica gel sources were purchased as high-purity synthetic amorphous silica with negligible metal cation content.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eTotal elemental composition of Ca-Mg silicate slag and switchgrass char as determined by X-ray Fluorescence (XRF). Silicic acid and silica gel were technical grade compounds purchased from Sigma-Aldrich; values are stoichiometric estimates based on formula weight and an assumed purity of 99%.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElement (g/kg)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCa-Mg Silicate Slag\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSwitchgrass\u003c/p\u003e \u003cp\u003eChar\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSilicic Acid (H\u003csub\u003e4\u003c/sub\u003eSiO\u003csub\u003e2\u003c/sub\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSilica Gel (SiO\u003csub\u003e2\u003c/sub\u003e\u0026bull;1.2H\u003csub\u003e2\u003c/sub\u003eO)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e120.55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e118.74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e292\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e467\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e267.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e51.72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMg\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e97.47\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e19.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.58\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e25.49\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eK\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e11.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.84\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e8.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e69.46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e32.32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e18.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e13.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMn\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e9.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNb\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e8.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZn\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCu\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003ePhase I: Laboratory Incubation\u003c/h2\u003e \u003cdiv id=\"Sec16\" class=\"Section3\"\u003e \u003ch2\u003eWater Extractable Phosphorus Mobilization\u003c/h2\u003e \u003cp\u003eA significant treatment \u0026times; date interaction was reported for soil WEP across all three soils (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001). Phosphorus desorption dynamics were strongly influenced by the solubility and chemical composition of the Si source, total Si rate, and soil chemistry. The application of switchgrass char consistently led to a significant reduction in soil WEP concentrations compared with unamended control across all these soils (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e; Table S2). In the coarse-textured Fort Mott soil, WEP concentrations were highly dependent on both Si source and rate, silicic acid provided the greatest WEP mobilization at lower rates. However, the Ca-Mg silicate slag resulted in the highest soil WEP concentrations when applied at the 2 Mg/ha total Si rate (7.98 mg/kg) by the end of the incubation, significantly exceeding the silicic acid at the same rate (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.0001). In contrast, the silicic acid significantly outperformed the Ca-Mg silicate slag in the Ingleside and Mullica-Berryland soils. In fact, the Ca-Mg silicate slag often resulted in a decrease in soil WEP compared to the control (as was seen with switchgrass char), especially when applied at the higher total Si rates.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe treatment \u0026times; date interaction was significant for soil M3-P concentrations in the Ingleside soils (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.012) but not for the Fort Mott (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.28) or Mullica-Berryland (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.08) soils (Table S3); however, the treatment main effect was significant for these soils (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.007 and 0.009 for Fort Mott and Mullica-Berryland, respectively). By the end of the study, there were no significant differences in M3-P concentrations for any treatment in the Ingleside soils. Soil M3-P concentrations in the Fort Mott soil at Day 154 were highest for the control soil (238 mg/kg). For the Mullica-Berryland soil at Day 154, the soils receiving the silicic acid or switchgrass char at the 2 Mg/ha Si rate had significantly higher M3-P concentrations (\u0026gt;\u0026thinsp;845 mg/kg) than the control soils (752 mg/kg). All soil M3-P concentrations remained solidly above the agronomic optimum (50 mg/kg) throughout the study.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003eSoil WEP and Silicon Relationships\u003c/h2\u003e \u003cp\u003eA significant treatment \u0026times; date interaction was observed for AA-Si across all three soil series (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001). Consequently, the effects of Si amendments are presented by sampling date (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Silicic acid application resulted in a rapid increase in AA-Si, which remained elevated throughout the 154-day incubation for all three soils. Application of silicic acid significantly increased AA-Si concentrations relative to the control across all three soils (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). In the sandy Fort Mott and Ingleside soils, silicic acid (2 Mg/ha total Si) maintained AA-Si levels between 12.8 and 17.2 mg/kg between days 56 and 154. In contrast, Si release was more attenuated in the high-organic matter Mullica-Berryland soil, where AA-Si concentrations peaked at 11.7 mg/kg at the same application rate (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSwitchgrass char also increased AA-Si concentrations relative to the control, but soil-specific differences were evident. At the 2 Mg/ha Si rate, char application significantly increased AA-Si in all three soils. However, at the 1.0 Mg/ha Si rate, significant increases were observed only in the Fort Mott and Ingleside soils, whereas AA-Si levels in the Mullica-Berryland soil remained statistically similar to the control.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRegression analysis confirmed a significant positive linear relationship between AA-Si and WEP in soils treated with pure silicic acid (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.0001; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This relationship was strongest in the Fort Mott soils. In contrast, the relationship between AA-Si and WEP amended with Ca-Mg silicate slag was statistically significant, but nearly flat for the Fort Mott soil (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.0001) and negative for the Ingleside (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.0028) and Mullica-Berryland (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.0001) soils. Similarly, the relationship between AA-Si and WEP with switchgrass char application was not significant for the Fort Mott soils (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.32) and strongly negative for the Ingleside and Mullica-Berryland soils (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.001). In contrast, the relationship between soluble Si and WEP in soils treated with Ca-Mg silicate slag was inconsistent (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This reduction coincided with significant increases in soil pH and M3-Ca (Table S5).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003eImpact of Amendments on Soil pH\u003c/em\u003e and Mehlich 3-Ca\u003c/p\u003e \u003cp\u003eA significant treatment \u0026times; date interaction was reported for soil pH with all three soils (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001; Table S3). By day 154, the application of the Ca-Mg silicate slag at Si rates\u0026thinsp;\u0026gt;\u0026thinsp;1 Mg/ha rate resulted in significantly higher soil pH than for the control for all three soils; soil pH was 7.27, 7.4, and 5.73, respectively for Fort Mott, Ingleside, and Mullica-Berryland soils at the 2 Mg/ha Si rate, 6.7, 6.8, and 5.4 for the 1 Mg/ha Si rate, and 5.97, 5.90, and 4.73 for the control. Application of the Ca-Mg silicate slag at the 0.5 Mg/ha Si rate also increased soil pH (6.33) compared to control (5.97) for the Fort Mott soil.\u003c/p\u003e \u003cp\u003eA significant treatment \u0026times; date interaction was also reported for M3-Ca with all three soils (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.03; Table S3). By day 154, the M3-Ca concentrations were statistically higher than the control for all three soils when the Ca-Mg silicate slag was applied at rates of 0.5 Mg/ha total Si or higher; application of the Ca-Mg slag at the 0.25 Mg/ha Si rate also resulted in higher M3-Ca relative to control for the Fort Mott and Mullica-Berryland soils. Application of the switchgrass char at the 2 Mg/ha Si rate also increased soil M3-Ca relative to the control in the Ingleside and Mullica-Berryland soils (933 and 800 vs. 767 and 633 mg/kg, respectively)\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003ePhase II: In-field Pot Study\u003c/h2\u003e \u003cdiv id=\"Sec19\" class=\"Section3\"\u003e \u003ch2\u003eSoil Phosphorus Dynamics and Sorption Capacity (S\u003csub\u003emax\u003c/sub\u003e)\u003c/h2\u003e \u003cp\u003eThe treatment \u0026times; date interaction was significant for soil WEP across all three soils (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.007; Table S4). The effect on P solubility was heavily dependent on Si source (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In general, the highest rate of Si application (2 Mg/ha Si) was required to increase soil WEP compared to the control. Application of silicic acid at the high rate resulted in significantly higher soil WEP concentrations than the CaCO\u003csub\u003e3\u003c/sub\u003e control throughout the growing season in Fort Mott and Mullica soils (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In the Ingleside soil, silicic acid at the high Si rate increased WEP significantly at days 4 and 40, but differences were not significant compared to the control at later dates. The Ca-Mg silicate slag resulted in an immediate spike in WEP (Day 4) in Fort Mott and Ingleside soils (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05), increasing WEP by approximately 30% compared to the control, but this effect was transient and did not persist through the growing season (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Silica gel had variable influence on soil WEP compared to the control, with similar response as silicic acid at the high (2 Mg/ha) rate in the Mullica-Berryland soil (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.0001). In contrast, the effects of Si gel were better than the Ca-Mg silicate treatment, but less effective than the high rate silicic acid when applied to Ingleside soils; Si gel did not increase soil WEP in the Fort Mott soil.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAnalysis of P sorption isotherms at harvest revealed that Si amendments physically altered the soil\u0026rsquo;s capacity to bind P, but results were soil specific. In the Fort Mott soil, silicic acid (2 Mg/ha Si rate) significantly reduced the S\u003csub\u003emax\u003c/sub\u003e​ to 51 mg/kg, compared to 72 mg/kg in the control (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.0001; Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). A similar significant reduction in S\u003csub\u003emax\u003c/sub\u003e was observed in the Ingleside soil (31 mg/kg vs. 87 mg/kg in the control). However, in the high-organic matter Mullica soil, no significant treatment effects on S\u003csub\u003emax\u003c/sub\u003e​ were observed (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.77; Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), consistent with the lack of WEP response in the incubation phase. Unlike WEP, soil M3-P and DPS\u003csub\u003eM3\u003c/sub\u003e were largely unresponsive to the Si treatments (Table S5).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eEffect of silicon treatments on phosphorus sorption maxima (S\u003csub\u003emax\u003c/sub\u003e) and binding energy (\u003cem\u003ek\u003c/em\u003e) determined by Langmuir isotherms in the in-field pot study. Values represent means (SD) for n\u0026thinsp;=\u0026thinsp;3. Means within a column followed by the same letter within the same soil series are not significantly different (Tukey\u0026rsquo;s HSD, α\u0026thinsp;=\u0026thinsp;0.05).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSoil Series\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTreatment\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTotal Si Rate (Mg/ha)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eS\u003csub\u003emax\u003c/sub\u003e​ (mg/kg)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003ek\u003c/em\u003e (L mg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e \u003cp\u003eFort Mott\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eControl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e72.0 (2.7) b\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.358 (0.020) a\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCa-Mg Silicate Slag\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e84.4 (2.7) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.236 (0.015) b\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (Low)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e87.1 (0.5) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.175 (0.004) c\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (Med)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e63.3 (2.2) bc\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.271 (0.018) b\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (High)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e50.9 (2.3) c\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.313 (0.007) ab\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilica Gel\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e68.4 (4.6) b\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.178 (0.020) c\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e \u003cp\u003eIngleside\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eControl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e86.8 (3.5) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.080 (0.001) c\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCa-Mg Silicate Slag\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e43.1 (0.4) bc\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.256 (0.016) a\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (Low)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e31.2 (0.9) c\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.286 (0.035) a\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (Med)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e40.2 (0.8) bc\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.215 (0.009) ab\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (High)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e61.1 (18.6) b\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.103 (0.034) bc\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilica Gel\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e33.1 (2.5) c\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.206 (0.004) ab\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e \u003cp\u003eMullica-\u003c/p\u003e \u003cp\u003eBerryland\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eControl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e283.7 (7.9) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.136 (0.004) ab\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCa-Mg Silicate Slag\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e271.8 (4.9) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.152 (0.004) a\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (Low)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e289.7 (3.4) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.123 (0.003) ab\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (Med)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e282.6 (13.8) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.127 (0.018) ab\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (High)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e280.4 (4.7) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.109 (0.003) b\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilica Gel\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e332.3 (54.4) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.099 (0.018) b\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003eSoil Silicon Solubility\u003c/h2\u003e \u003cp\u003eTreatment effects on extractable silicon varied by extraction method (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). As reported for the soil incubation, AA-Si concentrations for the Ca-Mg silicate slag amended soils were inflated, consistently 2 to 3 times higher than AA-Si concentrations with silicic acid application (data not shown). The WESi treatment \u0026times; date interaction was significant (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.0046 and 0.015, respectively) for the Ingleside and Mullica-Berryland soils, but not significant for the Fort Mott soil (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.053; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Overall, both the Ca-Mg silicate slag and the silicic acid significantly increased WESi relative to the control (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.0001), but the magnitude depended on the source and rate (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The Ca-Mg silicate slag significantly increased WESi in all three soils (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.0001), with increases ranging from 2.8 to 5.6 mg/kg over the control. While silicic acid provided a rapid, rate-dependent increase in WESi (with the highest rates achieving\u0026thinsp;\u0026gt;\u0026thinsp;2\u0026times; the control levels), the Ca-Mg silicate slag was also an effective Si source, maintaining significantly elevated WESi levels comparable to the agronomic rates of silicic acid.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec21\" class=\"Section2\"\u003e \u003ch2\u003eSoil pH and Mehlich-3 Ca\u003c/h2\u003e \u003cp\u003eSoil pH and Mehlich-3 Ca (M3-Ca) concentrations responded to treatment application, with variations by soil type (Table S5). Soil pH management efficacy varied by soil type and time. In the Fort Mott soil, pH values were statistically equivalent among treatments at Day 40 (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.66), but significantly diverged by harvest (Day 209; \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.0001), with the slag treatment maintaining higher alkalinity than the CaCO\u003csub\u003e3\u003c/sub\u003e controls. In the Fort Mott soil, all Si-amended treatments resulted in soil pH values that exceeded the 6.5 target pH by the final harvest (Table S5). The Ca-Mg silicate slag treatment resulted in a soil pH of 7.2, which was significantly above the CaCO\u003csub\u003e3\u003c/sub\u003e control (6.5). Conversely, in the Ingleside soil, while differences were evident at Day 40 (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001), soil pH converged by harvest, resulting in no significant treatment differences (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.07). In the Mullica-Berryland soil, significant differences persisted at both sampling dates (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001), with the slag treatment consistently resulting in higher pH values than the CaCO\u003csub\u003e3\u003c/sub\u003e control.\u003c/p\u003e \u003cp\u003eIn the Fort Mott and Mullica soils, the treatment \u0026times; date interaction for M3-Ca was not significant (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.30 and 0.58, respectively; Table S5), while the main effect of treatment was significant (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.037 and 0.001, respectively). In these soils, slag and CaCO\u003csub\u003e3\u003c/sub\u003e applications resulted in higher M3-Ca concentrations averaged across sampling dates compared to the CaCO\u003csub\u003e3\u003c/sub\u003e only control. In the Ingleside soil, M3-Ca concentrations also differed by treatment (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.013), with no significant interaction with time (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.22).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec22\" class=\"Section2\"\u003e \u003ch2\u003eCrop Yield and Nutrient Uptake\u003c/h2\u003e \u003cp\u003eDespite significant increases in soil P solubility (as evidenced by soil WEP) and reductions in sorption capacity (S\u003csub\u003emax\u003c/sub\u003e) in silicic acid treatments, there were no significant differences in winter wheat grain yield or straw biomass between Si treatments and the CaCO\u003csub\u003e3\u003c/sub\u003e control across the three soil types (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.05; Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The only exception was a significant increase in straw biomass for the silica gel treatment in the Mullica-Berryland soil compared to the CaCO\u003csub\u003e3\u003c/sub\u003e control. Similarly, tissue P concentrations in grain and straw were not significantly affected by Si treatment (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eTotal P removal (grain\u0026thinsp;+\u0026thinsp;straw) did not differ between the CaCO\u003csub\u003e3\u003c/sub\u003e control and Si treatments (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.05). However, plant Si uptake was highly responsive; wheat straw Si concentrations were significantly higher in all Si-amended treatments compared to the control. While the high-rate silicic acid treatment resulted in the highest straw Si concentrations, it can be noted that plants grown in pots receiving the Ca-Mg silicate slag treatment had straw Si concentrations that were 3\u0026ndash;5 fold higher than the lime only control (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eWheat grain yield, straw biomass, and tissue nutrient concentrations from the outdoor pot study. Means (standard deviation) for n\u0026thinsp;=\u0026thinsp;3 within a column followed by the same letter are not significantly different (Tukey\u0026rsquo;s HSD, α\u0026thinsp;=\u0026thinsp;0.05).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSoil Series\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTreatment\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTotal Si Rate (Mg/ha)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eGrain Yield (g/pot)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eStraw Biomass (g/pot)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eGrain P (mg/kg)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eGrain Si (mg/kg)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eStraw P (mg/kg)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eStraw Si (mg/kg)\u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e \u003cp\u003e\u003cb\u003eFort Mott\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eControl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e26.50 (1.48)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e117.9 (4.3) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1421 (647)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e16 (8) b\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e684 (135) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e2741 (392) c\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSlag\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e23.17 (2.34)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e120.6 (3.1) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2210 (263)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e101 (37) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e656 (92) b\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e8378 (756) b\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (Low)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e21.47 (3.59)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e109.2 (2.8) b\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2272 (85)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e68 (52) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e793 (73) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e8344 (481) b\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (Med)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e22.64 (2.59)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e116.0 (2.5) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1567 (692)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e28 (14) b\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e990 (73) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e18259 (2570) a\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (High)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e19.51 (5.21)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e115.3 (3.4) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2402 (228)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e41 (10) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e889 (180) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e11151 (1275) b\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilica Gel\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e21.61 (1.91)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e116.8 (2.7) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2250 (71)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e46 (11) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e845 (118)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e15205 (603) a\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e \u003cp\u003e\u003cb\u003eIngleside\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eControl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e22.08 (2.82)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e114.4 (3.2)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2652 (394)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e20 (14)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1195 (54)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e1466 (344) d\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSlag\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e20.08 (1.26)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e117.8 (4.1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2475 (191)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e35 (7)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1247 (67)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e7143 (757) cd\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (Low)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e20.09 (3.13)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e114.6 (3.5)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2735 (60)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e29 (2)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1373 (355)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e6501 (1104) cd\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (Med)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e23.51 (3.24)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e122.8 (3.8)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2342 (428)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e36 (14)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1624 (571)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e16634 (4575) a\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (High)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e25.80 (3.08)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e120.7 (4.5)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2294 (110)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e36 (5)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1213 (233)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e9560 (2998) bc\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilica Gel\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e22.75 (0.27)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e122.6 (3.9)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2385 (269)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e41 (1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1173 (212)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e15361 (2188) ab\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e \u003cp\u003e\u003cb\u003eMullica-\u003c/b\u003e\u003c/p\u003e \u003cp\u003e\u003cb\u003eBerryland\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eControl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e24.75 (3.62)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e117.1 (2.9) b\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2469 (295)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e19 (3) b\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1150 (352)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e1231 (115)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSlag\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e24.47 (3.49)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e122.7 (3.6) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3367 (1361)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e57 (17) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1035 (132)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e9626 (150)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (Low)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e27.17 (3.46)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e126.1 (1.2) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2652 (245)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e27 (2) b\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1010 (158)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e5525 (120)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (Med)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e23.76 (0.39)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e127.5 (1.3) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2429 (733)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e38 (15) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1037 (44)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e12292 (160)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicic Acid (High)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e22.70 (0.95)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e121.3 (1.0) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2362 (197)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e32 (4) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1060 (143)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e11303 (190)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilica Gel\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e28.94 (6.27)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e131.3 (1.4) a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2296 (109)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e35 (8) ab\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1080 (187)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e9516 (170)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003csup\u003ea\u003c/sup\u003eStatistical analysis (Tukey\u0026rsquo;s HSD) was not performed for straw Si in the Mullica-Berryland soil series due to insufficient sample size (n\u0026thinsp;\u0026lt;\u0026thinsp;6) resulting from missing values.\u003c/p\u003e \u003cdiv id=\"Sec23\" class=\"Section3\"\u003e \u003ch2\u003ePhase III: On-Farm Field Trial\u003c/h2\u003e \u003cdiv id=\"Sec24\" class=\"Section4\"\u003e \u003ch2\u003eField Soil and Crop Response\u003c/h2\u003e \u003cp\u003eIn the field trial on Pepperbox loamy sand, no treatment \u0026times; date interaction (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.39) on soil pH was observed. Soils receiving the Ca-Mg silicate slag, calcitic lime only, and lime\u0026thinsp;+\u0026thinsp;starter P treatments had a significantly higher pH (6.13, 6.13, and 6.07, respectively) than the control soils (pH\u0026thinsp;=\u0026thinsp;5.83; \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.0003; Table S6).\u003c/p\u003e \u003cp\u003eDespite differences in soil pH, there was no significant treatment effect on soil P dynamics. The treatment \u0026times; date interaction was highly non-significant (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.936), indicating the treatments behaved consistently (and indistinguishably) throughout the season. Mean soil WEP concentrations did not differ significantly between the Ca-Mg silicate slag treatment (4.34 mg/kg), standard calcitic lime (4.62 mg/kg), lime with starter P (5.27 mg/kg), or control (5.32 mg/kg) treatments across the study (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.066; Table S6). Similarly, treatments had no effect on M3-P (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.40) or DPS\u003csub\u003eM3\u003c/sub\u003e (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.34) among treatments at either sampling date (Table S6).\u003c/p\u003e \u003cp\u003eMehlich-3 Ca concentrations showed a numerical increase in the slag-amended plots (reaching 733 mg/kg at harvest vs. 439 mg/kg in the control), resulting in a significant pairwise difference by the end of the season (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.036), although the overall treatment main effect for the trial was not significant (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.12; Table S6).\u003c/p\u003e \u003cp\u003eDespite similar soil pH and M3-P concentrations by the end of the study, we noted a significant treatment effect on wheat yield (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.0049), where the slag-amended soils had significantly higher yields (8.25 Mg/ha) than all other treatments (mean\u0026thinsp;=\u0026thinsp;7.35 Mg/ha). No significant differences in early-season vegetative biomass (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.64) were observed among treatments (Table S7). Furthermore, while tissue Si concentrations at the early green-up stage were significantly higher in the Ca-Mg silicate slag treatment (1.85 g/kg) compared to the calcitic lime treatment (1.72 g/kg), this difference was not significant at harvest (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.71). There were no significant treatment effects on grain P or Si concentrations (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.62)\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"DISCUSSION","content":"\u003cdiv id=\"Sec26\" class=\"Section2\"\u003e \u003ch2\u003eCompetitive Desorption vs. Precipitation as the Mechanisms of P Mobilization\u003c/h2\u003e \u003cp\u003eOur results confirm that Si amendments can mobilize legacy P in acidic agricultural soils, but the magnitude and direction of this effect are strictly governed by the chemical composition of the amendment. The strong positive linear relationship between AA-extractable Si and WEP in incubation soils treated with pure silicic acid (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.60 for Fort Mott; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) supports the \"competitive exchange\" hypothesis, where silicate anions (H\u003csub\u003e3\u003c/sub\u003eSiO\u003csup\u003e4\u0026minus;\u003c/sup\u003e) displace phosphate (H\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) from specific sorption sites on Fe and Al oxides via ligand exchange (Koski-V\u0026auml;h\u0026auml;l\u0026auml; et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Lee et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2004\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eYet, the discrepancy between AA-Si and WESi results in the pot study explains why the Ca-Mg silicate slag failed to mobilize P. While AA-Si indicated a large pool of 'available' Si in the slag treatment (approximately 44\u0026ndash;54 mg/kg higher than control), the WESi data, which more accurately reflects the soil solution equilibrium, showed no significant increase over the control (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In contrast, the AA-Si concentrations in silicic acid-amended soils were only 3\u0026ndash;4 times higher than the WESi concentrations. These results confirm that the Si applied in the Ca-Mg slag amendment was unreacted in the soil. Without a sustained elevation in dissolved H\u003csub\u003e3\u003c/sub\u003eSiO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, the competitive exchange with H\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e could not occur. This validates that AA-Si overestimates the agronomically active Si pool in slag-amended soils, acting as a methodological artifact rather than a predictor of P mobilization (Wu et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eCritically, the P sorption isotherm data (S\u003csub\u003emax\u003c/sub\u003e) from our pot study provides physical validation of this mechanism in a plant-soil system. The significant reduction in S\u003csub\u003emax\u003c/sub\u003e observed in Fort Mott and Ingleside soils treated with silicic acid (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) indicates that Si anions successfully occupied binding sites that would otherwise be available for P sorption. By physically blocking these sites, soluble Si shifted the equilibrium toward the solution phase. This aligns with findings by Schaller et al. (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), who observed that increasing Si availability mobilized P from Fe minerals in Arctic soils. Our results also corroborate the model of competitive adsorption described by Obihara and Russell (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1972\u003c/span\u003e) and Hiemstra et al. (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2007\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eHowever, the efficacy of the Ca-Mg silicate slag source was compromised by a \"Ca penalty.\" Unlike pure silicic acid, the slag lime alternative contained significant quantities of Ca and Mg in addition to Si (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The significant increase in M3-Ca observed in the incubation (Table S3) supports the formation of stable Ca-P precipitates. In the Mullica and Ingleside soils, where slag reduced WEP, the high Ca loading likely shifted the equilibrium toward the formation of Ca-P minerals or the formation of Ca-P-Fe ternary complexes that outweighed the desorption pressure of the silicate anion (Haynes 1982; Penn et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Ma and Takahashi (1991) similarly reported that while sodium silicate increased pH and Si, it did not always increase P availability in P-deficient soils due to complex soil chemical interactions. Our data suggests that in \"legacy P\" soils, the addition of Ca-rich silicate amendments may inadvertently stabilize soil P pools rather than mobilize them, effectively acting solely as a liming agent rather than a P-mobilizer.\u003c/p\u003e \u003cdiv id=\"Sec27\" class=\"Section3\"\u003e \u003ch2\u003ePhosphorus Sorption by Charred Biomass\u003c/h2\u003e \u003cp\u003eThe consistent reduction in WEP following switchgrass char application indicates that thermal processing of biomass can negate the benefits of phytogenic Si. Despite adding total Si to the system, the char acted as a net P sink. This is likely due to the increased surface area and porosity generated during combustion, a phenomenon well-documented in biochar literature where surface functional groups and porous structures sorb phosphate (Chen et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Park et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). This finding is critical for defining BMPs; while returning crop residues to soil is a primary pathway for Si recycling (Puppe et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Vandevenne et al. \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2012\u003c/span\u003e), our results suggest that incorporating charred residues may exacerbate P fixation in the short term compared to the use of un-charred phytolith-rich residues that release Si more readily (Seyfferth et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec28\" class=\"Section2\"\u003e \u003ch2\u003eMethodological Artifacts in Soil Silicon Testing\u003c/h2\u003e \u003cp\u003eOur study highlights a significant methodological limitation in assessing plant-available Si in amended soils. The AA-Si overestimated Si availability in slag-amended soils by up to 24-fold compared to the CaCl\u003csub\u003e2\u003c/sub\u003e extraction in the incubation study; a result that was also noted by Wu et al. (\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) for soils amended with Ca silicate materials. Similarly, the WESi concentrations for slag-amended soils in the pot study were typically 10-fold lower than AA-Si concentrations in the same soils (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Acetic acid is aggressive enough to dissolve non-labile Ca silicates in the slag that are not available to plants under ambient soil pH conditions (Nonaka and Takahashi \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Snyder \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). Conversely, the 0.01 M CaCl\u003csub\u003e2\u003c/sub\u003e extraction provided values that were consistent between source types and aligned with thermodynamic solubility limits. This confirms that for agronomic consulting, particularly where slag amendments are used, dilute salt extractions provide a more accurate index of the active monosilicic acid pool than acid-based extractants (Miles et al. 2014; Wu et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec29\" class=\"Section2\"\u003e \u003ch2\u003eRegulation of P Uptake Due to Physiochemical Disconnect\u003c/h2\u003e \u003cp\u003eThe transition from the laboratory to the pot study revealed a consistent decoupling of soil solution chemistry (WEP) from plant physiology (yield and P uptake). Silicic acid applied at the 2 Mg/ha total Si rate increased WEP by up to 70% and reduced soil sorption capacity (S\u003csub\u003emax\u003c/sub\u003e) in the pot study, yet this did not translate to increased biomass or total P uptake in winter wheat. We propose that this lack of response is not a failure of the amendment, but a physiological regulation mechanism described by Ma and Takahashi (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1990\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eMa and Takahashi (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1990\u003c/span\u003e) observed that while Si promotes P uptake in low-P conditions, it can suppress excessive P uptake in high-P environments to prevent P toxicity and maintain favorable intracellular inorganic P levels. The soils in our study were agronomically \"excessive\" in P (M3-P\u0026thinsp;\u0026gt;\u0026thinsp;100 mg/kg), well above the agronomic critical level for winter wheat (Shober et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Therefore, the wheat plants likely engaged homeostatic mechanisms to limit P influx, rendering the Si-mobilized P redundant for biomass production (Neu et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Schaller et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2012\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAlthough the pot study was conducted outdoors under ambient weather conditions, the soil environment was highly optimized. Fertilizers were uniformly incorporated to ensure non-limiting conditions, and the soils were thoroughly homogenized. Silicon is well-documented to enhance crop yield primarily by alleviating severe abiotic and biotic stresses, such as drought, nutrient imbalances, or fungal pathogens (Eneji et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Provance-Bowley et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). The optimal moisture conditions during the 2015\u0026ndash;2016 growing season, coupled with nutrient management in the pot study likely masked the potential stress-mitigation benefits of the applied Si. Additionally, the thorough mixing of liming agents in the confined volume of the pots may have exacerbated chemical interactions between Ca and P, further limiting agronomic responses compared to a heterogeneous field environment.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eAgronomic Implications\u003c/h3\u003e\n\u003cp\u003eWhile the Si-driven P mobilization mechanism is chemically valid, harnessing it for yield gain in legacy P soils is challenging. The transition from mechanistic pot studies to field validation trials reveals that soil buffering and environmental variability often diminish the temporary P solubilization effects observed in the lab. In our pot study, although Si amendments nearly doubled straw Si concentrations, this accumulation did not translate into higher grain yield under the buffered, low-stress conditions of the nested pot design.\u003c/p\u003e \u003cp\u003eConsequently, Ca-Mg silicate slag should be viewed primarily as an effective liming agent and a source of plant-available Si for structural fortification (Epstein 1999; Provance-Bowley et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), rather than a reliable tool for \"mining\" legacy soil P. While slag provided a significant yield advantage in the field (likely by preserving yield potential against regional biotic stressors like powdery mildew), our data suggests a plant-health response rather than a P-mobilization response. For producers, this implies that while Si amendments enhance tissue Si and resistance to lodging (Rodrigues and Datnoff \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2005\u003c/span\u003e), they do not justify the elimination of starter P fertilizers.\u003c/p\u003e \u003cp\u003eFurthermore, the physical scale and chemical reactivity of amendments dictate their efficacy. In the pot study, lab homogenization with highly reactive CaCO\u003csub\u003e3\u003c/sub\u003e created a \"Ca penalty,\" where sudden influxes of Ca likely neutralized localized P desorption through rapid Ca-P precipitation. Conversely, the coarse heterogeneity of the field trial allowed slag to neutralize soil acidity and deliver Si more effectively without widespread Ca-P precipitation.\u003c/p\u003e \u003cp\u003eSimilarly, the increased P sorption following incorporation of switchgrass char indicates that crop residues are unlikely to be a viable amendment for P solubilization. Finally, while silicic acid and silica gel showed the most promise for P mobilization in the lab, they remain cost-prohibitive for field-scale application. Moreover, evidence from our pot study suggests that high concentrations of silicic acid may induce physiological responses in small grains that negate the benefits of increased soil P solubility. As such, there are currently no viable amendments available to growers that simultaneously maximize soluble Si and enhance legacy soil P availability. Future research should focus on intermediate Si sources, such as amorphous silica-rich crop residues (Seyfferth et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), which may offer a middle ground between the high solubility of silicic acid and the heavy Ca-loading of industrial slags.\u003c/p\u003e \u003cdiv id=\"Sec31\" class=\"Section2\"\u003e \u003ch2\u003eEnvironmental and Safety Considerations\u003c/h2\u003e \u003cp\u003eDespite its agronomic efficacy as a value-added liming agent in grain production systems, the use of industrial byproducts like Ca-Mg silicate slag in production agriculture is not without potential environmental concern. Chemical characterizations of the Ca-Mg silicate slag used in this study (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) and related mineral byproducts indicate highly variable concentrations of trace metals, including Cr, Ni, and Mo (Qin \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Torlon \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The liming effect of the slag can result in immobilization of cationic heavy metals, thereby reducing their phytoavailability and translocation to grain (Deus et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), anionic elements such as Mo can become increasingly plant-available at higher soil pH. Given the measurable Mo content in the evaluated slag (\u0026asymp;\u0026thinsp;107 mg kg⁻\u0026sup1;), there is a distinct risk of Mo accumulation in plant tissues. As such, land managers should use caution when applying slag-based lime alternatives to forage or pasture crops where high Mo forage can induce molybdenosis (Cu deficiency) in grazing ruminants. Consequently, the use of steel slag lime alternatives is better suited to grain production systems where soils need pH adjustment and residues (straw) are incorporated into the soil or removed after grain harvest.\u003c/p\u003e \u003c/div\u003e"},{"header":"CONCLUSION","content":"\u003cp\u003eWhile Si can chemically mobilize legacy soil P, this mechanism does not translate into a viable agronomic strategy for winter wheat. Biological homeostasis, environmental variability, and the \"Ca penalty\" of common agricultural amendments effectively neutralize P-desorption at the field scale. Therefore, Si should not be promoted as a tool to mine legacy P or replace starter fertilizers.\u003c/p\u003e \u003cp\u003eInstead, the agronomic value of Ca-Mg silicate slag lies in its performance as a highly soluble, multi-nutrient liming agent that supplies plant-available Si to preserve yield under environmental stress. However, using these industrial byproducts requires rigorous batch testing due to variable trace metal content. Specifically, the risk of Mo-induced toxicity restricts their safe application strictly to grain production systems, precluding their use in pastures or forage. Future research on Si-driven P mobilization should pivot toward Ca-free, intermediate Si sources (e.g., silica-rich plant residues) to sustainably deliver soluble Si without the confounding effects of Ca interference or heavy metal contamination.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eAA-Si, acetic acid-extractable silicon; BMP, best management practice; DPS\u003csub\u003eM3\u003c/sub\u003e, degree of phosphorus saturation; M3, Mehlich-3; WEP, water extractable phosphorus.\u003c/p\u003e"},{"header":"STATEMENTS \u0026 DECLARATIONS","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements:\u003c/strong\u003e The authors thank Mr. Shawn Tingle for assistance with soil collection and Ms. Karen Gartley and the University of Delaware soil testing program staff for assistance with analysis. The authors also wish to thank Dr. Nicole Fiorellino from University of Maryland – College Park for her insightful review of our manuscript draft.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e This work was supported by a USDA Sustainable Agriculture Research and Education (SARE) graduate student grant (GNE15-111).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests:\u003c/strong\u003e The authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions:\u003c/strong\u003e A. Shober, Z. Qin, and A. Seyfferth contributed to the study conception and design. Material preparation and data collection were performed by Z. Qin and L. Mosesso. Data analysis was performed by Z. Qin, A. Shober, and S. Pokhrel. The first draft was written by Z. Qin and A. Shober. All authors commented on previous versions and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability:\u003c/strong\u003e The datasets generated during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eBolster CH, Hornberger GM (2007) On the use of linearized Langmuir equations. Soil Sci Soc Am J 71:1796\u0026ndash;1806. https://doi.org/10.2136/sssaj2006.0304\u003c/li\u003e\n \u003cli\u003eBouyoucos GJ (1962) Hydrometer method improved for making particle size analyses of soils 1. Agron J 54:464-465. https://doi.org/10.2134/agronj1962.00021962005400050028x\u003c/li\u003e\n \u003cli\u003eChen X, Chen G, Chen L et al (2011) Adsorption of copper and zinc by biochars produced from pyrolysis of hardwood and corn straw in aqueous solution. Bioresour Technol 102:8877\u0026ndash;8884. https://doi.org/10.1016/j.biortech.2011.06.078\u003c/li\u003e\n \u003cli\u003eDeus ACF, B\u0026uuml;ll LT, Guppy CN, Santos SdMC, Moreira LLQ (2020) Effects of lime and steel slag application on soil fertility and soybean yield under a no till-system. Soil Tillage Res 196:104422. https://doi.org/10.1016/j.still.2019.104422\u003c/li\u003e\n \u003cli\u003eEneji AE, Inanaga S, Muranaka S, Li J, Hattori T, An P, Tsuji W (2008) Growth and nutrient use in four grasses under drought stress as mediated by silicon fertilizers. J Plant Nutr 31:355\u0026ndash;365. https://doi.org/10.1080/01904160801895035\u003c/li\u003e\n \u003cli\u003eEpstein E (1994) The anomaly of silicon in plant biology. Proc Natl Acad Sci 91:11\u0026ndash;17. https://doi.org/10.1073/pnas.91.1.11\u003c/li\u003e\n \u003cli\u003eGao X, Zou C, Wang L, Zhang F (2005) Silicon improves water use efficiency in maize plants. J Plant Nutr 27:1457\u0026ndash;1470. https://doi.org/10.1081/PLN-200025865\u003c/li\u003e\n \u003cli\u003eGoos RJ, Johnson BE (2001) Response of spring wheat to phosphorus and sulphur starter fertilizers of differing acidification potential. J Agric Sci 136:283\u0026ndash;289. https://doi.org/10.1017/S0021859601008711\u003c/li\u003e\n \u003cli\u003eGrant CA, Flaten DN, Tomasiewicz DJ, Sheppard SC (2001) The importance of early season phosphorus nutrition. Can J Plant Sci 81:211\u0026ndash;224. https://doi.org/10.4141/P00-093\u003c/li\u003e\n \u003cli\u003eHallmark, C. T., Wilding, L. P., \u0026amp; Smeck, N. E. (1982). Silicon. In A. L. Page, R. H. Miller, \u0026amp; D. R. Keeney (Eds.), Methods of soil analysis, Part 2 (2nd ed., pp. 263\u0026ndash;273). American Society of Agronomy. https://doi.org/10.2134/agronmonogr9.2.2ed.c15\u003c/li\u003e\n \u003cli\u003eHiemstra T, Barnett MO, van Riemsdijk WH (2007) Interaction of silicic acid with goethite. 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Commun Soil Sci Plant Anal 42:633-644. doi: 10.1080/00103624.2011.550374\u003c/li\u003e\n \u003cli\u003ePiatak NM, Parsons MB, Seal RR (2015) Characteristics and environmental aspects of slag: A review. Appl Geochem 57:236\u0026ndash;266. https://doi.org/10.1016/j.apgeochem.2014.04.009\u003c/li\u003e\n \u003cli\u003ePuppe D, Kaczorek D, Schaller J, Barkusky D, Sommer M (2021) Crop straw recycling prevents anthropogenic desilication of agricultural soil-plant systems in the temperate zone \u0026ndash; Results from a long-term field experiment in NE Germany. Geoderma 403:115187. https://doi.org/10.1016/j.geoderma.2021.115187\u003c/li\u003e\n \u003cli\u003eProvance-Bowley MC, Heckman JR, Durner EF (2010) Calcium silicate suppresses powdery mildew and increases yield of field grown wheat. Soil Sci Soc Am J 74:1652-1661. https://doi.org/10.2136/sssaj2010.0134\u003c/li\u003e\n \u003cli\u003eQin Z (2017) Strategies for management of high phosphorus agricultural soils on the Delmarva Peninsula. Dissertation, University of Delaware.\u003c/li\u003e\n \u003cli\u003eRodrigues FA, Datnoff LE (2005) Silicon and rice disease management. Fitopatol Bras 30:457\u0026ndash;469. https://doi.org/10.1590/S0100-41582005000500001\u003c/li\u003e\n \u003cli\u003eSample EC, Soper RJ, Racz GJ (1980) Reactions of phosphate fertilizers in soils. In: Khasawneh FE et al (eds) The Role of Phosphorus in Agriculture. ASA and SSSA, Madison, WI, pp 263-310. https://doi.org/10.2134/1980.roleofphosphorus.c12\u003c/li\u003e\n \u003cli\u003eSAS Institute (2008) SAS/STAT 9.2 user\u0026rsquo;s guide. SAS Institute, Cary, NC\u003c/li\u003e\n \u003cli\u003eSchaller J, Brackhage C, Gessner MO, B\u0026auml;uker E, Dudel EG (2012) Silicon supply modifies C:N:P stoichiometry and growth of Phragmites australis. Plant Biol 14:392\u0026ndash;396. https://doi.org/10.1111/j.1438-8677.2011.00537.x\u003c/li\u003e\n \u003cli\u003eSchaller J, Faucherre S, Joss H, Obst M, Goeckede M, Planer-Friedrich B, Peiffer S, Gilfedder B, Elberling B (2019) Silicon increases the phosphorus availability of Arctic soils. Sci Rep 9: 449 . https://doi.org/10.1038/s41598-018-37104-6\u003c/li\u003e\n \u003cli\u003eSchaller J, Puppe D, Busse J, Paasch S, Katz, O, Brunner, E, Kaczoreck D, Sommer M (2022) Silicification patterns in wheat leaves related to ontogeny and soil silicon availability under field conditions. Plant Soil 477:9\u0026ndash;23. https://doi.org/10.1007/s11104-022-05385-6\u003c/li\u003e\n \u003cli\u003eSchulte A, Hoskins B (2011) Recommended soil organic matter test. In: Recommended soil testing procedures for the Northeastern United States, 3rd edn. Northeast Coordinating Committee for Soil Testing (NECC-1312), University of Delaware, Newark, pp 63-74\u003c/li\u003e\n \u003cli\u003eSelf-Davis ML, Moore PA Jr, Joern BC (2009) Determination of water- and/or dilute salt-extractable phosphorus. In: Kovar JL, Pierzynski GM (eds) Methods of Phosphorus Analysis for Soils, Sediments, Residuals, and Waters, 2nd edn. Southern Cooperative Series Bulletin No. 396, Virginia Tech University, Blacksburg, pp 24\u0026ndash;26\u003c/li\u003e\n \u003cli\u003eSeyfferth AL, Fendorf S (2012) Silicate mineral impacts on the uptake and storage of arsenic and plant nutrients in rice (\u003cem\u003eOryza sativa\u003c/em\u003e L.). Environ Sci Technol 46:13176-13183. https://doi.org/10.1021/es3025337\u003c/li\u003e\n \u003cli\u003eSeyfferth AL, Morris AH, Gill R et al (2016) Soil incorporation of silica-rich rice husk decreases inorganic arsenic in rice grain. J Agric Food Chem 64:3760\u0026ndash;3766. https://doi.org/10.1021/acs.jafc.6b01201\u003c/li\u003e\n \u003cli\u003eSharpley AN, McDowell RW, Kleinman PJA (2004) Amounts, forms, and solubility of phosphorus in soils receiving manure. Soil Sci Soc Am J 68:2048-2057. https://doi.org/10.2136/sssaj2004.2048\u003c/li\u003e\n \u003cli\u003eShober AL, Gartley KL, Miller JO, Taylor R (2020) Nutrient recommendations - agronomic crops. University of Delaware Cooperative Extension, Newark, DE. https://www.udel.edu/academics/colleges/canr/cooperative-extension/environmental-stewardship/soil-testing/nutrient-recommendations/.\u003c/li\u003e\n \u003cli\u003eShober AL, Gartley KL, Sims JT (2025) Calculating the lime requirement using the Adams-Evans buffer method. University of Delaware Cooperative Extension, Newark, DE. https://www.udel.edu/canr/cooperative-extension/fact-sheets/calculating-lime-adams-evans-soil-buffer/\u003c/li\u003e\n \u003cli\u003eShober AL, Gartley KL, Sims JT (2025) Interpreting soil phosphorus and potassium tests. University of Delaware Cooperative Extension. Newark, DE. https://www.udel.edu/academics/colleges/canr/cooperative-extension/fact-sheets/interpreting-soil-phosphorus-and-potassium-tests/\u003c/li\u003e\n \u003cli\u003eSims JT, Eckert D (2011) Recommended soil pH and lime requirement tests. In: Recommended soil testing procedures for the Northeastern United States, 3rd edn. Northeast Coordinating Committee for Soil Testing (NECC-1312), University of Delaware, Newark, pp 19\u0026ndash;26\u003c/li\u003e\n \u003cli\u003eSims JT, Edwards AC, Schoumans OF, Simard RR (2000) Integrating soil phosphorus testing into environmentally based agricultural management practices. 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Front Ecol Environ 10:243\u0026ndash;248. https://doi.org/10.1890/110046\u003c/li\u003e\n \u003cli\u003eWolf A, Beegle D (2011) Recommended soil test for macro and micronutrients. In: Recommended soil testing procedures for the Northeastern United States, 3rd edn. Northeast Coordinating Committee for Soil Testing (NECC-1312), University of Delaware, Newark, pp 39\u0026ndash;48\u003c/li\u003e\n \u003cli\u003eWu W, Limmer MA, Seyfferth AL (2020) Quantitative assessment of plant-available silicon extraction methods in rice paddy soils under different management. Soil Sci Soc Am J 84:618\u0026ndash;626. https://doi.org/10.1002/saj2.20013\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"plant-and-soil","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"plso","sideBox":"Learn more about [Plant and Soil](https://www.springer.com/journal/11104)","snPcode":"11104","submissionUrl":"https://submission.nature.com/new-submission/11104/3","title":"Plant and Soil","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Legacy phosphorus, Silicon fertilization, Phosphorus mobilization, Winter wheat, Calcium-magnesium silicate slag, Lime alternatives","lastPublishedDoi":"10.21203/rs.3.rs-8972522/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8972522/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eBackground and Aims\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eHistorical manure applications on the Delmarva Peninsula have created legacy soil phosphorus (P) accumulation, yet winter wheat (\u003cem\u003eTriticum aestivum\u003c/em\u003e L.) often faces early-season P deficiency due to fixation in acidic soils. This study evaluated whether silicon (Si) amendments can mobilize legacy P across three scales: laboratory chemical desorption, in-field pot plant uptake, and on-farm agronomic yield.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMethods\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA tiered approach used three legacy P soils. A 154-day incubation screened the desorption potential of silicic acid, Ca-Mg silicate slag, and switchgrass char. An in-field pot study isolated Si effects from liming effects using pH-balanced applications of silicic acid, silica gel, and slag. Finally, an on-farm trial compared Ca-Mg silicate slag against standard lime and starter P practices.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eResults\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn the incubation, silicic acid and slag increased water-extractable P (WEP), while switchgrass char reduced it. In the pot study, soluble Si increased soil WEP and tissue Si concentrations, but did not enhance biomass or P uptake, likely due to Ca-P precipitation and low ambient stress. Conversely, the field trial showed that while slag did not significantly alter soil P availability, it significantly increased wheat yield compared to standard management, driven by high slag solubility and Si-mediated plant health benefits.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConclusion\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSilicon amendments can chemically mobilize legacy P, but the mechanism is constrained by soil buffering, Ca-interference, and plant homeostasis. Silicate slag improves yield through stress resilience rather than P-mobilization. Due to variable heavy metal content, caution is advised for its use in forage systems.\u003c/p\u003e","manuscriptTitle":"Legacy Phosphorus Mobilization by Silicon Amendments: From Laboratory Mechanisms to Agronomic Effectiveness in Winter Wheat","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-12 10:37:23","doi":"10.21203/rs.3.rs-8972522/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revisions","date":"2026-04-14T05:45:55+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2026-03-10T06:05:26+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-03-09T15:20:16+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Plant and Soil","date":"2026-03-02T07:34:20+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-03-02T06:47:31+00:00","index":"","fulltext":""},{"type":"submitted","content":"Plant and Soil","date":"2026-02-25T21:43:23+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"plant-and-soil","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"plso","sideBox":"Learn more about [Plant and Soil](https://www.springer.com/journal/11104)","snPcode":"11104","submissionUrl":"https://submission.nature.com/new-submission/11104/3","title":"Plant and Soil","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"23cba73f-62fe-45e6-8153-3110ba9fe94d","owner":[],"postedDate":"March 12th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-29T11:19:07+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-12 10:37:23","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8972522","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8972522","identity":"rs-8972522","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}