Soil amendments and phosphorus fertilizer increase maize productivity and improve the fertility of acidic soils in Southwestern Ethiopia

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This challenge is widespread in the highlands of Southwestern Oromia, Ethiopia, where maize is a staple and cash crop. This study evaluated the effects of soil amendments and phosphorus (P) fertilization on post-harvest soil properties, maize physiology, and grain yield during the 2023 and 2024 growing seasons across 14 farmers’ fields in Kersa, Bedele, and Mettu districts. A split-plot design was used with farms as replicates, testing four soil amendments (control, calcitic lime, biochar, vermicompost) and four P rates (0, 15, 30, and 45 kg P ha⁻¹). Calcitic lime and vermicompost markedly improved soil fertility, enhanced maize physiological performance, and increased yields, whereas biochar was less effective. Lime was most effective in raising soil pH, lowering acidity saturation, and increasing P availability, resulting in 38–78% yield gains over the control. Vermicompost also achieved substantial gains (41–66%). Although P fertilization consistently increased yield, its efficiency declined under high acidity saturation. Findings indicate that P response is strongly constrained by soil acidity, and effective management in clay-rich acidic soils requires prior or concurrent soil acidity correction. Integrated strategies combining lime or vermicompost with P fertilization significantly enhanced nutrient availability, maize growth, and productivity. These results highlight the importance of site-specific soil fertility management tailored to acidity levels for improving maize yields in acidic tropical soils. lime vermicompost biochar exchangeable acidity tropical soils Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Soil acidification is a global challenge that threatens food and nutrition security (Du et al., 2024 ). This phenomenon is particularly pronounced in humid tropical regions, where soil productivity decline is exacerbated by aluminum (Al) and manganese (Mn) toxicity and severe nutrient imbalances (Sanchez, 2019 ; Agegnehu et al., 2021 ; Warner et al., 2023; Silva et al., 2025 ). A major consequence of soil acidification is the sorption of phosphorus (P), a process governed by soil pH, clay mineralogy, and the abundance of iron (Fe) and Al (sesqui)oxides (Elias & Agegnehu, 2020 ) which restrict P availability and uptake by crops (Tiecher et al., 2023 ; Lei et al., 2024 ). Beyond nutrient availability, soil acidification also affects crop performance (Enesi et al., 2023 , Gurmu et al., 2025 ). Al toxicity leads to poor root development and negatively affects photosynthesis through reduced stomatal conductance, chlorophyll content, and overall photosynthetic efficiency (Vasconcelos et al., 2020 ; Guo et al., 2024 ). Increasing the fertility of acidic tropical soils thus requires an integration of soil management practices to reduce exchangeable acidity and/or to increase soil nutrient availability and mitigate physiological stressors to crop productivity (Morel et al., 2021 ; Tiecher et al., 2023 ). Soil acidity is a recognized constraint to agricultural productivity in Ethiopia, particularly in the southwestern highlands of Oromia, where soil pH typically ranges between 4.5 (very strongly acidic) and 5.5 (strongly acidic) (Eyasu, 2016; Silva et al., 2025 ). Despite unfavorable soil conditions, farmers grow staple crops with minimal or no application of soil amendments. This has been further exacerbating nutrient depletion and limiting crop productivity (Takala, 2019 ; Sori, 2021). Maize, a key staple crop for food security and a major source of livelihoods in the region, is particularly vulnerable to soil acidity (Farina & Channon, 1991 ; Zingore et al., 2023 ; Oumer et al., 2023 ). The Southwestern part of Ethiopia is an important maize production region, accounting for 31% of the national maize harvested area and 53% of the national maize production. Maize yields in the region are on average 4.6 t ha − 1 (CSA, 2022), slightly higher than the national average (4.2 t ha − 1 ), but considerably lower than what is possible to achieve with best agronomic practices (Assefa et al., 2020 ). Recent studies point to the importance of soil management practices that remediate soil acidity towards improving crop productivity and fertilizer use efficiency in the country (Asfaw et al., 2024 ). Agricultural lime, biochar, and vermicompost are recognized soil amendments to manage soil acidity, improve P availability and soil fertility, ultimately leading to increased crop productivity (Zingore et al., 2023 ; Terefe et al., 2023; Tiecher et al., 2023 ). Agricultural lime is an effective input to neutralize exchangeable acidity and increase pH in the topsoil by supplying essential cations that reduce Al toxicity (Enesi et al., 2023 ; Ejigu et al., 2023 ). Biochar is an alkaline by-product of biomass pyrolysis, and an effective means to enrich soils with carbon. Its high surface charge density, extensive surface area, and internal porosity facilitate metal adsorption and nutrient retention, thereby improving soil pH and crop growth (Bolan et al., 2021; Huang et al., 2023 ; Pandian et al., 2024 ; Bhattacharyya et al., 2024 ). Lastly, vermicompost, i.e., a product rich in organic matter, available nutrients, humic acids, and plant growth-promoting hormones, is effective in increasing soil pH, nutrient availability, microbial activity, and carbon sequestration (Toor et al., 2024 ; Raza et al., 2024 ). The integration of these soil amendments with mineral fertilizers has demonstrated significant potential in the reclamation of acidic soils by improving soil chemical properties, nutrient cycling, and physiological traits critical to crop growth (Abeba et al., 2024; Iticha et al., 2024 ). The contribution of different soil amendments to increase crop productivity and P use efficiency varies with of the properties of acidic soils. Yield benefits can occur due to increases in soil P availability (higher intercept of yield response curves), increases in the yield response to P due to improved growing conditions (higher slope, i.e., yield per unit of applied P), and/or increases in the attainable yield at high levels of applied P (higher plateau). The effectiveness of soil amendments in neutralizing soil acidity and improving P use efficiency and crop productivity likely varies with inherent soil fertility, particularly in relation to exchangeable acidity. Soil amendments with high neutralizing capacity, such as lime, are essential for strongly acidic soils where toxic levels of exchangeable Al are a primary constraint to crop production (Basak et al., 2022 ; Enesi et al., 2023 ). Conversely, nutrient-rich amendments with lower neutralizing capacity, such as biochar and vermicompost, are likely most effective in soils where nutrient deficiencies, rather than exchangeable acidity, are the dominant productivity constraint (Terefe et al., 2023; Toor et al., 2024 ). Although substitution and complementarity effects between soil amendments and P fertilizer can be expected to some extent (e.g., Alemu et al., 2017 ; Kisinyo et al., 2014 ), their presence and magnitude remain poorly studied. Understanding such effects supports targeting interventions for acid soil management that can increase crop productivity effectively and in the most profitable way. The objective of this study was to evaluate the effects of soil amendments and P fertilizer rates on maize productivity and soil fertility on acidic soils of Southwestern Oromia, Ethiopia. Researcher managed on-farm trials were conducted over two consecutive growing seasons to assess first-year yield responses and residual effects to applied inputs under low and high acidity saturation. It was hypothesized that maize yield and P use efficiency are primarily constrained by topsoil exchangeable acidity, and that targeted nutrient management strategies are most effective when tailored to site-specific soil fertility constraints. Given the high and stable rainfall in the study area, it is expected that crop physiological responses are predominantly influenced by soil chemical properties rather than by water availability during the growing season. Our findings provide practical insights for optimizing soil nutrient availability and management in acidic soils of Southwestern Oromia and other comparable agroecosystems. 2. Materials and Methods 2.1 Description of the study area The study was conducted during the 2023 and 2024 cropping seasons in Kersa, Bedele, and Mettu districts of Southwestern Oromia, Ethiopia. The study area ranges in altitude from 617 to 3,231 m above sea level and is characterized by a bimodal rainfall pattern, with the main rainy season occurring from June to September. Annual rainfall ranges between 1,200 and 2,800 mm, with two rainy seasons (mid-February to May and June to September, often extending into October; Fig. 1 ). Mean monthly temperatures vary between 10.7°C and 28.5°C (Fig. 1 ). Nitisols are the dominant soil type in the study area with soil pH (H 2 O) commonly ranging between 4.5 and 5.5, contributing to large tracts of arable land affected by soil acidity (ATA, 2014; Eyasu, 2016). Maize is the most important staple crop grown in the study area by smallholders as part of rainfed mixed crop-livestock farming systems, being planted in May and harvested in November. Legumes such as soybean are also an important component of local cropping systems. 2.2 Experimental setup and trial design A soil survey was conducted on 38 farmers’ fields to support the selection of the farms hosting the field experiments. Accordingly, 14 farmers’ fields, with soil pH (H 2 O) ranging between 4.32 and 5.45, were selected and delineated for the field experiment. The soil properties of the selected fields prior to soil amendment and fertilizer application are provided in Table 1 . The selected fields were characterized by a clay soil texture, relatively high effective cation exchange capacity (ECEC) and soil organic matter, and low P availability regardless of the district. There was however a gradient in exchangeable acidity and acidity saturation across districts, with higher levels observed in Bedele, intermediate in Mettu, and lower in Kersa. Unlike in the other districts, sites in Bedele were also affected by high concentrations of exchangeable acidity in the subsoil (data not shown). The experiment tested 4 soil amendments in interaction with 4 P fertilizer rates, comprising a total of 16 treatments following a split-plot design considering farmers’ fields as replicates. Soil amendments, calcitic lime (CaCO₃), biochar, and vermicompost were allocated to the main plots. Lime rates were determined for each field based on exchangeable acidity (Eq. 1). Biochar was applied at a rate of 10 t ha − 1 as per local recommendation (Abeba et al., 2024). Vermicompost was also applied at the same 10 t ha − 1 rate determined based on its N fertilizer equivalents required to meet the recommended N rate of 92 kg N ha − 1 for maize in the study area. Soil amendments were only applied in the first year of the experiment, prior to sowing, hence results from the second year refer to residual effects of the amendments beyond the year of application. P fertilizer rates were allocated to the subplots and determined relative to the recommendation of 30 kg P ha − 1 yr − 1 for maize in the study area. The four P fertilizer rates tested corresponded to 0%, 50%, 100% and 150% of the recommended P rate, equivalent to 0, 15, 30 and 45 kg P ha − 1 , respectively. P was applied in both growing seasons to the respective plots treated with each amendment. 2.3 Field layout and crop management practices The experimental fields were prepared using the traditional Maresha plough to a depth of 30 cm. A field layout was established based on the experimental protocol and treatments were randomly assigned to experimental units within each block. Each plot measured 4.5 m × 4.0 m (18 m²), with 1.5 m between blocks and 0.75 m between plots. Each plot contained six rows, each 4.5 m long, with a row spacing of 0.75 m and intra row spacing of 0.3 m to achieve a target maize population of 44,444 plants ha − 1 . To minimize boundary effects, the central four rows were designated as the net plot area for data collection, crop monitoring and harvesting. A hybrid maize variety, BH 661 released in 2008, was planted in all experimental sites. This variety is well adapted to mid-altitude regions with annual rainfall of 1,000–1,500 mm yr − 1 and is characterized by high yield potential (Legesse et al., 2011 ). N fertilizer in the form of urea (46% N) was applied uniformly to all treatments at a rate of 92 kg N ha − 1 , the recommended N fertilizer rate for maize in the study area, using a split application method with half the rate applied at sowing and the other half at 35–40 days after sowing. P fertilizer rates were applied as a basal dose at sowing in the form of triple superphosphate (TSP, 46% P 2 O 5 ). Table 1 Initial soil properties of the experimental sites in Kersa, Bedele and Mettu districts of Southwestern Oromia, Ethiopia. Soil samples were collected prior to soil amendment and fertilizer application in 2023 and analyzed at Holeta Agricultural Research Center. Kersa Bedele Mettu Mean Particle size distribution Clay (%) 57.00 70.30 69.19 66.50 Silt (%) 32.30 19.50 20.01 23.93 Sand (%) 10.70 7.20 10.80 9.57 Textural class clay clay clay clay Exchangeable properties CEC (cmol (+) kg − 1 ) 18.07 22.99 19.83 20.30 Ca (cmol (+) kg − 1 ) 6.59 3.71 6.59 5.63 Mg (cmol (+) kg − 1 ) 1.94 1.28 2.27 1.83 K (cmol (+) kg − 1 ) 1.48 0.80 1.72 1.33 Na (cmol (+) kg − 1 ) 0.03 0.02 0.02 0.02 Soil acidity parameters Soil pH (H 2 O) 5.02 4.56 4.90 4.83 Exchangeable acidity (cmol (+) kg − 1 ) 0.74 3.73 2.17 2.21 Exchangeable Al (cmol (+) kg − 1 ) 0.54 3.40 1.79 1.91 Acid saturation (%) 5.18 15.75 6.19 9.04 Soil organic carbon and nutrients Organic C (%) 1.56 2.58 2.28 2.14 Total N (%) 0.15 0.22 0.22 0.20 P-Bray II (ppm) 8.64 4.27 8.80 7.23 Cu (ppm) 0.95 0.57 0.89 0.80 Zn (ppm) 1.91 0.61 0.73 1.08 Mn (ppm) 105.43 34.77 64.02 68.08 Fe (ppm) 39.33 19.05 20.62 26.33 Lime was uniformly broadcast across the limed plots two weeks before sowing in the first year of the experiment and thoroughly incorporated into the soil with a hand hoe to enhance its neutralization capacity. The calcitic lime was sourced from Holetta Agricultural Research Center and it had a CaCO₃-equivalent (CCE) of 93.7%. The CCE was used to adjust the field-specific lime rate estimated with Eq. 1 (Kamprath, 1970 ) to ensure the required amount of CaCO 3 was applied in each field. The lime requirement method of Kamprath ( 1970 ) establishes the relationship between the concentration of exchangeable acidity in the soil and the amount of CaCO 3 equivalent required to neutralize it (Eq. 1): $$\:LR,CaC{O}_{3}\hspace{0.33em}\left(\frac{kg}{ha}\right)\hspace{0.33em}=\hspace{0.33em}\frac{\hspace{0.33em}EA\left(\frac{\text{c}\text{m}\text{o}\text{l}(+)\:}{kg}\right)*\hspace{0.33em}SD\hspace{0.33em}\left(m\right)*\hspace{0.33em}1{0}^{4}\hspace{0.33em}{m}^{2}\hspace{0.33em}*\hspace{0.33em}BD\left(\frac{Mg}{{m}^{3}}\right)\hspace{0.33em}*\hspace{0.33em}1000}{2000\:}*EF\:(\text{E}\text{q}.1)$$ where EA refers to the exchangeable acidity concentration (in cmol (+) kg − 1 ), SD to the soil depth assumed to be 15 cm, BD to the soil bulk density (in Mg m − 3 ), and EF to an efficiency factor equal to 1.5 accounting for liming efficiency (not all applied lime reacts when applied), soil variability, or field losses. All other values depict unit conversions. The estimated lime rate was on average 3.57 t ha − 1 across all experimental fields. Laboratory analyses were carried out at Holeta Agricultural Research Center to determine the nutrient composition of the biochar and vermicompost used in the experiments prior to their application (Table 2 ). Alike lime, biochar and vermicompost were also applied two weeks before sowing in the first year of the experiment through broadcast and thoroughly incorporated in the soil with a hand hoe. Biochar was produced from locally available coffee husks whereas vermicompost was prepared from farmyard manure. Both were obtained from the Jimma Agricultural Research Center. Standard agronomic practices recommended for maize cultivation in the study area were applied to ensure optimal crop management throughout the experiment. Table 2 Selected properties of vermicompost and biochar amendments used for the experiments. Biochar was produced from coffee husks whereas vermicompost was prepared from farmyard manure. Both were sourced from the Jimma Agricultural Research Center. Properties Vermicompost Biochar pH (1:2.5) 8.78 9.54 Total N (%) 0.92 0.63 Available P (ppm) 230.00 190.00 Exchangeable K (ppm) 0.71 - Organic C (%) 6.73 31.50 C:N ratio 7.30 50.00 CEC (cmol (+) kg − 1 ) 45.52 - Exchangeable Ca (cmol (+) kg − 1 ) 3.15 - Exchangeable Mg (cmol (+) kg − 1 ) 1.63 - 2.4 Crop physiological measurements and yield determination Physiological traits were assessed at tasseling stage with non-destructive methods on five representative plants selected from the middle rows of each plot. Leaf area was estimated by measuring the fully expanded green leaves of five plants within a defined area. Leaf length and width were recorded, and the leaf area was calculated using the method described by McKee ( 1964 , Eq. 2): $$\:Leaf\:area\left(cm2\right)=\text{L}\text{e}\text{a}\text{f}\:\text{l}\text{e}\text{n}\text{g}\text{t}\text{h}\left(\text{c}\text{m}\right)*Leaf\:width\:\left(cm\right)*0.75\:(\text{E}\text{q}.2)$$ where leaf Length refers to the length of the leaf from the base to tip (in cm), leaf width to the maximum width of the leaf (in cm) and 0.75 is an empirical correction factor determined by McKee ( 1964 ). Then leaf area index (LAI, cm 2 cm − 2 ) was determined as the ratio of the total leaf area of the five plants to the area occupied by these plants based on the spacing between plants between two rows and within a row, following the method of Diwaker and Oswalt ( 1992 ). Total chlorophyll content was measured at the time of sampling using a SPAD chlorophyll meter (Konica Minolta Sensing, Inc., 2011 ). Stomatal conductance (mmol m⁻² s⁻¹) was assessed at midday using an AP4 porometer (Delta-T Devices, Cambridge, UK) on the abaxial surface of five fully expanded, attached leaves. Both chlorophyll content and stomatal conductance were measured at the time of sampling from the same five plants per plot where the leaf measurements were conducted, with five readings taken per plant. Maize grain yield was assessed 20 days after physiological maturity, allowing for adequate field drying. Maize ears were harvested and shelled manually, after which the total grain weight from the net plot area (12 m²) was recorded. The grain moisture content was measured with a grain moisture meter and used to adjust maize yields to a standard 12.5% moisture content. 2.5 Soil sampling, preparation and analysis Soil samples were collected prior to sowing with an auger from the corners and center of the 14 experimental fields in a zigzag pattern at a depth of 0–20 cm. After harvest, soil samples were collected from the corners and center of each plot, resulting in 5 subsamples per treatment. The subsamples were thoroughly homogenized to form a composite soil sample for each site-treatment combination. Labeled samples were placed in plastic bags and transported to the Holeta Agricultural Research Center for laboratory analysis. At the laboratory, soil samples were air dried, crushed, and ground before being sieved through a 2 mm mesh, except for those intended for total N and organic carbon analysis, which were also sieved through a 0.5 mm mesh to ensure a finer particle size for accurate assessment. Soil particle size distribution was analyzed using the hydrometer method according to Day ( 1965 ). Soil pH (H 2 O) was measured using a pH meter according to Rhoades et al. ( 1999 ). Organic carbon content was determined by the wet oxidation method of Walkley and Black ( 1934 ), in which carbon is oxidized under standard conditions using potassium dichromate in a sulphuric acid solution. Finally, soil organic matter content was calculated by multiplying the percent organic carbon by 1.724. Total N was analyzed by Kjeldahl digestion (Jackson, 2003 ), while available P was extracted with the Bray II method (Bray and Kurtz, 1945 ) and quantified spectrophotometrically after color development. Exchangeable cations (Ca 2+ , Mg 2+ , K + , and Na + ) were extracted with a 1M ammonium acetate (NH₄OAc) solution at pH 7.0 and analyzed by atomic absorption spectrometer (Okalebo et al., 2002 ). Cation exchange capacity (CEC) was determined at pH 7 from samples saturated with ammonium acetate, with ammonium quantified by the modified Kjeldahl method (Okalebo et al., 2002 ). Exchangeable acidity (i.e., the sum of Al³⁺ and H⁺ cations) and exchangeable aluminum (Al³⁺) were quantified by saturating soil samples with a 1N KCl solution followed by titration with 0.02N NaOH and 0.02N HCl, respectively (Rowell, 1994 ). Acidity saturation was calculated as the ratio of exchangeable acidity to the ECEC, determined by sum of exchangeable Ca, Mg, K, and Na and exchangeable acidity. Available micronutrients (Fe, Mn, Zn, and Cu) were extracted with diethylenetriamine pentaacetic acid (DTPA) according to Tan ( 1996 ) and measured by atomic absorption spectrometer. 2.6 Statistical analysis Maize yield response to soil amendments and P fertilizer was evaluated using linear mixed models. Linear mixed models account for fixed and random effects to represent non-independent data structures. Fixed effects indicate the average effect of the predictors on the response variable (maize yield) and were specified as the three-way interaction between soil amendments (main plot), P rate (subplot), and initial acidity saturation. Soil amendments and P rate were defined as factors and the initial acidity saturation as a continuous variable. Random effects capture the variability in maize yield at different levels of grouping within the data and were specified as the soil amendments (main plot) nested within experimental sites (farms) to account for the split-plot design of the experiment. Year-specific models were fitted to the data using the lmer() function of the lmerTest R package (Kuznetsova et al., 2017 ) to account for maize yield responses to soil amendments in the year of application and in the subsequent year to assess residual effects. The significance of the fixed effects was evaluated at 5% significance level with analysis of variance (ANOVA) using the anova() function in R. Marginal means were estimated for the different soil amendments and P rates, under low (10% of ECEC) and high (30% of ECEC) acidity saturation conditions, using the emmeans() function of the emmeans R package (Searle et al., 1980 ). Acidity saturation thresholds were defined based on the variation observed in the acidity saturation of the experimental sites prior to input application. Cumulative probability plots were developed to quantify the probability of achieving a given maize yield response to the different soil amendments and P rates. The effect of soil amendments and P rates on physiological traits at tasseling and on post-harvest soil properties was assessed following the same approach described for maize yield. The physiological traits included chlorophyll content, stomatal conductance, and LAI. The soil properties included pH (H 2 O), exchangeable acidity, available P, soil organic matter, ECEC, acidity saturation, Ca and Mg saturation. Linear mixed models fitted with physiological traits and post-harvest soil properties as dependent variables used the same fixed effects structure as described for the maize yield model. An additive year fixed effect was added to these models, which were fitted to the pooled data. Soil amendments (main plot) nested within experimental sites (farms) were defined as random effects in these mixed models, similarly to the models fitted for maize yield. The relationship between post-harvest soil properties and physiological traits at tasseling on the one hand and maize yield on the other were investigated through boundary line analysis. Boundary lines were fitted to the pooled data to identify the maximum yield for a given level of each physiological trait and soil property and to characterize the respective yield response. Boundary lines were identified with quantile regressions fitted to the 90th quantile of the data using the nlrq() function of the quantreg R package (Koenker, 2025 ). A non-linear functional form (y = a + b x + c0.99 x , where a, b, and c are parameters to be estimated) was fitted for stomatal conductance and all soil properties, except acidity saturation. A linear functional form (y = a + b x , where a and b are parameters to be estimated) was justified for chlorophyl content, LAI, and acidity saturation and adopted for these data instead. 3. Results 3.1 Maize yield response to P and soil amendments There was a statistically significant effect (p < 0.05) of soil amendments, P fertilizer rates, and initial acidity saturation on maize yields both in the first and the second year of the experiment. The two-way and three-way interactions between these factors were not statistically significant in either year. Maize yield showed a positive response to increasing P rates, which was similar across soil amendments, acidity saturation levels, and years (Fig. 2 ). Maize yields were on average 5.8 t ha − 1 with no P applied, 6.9 t ha − 1 with 15 kg P ha − 1 , 7.6 t ha − 1 with 30 kg P ha − 1 , and 8.1 t ha − 1 with 45 kg P ha − 1 . There were no statistically significant yield differences between the two highest P application rates in the first year under low acidity saturation. It is noteworthy that comparable maize yields were obtained with the highest P fertilizer rate without amendments and with lime and vermicompost without P fertilizer. This positive effect of lime and vermicompost on the availability of P was observed across acidity saturation levels and experimental years. The maize yield response to soil amendments was greater for lime, followed by vermicompost and biochar regardless of P rate, but there were slight differences between years and acidity saturation conditions (Fig. 2 ). In the first year, under low acidity saturation, lime application produced the highest mean grain yield (8.0 t ha − 1 ), closely followed by vermicompost (7.9 t ha − 1 ; Fig. 2 A). Biochar also increased maize yield (6.8 t ha − 1 ) compared to the control (5.7 t ha − 1 ) without amendments. Under high acidity saturation, vermicompost produced the highest yield (6.6 t ha − 1 ), followed by lime (6.0 t ha − 1 ) and biochar (5.5 t ha − 1 , Fig. 2 B), but yield differences in response to the soil amendments were not statistically significant. The lowest yield was observed in the unamended control, with an average maize yield of 5.7 t ha − 1 and 4.1 t ha − 1 under low and high acidity saturation, respectively. In the second year, lime again produced the highest yield in both low (9.2 t ha − 1 ) and high (6.8 t ha − 1 ) acidity saturation, followed by vermicompost (8.1 and 6.5 t ha − 1 ) and biochar (6.1 and 4.5 t ha − 1 ; Figs. 2 C and 2 D). Maize yield with biochar was significantly lower than that with lime and vermicompost, and there was no significant difference in maize yield between biochar and the unamended control (Figs. 2 C and 2 D). On average, the unamended control produced the lowest maize yield, 5.4 and 3.6 t ha − 1 under low and high acidity saturation, respectively. Maize yield under low acidity saturation was significantly higher than under high acidity saturation, 7.2 t ha − 1 and 5.5 t ha − 1 , respectively (Fig. 2 ). Moreover, the maximum maize yield was lower under high acidity saturation regardless of P rate and soil amendment. There were also clear differences in maize yield response to soil amendments between the first and the second year, with lime and vermicompost showing considerable residual effects and yield improvements in the second year as opposed to biochar, regardless of acidity saturation (Supplementary Table 1). This is indicative of residual effects from lime and vermicompost, but not from biochar, particularly in the second cropping season. There was a large variability in maize yield response to application soil amendments and P rates (Fig. 3 ). Overall, the yield response to P fertilizer was more consistent, i.e., less variable, than the yield response to soil amendments. Maize yield with P fertilizer was significantly greater than in the control without P across all experimental sites (Figs. 3 A and 3 B). Conversely, yield response to soil amendments was more variable, particularly biochar for which a 20% probability of negative yield response was observed (yet the magnitude of this effect was mostly below 1 t ha − 1 ; Figs. 3 C and 3 D). Among the amendments, the yield response to biochar was smaller and more variable than the yield response to lime and vermicompost. There was 50% probability of obtaining a yield response close to 1 t ha − 1 for biochar and between 2 and 3 t ha − 1 for lime and vermicompost (Fig. 3 D). The yield response to P increased with P rate but the variability in response was comparable across P rates. There was a 50% probability of obtaining a yield response to P of about 1 t ha − 1 with 15 kg P ha − 1 , 2 t ha − 1 with 30 kg P ha − 1 , and 2.5 t ha − 1 with 45 kg P ha − 1 . Overall, these results indicate that lime, vermicompost, and P fertilizer were most effective in increasing maize productivity across all experimental sites. 3.2 Effects of P fertilizer and soil amendments on physiological traits Soil amendments and P fertilizer impacted maize physiological traits at tasseling stage (Fig. 4 ) under different acidity saturation levels (Supplementary Table 1). ANOVA indicated that chlorophyll content, stomatal conductance, and LAI were significantly ( p < 0.05) influenced by the main effects of soil amendments and P rates. The two-way interaction between soil amendments and P rates did not have a statistically significant effect on these physiological traits. There was a statistically significant difference in the chlorophyll content of maize in response to soil amendments and P rates under both low and high acidity saturation (Fig. 4 A; Supplementary Table 1). Application of vermicompost and lime increased chlorophyll content of maize compared to biochar and the control, with the highest value in response to vermicompost, closely followed by lime. Yet, the difference between chlorophyll contents of maize in response to vermicompost and lime application was not statistically significant. Conversely, chlorophyll content of maize grown in the unamended control was the lowest. Biochar application increased chlorophyl content of maize relative to the control, but that remained lower than observed with vermicompost and lime. Total chlorophyll content of maize increased with increasing P rates with the highest value recorded at 45 kg P ha − 1 . The lowest chlorophyl content was recorded for maize grown without P application. The response of chlorophyl content to P was more pronounced under low acidity saturation. Stomatal conductance showed a similar trend to chlorophyll content, with significant variation between treatments (Fig. 4 B; Supplementary Table 1). The vermicompost and lime amendments produced the highest stomatal conductance values under both acidity conditions, with vermicompost treatments reaching 204.2 mmol m − ² s − ¹ under low acidity saturation and 185.5 mmol m − ² s − ¹ under high acidity saturation. These values were statistically comparable to those observed with lime application. In contrast, the lowest stomatal conductance was recorded for biochar and the unamended control. P application significantly increased stomatal conductance, with the highest values observed at 45 kg P ha − ¹ (205.6 and 190.2 mmol m − ² s − ¹ under low and high acidity saturation, respectively), followed by 30 kg P ha − ¹. The unfertilized control showed the lowest stomatal conductance. The magnitude of the response was slightly reduced under high acidity saturation. The LAI was affected by soil amendments under low and high acidity saturation (Fig. 4 C; Supplementary Table 1). Vermicompost and lime applications resulted in significantly higher LAI (3.36 and 3.32 m 2 m − 2 under low acidity saturation and 3.2 and 3.0 m 2 m − 2 under high acidity saturation, respectively) compared to biochar and control (2.97 and 2.7 m 2 m − 2 , respectively). P fertilizer increased LAI under low and high acidity saturation, with the highest values recorded for 45 kg P ha − 1 , while the lowest LAI was observed when P was not applied. Under high acidity saturation, the difference in LAI between the two highest P rates was not statistically significant, suggesting a plateau in the LAI beyond 30 kg P ha − 1 particularly under high acidity saturation. 3.3 Relationship between maize yield and physiological traits There was a positive linear relationship between chlorophyl content and LAI on the one hand and maize yield on the other (Figs. 5 A and 5 C). The highest maize yield was obtained with chlorophyl content above 50 SPAD units and with LAI above 3 m 2 m − 2 , reflecting the importance of light interception and photosynthetic capacity to maize yield. The relationship between stomatal conductance and maize yield was less evident partly due to the large variation in maize yield at low levels of stomatal conductance (Fig. 5 B). High maize yield was obtained with stomatal conductance of 100 mmol m² s⁻¹. The physiological benefits on maize yield were most pronounced on lime and vermicompost treatments, indicating the effect of these soil amendments on maize growth and yield. Conversely, the biochar and unamended control resulted in sub-optimal physiological performance and reduced yield. 3.4 Effects of P fertilizer and soil amendments on soil properties Soil amendments and P fertilizer contributed to soil fertility improvements in different ways and to different extents, as observed in the results of post-harvest soil properties (Fig. 6 ). Soil pH, organic matter, available P, and ECEC were significantly influenced by the main effects of soil amendments and P rates ( p < 0.05). Conversely, acidity saturation and Ca saturation were only influenced by the main effect of soil amendments. The two-way interaction effects of soil amendments and P rates on the evaluated soil properties was not significant. There were a statistically significant ( p < 0.05) residual effects of amendments on soil pH, organic matter, ECEC, Ca saturation and acidity saturation during the second cropping season but not on available P. Soil pH increased with the application of soil amendments and increasing P rates (Fig. 6 A). On average, lime raised soil pH by 13.5%, vermicompost by 6.7%, and biochar by 4.1% over the control. The most pronounced pH increase was observed with lime. Vermicompost and biochar resulted in moderate pH increases, with vermicompost showing slightly more neutralization effect than biochar. The lowest pH values from 4.6 to 4.9 were recorded in the unamended control. Increases in P rates did not increase soil pH considerably. Application of amendments significantly increased soil organic matter relative to the control (Fig. 6 B; Supplementary Table 2). Vermicompost and biochar, increased soil organic matter to 3.92%. Lime resulted in a comparable increase in soil organic matter to an average of 3.96 g kg − 1 . There were no statistically significant mean differences in soil organic matter between amendments, indicating similar efficacy of the soil amendments in increasing soil organic matter. Soil organic matter content was also affected by P rates, increasing from 3.72% at 0 kg P ha⁻¹ to 3.94% at 45 kg P ha⁻¹. The ECEC values in the lime treatments were higher, with median value 11.5 cmol (+) kg − 1 , slightly above the mean ECEC across the pooled data for all P rates (Fig. 6 C). Application of lime increased ECEC by 17.8% over the control. Conversely, application of vermicompost and biochar showed no significant change in ECEC of the soil highlighting the unique capacity of lime to increase cation retention. Median ECEC values were below the mean ECEC across the pooled data for soil amended with biochar, vermicompost and the control. The effect of P rates on ECEC was not significant and consistent across soil amendments. Acidity saturation was significantly reduced due to lime application to levels below 20% of the ECEC, the critical threshold for maize (Aramburu Merlos et al., 2023 ; Farina & Channon, 1991 ; Fig. 6 D). Indeed, nearly 75% of the soil samples from the lime treatments had acidity saturation below this threshold and median acidity saturation of about 5% of the ECEC, which indicates that lime was effective in neutralizing exchangeable acidity. Vermicompost also reduced acidity saturation, but not as consistently as lime. The median acidity saturation after vermicompost application was between 10 and 15% of the ECEC but some sites had levels above the critical threshold. No substantial reductions in acidity saturation were observed after biochar application (Fig. 6 E). Lime increased Ca saturation to median values of about 45% of the ECEC whereas vermicompost and biochar had little impact on Ca saturation, with median values of 25–35% of the ECEC. Reductions in acidity saturation and increases in Ca saturation were thus primarily driven by lime and, to a less extent, by vermicompost rather than by biochar. Biochar is thus not effective in neutralizing soil acidity within one year of application. Soil available P increased in response to the application of soil amendments and with P fertilizer rates (Fig. 6 F). Lime and vermicompost significantly increased soil available P, Median soil available P was 18 mg kg − 1 for the lime treatments, close to the sufficient threshold of 15 mg kg − 1 , 14 mg kg − 1 for vermicompost, 10 mg kg − 1 for biochar, and 9 mg kg − 1 for the control. P rates also showed a direct impact on soil available P, which increased with increasing rates regardless of soil amendments. Increases in soil available P were particularly evident at high P rates in combination with lime or vermicompost, indicating the efficacy of these soil amendments in increasing the availability of immobilized P for crop nutrition. 3.5 Maize grain yield response to improvements in soil properties Maize yield was highly responsive to soil pH, ECEC, acidity saturation, Ca saturation, and available P, and not as much to soil organic matter (Fig. 7 ). High maize yields were obtained with soil pH above 4.9, organic matter of about 4%, ECEC about 10 cmol (+) kg − 1 , acidity saturation below 20% of ECEC, Ca saturation above 30% of ECEC, and available P above 12 mg kg − 1 . Boundary lines depicting the relationship between maize yield on the one hand and soil pH, ECEC, Ca saturation, and available P on the other showed a positive response with diminishing returns and potential detrimental effects beyond a threshold for each soil property (except available P). The positive yield response observed for these soil properties was largely explained by the application of lime and, to a less extent, vermicompost. The boundary line depicting the relationship between maize yield and acidity saturation was linear negative, pointing to severe yield reductions at high acidity saturation particularly in the unamended control and after biochar or vermicompost. Lastly, the relationship between maize yield and soil organic matter revellead a quadratic functional form with a maximum at about 4%, indicating that increasing soil organic matter beyond this level is not required to increase crop productivity in the study area. These findings underscore the need for integrated soil fertility management strategies that combine the correction of soil pH and exchangeable acidity with nutrient enrichment towards increasing crop productivity on acidic soils. 4. Discussion 4.1 Yield response to P on acidic soils of Southwestern Oromia P fertilizer and soil amendments are essential inputs to sustain and increase crop productivity and soil fertility, particularly in acidic soils (Zhang et al., 2023 ; Zingore et al., 2023 ; Howe et al., 2024 ). Our results revealed strong maize yield responses to P in the absence of soil amendments, highlighting considerable maize yield response to P fertilizer in the study area. P deficiency is a well-known constraint to cereal production in Ethiopia, particularly in highland regions dominated by low pH with high P sorption (Agegnehu et al., 2021 ; Dinssa and Elias, 2021 ; Kenea et al., 2021 ; Agegnehu et al., 2023 ; Yimer et al., 2024 ). In Southwestern Ethiopia, where Nitisols and Acrisols prevail, P is rapidly immobilized by Fe and Al (sesqui)oxides, severely limiting its availability to crops (Elias and Agegnehu, 2020 ). The yield response to P observed in our study thus aligns with this broader evidence characteristic of high rainfall, clay-rich agroecosystems of Southwestern Oromia and other parts of East Africa (Agegnehu et al., 2021 ; Kenea et al., 2021 ) We also found that maize yield and P use efficiency were limited by topsoil acidity saturation and that soil amendments are important when combined with P inputs. For instance, maize yield increased by up to 40% following diminishing returns to applied P, particularly in the first year of the experiment, yet it was consistently lower (1.7 t ha − 1 on average) under high acidity saturation when soil amendments were not applied. The impact of soil acidity on crop yield response to P is well documented (Salinas & Sanchez, 1980; Hao et al., 2020 ; Omenda et al., 2021 ; Lei et al., 2024 ) and can be explained by a combination of factors associated with low soil pH, particularly Al toxicity and P sorption, which were indeed detectable in some of the experimental sites. These factors are known to pose major chemical barriers to nutrient uptake and root development by crops and to have a negative impact on P use efficiency. A positive additive effect was found between P fertilizer and soil amendments on maize yield and post-harvest soil properties indicating that P fertilizer alone increased crop productivity on acid soils, but not to its highest levels. P fertilizer resulted in considerable increases in soil available P, which likely explains the maize yield response to P observed when soil amendments were not applied. As expected, P fertilizer had a more modest effect on other important soil properties such as soil pH, soil organic matter, ECEC, and acidity saturation. Conversely, soil amendments created more favorable conditions for crop growth, as observed in the relationships between maize yield and post-harvest soil properties (see also Pandit et al., 2018 ) and reduced the variability in the maize yield response to P (except for biochar). These findings underline the effectiveness of combining P fertilizers with lime or vermicompost to enhance maize productivity in the acidic soils of Southwestern Oromia. 4.2 Effectiveness of soil amendments for acid soil management Soil amendments are essential for improving soil fertility and optimizing maize yields, particularly in acidic soils. Recent interventions, such as the use of lime, vermicompost and biochar, have shown promise in alleviating soil acidity, improving soil fertility, and increasing crop productivity (Agegnehu et al., 2021 ; Zhang et al., 2023 ; Agumas et al., 2025 ). However, their effectiveness depends on soil fertility constraints that are site-specific, particularly in relation to acidity saturation across the soil profile. Although lime and vermicompost outperformed biochar, maize yield response to these soil amendments varied greatly, emphasizing the need for tailored nutrient management. These results highlight the limitations of uniform, blanket strategies and the importance of site-specific diagnosis and targeted interventions for improving crop productivity in acidic soils (see also Silva et al., 2025 ). Our results showed that lime increased maize yield by about 40% in soils with low acidity saturation and by about 70% in soils with high acidity saturation, underscoring its suitability for acid soil management. While other amendments also provide agronomic benefits, lime was most effective in correcting soil acidity by improving pH and nutrient availability (Enesi et al., 2023 ; Agegnehu et al., 2023 ). Changes in soil properties due to liming can be explained by the release of cations that reduce of the concentration of exchangeable acidity, raise soil pH and enhance P solubility (Minato et al., 2023 ; Ying et al., 2024; Wakwoya et al., 2022 ; Alemu et al., 2022 ; Kibet et al., 2023 ; Ejigu et al., 2023 ). Importantly, our results also show that lime can substitute P to some extent, as yield gains from lime alone matched or exceeded those obtained with high P rates without lime. This was true under and low and high acidity saturation and in the year of lime application and in the first year of residual effects. This finding suggests current P recommendations may be insufficient to overcome P limitations on crop growth and suggests that P sorbed in clay-rich soils can be made available through liming in the short-term (Bolo et al., 2021 ; Tiecher et al., 2023 ). It is not clear whether this substitution effect between lime and P would occur at P rates higher than the maximum 45 kg P ha − 1 tested in our experiment, the point at which P may no longer be limiting. Lastly, the residual effects observed in the second year of the experiment, i.e., sustaining yields without re-application, emphasize the potential economic value of lime for resource-constrained farmers, a topic which merits further research. By increasing fertilizer use efficiency, lime can offer a cost-effective solution for acidic soils, but its current high market price makes it a prohibitive investment for most smallholders (see Oumer et al. 2023 for further details about the lime value chain in Ethiopia). Vermicompost performed similarly to lime across cropping seasons and acidity saturation levels, increasing maize yield by about 40% under low acidity saturation and 65% under high acidity saturation. Vermicompost thus stands out as an effective option to improve the fertility of acidic soils through increases in soil pH and nutrient availability (Wegene & Wogi, 2023). The positive effects of vermicompost on maize yield across low and high acidity saturation can be explained in two ways. First, vermicompost is rich in N such that the N available for crop growth in vermicompost treatments was about double that of the other treatments. This being the case indicates that current N recommendation rates are likely insufficient to overcome N limitations to maize growth in the region. Second, vermicompost also increases soil fertility in several ways (i.e., increasing soil organic matter and nutrient availability, stimulating microbial activity, and buffering of soil pH; Zhang et al., 2023 ; Terefe et al., 2024 ) and can neutralize exchangeable acidity to some extent (Iqbal et al., 2024 ; Mulatu & Bayata, 2024 ). Future research is required to Disentangle the mechanisms through which vermicompost increases maize yield and the fertility of acidic soils. Vermicompost’s dual role as a nutrient source and a partial liming agent makes it valuable for resource-limited farms, yet its profitability must be assessed in future studies. Another aspect to consider when promoting vermicompost is whether it can be made available to farmers in the amount and at the cost needed for it to be effective at the field level in a profitable way. Despite lime and vermicompost application, maize yields remained lower under high acidity saturation due to a combination of soil and physiological constraints. The efficacy of lime and vermicompost under such conditions can be constrained by insufficient application rates, poor incorporation and neutralization efficiency, and the persistence of subsoil acidity, particularly in high rainfall environments where leaching is prevalent. Yet, the high acidity saturation sites in which our experiments were conducted (Bedele district) were also affected by subsoil acidity, particularly high exchangeable acidity concentrations (3.98–5.20 cmol (+) kg − 1 ) at 31-200cm depth. Due to its low mobility in the soil profile, calcitic lime was likely not able to remediate subsoil acidity, which might explain why the maize yield response to lime and vermicompost (and biochar in the first year) was comparable under high acidity saturation, a hypothesis that requires validation in future studies. Biochar was found to moderately increase maize yield under low and high acidity saturation, particularly in the first year. The benefits of biochar stem from its alkalinity and reactive surfaces, contributing to the long-term stabilization of soil organic matter (Huang et al., 2023 ) and to improving soil structure, water retention, and carbon sequestration (Yu et al., 2019 ; Pandian et al., 2024 ). However, its efficacy in acidic soils was limited due to its weak pH buffering capacity and low ability to reduce Al toxicity and to increase P availability. Compared to the other amendments, biochar demonstrated limited potential to increase crop yield and fertilizer use efficiency in acidic soils due to its small impact on acidity saturation, ECEC, and available P. Indeed, biochar’s performance was comparable to that of the unamended control for most of soil properties and physiological traits assessed, with yield responses to biochar also showing a 20% likelihood of yield reduction. These findings suggest that, due to its low solubility and limited interaction with the “acid soil complex”, i.e., the broader soil chemical constraints associated with low pH, biochar lacks the chemical reactivity and nutrient composition required to address the fertility constraints of acidic tropical soils. 4.3 Recommendations and scope for future research Soil acidity has been recognized as an obstacle to agricultural productivity in Ethiopia (Oumer et al., 2023 ; Warner et al., 2023) and liming has been prioritized as a key intervention for acid soil remediation by the Ministry of Agriculture and its partners (Tilahun, 2019 ). Yet, its adoption remains limited due to logistical, economic, and awareness challenges, highlighting the need for cost-effective, site-specific and evidence-based solutions (e.g., Oumer et al., 2023 ). There have also been challenges with targeting investments in acid soil management across the country (Agumas et al., 2025 ), despite the increasing awareness of the need to monitor acidity saturation for that purpose (Silva et al., 2025 ; Aramburu Merlos et al., 2023 ; Sanchez, 2019 ). This study highlights the importance of understanding soil chemical constraints for acid soil management and of acidity saturation as a major driver yield response to P fertilizer and soil amendments. In doing so, it provides robust empirical evidence that integrated soil fertility management, combining lime and/or organic amendments such as vermicompost with P fertilizers, can increase maize productivity (along with fertilizer use efficiency) and improve the fertility in acidic soils. Our results define nutrient management strategies for acidic soils and demonstrate the value of combining organic and inorganic inputs towards sustainable crop production in the Ethiopia's croplands. Future research is needed to better unpack interactions between soil amendments and nutrients, particularly N and P, and their impact on crop productivity and fertilizer use efficiency. We were unable to quantify nutrient use efficiency and its components (recovery and physiological efficiencies) in our study, due to the large number of experimental plots and sites. More targeted experiments are required in the future to refine our results and recommendations in this regard. This is particularly important in the case of lime and vermicompost, as these amendments have a direct effect on soil N and P availability to crops. In the case of vermicompost, we recommended assessing its effects against N treatments without its N-equivalent N supply since our study confound N inputs with other vermicompost’ benefits on soil fertility. This requires full factorial experimental designs combining varying rates of N, P and soil amendments and deployed across contrasting soil conditions. Non-limiting nutrient rates are particularly important in such assessment to unpack additive and/or substitutive effects between inputs in a consistent way. It is also important to run such experiments in the medium- to long-term to assess residual effects of different strategies on crop productivity and soil fertility (particularly P saturation levels). Future assessments could further complement our empirical assessment with modelling tools that capture crop-soil interactions in an integrated way. Advancing scientific understanding and ensuring sustainable management of acidic soils in smallholder farming systems requires targeted research in two key areas. Firstly, the agronomic performance, economic viability, and adoption potential of alternative soil amendments, such as lime substitutes derived from fertilizer and other by-products, and integrated organic-mineral formulations, must be rigorously evaluated in Ethiopia. This will guide the rigorous testing of alternative products to lime and unravel their agronomic and economic feasibility for resource-constrained farmers. Secondly, further research is required to better understand liming effects on P use efficiency and to which extent the substitutive effects between lime and P found in our results can be managed over time. Lime improves P uptake and the recovery and internal use efficiencies of P, but the magnitude of such improvements under on-farm conditions remains unquantified. It is also not clear how residual effects from lime and P evolve over time and more research is needed to understand these effects from a cropping systems perspective and in the long-term. These efforts will support designing effective, economically viable, site-specific nutrient management strategies attuned to acidic soil conditions across Ethiopia’s smallholder farming systems. 5. Conclusion The study evaluated the effect of P fertilizer rates and soil amendments on grain yield, physiological performance, and soil fertility for maize crops in Southwestern Oromia, Ethiopia. The region is strategic for maize production in the country, due to its large number of smallholder farms and fertile yet acidic soils. Researcher managed on-farm experiments evaluating the productivity and soil fertility merits of P fertilizer in interaction with soil amendments revealed strong maize yield response to P, pointing to the importance of P supplementation in the region. Yet, maize yield response to P was limited by topsoil acidity saturation, which remains an important metric for targeting investments in acid soil management. Soil amendments improved soil fertility by lowering soil acidity and increasing nutrient availability, which in turn had a positive effect on maize growth and maize yield response to P. Our results thus indicate there is scope for targeting soil fertility management strategies to soil chemical properties. Consistent with regional research, we conclude that effective P management in clay-rich acidic soils depends on prior or concurrent soil fertility correction measures to fully realize maize yield potential. Lime was the most effective amendment in increasing crop yield and P use efficiency due to its soil fertility improvements, most notably lowering Al toxicity and increasing P availability, across cropping seasons and experimental sites. Vermicompost provided comparable results to lime on maize yield and soil fertility, due to its dual role as a nutrient source and a partial liming agent. We also cannot exclude that subsoil acidity constrained the yield response to soil amendments in high acidity saturation sites. Substitutive effects were found between lime and vermicompost on the one hand and high rates of P fertilizer on the other, justifying more focus on understanding interactions between nutrients and soil amendments in future research. Biochar contributed to small improvements in maize yield and soil fertility, and caution is needed when recommending it as a standalone soil amendment. An integrated approach combining soil amendments with P fertilizer and guided by routine monitoring of key soil properties is essential to maintain soil fertility and optimize maize productivity. The economic viability of alternative nutrient management options for resource-constrained farmers remains to be evaluated in future research. Declarations Declaration of Competing Interest The authors declare they have no conflicting interest. Author Contribution Getahun Dereje: Conceptualization; Data curation; Software, Formal analysis; Methodology; Resources; Investigation, Visualization; Writing-original draft. Lemma Wogi: Supervision; Validation; Writing-review and editing. Temesgen Desalegn: Supervision; Validation; Writing-review and editing. Tesfaye Shiferaw Sida: Funding acquisition, Supervision; Validation; Writing-review and editing. Joao Vasco Silva: Software, Formal analysis, Writing-review and editing. Acknowledgement The authors extend their gratitude to the Bill & Melinda Gates Foundation, which funded this research through the project Guiding Acid Soil Investment in Africa (GAIA), grant number INV-002829, as part of a collaboration between the International Maize and Wheat Improvement Center (CIMMYT) and the Ethiopian Institute of Agricultural Research (EIAR). Special acknowledgment goes to the soil laboratories of the Department of Natural Resource Management at Holeta Agricultural Research Center and Jimma Agricultural Research Center for their invaluable assistance in the analysis of soil characteristics and to the farmers who availed their land for our experiments and supported the fields activities. Data Availability The data used to support this study are available from the corresponding author upon request. References Abeba Kenea, S., Abera Goshu, T. and Chimdessa, K. (2024) Examining the effect of combined biochar and lime rates on selected soil physicochemical properties of acid soils in Gimbi district, western Ethiopia. Applied and Environmental Soil Science , 2024 (1), p.4440448. https://doi.org/10.1155/2024/4440448. Adekiya, A.O., Ayorinde, B.B. and Ogunbode, T., (2024) Combined lime and biochar application enhances cowpea growth and yield in tropical Alfisol. 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Supplementary Files SupplementaryTables.docx Cite Share Download PDF Status: Published Journal Publication published 14 Feb, 2026 Read the published version in Nutrient Cycling in Agroecosystems → Version 1 posted Editorial decision: Revision requested 13 Oct, 2025 Reviews received at journal 06 Oct, 2025 Reviews received at journal 29 Sep, 2025 Reviewers agreed at journal 08 Sep, 2025 Reviewers agreed at journal 05 Sep, 2025 Reviewers invited by journal 03 Sep, 2025 Editor assigned by journal 28 Aug, 2025 Submission checks completed at journal 28 Aug, 2025 First submitted to journal 25 Aug, 2025 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. 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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-7456620","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":511644615,"identity":"b42ff682-8b54-47a4-9ec1-10b7ec7133ac","order_by":0,"name":"Getahun Dereje","email":"data:image/png;base64,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","orcid":"","institution":"Ethiopia Institute of Agricultural Research (EIAR)","correspondingAuthor":true,"prefix":"","firstName":"Getahun","middleName":"","lastName":"Dereje","suffix":""},{"id":511644617,"identity":"8f292656-b1ba-4d56-afb7-b58412dab80a","order_by":1,"name":"Lemma Wogi","email":"","orcid":"","institution":"Haramaya University","correspondingAuthor":false,"prefix":"","firstName":"Lemma","middleName":"","lastName":"Wogi","suffix":""},{"id":511644619,"identity":"12c9efe6-1a6a-49b7-bca1-0e530472f448","order_by":2,"name":"Temesgen Desalegn","email":"","orcid":"","institution":"Ethiopia Institute of Agricultural Research (EIAR)","correspondingAuthor":false,"prefix":"","firstName":"Temesgen","middleName":"","lastName":"Desalegn","suffix":""},{"id":511644620,"identity":"76b963ac-2245-4577-999f-4d44bc095724","order_by":3,"name":"Tesfaye Shiferaw Sida","email":"","orcid":"","institution":"International Maize and Wheat Improvement Center (CIMMYT)","correspondingAuthor":false,"prefix":"","firstName":"Tesfaye","middleName":"Shiferaw","lastName":"Sida","suffix":""},{"id":511644621,"identity":"14627ffe-1473-4c77-86fa-486308e12ae3","order_by":4,"name":"João Vasco Silva","email":"","orcid":"","institution":"International Maize and Wheat Improvement Center (CIMMYT)","correspondingAuthor":false,"prefix":"","firstName":"João","middleName":"Vasco","lastName":"Silva","suffix":""}],"badges":[],"createdAt":"2025-08-25 19:38:12","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7456620/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7456620/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10705-026-10471-7","type":"published","date":"2026-02-14T15:58:37+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":90990814,"identity":"d5c0d3eb-879b-40c5-b5e2-1d8230565e4e","added_by":"auto","created_at":"2025-09-10 11:10:21","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":28915,"visible":true,"origin":"","legend":"\u003cp\u003eMean monthly temperature and precipitation in (A) Kersa, (B) Bedele, and (C) Mettu districts, Southwestern Oromia, Ethiopia, from 2009-2023 (Ethiopian Meteorological Institute, Jimma Station).\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7456620/v1/9bf1cec2d4f46aec66a81825.png"},{"id":90990812,"identity":"cd3434ff-3c4d-4e7e-b539-feb454cef716","added_by":"auto","created_at":"2025-09-10 11:10:21","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":475350,"visible":true,"origin":"","legend":"\u003cp\u003eMaize yield response to P fertilizer and soil amendments under low and high acidity saturation (AS) in Southwestern Oromia, Ethiopia. Data refer to estimated marginal means from linear mixed models predicted for low (10% of ECEC) and high (30% of ECEC) acidity saturation defined based on the distribution of the initial acidity saturation across experimental fields. Year-specific models were fitted to the data to assess the effect of soil amendments in the year of application (year 1) and the associated residual effects in the subsequent year (year 2). Horizontal dashed lines indicated the maize yield obtained with 45 kg P ha\u003csup\u003e-1\u003c/sup\u003e without soil amendments.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7456620/v1/3bd4ac96fc138211e78df60b.png"},{"id":90990817,"identity":"d1ac7a6f-0a1f-441f-aff7-cbda0ec3e66d","added_by":"auto","created_at":"2025-09-10 11:10:21","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":227901,"visible":true,"origin":"","legend":"\u003cp\u003eMaize yield response to soil amendments and P fertilizer in Southwestern Oromia, Ethiopia. Panels on the right display the probability of achieving a given yield response with increased P rates (B) or lime, vermicompost or biochar (D). The vertical line in these panels indicates no yield response, and the horizontal dashed line indicates 50% probability.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7456620/v1/a36ca314ef4303f0d9e31255.png"},{"id":90990966,"identity":"8e398b22-59d2-48fc-9c7a-d5d084e9b650","added_by":"auto","created_at":"2025-09-10 11:18:21","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1031801,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of soil amendments and P fertilizer on chlorophyl content, stomatal conductance, and leaf area index measured at tasseling stage.\u003cstrong\u003e \u003c/strong\u003eData is pooled across the two experimental years. Horizontal red lines display the mean value of each physiological trait across the pooled data. The statistical significance of the main effects, soil amendment, P rate, and initial acidity saturation, on each trait was estimated with linear mixed models and the results are provided in Supplementary Table 1. P1 = no P applied, P2 = 15 kg P ha\u003csup\u003e-1\u003c/sup\u003e, P3 = 30 kg P ha\u003csup\u003e-1\u003c/sup\u003e, P4 = 45 kg P ha\u003csup\u003e-1\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7456620/v1/acc32c10e86f989fd8a32840.png"},{"id":90990964,"identity":"97a9604a-956a-49d4-a249-80a1588cc030","added_by":"auto","created_at":"2025-09-10 11:18:21","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":254674,"visible":true,"origin":"","legend":"\u003cp\u003eRelationships between maize yield and physiological traits measured at tasseling stage.\u003cstrong\u003e \u003c/strong\u003eData were pooled across the two experimental years and across P rates. Vertical lines display the mean value of each physiological trait and horizontal lines display the mean maize yield across the pooled data. Square symbols represent the mean value of each physiological trait and the respective mean maize yield for each soil amendment. Solid lines display quantile regressions fitted to the 90\u003csup\u003eth\u003c/sup\u003e quantile of the data, with a linear functional form in (A) and (C) and a non-linear functional form in (B).\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7456620/v1/226b49a9e767dca9405920ee.png"},{"id":90990822,"identity":"e4d2131c-2bc7-40c8-9ea7-86164eabf4b8","added_by":"auto","created_at":"2025-09-10 11:10:21","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1057322,"visible":true,"origin":"","legend":"\u003cp\u003eEffects of soil amendments and P rates on selected soil properties after harvesting.\u003cstrong\u003e \u003c/strong\u003eData are pooled across the two experimental years, i.e., soil samples were collected and analyzed after the harvest of one or two maize crops. Horizontal red lines display the mean value of each soil property across the pooled data. The statistical significance of the main effects, soil amendment and P fertilizer, on each soil property was estimated with linear mixed models and the results are provided in Supplementary Table 2. P1 = no P applied, P2 = 15 kg P ha\u003csup\u003e-1\u003c/sup\u003e, P3 = 30 kg P ha\u003csup\u003e-1\u003c/sup\u003e, P4 = 45 kg P ha\u003csup\u003e-1\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7456620/v1/37ad55929b01656b0868b652.png"},{"id":90991772,"identity":"e5f9835d-4a44-4000-b29f-13070d5392bf","added_by":"auto","created_at":"2025-09-10 11:26:21","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":199711,"visible":true,"origin":"","legend":"\u003cp\u003eMaize yield response to post-harvest soil properties.\u003cstrong\u003e \u003c/strong\u003eData is pooled across the two experimental years and across P rates. Vertical lines display the mean value of each soil property and horizontal lines display the mean maize yield across the pooled data. Square symbols represent the mean value of each soil property and the respective mean maize yield for each soil amendment. Solid lines display quantile regressions fitted to the 90\u003csup\u003eth\u003c/sup\u003e quantile of the data, with a non-linear functional form for all soil properties except for acidity saturation for which a linear functional form was fitted.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-7456620/v1/faeeb1da57920fafaf5d4ed9.png"},{"id":102785292,"identity":"48b04b8c-2657-4342-b725-db9c5619f87a","added_by":"auto","created_at":"2026-02-16 16:04:33","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6031011,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7456620/v1/03d6a752-e517-451b-9d76-7b7da58565c2.pdf"},{"id":90990810,"identity":"fa593159-58e1-4371-a2d0-3cb90b5938f2","added_by":"auto","created_at":"2025-09-10 11:10:21","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":20909,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryTables.docx","url":"https://assets-eu.researchsquare.com/files/rs-7456620/v1/efa672a4d6612da2a1767a1c.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Soil amendments and phosphorus fertilizer increase maize productivity and improve the fertility of acidic soils in Southwestern Ethiopia","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSoil acidification is a global challenge that threatens food and nutrition security (Du et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). This phenomenon is particularly pronounced in humid tropical regions, where soil productivity decline is exacerbated by aluminum (Al) and manganese (Mn) toxicity and severe nutrient imbalances (Sanchez, \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Agegnehu et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Warner et al., 2023; Silva et al., \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). A major consequence of soil acidification is the sorption of phosphorus (P), a process governed by soil pH, clay mineralogy, and the abundance of iron (Fe) and Al (sesqui)oxides (Elias \u0026amp; Agegnehu, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) which restrict P availability and uptake by crops (Tiecher et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Lei et al., \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Beyond nutrient availability, soil acidification also affects crop performance (Enesi et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e, Gurmu et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Al toxicity leads to poor root development and negatively affects photosynthesis through reduced stomatal conductance, chlorophyll content, and overall photosynthetic efficiency (Vasconcelos et al., \u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Guo et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Increasing the fertility of acidic tropical soils thus requires an integration of soil management practices to reduce exchangeable acidity and/or to increase soil nutrient availability and mitigate physiological stressors to crop productivity (Morel et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Tiecher et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eSoil acidity is a recognized constraint to agricultural productivity in Ethiopia, particularly in the southwestern highlands of Oromia, where soil pH typically ranges between 4.5 (very strongly acidic) and 5.5 (strongly acidic) (Eyasu, 2016; Silva et al., \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Despite unfavorable soil conditions, farmers grow staple crops with minimal or no application of soil amendments. This has been further exacerbating nutrient depletion and limiting crop productivity (Takala, \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Sori, 2021). Maize, a key staple crop for food security and a major source of livelihoods in the region, is particularly vulnerable to soil acidity (Farina \u0026amp; Channon, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1991\u003c/span\u003e; Zingore et al., \u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Oumer et al., \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The Southwestern part of Ethiopia is an important maize production region, accounting for 31% of the national maize harvested area and 53% of the national maize production. Maize yields in the region are on average 4.6 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (CSA, 2022), slightly higher than the national average (4.2 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), but considerably lower than what is possible to achieve with best agronomic practices (Assefa et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Recent studies point to the importance of soil management practices that remediate soil acidity towards improving crop productivity and fertilizer use efficiency in the country (Asfaw et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eAgricultural lime, biochar, and vermicompost are recognized soil amendments to manage soil acidity, improve P availability and soil fertility, ultimately leading to increased crop productivity (Zingore et al., \u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Terefe et al., 2023; Tiecher et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Agricultural lime is an effective input to neutralize exchangeable acidity and increase pH in the topsoil by supplying essential cations that reduce Al toxicity (Enesi et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Ejigu et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Biochar is an alkaline by-product of biomass pyrolysis, and an effective means to enrich soils with carbon. Its high surface charge density, extensive surface area, and internal porosity facilitate metal adsorption and nutrient retention, thereby improving soil pH and crop growth (Bolan et al., 2021; Huang et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Pandian et al., \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Bhattacharyya et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Lastly, vermicompost, i.e., a product rich in organic matter, available nutrients, humic acids, and plant growth-promoting hormones, is effective in increasing soil pH, nutrient availability, microbial activity, and carbon sequestration (Toor et al., \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Raza et al., \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The integration of these soil amendments with mineral fertilizers has demonstrated significant potential in the reclamation of acidic soils by improving soil chemical properties, nutrient cycling, and physiological traits critical to crop growth (Abeba et al., 2024; Iticha et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe contribution of different soil amendments to increase crop productivity and P use efficiency varies with of the properties of acidic soils. Yield benefits can occur due to increases in soil P availability (higher intercept of yield response curves), increases in the yield response to P due to improved growing conditions (higher slope, i.e., yield per unit of applied P), and/or increases in the attainable yield at high levels of applied P (higher plateau). The effectiveness of soil amendments in neutralizing soil acidity and improving P use efficiency and crop productivity likely varies with inherent soil fertility, particularly in relation to exchangeable acidity. Soil amendments with high neutralizing capacity, such as lime, are essential for strongly acidic soils where toxic levels of exchangeable Al are a primary constraint to crop production (Basak et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Enesi et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Conversely, nutrient-rich amendments with lower neutralizing capacity, such as biochar and vermicompost, are likely most effective in soils where nutrient deficiencies, rather than exchangeable acidity, are the dominant productivity constraint (Terefe et al., 2023; Toor et al., \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Although substitution and complementarity effects between soil amendments and P fertilizer can be expected to some extent (e.g., Alemu et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Kisinyo et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), their presence and magnitude remain poorly studied. Understanding such effects supports targeting interventions for acid soil management that can increase crop productivity effectively and in the most profitable way.\u003c/p\u003e\u003cp\u003eThe objective of this study was to evaluate the effects of soil amendments and P fertilizer rates on maize productivity and soil fertility on acidic soils of Southwestern Oromia, Ethiopia. Researcher managed on-farm trials were conducted over two consecutive growing seasons to assess first-year yield responses and residual effects to applied inputs under low and high acidity saturation. It was hypothesized that maize yield and P use efficiency are primarily constrained by topsoil exchangeable acidity, and that targeted nutrient management strategies are most effective when tailored to site-specific soil fertility constraints. Given the high and stable rainfall in the study area, it is expected that crop physiological responses are predominantly influenced by soil chemical properties rather than by water availability during the growing season. Our findings provide practical insights for optimizing soil nutrient availability and management in acidic soils of Southwestern Oromia and other comparable agroecosystems.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Description of the study area\u003c/h2\u003e\u003cp\u003eThe study was conducted during the 2023 and 2024 cropping seasons in Kersa, Bedele, and Mettu districts of Southwestern Oromia, Ethiopia. The study area ranges in altitude from 617 to 3,231 m above sea level and is characterized by a bimodal rainfall pattern, with the main rainy season occurring from June to September. Annual rainfall ranges between 1,200 and 2,800 mm, with two rainy seasons (mid-February to May and June to September, often extending into October; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Mean monthly temperatures vary between 10.7\u0026deg;C and 28.5\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Nitisols are the dominant soil type in the study area with soil pH (H\u003csub\u003e2\u003c/sub\u003eO) commonly ranging between 4.5 and 5.5, contributing to large tracts of arable land affected by soil acidity (ATA, 2014; Eyasu, 2016). Maize is the most important staple crop grown in the study area by smallholders as part of rainfed mixed crop-livestock farming systems, being planted in May and harvested in November. Legumes such as soybean are also an important component of local cropping systems.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e\u003cb\u003e2.2 Experimental setup and trial design\u003c/b\u003e\u003c/h2\u003e\u003cp\u003eA soil survey was conducted on 38 farmers\u0026rsquo; fields to support the selection of the farms hosting the field experiments. Accordingly, 14 farmers\u0026rsquo; fields, with soil pH (H\u003csub\u003e2\u003c/sub\u003eO) ranging between 4.32 and 5.45, were selected and delineated for the field experiment. The soil properties of the selected fields prior to soil amendment and fertilizer application are provided in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The selected fields were characterized by a clay soil texture, relatively high effective cation exchange capacity (ECEC) and soil organic matter, and low P availability regardless of the district. There was however a gradient in exchangeable acidity and acidity saturation across districts, with higher levels observed in Bedele, intermediate in Mettu, and lower in Kersa. Unlike in the other districts, sites in Bedele were also affected by high concentrations of exchangeable acidity in the subsoil (data not shown).\u003c/p\u003e\u003cp\u003eThe experiment tested 4 soil amendments in interaction with 4 P fertilizer rates, comprising a total of 16 treatments following a split-plot design considering farmers\u0026rsquo; fields as replicates. Soil amendments, calcitic lime (CaCO₃), biochar, and vermicompost were allocated to the main plots. Lime rates were determined for each field based on exchangeable acidity (Eq.\u0026nbsp;1). Biochar was applied at a rate of 10 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e as per local recommendation (Abeba et al., 2024). Vermicompost was also applied at the same 10 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e rate determined based on its N fertilizer equivalents required to meet the recommended N rate of 92 kg N ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for maize in the study area. Soil amendments were only applied in the first year of the experiment, prior to sowing, hence results from the second year refer to residual effects of the amendments beyond the year of application. P fertilizer rates were allocated to the subplots and determined relative to the recommendation of 30 kg P ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for maize in the study area. The four P fertilizer rates tested corresponded to 0%, 50%, 100% and 150% of the recommended P rate, equivalent to 0, 15, 30 and 45 kg P ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. P was applied in both growing seasons to the respective plots treated with each amendment.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Field layout and crop management practices\u003c/h2\u003e\u003cp\u003eThe experimental fields were prepared using the traditional \u003cem\u003eMaresha\u003c/em\u003e plough to a depth of 30 cm. A field layout was established based on the experimental protocol and treatments were randomly assigned to experimental units within each block. Each plot measured 4.5 m \u0026times; 4.0 m (18 m\u0026sup2;), with 1.5 m between blocks and 0.75 m between plots. Each plot contained six rows, each 4.5 m long, with a row spacing of 0.75 m and intra row spacing of 0.3 m to achieve a target maize population of 44,444 plants ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. To minimize boundary effects, the central four rows were designated as the net plot area for data collection, crop monitoring and harvesting. A hybrid maize variety, \u003cem\u003eBH 661\u003c/em\u003e released in 2008, was planted in all experimental sites. This variety is well adapted to mid-altitude regions with annual rainfall of 1,000\u0026ndash;1,500 mm yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and is characterized by high yield potential (Legesse et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). N fertilizer in the form of urea (46% N) was applied uniformly to all treatments at a rate of 92 kg N ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the recommended N fertilizer rate for maize in the study area, using a split application method with half the rate applied at sowing and the other half at 35\u0026ndash;40 days after sowing. P fertilizer rates were applied as a basal dose at sowing in the form of triple superphosphate (TSP, 46% P\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\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\u003eInitial soil properties of the experimental sites in Kersa, Bedele and Mettu districts of Southwestern Oromia, Ethiopia. Soil samples were collected prior to soil amendment and fertilizer application in 2023 and analyzed at Holeta Agricultural Research Center.\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=\"left\" 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\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eKersa\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eBedele\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eMettu\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eMean\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eParticle size distribution\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eClay (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e57.00\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e70.30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e69.19\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e66.50\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSilt (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e32.30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e19.50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e20.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e23.93\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSand (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e10.70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e7.20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e10.80\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e9.57\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTextural class\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eclay\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eclay\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eclay\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eclay\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eExchangeable properties\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCEC (cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e18.07\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e22.99\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e19.83\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e20.30\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCa (cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e6.59\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3.71\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e6.59\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e5.63\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMg (cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.94\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.28\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2.27\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.83\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eK (cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.48\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.80\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.72\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.33\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNa (cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.02\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eSoil acidity parameters\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSoil pH (H\u003csub\u003e2\u003c/sub\u003eO)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e5.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e4.56\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e4.90\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e4.83\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eExchangeable acidity (cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.74\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3.73\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2.17\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.21\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eExchangeable Al (cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.54\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3.40\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.79\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.91\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAcid saturation (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e5.18\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e15.75\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e6.19\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e9.04\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eSoil organic carbon and nutrients\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eOrganic C (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.56\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.58\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2.28\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.14\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTotal N (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.20\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eP-Bray II (ppm)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8.64\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e4.27\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e8.80\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e7.23\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCu (ppm)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.57\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.89\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.80\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eZn (ppm)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.91\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.61\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.73\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.08\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMn (ppm)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e105.43\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e34.77\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e64.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e68.08\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFe (ppm)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e39.33\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e19.05\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e20.62\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e26.33\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 was uniformly broadcast across the limed plots two weeks before sowing in the first year of the experiment and thoroughly incorporated into the soil with a hand hoe to enhance its neutralization capacity. The calcitic lime was sourced from Holetta Agricultural Research Center and it had a CaCO₃-equivalent (CCE) of 93.7%. The CCE was used to adjust the field-specific lime rate estimated with Eq.\u0026nbsp;1 (Kamprath, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e1970\u003c/span\u003e) to ensure the required amount of CaCO\u003csub\u003e3\u003c/sub\u003e was applied in each field. The lime requirement method of Kamprath (\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e1970\u003c/span\u003e) establishes the relationship between the concentration of exchangeable acidity in the soil and the amount of CaCO\u003csub\u003e3\u003c/sub\u003e equivalent required to neutralize it (Eq.\u0026nbsp;1):\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:LR,CaC{O}_{3}\\hspace{0.33em}\\left(\\frac{kg}{ha}\\right)\\hspace{0.33em}=\\hspace{0.33em}\\frac{\\hspace{0.33em}EA\\left(\\frac{\\text{c}\\text{m}\\text{o}\\text{l}(+)\\:}{kg}\\right)*\\hspace{0.33em}SD\\hspace{0.33em}\\left(m\\right)*\\hspace{0.33em}1{0}^{4}\\hspace{0.33em}{m}^{2}\\hspace{0.33em}*\\hspace{0.33em}BD\\left(\\frac{Mg}{{m}^{3}}\\right)\\hspace{0.33em}*\\hspace{0.33em}1000}{2000\\:}*EF\\:(\\text{E}\\text{q}.1)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere EA refers to the exchangeable acidity concentration (in cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), SD to the soil depth assumed to be 15 cm, BD to the soil bulk density (in Mg m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e), and EF to an efficiency factor equal to 1.5 accounting for liming efficiency (not all applied lime reacts when applied), soil variability, or field losses. All other values depict unit conversions. The estimated lime rate was on average 3.57 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e across all experimental fields.\u003c/p\u003e\u003cp\u003eLaboratory analyses were carried out at Holeta Agricultural Research Center to determine the nutrient composition of the biochar and vermicompost used in the experiments prior to their application (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Alike lime, biochar and vermicompost were also applied two weeks before sowing in the first year of the experiment through broadcast and thoroughly incorporated in the soil with a hand hoe. Biochar was produced from locally available coffee husks whereas vermicompost was prepared from farmyard manure. Both were obtained from the Jimma Agricultural Research Center. Standard agronomic practices recommended for maize cultivation in the study area were applied to ensure optimal crop management throughout the experiment.\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\u003eSelected properties of vermicompost and biochar amendments used for the experiments. Biochar was produced from coffee husks whereas vermicompost was prepared from farmyard manure. Both were sourced from the Jimma Agricultural Research Center.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\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=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eProperties\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eVermicompost\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eBiochar\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003epH (1:2.5)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e8.78\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e9.54\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTotal N (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e0.92\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.63\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAvailable P (ppm)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e230.00\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e190.00\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eExchangeable K (ppm)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e0.71\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eOrganic C (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e6.73\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e31.50\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC:N ratio\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e7.30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e50.00\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCEC (cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e45.52\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eExchangeable Ca (cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e3.15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eExchangeable Mg (cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1.63\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\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=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Crop physiological measurements and yield determination\u003c/h2\u003e\u003cp\u003ePhysiological traits were assessed at tasseling stage with non-destructive methods on five representative plants selected from the middle rows of each plot. Leaf area was estimated by measuring the fully expanded green leaves of five plants within a defined area. Leaf length and width were recorded, and the leaf area was calculated using the method described by McKee (\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e1964\u003c/span\u003e, Eq.\u0026nbsp;2):\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:Leaf\\:area\\left(cm2\\right)=\\text{L}\\text{e}\\text{a}\\text{f}\\:\\text{l}\\text{e}\\text{n}\\text{g}\\text{t}\\text{h}\\left(\\text{c}\\text{m}\\right)*Leaf\\:width\\:\\left(cm\\right)*0.75\\:(\\text{E}\\text{q}.2)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere leaf Length refers to the length of the leaf from the base to tip (in cm), leaf width to the maximum width of the leaf (in cm) and 0.75 is an empirical correction factor determined by McKee (\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e1964\u003c/span\u003e). Then leaf area index (LAI, cm\u003csup\u003e2\u003c/sup\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) was determined as the ratio of the total leaf area of the five plants to the area occupied by these plants based on the spacing between plants between two rows and within a row, following the method of Diwaker and Oswalt (\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1992\u003c/span\u003e). Total chlorophyll content was measured at the time of sampling using a SPAD chlorophyll meter (Konica Minolta Sensing, Inc., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Stomatal conductance (mmol m⁻\u0026sup2; s⁻\u0026sup1;) was assessed at midday using an AP4 porometer (Delta-T Devices, Cambridge, UK) on the abaxial surface of five fully expanded, attached leaves. Both chlorophyll content and stomatal conductance were measured at the time of sampling from the same five plants per plot where the leaf measurements were conducted, with five readings taken per plant. Maize grain yield was assessed 20 days after physiological maturity, allowing for adequate field drying. Maize ears were harvested and shelled manually, after which the total grain weight from the net plot area (12 m\u0026sup2;) was recorded. The grain moisture content was measured with a grain moisture meter and used to adjust maize yields to a standard 12.5% moisture content.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.5 Soil sampling, preparation and analysis\u003c/h2\u003e\u003cp\u003eSoil samples were collected prior to sowing with an auger from the corners and center of the 14 experimental fields in a zigzag pattern at a depth of 0\u0026ndash;20 cm. After harvest, soil samples were collected from the corners and center of each plot, resulting in 5 subsamples per treatment. The subsamples were thoroughly homogenized to form a composite soil sample for each site-treatment combination. Labeled samples were placed in plastic bags and transported to the Holeta Agricultural Research Center for laboratory analysis. At the laboratory, soil samples were air dried, crushed, and ground before being sieved through a 2 mm mesh, except for those intended for total N and organic carbon analysis, which were also sieved through a 0.5 mm mesh to ensure a finer particle size for accurate assessment.\u003c/p\u003e\u003cp\u003eSoil particle size distribution was analyzed using the hydrometer method according to Day (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1965\u003c/span\u003e). Soil pH (H\u003csub\u003e2\u003c/sub\u003eO) was measured using a pH meter according to Rhoades et al. (\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). Organic carbon content was determined by the wet oxidation method of Walkley and Black (\u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e1934\u003c/span\u003e), in which carbon is oxidized under standard conditions using potassium dichromate in a sulphuric acid solution. Finally, soil organic matter content was calculated by multiplying the percent organic carbon by 1.724. Total N was analyzed by Kjeldahl digestion (Jackson, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2003\u003c/span\u003e), while available P was extracted with the Bray II method (Bray and Kurtz, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1945\u003c/span\u003e) and quantified spectrophotometrically after color development. Exchangeable cations (Ca\u003csup\u003e2+\u003c/sup\u003e, Mg\u003csup\u003e2+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, and Na\u003csup\u003e+\u003c/sup\u003e) were extracted with a 1M ammonium acetate (NH₄OAc) solution at pH 7.0 and analyzed by atomic absorption spectrometer (Okalebo et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). Cation exchange capacity (CEC) was determined at pH 7 from samples saturated with ammonium acetate, with ammonium quantified by the modified Kjeldahl method (Okalebo et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). Exchangeable acidity (i.e., the sum of Al\u0026sup3;⁺ and H⁺ cations) and exchangeable aluminum (Al\u0026sup3;⁺) were quantified by saturating soil samples with a 1N KCl solution followed by titration with 0.02N NaOH and 0.02N HCl, respectively (Rowell, \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e1994\u003c/span\u003e). Acidity saturation was calculated as the ratio of exchangeable acidity to the ECEC, determined by sum of exchangeable Ca, Mg, K, and Na and exchangeable acidity. Available micronutrients (Fe, Mn, Zn, and Cu) were extracted with diethylenetriamine pentaacetic acid (DTPA) according to Tan (\u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) and measured by atomic absorption spectrometer.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e2.6 Statistical analysis\u003c/h2\u003e\u003cp\u003eMaize yield response to soil amendments and P fertilizer was evaluated using linear mixed models. Linear mixed models account for fixed and random effects to represent non-independent data structures. Fixed effects indicate the average effect of the predictors on the response variable (maize yield) and were specified as the three-way interaction between soil amendments (main plot), P rate (subplot), and initial acidity saturation. Soil amendments and P rate were defined as factors and the initial acidity saturation as a continuous variable. Random effects capture the variability in maize yield at different levels of grouping within the data and were specified as the soil amendments (main plot) nested within experimental sites (farms) to account for the split-plot design of the experiment. Year-specific models were fitted to the data using the \u003cem\u003elmer()\u003c/em\u003e function of the lmerTest R package (Kuznetsova et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) to account for maize yield responses to soil amendments in the year of application and in the subsequent year to assess residual effects. The significance of the fixed effects was evaluated at 5% significance level with analysis of variance (ANOVA) using the \u003cem\u003eanova()\u003c/em\u003e function in R. Marginal means were estimated for the different soil amendments and P rates, under low (10% of ECEC) and high (30% of ECEC) acidity saturation conditions, using the \u003cem\u003eemmeans()\u003c/em\u003e function of the emmeans R package (Searle et al., \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e1980\u003c/span\u003e). Acidity saturation thresholds were defined based on the variation observed in the acidity saturation of the experimental sites prior to input application.\u003c/p\u003e\u003cp\u003eCumulative probability plots were developed to quantify the probability of achieving a given maize yield response to the different soil amendments and P rates. The effect of soil amendments and P rates on physiological traits at tasseling and on post-harvest soil properties was assessed following the same approach described for maize yield. The physiological traits included chlorophyll content, stomatal conductance, and LAI. The soil properties included pH (H\u003csub\u003e2\u003c/sub\u003eO), exchangeable acidity, available P, soil organic matter, ECEC, acidity saturation, Ca and Mg saturation. Linear mixed models fitted with physiological traits and post-harvest soil properties as dependent variables used the same fixed effects structure as described for the maize yield model. An additive year fixed effect was added to these models, which were fitted to the pooled data. Soil amendments (main plot) nested within experimental sites (farms) were defined as random effects in these mixed models, similarly to the models fitted for maize yield.\u003c/p\u003e\u003cp\u003eThe relationship between post-harvest soil properties and physiological traits at tasseling on the one hand and maize yield on the other were investigated through boundary line analysis. Boundary lines were fitted to the pooled data to identify the maximum yield for a given level of each physiological trait and soil property and to characterize the respective yield response. Boundary lines were identified with quantile regressions fitted to the 90th quantile of the data using the \u003cem\u003enlrq()\u003c/em\u003e function of the quantreg R package (Koenker, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). A non-linear functional form (y\u0026thinsp;=\u0026thinsp;a\u0026thinsp;+\u0026thinsp;b\u003cem\u003ex\u003c/em\u003e\u0026thinsp;+\u0026thinsp;c0.99\u003csup\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sup\u003e, where a, b, and c are parameters to be estimated) was fitted for stomatal conductance and all soil properties, except acidity saturation. A linear functional form (y\u0026thinsp;=\u0026thinsp;a\u0026thinsp;+\u0026thinsp;b\u003cem\u003ex\u003c/em\u003e, where a and b are parameters to be estimated) was justified for chlorophyl content, LAI, and acidity saturation and adopted for these data instead.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Maize yield response to P and soil amendments\u003c/h2\u003e\u003cp\u003eThere was a statistically significant effect (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05) of soil amendments, P fertilizer rates, and initial acidity saturation on maize yields both in the first and the second year of the experiment. The two-way and three-way interactions between these factors were not statistically significant in either year. Maize yield showed a positive response to increasing P rates, which was similar across soil amendments, acidity saturation levels, and years (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Maize yields were on average 5.8 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with no P applied, 6.9 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with 15 kg P ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 7.6 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with 30 kg P ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and 8.1 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with 45 kg P ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. There were no statistically significant yield differences between the two highest P application rates in the first year under low acidity saturation. It is noteworthy that comparable maize yields were obtained with the highest P fertilizer rate without amendments and with lime and vermicompost without P fertilizer. This positive effect of lime and vermicompost on the availability of P was observed across acidity saturation levels and experimental years.\u003c/p\u003e\u003cp\u003eThe maize yield response to soil amendments was greater for lime, followed by vermicompost and biochar regardless of P rate, but there were slight differences between years and acidity saturation conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). In the first year, under low acidity saturation, lime application produced the highest mean grain yield (8.0 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), closely followed by vermicompost (7.9 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA). Biochar also increased maize yield (6.8 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) compared to the control (5.7 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) without amendments. Under high acidity saturation, vermicompost produced the highest yield (6.6 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), followed by lime (6.0 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and biochar (5.5 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB), but yield differences in response to the soil amendments were not statistically significant. The lowest yield was observed in the unamended control, with an average maize yield of 5.7 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003eand 4.1 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e under low and high acidity saturation, respectively. In the second year, lime again produced the highest yield in both low (9.2 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and high (6.8 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) acidity saturation, followed by vermicompost (8.1 and 6.5 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and biochar (6.1 and 4.5 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eC and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eD). Maize yield with biochar was significantly lower than that with lime and vermicompost, and there was no significant difference in maize yield between biochar and the unamended control (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eC and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eD). On average, the unamended control produced the lowest maize yield, 5.4 and 3.6 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e under low and high acidity saturation, respectively.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eMaize yield under low acidity saturation was significantly higher than under high acidity saturation, 7.2 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 5.5 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Moreover, the maximum maize yield was lower under high acidity saturation regardless of P rate and soil amendment. There were also clear differences in maize yield response to soil amendments between the first and the second year, with lime and vermicompost showing considerable residual effects and yield improvements in the second year as opposed to biochar, regardless of acidity saturation (Supplementary Table\u0026nbsp;1). This is indicative of residual effects from lime and vermicompost, but not from biochar, particularly in the second cropping season.\u003c/p\u003e\u003cp\u003eThere was a large variability in maize yield response to application soil amendments and P rates (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Overall, the yield response to P fertilizer was more consistent, i.e., less variable, than the yield response to soil amendments. Maize yield with P fertilizer was significantly greater than in the control without P across all experimental sites (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eA and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eB). Conversely, yield response to soil amendments was more variable, particularly biochar for which a 20% probability of negative yield response was observed (yet the magnitude of this effect was mostly below 1 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eC and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eD). Among the amendments, the yield response to biochar was smaller and more variable than the yield response to lime and vermicompost. There was 50% probability of obtaining a yield response close to 1 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for biochar and between 2 and 3 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for lime and vermicompost (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eD). The yield response to P increased with P rate but the variability in response was comparable across P rates. There was a 50% probability of obtaining a yield response to P of about 1 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with 15 kg P ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 2 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with 30 kg P ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and 2.5 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with 45 kg P ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Overall, these results indicate that lime, vermicompost, and P fertilizer were most effective in increasing maize productivity across all experimental sites.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Effects of P fertilizer and soil amendments on physiological traits\u003c/h2\u003e\u003cp\u003eSoil amendments and P fertilizer impacted maize physiological traits at tasseling stage (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) under different acidity saturation levels (Supplementary Table\u0026nbsp;1). ANOVA indicated that chlorophyll content, stomatal conductance, and LAI were significantly (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) influenced by the main effects of soil amendments and P rates. The two-way interaction between soil amendments and P rates did not have a statistically significant effect on these physiological traits.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThere was a statistically significant difference in the \u003cem\u003echlorophyll content\u003c/em\u003e of maize in response to soil amendments and P rates under both low and high acidity saturation (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA; Supplementary Table\u0026nbsp;1). Application of vermicompost and lime increased chlorophyll content of maize compared to biochar and the control, with the highest value in response to vermicompost, closely followed by lime. Yet, the difference between chlorophyll contents of maize in response to vermicompost and lime application was not statistically significant. Conversely, chlorophyll content of maize grown in the unamended control was the lowest. Biochar application increased chlorophyl content of maize relative to the control, but that remained lower than observed with vermicompost and lime. Total chlorophyll content of maize increased with increasing P rates with the highest value recorded at 45 kg P ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The lowest chlorophyl content was recorded for maize grown without P application. The response of chlorophyl content to P was more pronounced under low acidity saturation.\u003c/p\u003e\u003cp\u003e\u003cem\u003eStomatal conductance\u003c/em\u003e showed a similar trend to chlorophyll content, with significant variation between treatments (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB; Supplementary Table\u0026nbsp;1). The vermicompost and lime amendments produced the highest stomatal conductance values under both acidity conditions, with vermicompost treatments reaching 204.2 mmol m\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup2; s\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup1; under low acidity saturation and 185.5 mmol m\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup2; s\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup1; under high acidity saturation. These values were statistically comparable to those observed with lime application. In contrast, the lowest stomatal conductance was recorded for biochar and the unamended control. P application significantly increased stomatal conductance, with the highest values observed at 45 kg P ha\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup1; (205.6 and 190.2 mmol m\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup2; s\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup1; under low and high acidity saturation, respectively), followed by 30 kg P ha\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup1;. The unfertilized control showed the lowest stomatal conductance. The magnitude of the response was slightly reduced under high acidity saturation.\u003c/p\u003e\u003cp\u003eThe \u003cem\u003eLAI\u003c/em\u003e was affected by soil amendments under low and high acidity saturation (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eC; Supplementary Table\u0026nbsp;1). Vermicompost and lime applications resulted in significantly higher LAI (3.36 and 3.32 m\u003csup\u003e2\u003c/sup\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e under low acidity saturation and 3.2 and 3.0 m\u003csup\u003e2\u003c/sup\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e under high acidity saturation, respectively) compared to biochar and control (2.97 and 2.7 m\u003csup\u003e2\u003c/sup\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, respectively). P fertilizer increased LAI under low and high acidity saturation, with the highest values recorded for 45 kg P ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, while the lowest LAI was observed when P was not applied. Under high acidity saturation, the difference in LAI between the two highest P rates was not statistically significant, suggesting a plateau in the LAI beyond 30 kg P ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e particularly under high acidity saturation.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Relationship between maize yield and physiological traits\u003c/h2\u003e\u003cp\u003eThere was a positive linear relationship between chlorophyl content and LAI on the one hand and maize yield on the other (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eC). The highest maize yield was obtained with chlorophyl content above 50 SPAD units and with LAI above 3 m\u003csup\u003e2\u003c/sup\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, reflecting the importance of light interception and photosynthetic capacity to maize yield. The relationship between stomatal conductance and maize yield was less evident partly due to the large variation in maize yield at low levels of stomatal conductance (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB). High maize yield was obtained with stomatal conductance of 100 mmol m\u0026sup2; s⁻\u0026sup1;. The physiological benefits on maize yield were most pronounced on lime and vermicompost treatments, indicating the effect of these soil amendments on maize growth and yield. Conversely, the biochar and unamended control resulted in sub-optimal physiological performance and reduced yield.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Effects of P fertilizer and soil amendments on soil properties\u003c/h2\u003e\u003cp\u003eSoil amendments and P fertilizer contributed to soil fertility improvements in different ways and to different extents, as observed in the results of post-harvest soil properties (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Soil pH, organic matter, available P, and ECEC were significantly influenced by the main effects of soil amendments and P rates (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05). Conversely, acidity saturation and Ca saturation were only influenced by the main effect of soil amendments. The two-way interaction effects of soil amendments and P rates on the evaluated soil properties was not significant. There were a statistically significant (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) residual effects of amendments on soil pH, organic matter, ECEC, Ca saturation and acidity saturation during the second cropping season but not on available P.\u003c/p\u003e\u003cp\u003e\u003cem\u003eSoil pH\u003c/em\u003e increased with the application of soil amendments and increasing P rates (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eA). On average, lime raised soil pH by 13.5%, vermicompost by 6.7%, and biochar by 4.1% over the control. The most pronounced pH increase was observed with lime. Vermicompost and biochar resulted in moderate pH increases, with vermicompost showing slightly more neutralization effect than biochar. The lowest pH values from 4.6 to 4.9 were recorded in the unamended control. Increases in P rates did not increase soil pH considerably.\u003c/p\u003e\u003cp\u003eApplication of amendments significantly increased \u003cem\u003esoil organic matter\u003c/em\u003e relative to the control (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eB; Supplementary Table\u0026nbsp;2). Vermicompost and biochar, increased soil organic matter to 3.92%. Lime resulted in a comparable increase in soil organic matter to an average of 3.96 g kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. There were no statistically significant mean differences in soil organic matter between amendments, indicating similar efficacy of the soil amendments in increasing soil organic matter. Soil organic matter content was also affected by P rates, increasing from 3.72% at 0 kg P ha⁻\u0026sup1; to 3.94% at 45 kg P ha⁻\u0026sup1;.\u003c/p\u003e\u003cp\u003eThe \u003cem\u003eECEC\u003c/em\u003e values in the lime treatments were higher, with median value 11.5 cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, slightly above the mean ECEC across the pooled data for all P rates (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eC). Application of lime increased ECEC by 17.8% over the control. Conversely, application of vermicompost and biochar showed no significant change in ECEC of the soil highlighting the unique capacity of lime to increase cation retention. Median ECEC values were below the mean ECEC across the pooled data for soil amended with biochar, vermicompost and the control. The effect of P rates on ECEC was not significant and consistent across soil amendments.\u003c/p\u003e\u003cp\u003e\u003cem\u003eAcidity saturation\u003c/em\u003e was significantly reduced due to lime application to levels below 20% of the ECEC, the critical threshold for maize (Aramburu Merlos et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Farina \u0026amp; Channon, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1991\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eD). Indeed, nearly 75% of the soil samples from the lime treatments had acidity saturation below this threshold and median acidity saturation of about 5% of the ECEC, which indicates that lime was effective in neutralizing exchangeable acidity. Vermicompost also reduced acidity saturation, but not as consistently as lime. The median acidity saturation after vermicompost application was between 10 and 15% of the ECEC but some sites had levels above the critical threshold. No substantial reductions in acidity saturation were observed after biochar application (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eE). Lime increased Ca saturation to median values of about 45% of the ECEC whereas vermicompost and biochar had little impact on Ca saturation, with median values of 25\u0026ndash;35% of the ECEC. Reductions in acidity saturation and increases in Ca saturation were thus primarily driven by lime and, to a less extent, by vermicompost rather than by biochar. Biochar is thus not effective in neutralizing soil acidity within one year of application.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSoil \u003cem\u003eavailable P\u003c/em\u003e increased in response to the application of soil amendments and with P fertilizer rates (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eF). Lime and vermicompost significantly increased soil available P, Median soil available P was 18 mg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for the lime treatments, close to the sufficient threshold of 15 mg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 14 mg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for vermicompost, 10 mg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for biochar, and 9 mg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for the control. P rates also showed a direct impact on soil available P, which increased with increasing rates regardless of soil amendments. Increases in soil available P were particularly evident at high P rates in combination with lime or vermicompost, indicating the efficacy of these soil amendments in increasing the availability of immobilized P for crop nutrition.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003e3.5 Maize grain yield response to improvements in soil properties\u003c/h2\u003e\u003cp\u003eMaize yield was highly responsive to soil pH, ECEC, acidity saturation, Ca saturation, and available P, and not as much to soil organic matter (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). High maize yields were obtained with soil pH above 4.9, organic matter of about 4%, ECEC about 10 cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, acidity saturation below 20% of ECEC, Ca saturation above 30% of ECEC, and available P above 12 mg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Boundary lines depicting the relationship between maize yield on the one hand and soil pH, ECEC, Ca saturation, and available P on the other showed a positive response with diminishing returns and potential detrimental effects beyond a threshold for each soil property (except available P). The positive yield response observed for these soil properties was largely explained by the application of lime and, to a less extent, vermicompost. The boundary line depicting the relationship between maize yield and acidity saturation was linear negative, pointing to severe yield reductions at high acidity saturation particularly in the unamended control and after biochar or vermicompost. Lastly, the relationship between maize yield and soil organic matter revellead a quadratic functional form with a maximum at about 4%, indicating that increasing soil organic matter beyond this level is not required to increase crop productivity in the study area. These findings underscore the need for integrated soil fertility management strategies that combine the correction of soil pH and exchangeable acidity with nutrient enrichment towards increasing crop productivity on acidic soils.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003e4.1 Yield response to P on acidic soils of Southwestern Oromia\u003c/h2\u003e\u003cp\u003eP fertilizer and soil amendments are essential inputs to sustain and increase crop productivity and soil fertility, particularly in acidic soils (Zhang et al., \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Zingore et al., \u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Howe et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Our results revealed strong maize yield responses to P in the absence of soil amendments, highlighting considerable maize yield response to P fertilizer in the study area. P deficiency is a well-known constraint to cereal production in Ethiopia, particularly in highland regions dominated by low pH with high P sorption (Agegnehu et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Dinssa and Elias, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Kenea et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Agegnehu et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Yimer et al., \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). In Southwestern Ethiopia, where Nitisols and Acrisols prevail, P is rapidly immobilized by Fe and Al (sesqui)oxides, severely limiting its availability to crops (Elias and Agegnehu, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The yield response to P observed in our study thus aligns with this broader evidence characteristic of high rainfall, clay-rich agroecosystems of Southwestern Oromia and other parts of East Africa (Agegnehu et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Kenea et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2021\u003c/span\u003e)\u003c/p\u003e\u003cp\u003eWe also found that maize yield and P use efficiency were limited by topsoil acidity saturation and that soil amendments are important when combined with P inputs. For instance, maize yield increased by up to 40% following diminishing returns to applied P, particularly in the first year of the experiment, yet it was consistently lower (1.7 t ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e on average) under high acidity saturation when soil amendments were not applied. The impact of soil acidity on crop yield response to P is well documented (Salinas \u0026amp; Sanchez, 1980; Hao et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Omenda et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Lei et al., \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) and can be explained by a combination of factors associated with low soil pH, particularly Al toxicity and P sorption, which were indeed detectable in some of the experimental sites. These factors are known to pose major chemical barriers to nutrient uptake and root development by crops and to have a negative impact on P use efficiency.\u003c/p\u003e\u003cp\u003eA positive additive effect was found between P fertilizer and soil amendments on maize yield and post-harvest soil properties indicating that P fertilizer alone increased crop productivity on acid soils, but not to its highest levels. P fertilizer resulted in considerable increases in soil available P, which likely explains the maize yield response to P observed when soil amendments were not applied. As expected, P fertilizer had a more modest effect on other important soil properties such as soil pH, soil organic matter, ECEC, and acidity saturation. Conversely, soil amendments created more favorable conditions for crop growth, as observed in the relationships between maize yield and post-harvest soil properties (see also Pandit et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) and reduced the variability in the maize yield response to P (except for biochar). These findings underline the effectiveness of combining P fertilizers with lime or vermicompost to enhance maize productivity in the acidic soils of Southwestern Oromia.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003e4.2 Effectiveness of soil amendments for acid soil management\u003c/h2\u003e\u003cp\u003eSoil amendments are essential for improving soil fertility and optimizing maize yields, particularly in acidic soils. Recent interventions, such as the use of lime, vermicompost and biochar, have shown promise in alleviating soil acidity, improving soil fertility, and increasing crop productivity (Agegnehu et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Agumas et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). However, their effectiveness depends on soil fertility constraints that are site-specific, particularly in relation to acidity saturation across the soil profile. Although lime and vermicompost outperformed biochar, maize yield response to these soil amendments varied greatly, emphasizing the need for tailored nutrient management. These results highlight the limitations of uniform, blanket strategies and the importance of site-specific diagnosis and targeted interventions for improving crop productivity in acidic soils (see also Silva et al., \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eOur results showed that lime increased maize yield by about 40% in soils with low acidity saturation and by about 70% in soils with high acidity saturation, underscoring its suitability for acid soil management. While other amendments also provide agronomic benefits, lime was most effective in correcting soil acidity by improving pH and nutrient availability (Enesi et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Agegnehu et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Changes in soil properties due to liming can be explained by the release of cations that reduce of the concentration of exchangeable acidity, raise soil pH and enhance P solubility (Minato et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Ying et al., 2024; Wakwoya et al., \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Alemu et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Kibet et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Ejigu et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Importantly, our results also show that lime can substitute P to some extent, as yield gains from lime alone matched or exceeded those obtained with high P rates without lime. This was true under and low and high acidity saturation and in the year of lime application and in the first year of residual effects. This finding suggests current P recommendations may be insufficient to overcome P limitations on crop growth and suggests that P sorbed in clay-rich soils can be made available through liming in the short-term (Bolo et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Tiecher et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). It is not clear whether this substitution effect between lime and P would occur at P rates higher than the maximum 45 kg P ha\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e tested in our experiment, the point at which P may no longer be limiting. Lastly, the residual effects observed in the second year of the experiment, i.e., sustaining yields without re-application, emphasize the potential economic value of lime for resource-constrained farmers, a topic which merits further research. By increasing fertilizer use efficiency, lime can offer a cost-effective solution for acidic soils, but its current high market price makes it a prohibitive investment for most smallholders (see Oumer et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2023\u003c/span\u003e for further details about the lime value chain in Ethiopia).\u003c/p\u003e\u003cp\u003eVermicompost performed similarly to lime across cropping seasons and acidity saturation levels, increasing maize yield by about 40% under low acidity saturation and 65% under high acidity saturation. Vermicompost thus stands out as an effective option to improve the fertility of acidic soils through increases in soil pH and nutrient availability (Wegene \u0026amp; Wogi, 2023). The positive effects of vermicompost on maize yield across low and high acidity saturation can be explained in two ways. First, vermicompost is rich in N such that the N available for crop growth in vermicompost treatments was about double that of the other treatments. This being the case indicates that current N recommendation rates are likely insufficient to overcome N limitations to maize growth in the region. Second, vermicompost also increases soil fertility in several ways (i.e., increasing soil organic matter and nutrient availability, stimulating microbial activity, and buffering of soil pH; Zhang et al., \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Terefe et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) and can neutralize exchangeable acidity to some extent (Iqbal et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Mulatu \u0026amp; Bayata, \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Future research is required to Disentangle the mechanisms through which vermicompost increases maize yield and the fertility of acidic soils. Vermicompost\u0026rsquo;s dual role as a nutrient source and a partial liming agent makes it valuable for resource-limited farms, yet its profitability must be assessed in future studies. Another aspect to consider when promoting vermicompost is whether it can be made available to farmers in the amount and at the cost needed for it to be effective at the field level in a profitable way.\u003c/p\u003e\u003cp\u003eDespite lime and vermicompost application, maize yields remained lower under high acidity saturation due to a combination of soil and physiological constraints. The efficacy of lime and vermicompost under such conditions can be constrained by insufficient application rates, poor incorporation and neutralization efficiency, and the persistence of subsoil acidity, particularly in high rainfall environments where leaching is prevalent. Yet, the high acidity saturation sites in which our experiments were conducted (Bedele district) were also affected by subsoil acidity, particularly high exchangeable acidity concentrations (3.98\u0026ndash;5.20 cmol\u003csub\u003e(+)\u003c/sub\u003e kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) at 31-200cm depth. Due to its low mobility in the soil profile, calcitic lime was likely not able to remediate subsoil acidity, which might explain why the maize yield response to lime and vermicompost (and biochar in the first year) was comparable under high acidity saturation, a hypothesis that requires validation in future studies.\u003c/p\u003e\u003cp\u003eBiochar was found to moderately increase maize yield under low and high acidity saturation, particularly in the first year. The benefits of biochar stem from its alkalinity and reactive surfaces, contributing to the long-term stabilization of soil organic matter (Huang et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) and to improving soil structure, water retention, and carbon sequestration (Yu et al., \u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Pandian et al., \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). However, its efficacy in acidic soils was limited due to its weak pH buffering capacity and low ability to reduce Al toxicity and to increase P availability. Compared to the other amendments, biochar demonstrated limited potential to increase crop yield and fertilizer use efficiency in acidic soils due to its small impact on acidity saturation, ECEC, and available P. Indeed, biochar\u0026rsquo;s performance was comparable to that of the unamended control for most of soil properties and physiological traits assessed, with yield responses to biochar also showing a 20% likelihood of yield reduction. These findings suggest that, due to its low solubility and limited interaction with the \u0026ldquo;acid soil complex\u0026rdquo;, i.e., the broader soil chemical constraints associated with low pH, biochar lacks the chemical reactivity and nutrient composition required to address the fertility constraints of acidic tropical soils.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\u003ch2\u003e4.3 Recommendations and scope for future research\u003c/h2\u003e\u003cp\u003eSoil acidity has been recognized as an obstacle to agricultural productivity in Ethiopia (Oumer et al., \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Warner et al., 2023) and liming has been prioritized as a key intervention for acid soil remediation by the Ministry of Agriculture and its partners (Tilahun, \u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Yet, its adoption remains limited due to logistical, economic, and awareness challenges, highlighting the need for cost-effective, site-specific and evidence-based solutions (e.g., Oumer et al., \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). There have also been challenges with targeting investments in acid soil management across the country (Agumas et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2025\u003c/span\u003e), despite the increasing awareness of the need to monitor acidity saturation for that purpose (Silva et al., \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Aramburu Merlos et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Sanchez, \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). This study highlights the importance of understanding soil chemical constraints for acid soil management and of acidity saturation as a major driver yield response to P fertilizer and soil amendments. In doing so, it provides robust empirical evidence that integrated soil fertility management, combining lime and/or organic amendments such as vermicompost with P fertilizers, can increase maize productivity (along with fertilizer use efficiency) and improve the fertility in acidic soils. Our results define nutrient management strategies for acidic soils and demonstrate the value of combining organic and inorganic inputs towards sustainable crop production in the Ethiopia's croplands.\u003c/p\u003e\u003cp\u003eFuture research is needed to better unpack interactions between soil amendments and nutrients, particularly N and P, and their impact on crop productivity and fertilizer use efficiency. We were unable to quantify nutrient use efficiency and its components (recovery and physiological efficiencies) in our study, due to the large number of experimental plots and sites. More targeted experiments are required in the future to refine our results and recommendations in this regard. This is particularly important in the case of lime and vermicompost, as these amendments have a direct effect on soil N and P availability to crops. In the case of vermicompost, we recommended assessing its effects against N treatments without its N-equivalent N supply since our study confound N inputs with other vermicompost\u0026rsquo; benefits on soil fertility. This requires full factorial experimental designs combining varying rates of N, P and soil amendments and deployed across contrasting soil conditions. Non-limiting nutrient rates are particularly important in such assessment to unpack additive and/or substitutive effects between inputs in a consistent way. It is also important to run such experiments in the medium- to long-term to assess residual effects of different strategies on crop productivity and soil fertility (particularly P saturation levels). Future assessments could further complement our empirical assessment with modelling tools that capture crop-soil interactions in an integrated way.\u003c/p\u003e\u003cp\u003eAdvancing scientific understanding and ensuring sustainable management of acidic soils in smallholder farming systems requires targeted research in two key areas. Firstly, the agronomic performance, economic viability, and adoption potential of alternative soil amendments, such as lime substitutes derived from fertilizer and other by-products, and integrated organic-mineral formulations, must be rigorously evaluated in Ethiopia. This will guide the rigorous testing of alternative products to lime and unravel their agronomic and economic feasibility for resource-constrained farmers. Secondly, further research is required to better understand liming effects on P use efficiency and to which extent the substitutive effects between lime and P found in our results can be managed over time. Lime improves P uptake and the recovery and internal use efficiencies of P, but the magnitude of such improvements under on-farm conditions remains unquantified. It is also not clear how residual effects from lime and P evolve over time and more research is needed to understand these effects from a cropping systems perspective and in the long-term. These efforts will support designing effective, economically viable, site-specific nutrient management strategies attuned to acidic soil conditions across Ethiopia\u0026rsquo;s smallholder farming systems.\u003c/p\u003e\u003c/div\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eThe study evaluated the effect of P fertilizer rates and soil amendments on grain yield, physiological performance, and soil fertility for maize crops in Southwestern Oromia, Ethiopia. The region is strategic for maize production in the country, due to its large number of smallholder farms and fertile yet acidic soils. Researcher managed on-farm experiments evaluating the productivity and soil fertility merits of P fertilizer in interaction with soil amendments revealed strong maize yield response to P, pointing to the importance of P supplementation in the region. Yet, maize yield response to P was limited by topsoil acidity saturation, which remains an important metric for targeting investments in acid soil management. Soil amendments improved soil fertility by lowering soil acidity and increasing nutrient availability, which in turn had a positive effect on maize growth and maize yield response to P. Our results thus indicate there is scope for targeting soil fertility management strategies to soil chemical properties. Consistent with regional research, we conclude that effective P management in clay-rich acidic soils depends on prior or concurrent soil fertility correction measures to fully realize maize yield potential.\u003c/p\u003e\u003cp\u003eLime was the most effective amendment in increasing crop yield and P use efficiency due to its soil fertility improvements, most notably lowering Al toxicity and increasing P availability, across cropping seasons and experimental sites. Vermicompost provided comparable results to lime on maize yield and soil fertility, due to its dual role as a nutrient source and a partial liming agent. We also cannot exclude that subsoil acidity constrained the yield response to soil amendments in high acidity saturation sites. Substitutive effects were found between lime and vermicompost on the one hand and high rates of P fertilizer on the other, justifying more focus on understanding interactions between nutrients and soil amendments in future research. Biochar contributed to small improvements in maize yield and soil fertility, and caution is needed when recommending it as a standalone soil amendment. An integrated approach combining soil amendments with P fertilizer and guided by routine monitoring of key soil properties is essential to maintain soil fertility and optimize maize productivity. The economic viability of alternative nutrient management options for resource-constrained farmers remains to be evaluated in future research.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eDeclaration of Competing Interest\u003c/h2\u003e\u003cp\u003eThe authors declare they have no conflicting interest.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eGetahun Dereje: Conceptualization; Data curation; Software, Formal analysis; Methodology; Resources; Investigation, Visualization; Writing-original draft. Lemma Wogi: Supervision; Validation; Writing-review and editing. Temesgen Desalegn: Supervision; Validation; Writing-review and editing. Tesfaye Shiferaw Sida: Funding acquisition, Supervision; Validation; Writing-review and editing. Joao Vasco Silva: Software, Formal analysis, Writing-review and editing.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe authors extend their gratitude to the Bill \u0026amp; Melinda Gates Foundation, which funded this research through the project Guiding Acid Soil Investment in Africa (GAIA), grant number INV-002829, as part of a collaboration between the International Maize and Wheat Improvement Center (CIMMYT) and the Ethiopian Institute of Agricultural Research (EIAR). Special acknowledgment goes to the soil laboratories of the Department of Natural Resource Management at Holeta Agricultural Research Center and Jimma Agricultural Research Center for their invaluable assistance in the analysis of soil characteristics and to the farmers who availed their land for our experiments and supported the fields activities.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data used to support this study are available from the corresponding author upon request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAbeba Kenea, S., Abera Goshu, T. and Chimdessa, K. 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Global Change Biol. 28, 154\u0026ndash;166. https://doi.org/10.1111/gcb.15930.\u003c/li\u003e\n\u003cli\u003eZingore, S., Desalegn, T., Diallo, A., Amede, T., Njoroge, S., Diallo, M., Wanjiru, L., Botillen, \u0026Oslash;. (2023) The Implication of Soil Acidity and Management Options for Sustainable Crop Production in Africa, Growing Africa 2(1), 32-38. https://doi.org/10.55693/ga21.IFCZ1970. https://doi.org/10.55693/ga21.IFCZ1970.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nutrient-cycling-in-agroecosystems","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"fres","sideBox":"Learn more about [Nutrient Cycling in Agroecosystems](http://link.springer.com/journal/10705)","snPcode":"10705","submissionUrl":"https://submission.nature.com/new-submission/10705/3","title":"Nutrient Cycling in Agroecosystems","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"lime, vermicompost, biochar, exchangeable acidity, tropical soils","lastPublishedDoi":"10.21203/rs.3.rs-7456620/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7456620/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSoil acidity is a major constraint to crop production in tropical regions, primarily due to aluminum toxicity and reduced nutrient availability. This challenge is widespread in the highlands of Southwestern Oromia, Ethiopia, where maize is a staple and cash crop. This study evaluated the effects of soil amendments and phosphorus (P) fertilization on post-harvest soil properties, maize physiology, and grain yield during the 2023 and 2024 growing seasons across 14 farmers\u0026rsquo; fields in Kersa, Bedele, and Mettu districts. A split-plot design was used with farms as replicates, testing four soil amendments (control, calcitic lime, biochar, vermicompost) and four P rates (0, 15, 30, and 45 kg P ha⁻\u0026sup1;). Calcitic lime and vermicompost markedly improved soil fertility, enhanced maize physiological performance, and increased yields, whereas biochar was less effective. Lime was most effective in raising soil pH, lowering acidity saturation, and increasing P availability, resulting in 38\u0026ndash;78% yield gains over the control. Vermicompost also achieved substantial gains (41\u0026ndash;66%). Although P fertilization consistently increased yield, its efficiency declined under high acidity saturation. Findings indicate that P response is strongly constrained by soil acidity, and effective management in clay-rich acidic soils requires prior or concurrent soil acidity correction. Integrated strategies combining lime or vermicompost with P fertilization significantly enhanced nutrient availability, maize growth, and productivity. These results highlight the importance of site-specific soil fertility management tailored to acidity levels for improving maize yields in acidic tropical soils.\u003c/p\u003e","manuscriptTitle":"Soil amendments and phosphorus fertilizer increase maize productivity and improve the fertility of acidic soils in Southwestern Ethiopia","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-10 11:10:16","doi":"10.21203/rs.3.rs-7456620/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-10-13T20:35:16+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-06T09:25:59+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-29T08:36:28+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"30787165078898466075085841073569346980","date":"2025-09-08T05:52:37+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"51352560169118985787391627545013092179","date":"2025-09-06T00:19:59+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-03T13:58:28+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-29T01:49:44+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-29T01:49:00+00:00","index":"","fulltext":""},{"type":"submitted","content":"Nutrient Cycling in Agroecosystems","date":"2025-08-25T19:28:32+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nutrient-cycling-in-agroecosystems","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"fres","sideBox":"Learn more about [Nutrient Cycling in Agroecosystems](http://link.springer.com/journal/10705)","snPcode":"10705","submissionUrl":"https://submission.nature.com/new-submission/10705/3","title":"Nutrient Cycling in Agroecosystems","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"05f7aff8-34d6-4519-bd05-d22c804e458e","owner":[],"postedDate":"September 10th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-02-16T16:01:34+00:00","versionOfRecord":{"articleIdentity":"rs-7456620","link":"https://doi.org/10.1007/s10705-026-10471-7","journal":{"identity":"nutrient-cycling-in-agroecosystems","isVorOnly":false,"title":"Nutrient Cycling in Agroecosystems"},"publishedOn":"2026-02-14 15:58:37","publishedOnDateReadable":"February 14th, 2026"},"versionCreatedAt":"2025-09-10 11:10:16","video":"","vorDoi":"10.1007/s10705-026-10471-7","vorDoiUrl":"https://doi.org/10.1007/s10705-026-10471-7","workflowStages":[]},"version":"v1","identity":"rs-7456620","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7456620","identity":"rs-7456620","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}