Estimation of Effective Cation Exchange Capacity and Exchangeable Iron in Paddy Fields After Soil Flooding

In flooded soils, the concentrations of exchangeable Mn2+ and, mainly, Fe2+ can be high and need to be considered in determining the cation exchange capacity (CEC) of the soil under flooded conditions. However, these reduced forms of Mn and Fe are oxidized and precipitated during the extraction process used by traditional methods for determining CEC. This procedure underestimates the exchangeable portion of these cations and, consequently, the CEC value of the flooded soil. The objective of this study is to propose an alternative to estimate the exchangeable Fe2+ and the effective CEC (ECEC) of flooded soils. To achieve this goal, 21 surface samples (0-20 cm) of soils from rice fields were collected, distributed in the cultivation regions of Southern Brazil. The soils were flooded for 50 days. The soil solution was collected on the first day and after 50 days of flooding. and, pH, Na, K, Ca, Mg, Fe and Mn were determined. In these samples, exchangeable cations (K, Na, Ca, Mg, Mn, Al and H+Al) were determined to calculate ECEC and CEC at pH 7 of unflooded soil and after 50 days of flooding. The results were used to develop models to predict ECEC and exchangeable Fe content after 50 days of flooding. The estimation of the ECEC after flooding by the gradient of pH increase before and after flooding generated values closer to CEC pH 7.0, correcting the possible underdetermination of the ECEC during flooding. The amount of exchangeable Fe estimated was higher than the exchangeable Fe determined, correcting the possible underestimation of these quantities determined during flooding. It is concluded that the estimatives of ECEC after flooding through the equation ECECafter=ECEC+pHsol.after- pHsol.before . (CECpH7- ECEC)(7- pHsol.before) and the exchangeable Fe2+ after flooding through the equation Feexc.after.estimated=ECECafter- Ca+Mg+K+Na+Mn corrected the problem of underestimating the values of these variables by analythical methods, demonstrating its viability for the use in flood prone soils.