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, which 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 of the flooded soil. To achieve the objective of the study, 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 of flooding and after 50 days, and pH, Na, K, Ca, Mg, Fe and Mn were determined. Soil subsamples were collected at two times: before flooding and after 50 days of flooding. In these samples, exchangeable cations (K, Na, Ca, Mg, Mn, Al and H+Al) were determined to calculate effective CEC and CEC at pH 7 of dry soil and after 50 days of flooding. The results were used to develop models to predict effective CEC and exchangeable Fe content after 50 days of flooding. The estimation of the effective CEC 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 effective CEC 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 estimation of the effective CEC and exchangeable Fe2+ after flooding by the proposed method proved to be efficient.