The cation exchange capacity of a sandy soil in southern Brazil: an estimation of permanent and pH‐dependent charges

Abstract

SummaryIn soils in general, the cation exchange capacity is an important factor in soil fertility because it determines both cation retention and selectivity, and in turn controls most of the other soil properties. The main goal of this paper was to estimate the cation exchange capacity (CEC) and the contribution of permanent charges and pH‐dependent charges and their respective importance in soil CEC as a function of pH, clay and organic matter content in soils. A multiple regression method was developed from 75 samples under different soil management practices. The permanent charge (∼ 140 mmolc kg−1 of clay) estimated by this approach was independent of the analytical method used, i.e. a French method (MF) and a Brazilian method (MB), and is consistent with the presence of 2:1 clay minerals. However, the pH‐dependent charges were closely related to the buffering capacity inherent in each protocol (MF protocol using NH4+, and MB using a SMP solution proposed earlier by other workers, both at pH 7). For the MF protocol, the proposed method appeared to be well adapted for reproducing soil component properties under different management conditions. In the soil samples studied, the contribution of organic carbon to negative charges ranged from 12.3 to 55.8% of effective CEC and from 14.1 to 59.2% of CEC at pH 7. However, the clay contribution, notably its permanent charges, can contribute to 31% and more of the CEC in the lower horizons of soil profiles. The proposed approach can accurately evaluate the effective and pH 7 CECs when the MF protocol is used but evaluation is less accurate with the MB protocol.