ACTUAL CATION EXCHANGE CAPACITY OF AGRICULTURAL SOILS AND ITS RELATIONSHIP WITH pH AND CONTENT OF ORGANIC CARBON AND CLAY

Abstract

For the set up of a multinutrient 0.01 M calcium chloride (CaCl₂) soil testing program a conversion from conventional soil testing programs to a CaCl₂ program has been proposed in literature. Such conversion should be based on the relationship between test values of the conventional method and the CaCl₂ method. For magnesium (Mg) it was shown in earlier work that the conversion could be improved when the actual cation exchange capacity (CEC) of the soil, CEC_act, was taken into account. However, determination of CEC_act necessitates an extra analytical procedure. The goal of this study was to test a procedure for estimating CEC_act of a soil. In this procedure, CEC_act was calculated as the summation of the estimated charge of organic carbon (C) and clay in the soil at pH_act, the actual pH of the soil. A series of 39 test soils representing agricultural soils in The Netherlands was used to derive the pH dependency of the negative charge of organic C and clay. The following relationship was found: CEC_act: [M(1)× 0.0624] + [M(2)× (0.295−D(2)_pHact)]. In this relationship, M(1) and M(2) represent clay and organic C in g kg⁻¹ dry soil, respectively, and D(2)_pHact the difference in negative charge of organic C at pH_act and pH 8.1. The pH_act equals pH measured in 0.01 M CaCl₂. The relationship was tested on another dataset of 38 agricultural soils. There was good agreement between the calculated and measured CEC_act (R ²=0.89). It was concluded that the procedure can be used for estimation of CEC_act.

Acknowledgments