Actual cation exchange capacity of agricultural soils and its relationship with pH and content of organic carbon and clay

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, CECact, was taken into account. However, determination of CECact necessitates an extra analytical procedure. The goal of this study was to test a procedure for estimating CECact of a soil. In this procedure, CECact was calculated as the summation of the estimated charge of organic carbon (C) and clay in the soil at pHact, 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: CECact: [M(1)x 0.0624] [M(2)x (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 pHact and pH 8.1. The pHact 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 CECact (R2=0.89). It was concluded that the procedure can be used for estimation of CECact.