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

For the set up of a multinutrient 0.01 M calcium chloride (CaCl2) soil test ing program a conversion from conventional soil testing programs to a CaCl2 program has been proposed in literature. Such conversion should be based o n the relationship between test values of the conventional method and the C aCl2 method. For magnesium (Mg) it was shown in earlier work that the conve rsion could be improved when the actual cation exchange capacity (CEC) of t he soil, CECact, was taken into account. However, determination of CECact n ecessitates an extra analytical procedure. The goal of this study was to te st a procedure for estimating CECact of a soil. In this procedure, CECact w as 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 3 9 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)X0.0624] + [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 neg ative charge of organic C at pH(act) and pH 8.1. The pH(act) equals pH meas ured in 0.01 M CaCl2. The relationship was tested on another dataset of 38 agricultural soils. There was good agreement between the calculated and mea sured CECact (R-2=0.89). It was concluded that the procedure can be used fo r estimation of CECact..