Indophenol blue colorimetric method for measuring cation exchange capacityin sandy soils

In the determination of the cation exchange capacity (CEC) by the NH4-aceta te-NaCl method. the evaluation of the quantity of ammonium exchanged by the soil is often carried out by steam distillation and titration. While this procedure gives reliable results when applied to soils with medium to high amounts of available sites for exchange, in the case of sandy or desert soi ls it may not be as precise as required for a good interpretation of the re sults. Colorimetric methods are instead well known for their high sensitivi ty, although their application may require a more complicated procedure. Th e aim of this work was to assess the suitability of a well known colorimetr ic measurement of ammonium content for the determination of the CEC in sand y soils, to evaluate its range of applicability and to propose a standardiz ed procedure. Eight sandy soils from Southern Africa were used for the eval uation of the method suitability. The procedure gave very acceptable result s, with low internal variability and the data obtained were comparable to t hose obtained with the standard steam distillation procedure. However. its application requires certain precautions as the equilibrium for the reactio n of color development is reached only after 90 minutes. With this method t he determination of CEC values from as low as 1 cmol(+) Kg(-1) to about 20 cmol(+) Kg(-1) was possible using 0.1-0.5 mL of NaCl solution. but a wider range may be measured either by using lower sample weights or by diluting t he NaCl solutions.