Indophenol Blue Colorimetric Method for Measuring Cation Exchange Capacity in Sandy Soils

In the determination of the cation exchange capacity (CEC) by the NH4-acetate-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 soils it may not be as precise as required for a good interpretation of the results. Colorimetric methods are instead well known for their high sensitivity, although their application may require a more complicated procedure. The aim of this work was to assess the suitability of a well known colorimetric measurement of ammonium content for the determination of the CEC in sandy soils, to evaluate its range of applicability and to propose a standardized procedure. Eight sandy soils from Southern Africa were used for the evaluation of the method suitability. The procedure gave very acceptable results, with low internal variability and the data obtained were comparable to those obtained with the standard steam distillation procedure. However. its application requires certain precautions as the equilibrium for the reaction of color development is reached only after 90 minutes. With this method the 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 the NaCl solutions.