Comparison of three cation extraction methods and their use in determination of sodium adsorption ratios of some sodic soils

Cation concentrations are extensively used in soil science to determine a soil's potential for dispersion. A number of extraction methods were compared using either ammonium acetate or various ratios of soil:water. The ammonium acetate method extracted the largest quantities of all the cations compared with water extractions and this was especially true for calcium. However, it was concluded that this extraction technique is not appropriate for dispersion studies since it is lengthy and does not mimic field conditions. The water extraction methods were rapid and simple. However, inconsistency was found in the results and subsequent sodium adsorption rate (SAR) calculations where these were expressed using water concentrations that had not been corrected for soil mass. The concentration of cations extracted also changed as the volume of water extractant increased which in turn had a significant effect on SAR. The variation in extraction however was not incremental or linear and diminished above a 1:10 soil:water ratio. It was concluded that for dispersion studies, extraction results should be expressed in terms of concentration per mass of soil, and that extraction ratios of at least 1:10 soil:water should be used.