Analytical methods and quality assurance

Abstract

Soil acidification under tropical and sub‐tropical conditions is a well known phenomenon. Because of cost implications in practice it is important that the best available liming materials be used. Various methods have been employed to distinguish between the effectiveness of such materials, most based on the principle of calcium carbonate equivalent. Many of these use relatively strong acids for dissolution of lime, at concentrations far greater than ever actually occur in even the most acid soils. Because contact exchange between the acidified sites and the liming material is considered to be vital, a method in which a slightly acidified cation exchange resin is used was developed (the resin exchange method). This method has been used by several laboratories in South Africa that take part in the quality control scheme run by ALASA. Several problems were experienced in achieving reproducibility between laboratories, that were diagnosed as being due to variations in applications of the method and especially lack of discipline in using a standardized resin. This resulted in a number of independent investigations into reasons for this apparent lack of reproducibility. Reasons for this included inherent particle size distribution in the sub‐samples of liming material, resin properties, water quality and electrode maintenance and calibration. Under standardized and controlled conditions the method proved to be eminently satisfactory. An alternative possibility that is being investigated is the use of an unbuffered acid topsoil to replace the resin. A bulk sample could be homogenized and subdivided to provide an adequate supply.