The determination of the Al concentration in soil solution is an important diagnostic tool in studies of Al toxicity in acid soils. Where organic residues have been applied as améliorants of Al toxicity, it is necessary to measure both the total soluble and monomeric Al concentrations in order to estimate the degree of Al complexation. Determination by ICPAES provides a measure of total Al without discrimination between chemical forms, and thus may include solid-phase Al present as micro-particulates that have passed through a 0.22 pm filter. Colorimetric methods, which do not include an acidification step, discriminate against micro-particulates, but do not measure all Al present as soluble organic forms and thus underestimate total soluble Al. Acidification of soil solutions to decomplex organically bound Al, and thus permit its measurement by colorimetric methods, results in dissolution of micro-particulates and overestimation of total soluble Al. Filtering to 0.025 pm eliminated microparticulates, thereby removing a source of error. However, this process is time consuming and thus unsuitable for routine use
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