Abstract
Abstract The effects of exchangeable calcium on amounts of inorganic phosphate, organic phosphorus and organic matter extracted from soil by sequential extraction with alkali and acid were investigated. Where exchangeable calcium was present in soil samples, precipitation of calcium phosphate, and calcium‐organic matter complexes occurred during the alkali extraction. Dissolution of precipitated calcium phosphate during subsequent acid extraction caused overestimation of apatite phosphorus in soil and soil/phosphate rock mixtures. Release of exchangeable acidity when exchangeable calcium was removed by prewashing with 1 M NaCl caused dissolution of phosphate rock in freshly prepared soil/rock mixtures. This resulted in underestimation of apatite phosphorus and overestimation of dissolved phosphorus. Buffered NaCl solutions (pH 7.8) were less efficient than unbuffered NaCl at removing exchangeable calcium. However, inclusion of EDTA in the buffered NaCl solution markedly improved the extraction of Ca. Some dissolution of phosphate rock may have occurred during prewashing with buffered NaCl/EDTA solutions. Using EDTA concentrations no greater than that required to complex the exchangeable Ca present in unfertilised soil samples would limit this.
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