The reactivity of seven phosphate rocks was estimated by five methods (neutral ammonium citrate first and second extractions, 2% citric acid, 2% formic acid, ammonium citrate pH 3, and absolute citrate solubility). These measurements were evaluated by crop response data obtained from a greenhouse experiment with guinea grass ( Panicum maximum ) and a field experiment with beans ( Phaseolus Vulgaris L.), both on acid Colombian soils. Neutral ammonium citrate (second extraction), 2% formic acid, and ammonium citrate pH 3 solubilities were best correlated with these agronomic data. Factors such as: (i) calcite depression of apatite solubility in neutral ammonium citrate, (ii) the “grade” effect (total P content) on the apparent solubility, and (iii) the textural effect of apatite‐silica intermixing, need to be considered when correlations of chemical reactivity of the various phosphate rocks and their agronomic effectiveness are compared.
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Chien et al. (1978) studied this question.