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A Comparison of Various Laboratory Methods for Predicting the Agronomic Potential of Phosphate Rocks for Direct Application¹

ABSTRACT

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.

¹ Contribution from the Fertilizer Technology and Agro-Economic Divisions, International Fertilizer Development Center, Muscle Shoals, AL 35660

² Research Chemist (Soils) and Soil Scientist, respectively.

Received for publication March 28, 1978. Accepted for publication June 21, 1978.

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