AVAILABILITY OF PHOSPHORUS FROM 17 ROCK PHOSPHATES IN MODERATELY AND WEAKLY ACIDIC SOILS AS DETERMINED BY 32P DILUTION, A VALUE, AND TOTAL P UPTAKE METHODS

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Abstract
Isotope dilution, A value determination and total P uptake methods were used to measure the effectiveness of 17 rock phosphates as sources of P for wheat, following direct application to moderately and slightly acidic soils. The measured P uptake values were compared with values determined by chemical extraction methods used to predict availability of rock phosphates. Tracer techniques provided more reliable indexes of availability than did the nontracer method and were more highly correlated with chemical extraction measurements. The most effective rock phosphate was approximately 88% as effective as triple superphosphate (TSP) in the moderately acidic soil and 68% as effective as TSP in the slightly acidic soil when rock phosphate was added at 10 times the TSP rate. Several rock phosphates had no positive effect on wheat growth or P uptake. The main advantage of the isotope dilution method over A value determinations is that inexpensive ortho 32P is used instead of specially labeled phosphate compounds. Other advantages are discussed.

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