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Fertilizer Evaluation: III. Availability Coefficient of Water-Soluble and Citrate-Soluble Phosphorus in Acidulated Phosphate Fertilizers¹

ABSTRACT

A method is described by means of which the availability coefficient of citrate-soluble relative to that of water-soluble phosphorus may be evaluated when mixtures of the two phosphorus fractions are present in different proportions in two or more fertilizers. Knowledge of the relative availability coefficients of these two fractions, together with knowledge of the percentage content of the fractions, permits prediction of the relative availability coefficients of phosphorus in the different fertilizers. Application of the method to data obtained from experiments in which superphosphate and various nitric phosphates were compared as sources of phosphorus for corn showed that the ratio of the availability coefficient of citrate-soluble phosphorus to that of water-soluble phosphorus decreased from 0.85 to 0.16 as the fraction of the fertilizer phosphorus in citrate-soluble form increased from 0.56 to 0.95.

¹ Joint contribution from the Iowa Agr. Exp. Sta. and the Tennessee Valley Authority. Journal Paper No. J-2600 of the Iowa Agr. Exp. Sta., Ames, Iowa. Projects 1183 and 1189.

² Professor of Soils, Assistant Professor of Soils, and Professor of Statistics, respectively.

Received for publication April 15, 1955.