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Calcium Phosphate Fertilizers: IV. The Relation Between Solubility in Soils and Availability Coefficients of Dicalcium and Fused Tricalcium Phosphates¹

ABSTRACT

The availability coefficient indexes per unit of P of 2 granule sizes of fused tricalcium phosphate (FTP), anhydrous dicalcium phosphate (DCPA), and dicalcium phosphate dihydrate (DCPD) were measured with 3 soils in greenhouse experiments. Availability coefficient indexes increased as granule size was reduced. Per unit of geometric surface area, availability coefficient indexes increased in the order FTP < DCPA < DCPD for all 3 soils.

The composition of solutions after 2 weeks' reaction between 1 g. soil and 1 g. of phosphate in 40 ml. of water was measured. The product of the concentration of P in these solutions and geometric surface area of the respective fertilizers explained 86% of the variation in availability coefficient indexes of the 6 fertilizers in the 3 soils.

¹ Contribution from the Soils and Fertilizer Research Branch, Division of Agricultural Relations, Tennessee Valley Authority, Wilson Dam, Alabama.

² Soil Chemist and Analytical Chemist, respectively.

Received for publication October 21, 1958. Accepted for publication March 2, 1959.