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Calcium Phosphate Fertilizers: III. The Effect of Surface Area on the Availability Coefficients of the Dicalcium Phosphates¹

ABSTRACT

Several granule sizes (ranging from 20 to 40 mesh to < 325 mesh) of dicalcium phosphate dihydrate (DCPD) and anhydrous dicalcium phosphate (DCPA) were compared in a greenhouse experiment with corn forage grown on Mountview silt loam, pH 6.6. Several comparable screen-size separates varied with respect to the size of the component crystals. Plant response increased with decreasing granule size.

Geometric surface areas of the fertilizers were estimated from the counted number of granules and the size of the granules. Relative rates of solution, which are roughly proportional to geometric surface area, were also measured.

The linear correlation coefficients between (a) availability coefficient indexes and geometric surface area were 0.94 and 0.69 for the DCPA and DCPD fertilizers, respectively, and (b) availability coefficient indexes and relative rates of solution were 0.98 and 0.85 for the DCPA and DCPD fertilizers, respectively.

The results of these experiments indicate that the availability coefficients of different granule sizes of DCPD and DCPA are primarily dependent upon geometric surface area of the granules and are not much influenced by variations in size of the component crystals in the granule. Per unit of surface area, the availability coefficient of the DCPD is about 2.3 times that of the DCPA.

¹ Contribution from the Soils and Fertilizer Research Branch, Division of Agricultural Relations, Tennessee Valley Authority, Wilson Dam, Alabama. Presented before Div. II, Soil Science Society of America, Aug. 5, 1958, at Lafayette, Ind.

² Soil Chemist and Analytical Chemist, respectively.

Received for publication October 21, 1958. Accepted for publication February 3, 1959.