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Calcium Phosphate Fertilizers: I. Availability to Plants and Solubility in Soils Varying in pH¹

ABSTRACT

Availabilities of the phosphorus in six pure crystalline calcium phosphates to ryegrass and Sudangrass were determined in the greenhouse. The crops were grown on three acid soils, Clarksville silt loam, Hartsells fine sandy loam, and Edina silt loam; and on two alkaline soils, Rosebud loam and Webster silty clay loam. Dicalcium phosphate dihydrate was appreciably more available than anhydrous dicalcium phosphate on all soils. Monocalcium phosphate was slightly more available than the dicalcium phosphates on the alkaline soils but was less available on the acid soils. Alpha tricalcium phosphate was nearly as available as anhydrous dicalcium phosphate on the acid soils, but was of very low availability on the alkaline soils. Hydroxyapatite was of very low availability on all soils. Availability of octocalcium phosphate was less than for the dicalcium phosphates and decreased with increasing liming rates on Hartsells fine sandy loam. A significant correlation was found between the phosphorus extracted by 0.5N NaHCO₃ and the phosphorus uptake by the two crops from the various phosphates. Placing the phosphates in a layer 1 inches below the surface markedly increased availability to the crops grown in low-phosphorus Clarksville and Hartsells soils, while mixing with the soil was equally effective in the higher-phosphorus Edina, Rosebud, and Webster soils.

¹ Contribution of the Soils and Fertilizer Research Branch, T.V.A., Wilson Dam, Alabama. Presented before Div. II, Soil Science Society of America, Nov. 21, 1957 at Atlanta, Georgia.

² Agronomist, Soil Chemist, and Research Chemist, respectively, Tennessee Valley Authority, Wilson Dam, Alabama. L. B. Clements assisted with the greenhouse experiments and E. C. Sample and C. M. Hunt analyzed soil and plant samples.

Received for publication May 24, 1957. Accepted for publication July 11, 1957.