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Influence of Soluble Salts on the Solubility of and Plant Response to Dicalcium Phosphate¹

ABSTRACT

A considerable part of the phosphorus in most mixed fertilizers is carried as dicalcium phosphate produced as a consequence of ammonia neutralization of the mixture. Agronomic experience with such mixtures suggests that the phosphate produced in situ has a higher nutritive value than separate preparations of dicalcium phosphate. Any increased nutritive value of the dicalcium phosphate must, it would seem, stem mainly from enhancement of phosphorus solubility in the presence of soluble salts. Accordingly, the solubility of a laboratory preparation of dicalcium phosphate was measured in water and several salt solutions. Greenhouse techniques were designed to minimize responses to nutrient constitutents of the salts, change in available soil phosphorus status and physiological effects due to the salts, so that differences in plant response to binary mixtures of dicalcium phosphate and several salts were indicative of differences in solubility of dicalcium phosphate. Alfalfa was used as the test crop on Nunn, Carrington, and Chester soils. In general, salts such as ammonium sulfate, which in laboratory tests markedly increase the solubility of dicalcium phosphate with increasing ionic strength, also gave the greatest crop responses in the greenhouse. Those salts such as the ammonium nitrate type, which increase the solubility only moderately and level off at ionic strengths between 2 and 4, only occasionally gave increased crop response, while salts such as the calcium nitrate type, which decrease the solubility sharply with increasing ionic strengths, usually gave a negative crop response.

¹ Contribution from Fertilizer and Agricultural Lime Section, S.W.C.R.B., A.R.S., U.S.D.A., Beltsville, Md. Presented before Division IV, Soil Science Society of America, Dallas, Texas, Nov. 18, 1953.

² Soil Scientist and Principal Chemist, respectively.

Received for publication August 16, 1954.