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The Effect of Associated Salts on the Availability of Concentrated Superphosphate¹

ABSTRACT

Phosphorus uptake by oat plants grown in the greenhouse on 2 soils fertilized with pellets of concentrated superphosphate (CSP) alone or of mixtures of CSP and each of 5 salts was measured. With Hartsells fine sandy loam (pH 5.2) yields of P were in the order: CSP < CSP + KCl CSP + NH₄Cl CSP + NH₄NO₃ < CSP + (NH₄)₂SO₄ CSP + KNO₃. With Mountview silt loam (pH 6.6) yields of P were in the order: CSP < CSP + KCl < CSP + (NH₄)₂SO₄ CSP + NH₄Cl CSP + NH₄NO₃ CSP + KNO₃.

After pellets of CSP, CSP + KNO₃, and CSP + NH₄NO₃ had reacted with the moist soils for 3 weeks, samples of soil were taken at increments of distance from the center of the pellet residue. Both water-soluble P (1:15 soil-water extracts) and P extractable in 6N HCl were linear functions of distance of the sample from the center of the pellet residue. The constants in the linear equations depended upon the composition of the pellet. The summation, over the total volume of soil influenced by fertilizer, of the products of water-soluble P x volume of soil containing this concentration of water-soluble P was well correlated with the uptake of P by oats in the greenhouse experiment. Thus, the effect of the associated salts on the availability of the CSP was largely explained on the basis of measured chemical differences. Any effects on physiology of the plants were of secondary importance in this experiment.

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, Ga.

² Soil Chemist and Chemist, respectively. L. B. Clements assisted in greenhouse studies and C. M. Hunt analyzed plant samples for phosphorus.

Received for publication October 3, 1957. Accepted for publication December 5, 1957.