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Salt Concentration a Factor in the Availability of Phosphorus from Rock Phosphate as Revealed by the Growth and Composition of Alfalfa¹

ABSTRACT

A study of the availability of phosphorus held in rock phosphate in relation to the soluble salt content of sandclay cultures was made. An effort was made to relate the availability of the phosphorus to the relative bonding energies of Putnam clay for calcium, magnesium, potassium, and hydrogen. Alfalfa was grown as the test crop. The less than 0.2 µ Putnam clay was brought to desired cation saturation by making the system unsaturated by treating the Na suspended clay with hydrogen-Amberlite (IR-120), electrodialyzing to reconcentrate, and then adding carbonates or hydroxides of Ca, Mg, and K. Trace elements and nitrogen as ammonium nitrate were added in small amounts. Phosphorus was added as either rock phosphate or phosphoric acid. Calcium, magnesium, potassium, and sodium salts were added. Calcium and magnesium salts were added in series at 0.010, 0.020, 0.041, 0.081, and 0.163 moles per liter and potassium and sodium salts at 0.020, 0.041, 0.081, and 0.163 moles per liter of soil water.

The results showed all salts influenced the availability of phosphorus held in rock phosphate. However, calcium salts reduced phosphorus uptake with small reduction in plant yields. Variable effects on plant yields and phosphorus uptake resulted when magnesium salts were used. Potassium salts of different anions produced different degrees of response, with the first increments of the carbonate and sulfate giving a positive yield response while the chloride depressed growth and phosphorus uptake. Sodium sulfate gave a reduction in crop yields and phosphorus uptake indicating an influence of excessive sodium.

¹ Contribution from the Missouri Agr. Exp. Sta. Journal Series No. 1589. Approved by the Director. Presented before Div. II. Soil Science Society of America, Davis, Calif., August 17, 1955. Encouragement and support by the International Minerals and Chemical Corporation, Chicago, Illinois, is gratefully acknowledged.

² Assistant in Soils and Professor of Soils, respectively.

Received for publication December 15, 1955. Accepted for publication June 14, 1956.