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A Study of Isotopic Dilution as a Method for Relating Phosphorus Retention to Availability of Phosphorus in Widely Different Soils¹

ABSTRACT

Six soils which were considered different with respect to release and retention of P were limed and allowed to equilibrate with a constant amount of P³² and 25, 100, and 400 ppm. of P³¹ by alternate wetting and drying 4 times prior to seeding Turghi millet and Tokyo buckwheat. The objective was to relate P retention characteristics to "availability of P" in different soils. "Active solid phase P" as determined by isotopic dilution was found to be an index of the amount of "available P" contained in a soil. A constant proportion of the active solid phase P was not in solution or removed from all soils by any of the extracting reagents. The percent active solid phase P removed by all extractants was affected by the rates of added P. From the results it was concluded that the percent active solid phase P removed by Bray's No. 1 extractant from each soil type was an index of its retention characteristics, if the native active solid phase P was in equilibrium with the added P³².

¹ Contribution of the Pennsylvania Agr. Exp. Sta., University Park, Pa. Authorized for publication on Oct. 10, 1963 as Paper Number 2831 in the Journal Series.

² Assistant Professor of Soil Technology. Acknowledgment is made to Leon Marshall for analytical assistance.

Received for publication November 18, 1963. Accepted for publication March 23, 1964.