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Isotopic Exchange Characteristics of Aluminum- and Iron-bound Fractions of Soil Phosphorus¹

ABSTRACT

The source of most of the isotopically exchangeable P in widely different soils was found to be in the Al- and Fe-P fractions. The relative contribution of Fe-P to exchangeable P was greater at longer exchange periods, showing that Fe-P had more slowly-exchangeable components than Al-P. The rate of exchange of Al- and Fe-P varied from 28 to 162 ppm/hr and 22 to 72 ppm/hr, respectively, in the first half-hour and was in no case greater than 2 ppm/hr after 24 hours of exchange. Ratio of specific activities of Al- or Fe-P were used to compare the surface activities of the P fractions. The term "Relative Activity Number" (ratio of specific activities of Al- to Fe-P in a soil) is proposed as an index of the relative activity of Al- and Fe-P. RAN for soils studied varied from 1.11 to 2.80 showing that Al-P was 1.11 to 2.80 times as active as Fe-P. RAN in soils which had received P was smaller than RAN in untreated soils showing greater exchangeability of the reaction products in the Fe-P form. In a few soils as much as to of Al- and Fe-P was not exchanged in 147 hours. Thus, a considerable proportion of these P fractions is relatively "inactive.".

¹ Contribution from the Dep. of Agronomy, University of Illinois, Urbana. Part of senior author's Ph.D. Thesis. Presented before Div. S-2, Soil Science Society of America, Washington, D.C., November 1967. The authors sincerely thank the following persons who supplied soil samples mentioned in this report: C. Chavengsaksongkram, Dr. S. A. Barber, Dr. T. L. Jackson, Dr. D. C. Martens, and Dr. M. Salomon.

² Research Assistant, and Professor of Soil Fertility, respectively.

Received for publication April 22, 1968. Accepted for publication August 6, 1968.