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Availability and Fractionation of Residual Phosphorus in Soils High in Aluminum and Iron¹

ABSTRACT

The fate of applied P has been studied on two soils high in Al and Fe using the method of Chang and Jackson (2). Water soluble-P, Ca-P, Al-P, and Fe-P fractions were determined. The "occluded" and organic P were not determined but were considered to be the difference between the above forms and the total P.

Single applications of concentrated superphosphate (CSP) and rock phosphate (RP) up to 175 and 349 lb of P/acre respectively were made to Norfolk loamy fine sand, and corn (Zea mays L.) was used as an indicator crop. Annual applications of ordinary superphosphate (OSP) to the same plots using the split-plot technique gave significant yield responses in all but 3 of the 5 years following P application. Only a small part of the increase in total P due to the initial P application could be accounted for in the Al-P, Fe-P and Ca-P fractions after 8, 11 and 13 years cropping. Apparently much of the P had been converted to occluded P. This may account for the need for currently applied P.

After 2 years 220 lb of P/acre from CSP applied to Red Bay fine sandy loam raised the total P 95 lb/acre above that of a 110 lb/acre application. Of this amount 54, 28 and 5 lb/acre (92%) were in the Al-P, Fe-P and Ca-P forms, respectively. An 108 lb of P/acre increase in total P due to an application of OSP 8 years before raised the Al-P, Fe-P and Ca-P, 17, 15 and 3 lb/acre (32%), respectively. Assuming that CSP reacted in the soil like OSP, it was apparent that P was converted to other forms with time similar to Norfolk loamy fine sand. The formation of Fe-P from Ca-P in Red Bay fine sandy loam was related to total P 2 and 8 years after application.

After 2 years there was 4.1 and 1.5 times as much Al-P and Fe-P, respectively, in the soil following CSP than RP treatments for Red Bay fine sandy loam. Plant uptake data using P³² also showed P from CSP was more available than from RP.

¹ Florida Agr. Exp. Sta. Journal Ser. no. 2258.

² Soil Chemist, Soil Chemist, and Soil Chemist in Charge West Florida Exp. Sta., respectively, of the Florida Agr. Exp. Sta.

Received for publication November 10, 1965. Accepted for publication April 8, 1966.