RELEASE OF SOIL PHOSPHATE BY SEQUENTIAL EXTRACTIONS AS A FUNCTION OF SOIL PROPERTIES AND ADDED PHOSPHORUS

A study of sequential phosphate (P) extraction by water and iron oxide -impregnated paper strip procedures was carried out on three Italian s oils ranging widely in soil characteristics and enriched with three ra tes of fertilizer P. The degree of change was dependant on P addition, soil P properties, and type of extraction. For the Fe-oxide strip pro cedure, a greater release of P than for water extraction was observed for soils with and without added P. At a given level of added P, more P was released from the soil with the lowest P sorption index (SI). Ho wever, at a given level of NaHCO3-extractable P, less P was released f rom the soil with lower SI than from soil with a higher SI, indicating that a greater available P content was necessary for low P sorbing so ils to maintain a given rate of P release, The variation of SI account ed for 96% and 92% of the variation in amount of water-extractable and Fe-oxide strip P at a given P addition. Furthermore, SI accounted for 97% and 98% of the variation in water-extractable and Fe-oxide P at a given increase in available soil P. Inclusion in a soil testing progr am of an estimate of the P Sorption Index, that accounts for the overa ll effect of soil properties affecting sorption in soils (clay content and type, iron and aluminum oxide content, surface area, etc.), may i mprove fertilizer P requirements for optimum crop growth for certain s oils.