SOIL-PHOSPHORUS SORPTION AND AVAILABILITY AS A FUNCTION OF HIGH PHOSPHORUS-FERTILIZER ADDITIONS

After a 3-months equilibration of soil with phosphorus (P) (up to four times the respective P sorption capacity), equilibrium P concentratio n (EPCo), standard P requirement (SPR), P sorption index (SI), and P a vailability by Bray I, Olsen, water and iron-oxide paper strip methods were determined on three soils of the Latium region of Italy, widely ranging in their affinity for P. Soil P addition increased EPCo and av ailability P content and decreased SPR and SI values for all soils wit h differences between soil types a ffinction of P sorption maximum. Th e tractional increase of available NaHCO3-P with added P, i.e. P avail ability index (F) was 0.486 for the soil with the lowest P sorption ma ximum, 0.217 for the soil with the highest P sorption maximum, and 0.3 69 for the third soil presenting an intermediate P sorption (r = 0.997 ; P<0.01). The results indicate that soil type, in addition to the amo unt of P added, will determine the potential for a soil to release P t o runoff.