PHOSPHORUS SUPPLY TO PLANTS BY SOILS WITH VARIABLE PHOSPHORUS EXCHANGE

Soil phosphorus (P) supply to plants increases with concurrent increas es in both soil solution P (C-P) and P exchanged between the solution and the soil solid phase that rapidly equilibrates with C-P. For many temperate region soils, the amount of P isotopically exchanged in 1 mi n (E(1)) acts as an immediate source of replenishment for C-P. Relatio nships between E(1) and C-P were investigated to characterize the Inte nsity (C-P), Quantity (E(1)) and Capacity (dE(1)/dC(P)) factors of soi l P supply to plants. Soil (Alfisol) samples were collected from seven P fertilization field experiments conducted in France over a period o f 4 to 32 years. The E(1)/C-P curves are described by the same equatio n for the seven experiments: E(1) = vC(P)(0.78) where v is the E(1) va lue at 1 mg P L(-1). The v constant is soil dependent and ranged from 14.7 to 28.3 mg P kg(-1). The above equation was validated by testing a previous published data set. The buffer power of P in solution contr olled by E(1) (dE(1)/dC(P)) decreased when C-P increased. At a given C -P level, dE(1)/dC(P) increased when v constant increased. The E(1)/C- P curves might be used as a basis for transferring information on soil P supply from one soil to another. The present work proposed an equat ion based on the use of E(1)/C-P curves to predict the C-P value assoc iated with an optimum crop yield in different soils (OPtCP). The optC( P) value is inversely related to the v value, indicating that the lowe r the P buffer power of soils, the higher the optC(P) value.