AVAILABLE SOIL PHOSPHATE - ITS REPRESENTATION BY A FUNCTIONAL MULTIPLE COMPARTMENT MODEL

Analysis of the isotopic exchange kinetics of phosphate ions which occurs in a soil-solution system maintained in steady-state shows that the available soil phosphate is a heterogeneous system that lends itself to compartmentalization. Available soil P has been subdivided into 5 pools in relation to the functioning of roots and root systems. The central and most important pool for P uptake by crops is the pool P(L) of free phosphate ions: this pool contains ions which can be transferred into the soil solution in < 15 s. The 4 other pools, B, C, D and F, branch directly off the central pool. In this system studied in steady-state, the pool B contains ions which can exchange with the phosphate ions of the soil solution in between 1 min and 1 d. The pool C contains the ions able to enter the solution in steady-state between 1 d and 3 months. The pool D contains ions which can exchange with phosphate ions in the solution in 3 months to 1 yr. The pool F contains phosphate ions exchangeable after > 1 yr. This model concerns only inorganic P, organic P following other rules of transformation, such as those of microbial mineralization. A schematic functional model has been developed to summarize this information. The model applied to various soil samples taken from 2 long-term field experiments showed that repeated fertilization or exhaustive cropping can modify all the pools. This scheme shows 2 kinds of phosphate buffering capacity (PBC) for soils. The first kind, which depends on the P(L) pool, is called 'instantaneous PBC'; the second kind, which depends on the 4 other pools, is called 'delayed PBC': its effect on the maintenance of phosphate ion concentration in the soil solution is time-dependent.