Transport and fixation of phosphate in acid, homogeneous soils I. Physico-mathematical model

A physico-mathematical model describing convective and diffusive transport, nonlinear reversible adsorption, and irreversible fixation is formulated. The rate of fixation is assumed to be proportional to the amounts of mobile phosphate above an equilibrium level and to the amount of Al + Fe available for binding phosphate. The finite fixation capacity implies that, with a continuous supply of phosphate at the soil surface, all fixation occurs in a front moving downward at a certain velocity and having a constant shape. The fixation front separates a transmission zone near the soil surface in which all available aluminum and iron has been used and a zone in which the equilibrium values prevail. If the fixation capacity were unlimited then all fixation would occur in a zone near the soil surface. In the absence of fixation, the model reduces to one dealt with extensively in the literature.