Soil Phosphorus

Publisher Summary

This chapter describes phosphorus in the lithosphere that always occurs in the quinquevalent oxidation state. Nearly 200 phosphorus minerals have been found in nature, but of these only one group is of significance— namely, the apatites. The inorganic phosphorus in the soil solution takes part in many equilibria; some of these are within the liquid phase only, the homogeneous equilibria, and others occur between the solid and liquid phases, the heterogeneous equilibria. The bulk of phosphorus in the solid phase of soil is customarily considered to be present in three forms: (a) adsorbed to the surface of soil particles, (b) sparingly soluble minerals, and (c) organic compounds. In addition, phosphorus may be present in the lattice of clays and other silicate minerals. Soil phosphorus may be moved in three ways: (a) by the action of soil organisms, (b) with flowing water (mass flow), and (c) by thermal movement along a concentration gradient (diffusion). The magnitude of the movement depends on the fraction of soil phosphorus that is involved and the rate of movement of that fraction. The immediate source of phosphorus for plants is the small amount in the soil solution. The important factors in phosphorus supply to the plant includes: the intensity, kinetic, capacity factors of Wiklander, and the diffusion factor.