Phosphorus retention in calcareous soils and the effect of organic matter on its mobility

A survey of the interactions between phosphorus (P) species and the components of calcareous soils shows that both surface reactions and precipitation take place, especially in the presence of calcite and limestone. The principal products of these reactions are dicalcium phosphate and octacalcium phosphate, which may interconvert after formation. The role of calcium carbonate in P retention by calcareous soils is, however, significant only at relatively high P concentrations - non-carbonate clays play a more important part at lower concentrations. In the presence of iron oxide particles, occlusion of P frequently occurs in these bodies, especially with forms of the element that are pedogenic in origin. Progressive mineralization and immobilization, often biological in nature, are generally observed when P is added as a fertilizer. Manure serves both as a source of subsurface P and an effective mobilizing agent. Blockage of P sorption sites by organic acids, as well as complexation of exchangeable Al and Fe in the soil, are potential causes of this mobilization. Swine and chicken manure are especially rich P sources, largely due the practice of adding the element to the feed of nonruminants. Humic materials, both native and added, appear to increase recovery of Olsen P. In the presence of metal cations, strong complexes between inorganic P and humates are formed. The influence of humic soil amendments on P mobility warrants further investigation.