The largest global reservoir of P is the ocean sediment pool (Table 15.1), which represents a small but steady sink for terrestrial P released by the weathering of minerals or released by the biota. Phosphorus occurs mainly in inorganic phosphates and in organic phosphate derivatives. The primary mineral form of P is apatite, with the basic formula M10(PO4)6X2. Commonly, the mineral (M) is Ca and less often Al or Fe. The anion (X) is F-, Cl-, OH-, or CO3-, thus creating fluor-, chloro-, hydroxyl-, or carbonate apatites. Diverse substitutions and combinations of M and X result in some 200 forms of inorganic P. Total P concentrations in soils range from 35 to 5300 ^gg-1 with an average of 800 ^gg-1 (Bowen, 1979; Table 15.2). Highly weathered soils may have very little P, while weakly weathered soils and those soils formed from alkaline parent materials may have high P contents. The chemical weathering of apatite results in the release of orthophosphate (H2PO4- is the dominant species at pH < 7.2, while HPO4- dominates at pH > 7.2).
Very little orthophosphate is present in soil solution at any one time. The concentration of P in the soil solution rarely exceeds 0.1 to 1 ^gg -1 , representing < 1% of the total P. Solubility of P is complicated by common ion-ion association and pH effects and the amount of P adsorbed on clay mineral surfaces. Soluble P rapidly precipitates as iron and aluminum phosphates in acid soils or as calcium phosphates in alkaline soils or is adsorbed to iron and aluminum oxides or clay mineral surfaces. The optimum availability of orthophosphate occurs at a soil pH of approximately 6.5, at which aluminum and calcium precipitation is at a minimum.
The hundreds of structural forms of inorganic P in nature necessitate their discussion on the basis of extractability and availability to plants and microorganisms.
Soil microbial P
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