Forms of Phosphorus
i) Soil Solution Phosphorus:
H2PO4 and HPO4-2 are two forms of P which are present in the soil solution in the available form. At neutral PH range both forms of P present in equal proportion. While under acidic condition H2PO4 is dominant and above this PH range concentration of HPO4-2 increases. Processes of mass now and diffusion provides solution P when its concentration reduces due to absorption.
ii) Organic Phosphorus:
About 50% P from total P is in organic forms. Additions of organic matter to soil increase the proportion of organic P. Its availability decreases with depth.
iii) Inorganic Soil Phosphorous:
Mineralization of organic P gives rise to inorganic P in soil. This is available form of soil P and is available to plants only when it is not adsorbed to mineral surface of soil. Its fixation totally depends on the PH range of soil for example in acidic soil inorganic P precipitates as Fe /Al –P secondary mineral and or is adsorbed on the surface of Fe /Al oxides and clay mineral. While in neutral or calcareous soils inorganic P precipitate as Ca-P secondary mineral this adsorption and precipitation are collectively called as P fixation or retention.