Soil Fertility and Nutrient management (1+1)

Phosphorus transformation & availability in soils

Phosphorus Cycle in soil
In P cycle the phosphorus is not involved in any exchange process with the atmosphere. The amount of dissolved or solution P mostly H2PO₄^- and HPO₄^2- ions in the soil is very small and crops will utilize P from this source. The soil solution P can come from mineralization of organic matter, added fertilizers, adsorbed phosphate ions and solid P compounds (primary & secondary minerals).
Any P present in solution P form and it is available to plants is called Labile P, where as any P which is bounded in solid P compounds such as primary and secondary minerals is called Non Labile P which is not available to plants.

P- Transformation in aerobic soils:
P transformation depends on their solubility, interaction with other soil components. When a water soluble phosphate fertilizer, such as super phosphate is applied to a soil, P dissolves immediately and enters the soil solution and readily forms new compounds with Ca as Ca-PO₄ and carbonates of hydroxyapatite in calcareous and alkaline soils respectively. While in acid soils rich in Al, Mn and Fe, applied P is converted as Al-PO₄ , Mn –PO₄ and Fe-PO₄ which are are precipitated as newly formed insoluble compounds and thus reduce the availability of P to crops.

P Transformation in Anaerobic soils:
Submerged or flooded conditions/soils create reduced conditions (anaerobic soil) which lead to reduction of Ferric-PO₄ to ferrous PO₄ resulting in greater availability of P in the reduced soil. Organic acids formed under submerged conditions also solubilize PO4 there by P is available to plants.