Soil Science

Phosphorus

The main forms of P in the soil solution (HPO4^2- and H2PO4^-) are very strongly adsorbed by sesquioxides clays, and also form Fe, Al, and Ca phosphate minerals of poor solubility. The P concentration in soil solution is therefore low and is rapidly diminished by a growing plant. To some extent, this will be replenished from the solid phase. However, some adsorbed and precipitated P cannot be re-released into the soil solution and is said to be fixed.

Much of the fertiliser added to acid soils with high sesquioxide clay contents (e.g. krasnozems) becomes fixed and unavailable to plants. As 15-80% of soil P occurs in organic matter, mineralisation of organic P is particularly significant for P nutrition in such soils.

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