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Phosphorus is one of the immobile nutrients in the soil. It occurs in the soil in both organic and inorganic forms, the latter being of greater importance in mineral soils. Inorganic P is known to occur in three main categories, viz., apatite which is a discrete phase of P compounds, P sorbed on surface of Fe, Al and Ca as soil constituents and P present within materials of Fe and Al compounds. Sodic soils are rich in total and Olsen-P. The content in reclaimed sodic soils, on the other hand, decreased due to inactivation by added gypsum (soluble sodium-phosphates are converted to less soluble Ca-phosphates) and higher sorption of added P by soil. Most of the soils have the ability to retain soluble P against recovery by plants with the result that crop uptake of phosphorus rarely exceeds 20-25% in the first growing season. This is due to various types of reactions occurring in the soil such as adsorption, precipitation, chemical and biological transformations occurring in the soil that revert the added soluble P into various insoluble forms. The reversion rate, release and availability of P are influenced not only by contact period between P and the soil but also by factors such as temperature and moisture.
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