Phosphate is highly reactive with soil constituents in various mechanisms. Due to these reactions, a large excess of phosphorus fertilizers are applied to agricultural fields as compared with the amount of phosphate uptake by crops. This paper outlines the reactions of phosphate with different soil colloids.
Phosphate reacts with goethite forming a binuclear surface complex under acid to alkaline conditions. Phosphate reacts similarly with noncrystalline hydrous iron oxide under alkaline conditions. However, under the acidic conditions, the reaction product is partly converted into noncrystalline iron phosphate. In contrast, phosphate reacts with noncrystalline aluminum hydroxide, allophanic clays, and Andisols to give a material close to noncrystalline aluminum phosphate at wide pH range.
Although silicate layers of 2: 1 layer silicate minerals hardly react with phosphate, exchangeable Al, Ca and Mg react with phosphate depending on the reaction conditions. HPO
2-4is highly reactive with exchangeable Al than with Al-humus complex and ferrihydrite. On the other hand, H
2PO
-4 is more reactive with Al-humus and ferrihydrite than with exchangeable Al. The exchangeable Ca and Mg held by layer silicate minerals react with ammonium phosphate to give CaHPO
4· 2H
20 and MgNH
4PO
4·6H
20, respectively at pH 7.0. However, the exchangeable Ca and Mg in Andisols do not give CaHPO
4·2H
20 and MgNH
4PO
4·6H
20 under the same conditions, possibly due to the high affinity of Ca
2+ and Mg
2+ for phosphated Andisols.
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