Phosphate Reactions in Soils

In the laboratory, phosphate adsorption by layer silicates is rapid for a few hours and then continues more slowly for weeks. The initial rapid reaction can be envisioned as a combination of nonspecific adsorption and ligand exchange on mineral edges. The slower reaction probably consists of a complex combination of mineral dissolution and precipitation of added phosphate with exchangeable cations or cations within the lattices. 

Low and Black showed that phosphate retention by kaolinite increased with time and phosphate concentration. Silica concentrations in the bulk solution increased simultaneously. The reaction was thought to be a two-stage reaction  

FIGURE 9.5. Representation of H2POJ penetration into an iron oxide surface and subsequent formation of a stable binuclear bridge. (After J. C. Ryden, J. K. Syers, and F. R. Harris. 1973. Adv. Agron. 25 1 t5.) 

AI phosphate. When hectorite (a 2 1 layer silicate in which Mg2+ is the dominant octahedral cation rather than Al3+) is substituted for kaolinite, much less phosphate is retained. This points out the importance of Al to phosphate retention. In acid soils, phosphate retention is often closely related to the amounts of extractable Pe3+ and Al. 

The diagram also indicates that phosphate should precipitate in basic soils as one of several Ca phosphates, the least soluble of which are hydroxy- and fluoroapatite. Variscite and strengite are too soluble to exist under basic conditions, and they should not form in basic soils. Both variscite and strengite, in fact, would be good phosphate fertilizers in alkaline soils because of their solubility in basic soils, if they were applied as finely ground materials. Calcium phosphate ore ( rock phosphate, mostly hydroxy- and fluoroapatite) is similarly effective in acid soils. Rock phosphate is treated with sulfuric acid to make superphosphate, nominally CaHP04 treatment with phosphoric acid yields triple superphosphate, nominally Ca(H2P04)2. Both superphosphate and triple superphosphate are more soluble than rock phosphate and make phosphate more immediately available when added to soils at any pH. 

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