PHOSPHATE FIXATION—AN EXCHANGE OF PHOSPHATE AND HYDROXYL IONS

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Abstract
Most soils contain a considerable amt. of iron oxide distributed as a film over individual soil particles. This is especially true of the lateritic and podzolic soils (B horizon). Though this hydrated iron oxide is not in soln., its colloidal nature and its surface position on soil particles make it very active in fixing any soluble phosphate that comes in contact with it. Data presented in this paper indicate that phosphate fixation is a physicochemical exchange of phosphate irons for these exposed OH ions. Though this seems to be an equilibrium reaction, in soils it is greatly shifted toward phosphate fixation, because of the very low OH-cone, in the soil soln. and the high OH cone, of the hydrated ferric oxides on the surface of the soil particles. Kaolin fixes phosphate only when it is finely ground to expose active OH ions. Ferric hydroxide, finely ground kaolin, and a podzol B horizon fix large amts. of phosphate in an unavailable form. When a suspension of any of these materials was mixed with soluble phosphate of similar pH, the resultant mixture increased in pH. Evidently phosphate ions replaced the OH ions. Dehydration and removal of OH ions by high temps, greatly reduced the phosphate-fixing capacity of soils and of hydrated iron oxide. The reverse is true for hydrated aluminum oxide. Removal of active Fe and Al (R2O3) also greatly reduced the capacity of soils to fix phosphate. Both silicate and fluoride ions are capable of replacing OH ions, as evidenced by pH changes. The beneficial effect of silica on plant growth may be due to its ability to remove fixed phosphate or to replace the OH ions and thus decrease phosphate fixation.