SOIL PHOSPHORUS FRACTIONS IN SOME REPRESENTATIVE SOILS ¹

Abstract

Summary The distribution of soil inorganic phosphorus in soils was found to measure the degree of chemical weathering, the chemical weathering sequence being calcium phosphate, aluminium phosphate, iron phosphate, and occluded phosphate. The latter category includes reductant soluble iron phosphate and aluminium‐iron phosphate occluded in iron oxides. For example, three horizons of one Chernozem profile, a Dark Brown soil, and the calcareous C1 horizon of a Grey‐Brown Podzolic soil containqd 68–95 per cent. of their inorganic phosphorus in the form of calcium phosphate, the other forms decreasing exponentially in the sequence. The inorganic phosphorus of two Latosols increased exponentially in the order calcium phosphate (1%), aluminium phosphate (0–3%), iron phosphate (10–13 %), and occluded (reductant soluble) phosphate (66–78%). Three samples of podzolized soils contained intermediate, sigmoidal distributions of the four phosphates. In two Miami silt loam profiles, the 0–002 N H 2 SO 4 ‐extractable‐phosphorus test was much higher in the subsoils down to 31/2 ft. than in the surface soil, and was correlated mostly with the calcium‐phosphate content, somewhat with aluminium and iron phosphate, and none at all with occluded phosphates which constituted a high percentage of the total phosphorus. The higher content of available phosphorus in the subsoils and release of phosphorus in the surface soil by de‐ crease of A1 and Fe activity through liming most probably explain the lack of crop response to added phosphate fertilizer even though the surface soil had a low phosphorus test. Application of phosphate fertilizer to Almena silt loam greatly increased the amount of aluminium and iron phosphate at three lime levels: only a slight increase of calcium phosphate occurred; at tge highest lime level. The amount of occluded phosphate remained unchanged by liming or fertilization. The relatively small increase of calcium phosphate through phosphate application is attributable partly to crop removal but is shown to be due mostly to formation of the less soluble iron and aluminium phosphates.