SOIL PHOSPHORUS FRACTIONS IN SOME REPRESENTATIVE SOILS¹

Abstract

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 C, 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 (I yo), aluminium phosphate (0-3 yo), 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,SO,-extractable-phosphorus test was much higher in the subsoils down to 3+ 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 decrease 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 DhosDhate at three lime levels : onlv 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.

Soil Properties of a Tropical Savannah in the Eastern Ghats of India

“…Phosphates do not normally stay for very long in soils and are readily absorbed by plants [39]. Distribution and forms of phosphorous in soils help in evaluating the phosphorous status and degree of chemical weathering of the soils [13,46]. Phosphorous availability and soil acidity are closely related and different phosphorous fractions in soils depend on the magnitude and proportion of different forms of soil acidity.…”

Frequency Distribution of Clay Minerals in Major Great Soil Groups as Related to the Factors of Soil Formation

“…As the free CaCO3 is leached out, that phosphate becomes fairly soluble and weathers chemically, reacting with aluminum ions from aluminosilicates and iron ions from iron minerals (reviewed by Chang and Jackson, 1958). A weathering index or sequence is found for phosphate of various great groups of soils, measured by the percentage of the total inorganic phosphate that is chemically bonded with calcium, with aluminum, with iron, and (as a further step in the sequence) with iron and occluded in iron oxides (Chang and Jackson, 1958).…”

Phosphates in Some Missouri Refractory Clays

“…Composition of the veinlet and the reason for the designation "variscite-strengite" have been presented already. Variscite and strengite are likely to be found in acid soils (Chang and Jackson 1958;Hsu and Jackson 1960;Bache 1963). Reducing conditions favor variscite over strengite (Hsu and Jackson 1960).…”