Ph~sphorus Sorption as Influenced by Soil Clay Minerals in Major Soil Groups of India

Sorption envelopes (pH 4.2 to 8.0) of ten soils representative of various soil groups of India at 3.23, 6.45, 12.9 and 25.8 mmol P kg⁻¹ soil levels in 0.01M CaCl₂ matrix solution were studied. At low P levels, kaolinite dominant soils sorbed highest amount of P followed by illite and 2:1 expanding layer mineral dominated soils. This was attributed to spilling over of negative charge from basal planes to edge face region in 2:1 expanding layer minerals and to a lesser extent in illite, which weakened the attractive forces between the positively charged edge face of 2:1 minerals and phosphate. The above sequence of P adsorption changed at high P rates. This was ascribed to electrostatic adsorption on low affinity sites, which are abundant in 2:1 expansible layer minerals. The acidic soils sorbed less than 60% of added P at low pH and greater than 99% at high pH compared to non-acid soils (86% and 84%, respectively). This peculiar behaviour is ascribed to increasing polymerization of exch. A1³⁺ with increase in pH. P sorption was not influenced by any soil constituent other than free iron oxides, exch. Al³⁺ and clay minerals.