Phosphorus Release Characteristics of Partially Acidulated Phosphate Rocks in a Typic Ustochrept Soil

Three indigenous phosphate rocks, viz. Mussoorie, Udaipur and Purulia and one exogenous source from North Carolina (USA) were acidulated at 25, 50 and 75 per cent levels of complete conversion with either sulphuric or phosphoric acid and applied to soil @ 75 and 150 mg P₂O₅ kg⁻¹ soil. Treated soils were incubated for 112 days at a temperature of 35 ± 1°C and soil water content of 50 per cent of the field capacity. Throughout the period of incubation, phosphate treated soil samples maintained a higher level of P status as compared with control. Purulia sample, among the PR's was superior during the first two weeks of incubation over others and subsequently the North Carolina sample had higher P. Phosphoric acid treated products maintained higher extractable P status in soils over sulphuric acid treated ones. Every increase in level of acidulation or P dose, was accompanied by concurrent improvement of extractable P status of the soil. In general, after a certain period of incubation, the solution P concentration dropped rapidly, suggesting thereby precipitation of P in all the treatments, irrespective of sources of PRs, acids, acidulation levels and levels of P application.