Thermally Promoted Reaction Products of Purulia Phosphate Rock and Their Effect on P Availability in Rice Soils

The P availability in soils due to the application of products of phosphate rock (PR) from Purulia heated alone or with Na₂CO₃ or KCI in the ratio (by weight) of 1:0.5, 1:0.25 and 1:0.15 at 300, 500, 700 and 900°C was investigated. The products of PR alone, or PR-KCI mixtures heated at 300–900°C or the products of PR-Na₂CO₃ mixtures heated at 300–700°C did not improve the availability of phosphorus when applied to soils. However, due to hetating the PR-Na₂CO₃ mixture at 900°C, the reaction products contained substantial amount of available P. These products could release substantial amount of P to increase bicarbonate extractable P (Olsen P) in soils. The product of PR-Na₂CO₃ mixture in the ratio 1:0.5 (by weight) heated at 900°C was comparable to superphosphate in term of P availability. Probably the 37 per cent water and 58 per cent citrate soluble P of the total P in this product might have contributed to increase the P availability in soil. The presence of a water soluble phosphate (Na₃PO₄) and some citrate soluble phosphate CaNaPO₄, NaCaPO₄, FeNaPO₄, Ca₇(PO₄)₂ (SiO₄)₂ have been indicted from X-ray diflraction studies.