Phosphorus supplying power of some thermally promoted reaction products of phosphate rocks

Abstract

The P availability in soil and agronomic efficiency of the products of non-premium grade, unreactive Purulia phosphate rock (PPR) heated alone or with Na₂CO₃ or KCl at different temperatures were investigated in two P deficient soils. The heated products of PPR alone did not improve the P availability in soil or P utilisation by rice over the original PPR. The products of PPR-KCl mixtures heated at 300-900°C were not effective at all. Out of several products of PPR with Na₂CO₃, the product prepared from PPR and Na₂CO₃ mixture in the weight ratio 2:1 heated at 900°C was comparable to superphosphate (SP) with respect to P availability in soil, straw and grain yield and P uptake by rice. The effectiveness of the products of PPR-Na₂CO₃ mixtures heated at 700°C though inferior to SP were superior to that of the original PPR in the highly acidic P deficient soil from Choudwar. However, products of another phosphate rock (PR) from Jordan and NA₂CO₃ mixtures heated at 900°C were less effective in comparison to SP. The amount of inherent silica present in Jordan PR was inadequate to promote the apatite-NA₂CO₃-SiO₂ reaction towards completion thus leading to an inferior product. On the other hand, similar products of non-premium grade Kasipatnam and Mussoorie PRs which are not suitable for direct application were comparable to SP in their effectiveness when these PRs were fused with Na₂CO₃ in the weight ratio 2:1 at 900°C. X-ray diffraction studies indicated presence of water and citrate soluble phosphate phases viz., Na₃PO₄, NaCaPO₄ and possibly Ca₇ (PO₄)₂ (S10₄)₂ in these products of PR-Na₂CO₃ mixture heated at 900°C. The water and citrate soluble phases of these products could release adequate P for absorption by crop.