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

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 degrees 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 degrees 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 degrees 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 degrees 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 degrees 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 degrees C. The water and citrate soluble phases of these products could release adequate P for absorption by crop.