Journal of the Indian Society of Soil Science

In a greenhouse study, the direct and residual effectiveness of partially acidulated phosphate rocks (pAPRs) as P fertilizer were evaluated using a neutral soil (Typic Ustochrept). Finely ground phosphate rocks from Mussoorie, Udaipur and Purulia and a sample from North Carolina (USA) were acidulated to 25, 50 and 75 per cent of full acidulation with H₂SO₄ and H₃PO₄. The direct effect of PAPRs was compared with triple superphosphate (TSP) using cowpea (Vigna sinensis L.) as direct crop and the residual effect was examined on wheat (Triticum aestivum L.). Yield of and P uptake by cowpea and wheat increased significantly with increasing levels of acidulation up to 50 per cent. At a given degree of acidulation, products resulting from H₂SO₄ acidulation were less efficient to the direct crop of cowpea in respect of their dry matter yield and P uptake than those prepared with H₃O₄. However, the residual effects of both the groups of materials were comparable on wheat crop.