Thermally promoted phosphate rocks III. Direct and residual effects in a Typic Ustochrept soil

Abstract

Direct and residual effects of 16 thermally promoted phosphate rocks (TPPRs) were compared with DAP in a pot experiment conducted with P deficient Typic Ustochrept collected from the Indian Agricultural Research Institute farm, New Delhi. TPPRs were prepared by heating four phosphate rocks (PRs) viz., Jhabua, Mussoorie, Purulia and Udaipur with Na2CO3 or KCl at two temperatures (700 and 900°C) and applied to pots @ 50 and 100 mgP kg−1 soil. Cowpea was grown as a test crop to study the direct effect of TPPRs and subsequently mustard and wheat were grown in sequence to assess the residual effect. Application of TPPRs resulted in significantly higher dry matter yield and P uptake in cowpea compared to no-P control. Among the four PRs, TPPRs prepared with Mussoorie PR were found to be better than others in influencing the plant attributes of cowpea. Thermal products prepared with KCl (PR: additive ratio 1:0.25) were found to be superior in affecting the plant attributes of cowpea. Between the two temperatures, TPPRs of 900°C recorded significantly higher plant attributes in cowpea over those obtained at 700°C. The trend of residual effects of TPPRs on plant attributes of mustard and wheat was similar to that of direct effect, but the TPPRs obtained with either Na2CO3 or KCl were found to be on par in affecting drymatter yield of mustard and wheat. TPPRs also left substantial quantities of residual P in soil samples collected after harvest of each crop. However, both direct and residual effects of TPPRs were significantly lower as compared to DAP.