Potentiality of Indian rock phosphate as liming material in acid soil

Most of the Indian rock phosphates (RPs) are not suitable for production of commercial phosphate fertilizer because of their low phosphorus (P) content (low-grade). They are suitable as source of P in acid soil, but not effective for direct use in neutral to alkaline soil. The main aim of this study was to evaluate the potentiality of these RPs as liming material and subsequently their effect on P availability in acid soil. Four Indian RPs namely, Udaipur, Mussoorie, Jhabua and Purulia were evaluated for their liming potential through theoretical calculations, laboratory titration and soil incubation studies. The incubation experiment was carried out for 90 days to quantify more accurate per cent calcium carbonate equivalence (%CCE) values of RPs as well as the changes in pH, exchangeable aluminum (Al) and P availability in an acid soil having pH 4.53. Results emanated from the theoretical calculation showed that the %CCE of Indian RPs varied from 59 to 62. While, laboratory studies on quantification of %CCE (AOAC method 955.01) of RPs by titration ranged between 39.9 and 53.7 which were lesser than the theoretical values. The %CCE values obtained in soil incubation study at lower rate of RP application followed the similar pattern as those values obtained in the theoretical calculation. Though RP was less effective in increasing soil pH as compared to CaCO3, there was an increase in soil pH due to application of RPs, which is sufficient enough to decrease the content of Al much below the toxic and safe limit for supplying available P for plant growth. Thus, application of low-grade RPs could be a potential option as liming material in acid soil in addition to a source of P for crop production.