Phosphate rock and a phosphate rock/superphosphate mixture as fertilizers for crops grown on a calcareous soil

Abstract

The agronomic effectiveness of Mussoorie phosphate rock (MPR) from India and a 2:1 mixture of MPR and single superphosphate (SSP) was compared against SSP as phosphate fertilizers for crops. The experiment was conducted for three seasons and was designed to study the response of crops to current application as well as residual effects of the various P fertilizers. Three crops were grown in sequence: finger millet (Eleusine coracana), maize (Zea mays L.), and blackgram (Phaseolus mungo) on a calcareous soil under irrigated conditions. The phosphate fertilizers were applied to finger millet and/or maize but not to the blackgram. Soil samples were collected at intervals and analysed for Olsen bicarbonate‐extractable P. The agronomic effectiveness of fertilizers relative to SSP (RAE) were calculated from the fertilizer substitution ratios. When used on finger millet, the RAE of MPR, calculated at a yield which corresponded to 90% of calculated maximum yield on applying SSP, was 42%. For the mixture of MPR and SSP the RAE was 68%. On maize, yield in MPR treatment plateaued at too low a level (about 80% of calculated maximum yield for SSP) to calculate RAE but for MPR/SSP the RAE was 80%. The residual effectiveness of fertilizers on the second crop, compared against freshly applied SSP was 41% for SSP, 49% for MPR, and 73% for MPR/SSP. Olsen bicarbonate‐extractable P values determined one month after fertilizer application increased over control by about 55% in MPR and 86% in MPR/SSP treatments relative to SSP. Economic calculations indicated that, application of MPR is of equal value to SSP for the cropping sequence whereas MPR/SSP for the cropping sequence as well as for individual crops.