Published online by Cambridge University Press: 27 March 2009
Three nitric phosphate fertilizers with 30, 60 and 70% of their phosphorus in watersoluble form were evaluated relative to superphosphate and rock phosphate on neutral to alkaline soils. The test crops were wheat, paddy and berseem. Linear grain yield and dry-matter yield responses were obtained up to 100 kg P2O5/ha for wheat and berseem, respectively. Quadratic response functions were fitted to paddy grain yield. Yields of the various crops were well related to the water solubility of the P fraction in different sources. Availability coefficient ratios were calculated for each fertilizer. These indicated that the minimum water solubility of the phosphorus fraction in nitric phosphates required for high crop yields is dependent on the nature of the crop, on soil characteristics and on the method of application which together determine the shape of the response curve. Band placement of sources with higher water solubility of their P fraction (70% or more) resulted in fertilizer efficiency similar to wholly water-soluble sources. Nitric phosphates of 30 and 50% water-solubility were inferior to water-soluble sources when applied broadcast. However, where curvilinear responses were obtained, low water-solubility sources (30 or 50% water-solubility) performed better at higher than at lower rates of application.