Phosphorus supplying capacity of phosphate rocks as influenced by compaction with water-soluble P fertilizers

Greenhouse and incubation experiments were conducted on an alkaline soil in order to assess the phosphate (P) supplying capacity of eight sources of phosphatic fertilizers, viz., Jhamarkotra phosphate rock A and B [JPR(A) and JPR(B )] and the products manufactured by their compaction with single superphosphate (SSP), monoammonium phosphate (MAP) and MAP+ elemental sulphur (Sdegrees). Results obtained from the 100-day incubation study indicated an increase in 0.5 M NaHCO3 extractable P status of the soil in the first 2 weeks followed by a gradual decrease in the next 65 days and a sharp decline thereafter, across all the P fertilizer sources. The order of dissolution of the compacted phosphate rocks (PRs) was JPR + MAP + Sdegrees > JPR + MAP > JPR + SSP. In the greenhouse experiment, using wheat as the test crop, an increased dose of applied fertilizer P registered higher grain and straw yield of wheat, as did the application of compacted PRs when compared to non-compacted PRs. Compacted and non-compacted JPR(A) sources performed better compared with the JPR(B) sources. Apparent P recovery, N, P and S content were found to be maximum in treatments receiving JPR(A) + MAP + Sdegrees, the efficiency of which in terms of grain yield was found to be 80.5% in comparison to DAP (the standard fertilizer used in the experiment). The performance of compacted PRs in the greenhouse experiment closely followed the trend obtained during the incubation study.