Phosphorus Fractions of Soils Treated with Phosphate Rock and Monocalcium Phosphate after Ryegrass Growth

Abstract

A pot experiment was conducted in two acidic soils (pH 4.8) with similar properties except for phosphorus (P) status to investigate the influence of different P fertilizers [Kunyang phosphate rock (KPR), monocalcium phosphate (MCP)] on ryegrass growth and different soil P fractions in the two soils. The P treatments were KPR at 50, 100, and 250 mg P kg–1 and MCP at 50 mg P kg–1. Results showed that the application of P fertilizers produced significant increase of ryegrass growth compared with no P application on soil with low P status. Dry weight of ryegrass did not increase after P application, compared with no P application on soil with high P status. Through P fractionation as modified method of Hedley, Stewart, and Chauhan (1982), inorganic resin P, sodium bicarbonate (NaHCO3) P, and sodium hydroxide (NaOH) P increased after MCP treatment compared with no P application. The PR application only increased resin P and hydrochloric acid (HCl) P. Soil organic P fractions were stable and less affected by P application. The average of KPR P dissolved in the two soils across all P rates was low: only about 19% of the averaged amount of KPR P applied, which was due to its low reactivity. This explains the poor agronomic effectiveness of KPR with respect to MCP.