Changes in soil phosphorus and pH in a red earth soil during build-up and residual phases of a wheat clover ley farming system

Abstract

In a red earth soil at Wagga eight soil phosphorus pools, and pH, were measured. Positive linear relationships were found between the amount of fertilizer phosphorus applied prior to the start of the experiment (1972) and the inorganic (Pi) pools. Despite a range in total phosphorus of 130 ppm the plots had similar organic (Po) pools. Mehlich phosphorus (0.05 N hydrochloric acid + 0.025N sulphuric acid) and its major component, aluminium phosphorus, increased more rapidly with each unit of fertilizer phosphorus than other Pi pools. During 1972-1974 the soil carried grazed clover pasture but no phosphate fertilizer was applied. There were linear declines in soil total phosphorus, aluminium phosphorus and Mehlich phosphorus pools. The rates of decline were higher on plots which had higher phosphorus levels at the beginning of the residual phase. The Mehlich phosphorus pool declined by 1.8 to 9.3 ppm. 'Available' phosphorus (Bray and Kurtz 1) declined at a similar rate, but 'available' phosphorus (Colwell) declined more rapidly. Po increased at a mean rate of 5.4 ppm year-1. This was similar to the estimated return of Po in plant and faecal material on low phosphorus plots, but only about 30% of the return of Po and high phosphorus plots, indicating that a substantial quantity of Po can be mineralized in a pasture ecosystem. Some movement of phosphorus to the subsoil was noted during and immediately following phosphate application. Soil pH declined by 0.1-0.2 units during the residual phase.