Effect of agronomic management on pasture response and fertiliser phosphorus use efficiency over time: an Australian perspective

Keywords: phosphorus, pasture, P use efficiency, Australia.

For over 100 years, fertiliser phosphorus (P) has been applied to leguminous based pastures in the high rainfall zone of south-eastern Australia. However, little is known on the growth response and fertiliser

P use efficiency of pastures. The aims of this study were to understand the fate of fertiliser P in pastures over the short- and long-term, and the effect of agronomic management (i.e. fertiliser timing and placement, and initial concentration of ‘plant-available’ P in the soil) on pasture production and fertiliser P use efficiency. This involved a soil P audit of a long-term permanent pasture field experiment (13 years), and the use of 33P-labelled single superphosphate (SSP) that was applied to Trifolium subterraneum pastures at two field sites in south-eastern Australia.

Over the long-term (13 years), we found that the majority (~ 85%) of fertiliser P added to pastures could be recovered in the 0 – 20 cm soil layer. The chemical nature of the accumulated P was largely that of stable forms of inorganic and organic soil P. In contrast, over a single growing season, we found that a large proportion (up to 50%) of the fertiliser P could be recovered by the pasture in the year of application. The recovery of fertiliser P in the soil was relatively low (~ 25%) and found largely as pools of inorganic P in the 0 – 4 cm soil layer. Therefore, it appears that much of the P taken up by pastures in the year of application is returned to the soil surface. In general, the highest pasture production and recovery of fertiliser P occurred when fertiliser was applied at early-season to the soil surface, and with an initial concentration of plant-available P at the agronomic optimum level for maximum pasture growth. Therefore, our data provide supporting evidence that the current (‘best practice’) management of fertiliser P in Australian pastures should result in the greatest pasture growth and fertiliser P use efficiency.

Timothy I. McLaren, Group of Plant Nutrition, ETH Zürich, Switzerland.

16 pages, 4 figures, 4 tables, 19 references