Phosphorus Reactions and Fluxes in Pasture Soils

Summary

​Pasture growth is often limited by the availability of soil phosphorus (P). Adding P fertiliser can overcome this limitation but the balance efficiency of the fertiliser, i.e. the proportion of fertiliser P that is exported from the paddock in produce in the year of application, is generally low (e.g. < 20%). The objective of this project was to investigate the causes of this low efficiency. Field trials using superphosphate containing a radioactive label indicated relatively high (30-60%) P use efficiency of pasture species (i.e. the proportion of fertiliser P that is taken up by plants). Analysis of soils from long-term field experiments showed that over longer timescales P accumulates in both inorganic and organic forms, with much of the organic P accumulating in these soils apparently produced by a previously unrecognised process. Together these results indicate that an important contributor to low P balance efficiency in grazed pastures are reactions occurring over longer time scales sequestering some of the P cycling in the soil-plant-animal system. Incorporation of this knowledge into fertiliser P management and the selection and breeding of pasture plants holds much promise for reducing P fertiliser requirements in the red meat industry.