The effect of soil P sorption properties and phosphorus fertiliser application strategy on ‘incidental’ phosphorus fertiliser characteristics: a laboratory study
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The application of phosphorus (P) fertilisers to grazed pasture systems can increase the export of P in surface runoff. This increase can arise from interaction of recently applied fertiliser P with surface runoff (incidental effects) or the interaction between pseudo-equilibrated soil P and surface runoff (systematic effects). The former can represent a large proportion of annual exports. In this paper we investigate the effect of soil P buffering properties and fertiliser application strategy—split versus single applications—on incidental fertiliser effects, using laboratory studies. We used a weak electrolyte solution as a surrogate measure of runoff P and consequently defined ‘fertiliser half-life’ for six soils with widely differing P buffering properties. There was a significant (P < 0.01) exponential decay relationship between soil P buffering and fertiliser half life. For soils with low P buffering capacity, fertiliser half life was up to ~4 days, whereas for highly P buffered soils the half life was <0.5 day. There was also a highly significant (P < 0.01) effect of P buffering capacity on the magnitude of the incidental fertiliser effect, with the magnitude increasing as P buffering decreased. On one of our soils with buffering properties typical of soils used for dairying in SE Australia, we compared the effect on soluble P of a single application of 40 kg P ha−1 with three applications of 13 kg P ha−1. A simple comparative measure of the risk associated with the two fertiliser strategies—the area under the time by concentration curves—suggests that there is greater risk with a single application. Our results show that particular attention should be paid to timing of P fertiliser application on poorly buffered soils.
KeywordsFertiliser Phosphorus Runoff Incidental Half life
The authors would also like to acknowledge the funding support provided by an Australian Research Council Linkage grant, Dairy Australia, the Tasmanian Department of Primary Industries, Parks, Water and the Environment, Impact Fertilisers Ltd, Roberts Ltd, the DairyTas Board Inc, the Tasmanian Institute of Agricultural Research, the University of Tasmania, and Industry & Investment NSW. We gratefully acknowledge the constructive comments of reviewers and Ms E. Keskula in assisting with manuscript preparation.
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