The effect of soil P sorption properties and phosphorus fertiliser application strategy on 'incidental' phosphorus fertiliser characteristics: a laboratory study

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.