Effect of P fertiliser application strategy and soil P sorption properties on 'incidental' P fertiliser characteristics using laboratory techniques and long term Bayesian risk modelling

The occurrence of rainfall and runoff soon after phosphorus (P) fertiliser application (fertiliser ‘incidental’ effect) can increase P exports. In this paper we firstly use a laboratory study to examine the effect of soil properties on the duration of the incidental fertiliser effect. Secondly we examine the effect of P fertiliser strategy – single v split applications – on long-term runoff P risk using laboratory and catchment runoff data in a Bayesian risk model. We found that soil P buffering characteristics have a large effect on the duration of incidental fertiliser effect. Despite the disproportionately large effect of a single application on runoff P concentration compared to split applications, the increased likelihood of runoff and fertiliser application coinciding when fertiliser applications were split, mean that the overall risk of runoff P loss is increased with split fertiliser applications. Farmers should therefore apply P fertiliser in a single annual application and apply this when risk of runoff is low, the importance of this strategy being greatest for farmers on soils with moderate to low P buffering