Contrasting yield responses to phosphorus applications on mineral and organic soils from extensively managed grasslands: Implications for P management in high ecological status catchments

Phosphorus (P) loss from grassland production is one of the main causes affecting high status water bodies in Europe. Soils with a high content in organic matter (OM), even if extensively managed, are particularly vulnerable to P losses due to their poor P sorption capacities, and can affect the water quality of high status catchments if the dynamics of applied P in these soils is not well understood. The aim of this study was to assess dry matter yield, herbage P content, and P use efficiency in six soils deficient in P and ranging in OM content from 8.7% to 76.4% in a pot experiment under increasing P applications using the Mitscherlich equation. Of the six soils investigated, there was a better response in dry matter yield and greater P use efficiency in the soils with greater OM content than the mineral soils. The Mitscherlich model described grass response precisely in organic soils due to the higher plant availability of applied P as a consequence of the poor P sorption capacities of these soils. Despite the higher availability of applied P for plants in organic soils, the P requirements to meet the threshold herbage P content for dietary P supply to ruminants were still very high, which may pose a risk of P loss to the environment if P fertiliser is applied based on recommendations obtained from plant analysis. These results indicate that P fertilisation of organic soils in sensitive catchments poses a potentially high risk of P transfer to water bodies.