Availability of phosphorus in intensive used grassland soils

We investigated the sorption capacity and availability of phosphorus (P) in soils of the Lippenrütibach catchment (Canton Luzern, Switzerland) to assess the risk of P-losses from agricultural land in the long-term. The different available P-forms in soil were characterised using six P extraction methods. The soils in the study area revealed a high P-sorption capacity and medium to high P-contents. In the top 5 cm soil layer these were a few times larger than in the soil layer 5 to 20 cm. The P-saturation index, expressed as the ratio between P-content and P-sorption capacity in soil, was on average 0.48 (0 to 5 cm) and 0.36 (5 to 20 cm) and was closely related to both the soluble P and plant available P in soil. The P-saturation index varied strongly between sites of different farms as well as between sites of the same farm. Based on the P-sorption capacity and the P-saturation of the soils investigated, we presume that a reduced P-fertilization below the P-demand of the plants may decrease the potential risk of P-losses in the catchment only slowly, i.e. within 10 to 20 years.