Prediction of P concentrations in soil leachates: Results from 6 long term field trials on soils with a high P load

Abstract

We tested if a single soil P capacity test allows for a reliable forecast of P leaching from agricultural soils with a high P load. In regions with intensively managed arable soils, the soil P content has been dramatically increased by overfertilization with significant P leaching losses as a result. As a consequence, in order to control the P losses, P fertilization has been legally restricted. In several EU28 countries, the ammonium lactate extraction method (P-AL) is used as a soil test for P fertilizer advice, but sometimes also to determine the allowed P fertilizer dose to reduce leaching losses. We hypothesize that P-AL as an estimator of soil P capacity should be combined with 0.01M CaCl2 extractable P (P-CaCl2) and/or hot water extractable P (HWP), both estimators of the soil P intensity, to predict P leaching losses in soils with a high P load.Six long term field trials with a high P load (P-AL: 123 to 375mgPkg−1) on silt loam soils with a specific history of organic and inorganic fertilizer application were sampled for soil analysis and to conduct leaching experiments in the laboratory. P concentrations in the leachates served as a proxy for P leaching. Five field trials were used for model calibration and a sixth one for model validation. Two models, either with P-CaCl2 or with HWP as independent variables, have proven to be suited to distinguish soils with low and high risk for P leaching. In the range of P-AL in this study, P-AL proved to be a non-significant factor and was therefore not retained in either of the models. We conclude that for soils with large soil P stocks (high P-AL concentrations), both P-CaCl2 and HWP are suitable to detect the ones with a higher risk of P leaching losses. We suggest a threshold value for P-CaCl2 and HWP to be used in combination with P-AL, to select those soils where further P fertilization restrictions or other measures to reduce P leaching losses are needed most urgently. This threshold value however depends on what is considered as an acceptable P concentration in the leaching water of the tillage layer.