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

Decennia of imbalance between P fertilization and P uptake by crops in regions with intensive agriculture in Europa and North America have resulted in an increase in soil P content above those required for optimum plant growth [1]. 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. The objective of this study is to select a soil P test or a combination of soil P tests, which is also useful to predict the risk for P leaching.

Main results

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. We hypothesized that P-AL is well adapted to identify agricultural soils with excessive soil P stocks, but that within the category of soils with elevated P-AL levels (i.e., soils with a P-AL level > 180 mg P kg-1), the soil P availability parameters P-CaCl2 and hot water extractable P (HWP) are better suited to predict P leaching losses.

Six long term field trials with a high P load (P-AL: 123 to 375 mg P kg-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 (figure 1).

Figure 1: Diagram of the equipment for leaching experiment

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 (figure 2), 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.

Figure 2 : (left) Scatter plots of the relation between P-CaCl2, HWP and P-AL (mg P kg-1) and the TP concentrations (mg P L-1) in column leachates of the leaching experiments. PCaCl2, HWP and P-AL are calculated on dry soil basis. The dashed lines illustrate the medians of TP, P-CaCl2, HWP and P-AL. (right) Risk to exceed a TP concentration of 0.5, 1, 2 or 4 mg P L-1 in function of P-CaCl2, HWP and P-AL. The Y-value is calculated as the percentage of data points with respectively a P-CaCl2, HWP and P-AL level equal or below the X-value, that exceed the predefined TP concentration limit. (HWP: hot water extractable P, P-CaCl2: 0.01 M CaCl2 extractable P, P-AL: ammonium lactate extractable P, TP: total P concentration in leachates).

Reference

Jordan-Meille, L., Rubæk, G.H., Ehlert, P.A.I., Genot, V., Hofman, G., Goulding, K., Recknagel, J., Provolo, G., Barraclough, P., 2012. An overview of fertilizer-P recommandations in Europe: soil testing: calibration and fertilizer recommandations. Soil Use Manage. 28, 419–435.

Affiliations

a Institute for Agricultural and Fisheries Research (ILVO), Plant Sciences Unit, Crop Husbandry and Environment, Burg. Van Gansberghelaan 109, 9820 Merelbeke, Belgium
b Katholieke Universiteit Leuven, Department of Earth and Environmental Sciences, Division of Soil and Water Management, Kasteelpark Arenberg 20, 3001 Heverlee, Belgium