The relationship between water-soluble P and modified Morgan P: results based on data and chemical modelling

An improved understanding of potential soluble phosphorus (P) loss in run-off and leachate from agriculturally managed soils presents practical and theoretical challenges. Our study aimed to discover whether modified Morgan extractable P (MMP) can be used to predict water-soluble P (WSP). We first addressed the relationship between MMP and WSP, and whether MMP is useful for predicting the WSP concentrations demanded by water quality regulations. Secondly, we applied novel soil chemical models to explain why the relationship between MMP and WSP depends upon soil properties. Thirdly, we explain how soil properties relate to potential soluble P loss in situations in which soil is subjected to a wide liquid-to-soil ratio (e.g. run-off and rivers) compared with those with a narrow ratio (e.g. soil porewater). To address these P loss scenarios, 60 agricultural topsoils (0–10 cm) were collected from a mixed-farming catchment (Lunan catchment, northeast Scotland) and chemically characterized. Theoretical understanding of P solubility was obtained with a P sorption model. The data showed variability in the relationship between MMP and WSP. Modelling shows the MMP versus WSP relationship is nonlinear, depending on several confounding factors (P sorption capacity (PSC), Ca, pH) and the liquid-to-soil ratio (L:S) employed for WSP determination. Consequently, the slope of the relationship is not unique but depends subjectively on the set of soils surveyed. MMP versus WSP at large L:S (e.g. in run-off or rivers) is positively correlated to PSC, whereas at narrow L:S (e.g. porewater) there is a negative correlation with PSC. The study provides new ideas for the interpretation and extrapolation of agronomic soil test data for soils of varied properties and highlights the need to utilize insights from soil chemistry.