A common agricultural soil test can identify legacy P hotspots in a drainage ditch network

Agricultural soils have accumulated considerable phosphorus (P) reserves along the transport pathways within land-water continuum. Where P concentrations are excessive compared to the soil P sorption capacity, dissolved soluble P can leach to waterbodies. A phosphorus saturation ratio (PSR = P/(Fe + Al)) can be used to classify high and low risk soils based on a commonly applied Mehlich-3 soil test. PSR has been used for acid mineral soils, but in this study it was applied to sediments and drainage ditch bankside samples. Previous published data was converted to PSR and compared to P availability measurements. The results confirmed earlier findings, that a PSR threshold of 0.1 can delineate high and low P risk sites. By quantifying the amount of P in excess to the threshold, legacy P hotspots could be located in the network which would act as an additional source of P inputs to waters. In the study site, two soils contained over 80% of the excess legacy P, presenting a localized long-term risk to water quality. The findings support using the cost effective Mehlich-3 extraction to identify hotspots with most susceptible soil-P to losses and quantify the amount of potentially leachable legacy P.