Reliable techniques must be developed to predict phosphorus (P) storage and release from soils of uplands, ditches, streams, and wetlands in order to better understand the natural, anthropogenic, and legacy sources of P and their impact on water quality at a field/plot as well as larger scales. A concept called the “safe” soil phosphorus storage capacity (SPSC) that is based on a threshold phosphorus saturation ratio (PSR) has been developed; the PSR is the molar ratio of P to Fe and Al, and SPSC is a PSR-based calculation of the remaining soil P storage capacity that captures risks arising from previous loading as well as inherently low P sorption capacity of a soil. Zero SPSC amounts to a threshold value below which P runoff or leaching risk increases precipitously. In addition to the use of the PSR/SPSC concept for P risk assessment and management, and its ability to predict isotherm parameters such as the Langmuir strength of bonding, , and the equilibrium P concentration, EPC0, this simple, cost-effective, and quantitative approach has the potential to be used as an agronomic tool for more precise application of P for plant uptake. 1. Introduction Concern about the environmental fate of phosphorus (P) has arisen from evidence that P derived from land-applied animal wastes and fertilizers can potentially promote eutrophication of water bodies [1–9]. There is a need to develop ways of managing nutrients such that risks to the environment are minimized. This paper describes a field test approach that we call the “soil P storage capacity” (SPSC) which enables the on-site prediction of how much P can be added to a soil prior to development of significant risk of P leaching and/or runoff. “Soil-test P” (STP) procedures that were initially developed for crop nutrient recommendations have been adapted to environmental risk assessment for P loss [10–13]. Another approach used in P risk assessment involves a “change point” phenomenon, which corresponds to a threshold of P loading where a marked increase in solution P concentration is observed [14]. The change point threshold is expressed in terms of the molar ratio of extractable P to the sum of extractable Al and Fe [14–17]. The latter ratio is called the P saturation ratio (PSR). The PSR can be determined by various extractants, including those used for STP. The STP and PSR have two shortcomings stemming from failure to capture P retention capacity: (i) a low value is not indicative of a safe application site and (ii) neither provides a means of predicting the “safe lifespan” of an application site.
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