What is “legacy” phosphorus?

Phosphorus (P) is strongly held in soils and sediments due to covalent bonding with variably-charged/constant-potential minerals on valence-unsatisfied terminal hydroxide groups. These bonds ultimately arise from the nature of electron sharing between P and oxygen atoms within a phosphate molecule. Phosphorus adsorption onto mineral surfaces serves as a “seed” and exists in a continuum of P solubility with precipitated aluminum (Al), iron (Fe), magnesium (Mg), and calcium (Ca) P forms, where solubility is a function of the source cation mineral solubility and pH. The adsorption of P onto soil and its subsequent desorption from soil exhibits hysteretic behavior resulting in the general insolubility and immobility of P relative to other macronutrients (e.g., nitrogen). Very little soil-bound P is soluble; plants can only access a small percentage of soil's total P, and an even smaller percentage is lost from soils through surface runoff and subsurface leaching annually. These characteristics of P allow it to accumulate and remain in soils and sediments over time and have spurred the rebranding of accumulated P as “legacy P” in both the scientific literature and contemporary vernacular (A. Sharpley et al., 2013). Phosphorus accumulation in agricultural soils often arises due to the imbalance of P inputs (e.g., fertilizers and manures) and outputs (e.g., crop removal). As P accumulates in soil and sediments over time, it becomes more soluble, and, as a result, greater dissolved phosphorus (DP) concentrations in surface runoff and subsurface leaching accompany increasing soil P levels (A. N. Sharpley, 1995). However, not all soils are created equal; the concentration of soil P required for producing a given concentration of DP in solution varies as a function of soil properties, including mineralogy, pH, texture, and organic matter. For a given soil test P (STP) value, sandy soils of the Eastern US Coastal Plain, for example, tend to produce greater DP concentrations in solution compared to Piedmont soils (Penn et al., 2006). Many soils from Wisconsin also produce greater DP concentrations in solution at a given STP value compared to Mid-Atlantic and Ohio soils (Andraski & Bundy, 2003; Vadas et al., 2005). Legacy P is, therefore, a soil chemical condition that is defined by its solubility rather than a specific chemical form of P such as Ca- or Al-related P forms. How is legacy P defined? Legacy P is the ability of the soil, without recently added inorganic fertilizer or manure, to produce elevated P concentrations in drainage water (surface runoff and subsurface leaching). In this context, “elevated P concentrations” can be objectively defined but will vary among soils and the sensitivity of the downstream aquatic ecosystem to P inputs (Correll, 1999). Since DP concentration in solution varies as a function of soil properties and water bodies differ in their response to P inputs, it is difficult to assign a universal quantitative legacy P threshold level. It is therefore much easier to qualitatively describe legacy P than it is to define quantitative values. Communication of soil legacy P, as a result, should encompass a range of STP values, especially when information on specific soils or receiving water bodies is not known. Chad Penn: Writing—original draft. Mark Williams: Writing—review and editing. Kevin King: Writing—review and editing. William Osterholz: Writing—review and editing. The authors declare no conflicts of interest.