A New Tool Combining Assessment of Environmental Risk and Optimal Agronomic Application of Phosphorus.

We are proposing a management tool to minimize phosphorus (P)-related environmental risks while providing optimal agronomic P. Soil test P (STP) procedures, developed for crop fertilization recommendation, have two major shortcomings for environmental risk assessment: (i) a low value is not necessarily indicative of a safe P application site and (ii) they do not provide prediction of “safe lifespan” of application sites.  A recently-developed alternative, “safe” soil P storage capacity (SPSC), overcomes these shortcomings while also having potential to determine P fertilization needs more quantitatively than via STP. SPSC is calculated as: (0.15 – PSR)*Extractable (Fe + Al), where “PSR” is the molar ratio of P to the sum of Fe and Al. Concentrations of P, Fe and Al from routine analyses in soil testing labs (e.g. in Mehlich 1 or Mehlich 3 solutions) can be used with calibration, a practical advantage.  Positive SPSC indicates the soil is a P sink and P applications could be dictated by crop requirement.  Negative SPSC is linearly related to P release from soils that can offset fertilizer P requirements.  Slightly-negative SPSC would support plant growth while minimizing environmental impacts.  Validation of this approach may lead to routine adoption of a coupled agronomic-environmental protocol more rigorous than provided by fixed STP thresholds. Managing P-impacted sites (SPSC<0) to achieve “zero SPSC + crop requirement” is a means of protecting surface water while maintaining optimal plant growth. We believe it will be a major step toward sustainable land management practices, including an understanding of situations associated with manure applications based on N requirements, P source effects, and Fe/Al-based soil amendments for mitigating heavily P-impacted soils. Once the feasibility of the approach is established for both environmental and agronomic purposes, the approach can be readily used for a range of soils and cropping systems common to the southeastern USA, and other Al/Fe-based sandy soils.