Developing a Phosphorus (P) Credit to Support Illinois Farmer Precision Management of P


PI: Dr. Andrew Margenot, Assistant Professor, Dept. Crop Sciences

Co-PI: Dr. Jordon Wade, Post-doctoral Scholar, Dept. Crop Sciences

Co-PI: Dr. Chongyang Li, Post-doctoral Scholar, Dept. Crop Sciences


In constructing N recommendations for farmers, N credits from organic matter mineralization play a considerable role. As described in the Illinois Agronomy Handbook, these N credits are developed by combining the size of the soil organic matter “bank account” with inherent soil properties such as soil texture to predict the total release of plant-available N. Together with yield goals and pre-plant N content, the N credit or ‘release’ enables further fine tuning of N recommendations for greater use efficiency of N, with savings for farmers and benefits to water quality.

Why don’t we have a similar credit for P?

Soils also contain large reserves of P in organic forms bound up in organic matter (OM). Like N, organic P must first be decomposed or “mineralized” by microbes before becoming crop-available as orthophosphate. There is reason to anticipate agronomically relevant P credits in Illinois due to her OM-rich soils. Crediting P from mineralization can increase net economic gains by yielding savings on P inputs. In addition to missed economic opportunities of an unquantified P credit, not accounting for P mineralization also has implications for off-farm losses because release of orthophosphate from OM may not necessarily synchronize with crop demand (Figure 1).

Currently, no such credit exits for P, for several reasons. As an inherently biological process, estimation of P release is complex and is driven by the interactions of soil conditions and weather. Additionally, the size of the organic P “bank account” and the rates of release are likely to vary how soils are managed, such as tillage practices. Lastly, the approach to estimate P mineralization is much more complex than for N since P can be strongly influenced by both biological and non-biological processes in soils.

This DSynergy project will capitalize recent advances in isotopic chemistry of P to estimate P mineralization across a range of Illinois soils, a first and essential step toward the possibility of developing a P credit. To do so, our research team will build on ongoing experiments at the Dudley-Smith Farm as well two additional University of Illinois research and demonstration centers to capture soil, climate and management practices that are likely to influence the size of the hypothesized P credit. Sampling will be conducted in spring 2020, with preliminary results anticipated by early 2021. The research team looks forward to discussing this and other P-related management topics with farmers at upcoming events at the Dudley Smith Farm.

Please direct questions to Dr. Andrew Margenot at

Figure 1: The conceptual relationship between P mineralization and crop P uptake. Mismatches between crop demand and soil P release represents potential P losses, while areas of overlap between demand and supply is considered a potential P credit.