The Role of Phosphorus Stratification in Cropland Soils in Increasing Lake Erie Phosphorus Loading

Increasing dissolved reactive phosphorus (DRP) concentrations in runoff from cropland draining into the western basin of Lake Erie are thought to be a major cause of the return of harmful algal blooms to the lake. In 2007, the Great Lakes Protection Fund awarded a $1 million grant to the National Center for Water Quality Research (NCWQR) to explore the extent of phosphorus stratification in area soils and assess the link between stratification and increased DRP loading to the lake.

The work was founded on the hypothesis that many long-accepted practices like “no-till” or “conservation tillage” agriculture – which are effective in reducing erosion and particulate phosphorus loading– may actually contribute to increased DRP loading in the western Lake Erie basin. These tillage practices can result in increased levels of phosphorus at the surface of the soils (phosphorus stratification), leading to increased concentrations of DRP in cropland runoff.

The project team, consisting of NCWQR staff along with certified crops advisors, extension agents, farmers, fertilizer dealers, university researchers, and U.S. Department of Agriculture representatives, organized a study where stratified soil tests were collected during routine soil testing so that phosphorus levels in the top two inches of soil could be compared with the levels found in the standard 8-inch test. A total of 4,270 soil samples were analyzed for this project, creating one of the largest soil data sets available on the topic of phosphorus stratification. The results confirmed that phosphorus soil test levels in top two inches of soil were much higher than in the deeper portions of the soil. Since runoff water from cropland interacts primarily with the surficial soils (less than 2 inches) and DRP concentrations in runoff water increase with increasing phosphorus soil test levels, phosphorus stratification is definitely a contributing factor to increased DRP loading to Lake Erie.

The project team identified several potential causes of stratification. These included:

  1. the switch from moldboard plowing, which inverted the soil, to no-till and reduced till methods;
  2. increased broadcasting of fertilizer onto the soil surface; and
  3. increasing amounts of crop residue on the soil surface that releases phosphorus as the residue breaks down.

It soon became apparent to the project team that there will be no “silver bullet” for reducing DRP loading to Lake Erie. Instead, programs to reduce DRP loading while maintaining crop yields and farmer profitability will have to incorporate appropriate combinations of nutrient, tillage, and water and soil management BMPs (such as those offered by the 4‐R nutrient stewardship program), customized to the circumstances of individual farming operations. Working with partners across the agricultural supply chain, from certified crop advisors to farmers and the fertilizer industry, the team developed a draft BMP toolbox for reducing DRP runoff (PDF)  which incorporates extensive feedback from outside experts.

Based on the work of the NCWQR team, the International Plant Nutrition Institute and The Fertilizer Institute developed and distributed special materials to fertilizer dealers in the Sandusky River watershed to advance nutrient management programs relative to the DRP runoff problems. The Andersons formed an internal phosphorus task force and has committed the participation of their staff agronomists to help the team address the DRP runoff problem. Research is progressing on possible new BMPs, such as gypsum treatment of soils and end‐of‐tile-drain treatment systems, but their role within current management efforts remains uncertain.

The study results and draft toolbox have been presented at meetings with farmers, certified crop advisors, agricultural retailers, state and federal agency personnel, and environmental groups. These presentations have reached ever-widening audiences across and beyond Ohio, including a working team of NRCS researchers brought to northwest Ohio in December 2011 by Dave White, Chief, USDA NRCS headquarters in Washington, DC.

Learn more about Heidelberg’s work on Dissolved Reactive Phosphorus.

Watch NCWQR Emeritus Director David Baker describe the problems caused by a buildup of phosphorus at the soil’s surface.

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