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Reducing phosphorus losses from agricultural fields: Developing solutions for Ontario Farmers

Principal Investigator: Merrin Macrae

Research Institution: University of Waterloo

Timeline: October 2018 – May 2021


  • Synthesize edge-of-field runoff (surface and/or tile drainage) and P concentrations and loadings (soluble reactive P (SRP), total P (TP)) results from 8 sites in Ontario (ranging in soil texture and topography) to generate information regarding annual edge-of-field losses (3 sites), cover crops (2 sites), tillage (3 sites), P application strategies (3 sites) and preferential transport into tile drains (2 sites).
  • Support the development and validation of Phosphorus Loss Assessment Tool for Ontario (PLATO) by providing relevant Ontario-specific information from the data set developed (for the above objective) as well as plot and lab results to address areas of weakness in the tool.
  • Collaborate with farming organizations to educate Ontario farmers, certified crop advisors and other agri-environmental professionals on P transport.


  • Compilation of edge-of-field findings from Ontario sites shed light on regional differences between Ontario and areas in the USA and improved knowledge on the mechanisms driving these differences.
  • Ontario edge of field data helped to develop and validate PLATO.
  • PLATO allowed producers and agricultural organizations to estimate benefits of practices and demonstrated progress towards P loss reduction on their farms.
  • Public presentations to farmers regarding edge of field research and PLATO  helped increase adoption of BMPs, reduced mixed messages to farmers and increased acceptance of the PLATO tool.

Scientific Summary:

Agricultural phosphorus (P) losses are a significant economic and environmental concern in Ontario and throughout the Great Lakes Region, particularly in the Lake Erie watershed. Designing and implementing effective best management strategies (BMPs) to reduce P in runoff requires understanding of P loss pathways (i.e., surface versus tile), the form of P (particulate versus dissolved), availability of soil P, and timing of P loss (e.g., seasonality, time since P application). Grain production in Ontario is located on variable soil textures (loam-textured and clay textured) and topography (hummocky till moraines and flat clay-plain), which may result in regionally-specific water and P loss patterns currently not well constrained. Ontario farmers are looking at research and media reports from the USA for guidelines on how to manage P on their farms; however, some of the information may not be applicable to their farms. Thus, there is a clear need to integrate how, why and when P leaves fields across Ontario. This is essential to improve risk assessments and to recommend appropriate BMPs to mitigate P losses across the province.

The first objective of this project was to synthesize P loss data (i.e., concentrations, loads and P speciation (i.e., soluble reactive P versus total P)) in surface runoff and tile drainage from Ontario croplands (corn-soy-wheat) ranging in soil-texture (clay versus loam) and topography (flat plain versus hummocky). The Ontario data synthesis involved calculating seasonal and annual water and P budgets from three edge-of-field long-term water quality monitoring stations (2012-2017). In addition to this, data were synthesized and provided to OMAFRA for the advancement of the Phosphorus Loss Assessment Tool for Ontario (PLATO), and Ontario-specific recommendations for conservation practices (place-based) were given (objective 2). A third objective of this project was to provide information to farmers, CCAs and other specialists regarding P loss in the environment, but specific to Ontario farms. This was accomplished through a combination of live presentations, webinars and peer-reviewed journal articles.

Results show that the partitioning of both water and phosphorus loss differ seasonally across all sites, with the greatest losses in winter and spring. The partitioning of water and phosphorus between these pathways also differs between fields in the clay plain (i.e., Essex) and coarser-textured soils in the undulating landscapes of Huron County and the Upper Thames. Given that the pathways for P transfer vary across regions, the solutions to this may also differ within these landscapes.

An additional synthesis of these data, together with analyses of the efficacy of conservation practices across regions (some from our data, and some from the literature) has shown that the efficacy of conservation practices may differ across Ontario.  This was recently published in the Journal of Environmental Quality.

Numerous presentations and podcasts have been given on this topic to a wide range of audiences. We have also met several times with the PLATO team to assist in the development of the tool, but also to harmonize messaging between our team and the PLATO team to avoid confusion of those using the information.

External Funding Partners:

Fertilizer Canada

Project Related Publications:

Macrae, M.L., Jarvie, H., Brouwer, R., Gunn, G., Reid, K., Joosse, P., King, K., Kleinman, P.J., Smith, D., Williams, M. and Zonitzer, M. (2021). One Size Does Not Fit All: Towards Regional Conservation Practice Guidance to Reduce Phosphorus Loss Risk in the Lake Erie Watershed. Journal of Environmental Quality.

Macrae, M.L., Ali, G.A., King, K.W., Plach, J., Pluer, W.T., Williams, M., Morison, M.Q., and Tang, W.V. (2019). Evaluating hydrologic response in tile drained landscapes: Implications for phosphorus transport. Journal of Environmental Quality. 48(5): 1347-1355.

Plach, J.M., Pluer, W., Macrae, M.L., Kompanizare, M., Brunke, R.R., and McKague, K. (2019). Agricultural edge of field phosphorus losses in Ontario, Canada: Implications for land management efficacy in cold regions.Journal of Environmental Quality. 48: 4: 813-821, doi:10.2134/jeq2018.11.0418.

Macrae, M.L. 2021. Too much or too little water: Two Sides of the Same Coin // Trop d’eau ou pas assez: deux faces d’une même médaille. Living Soils Symposium – Regeneration Canada. Feb. 22-26, 2021 (Virtual).

Macrae, M.L., Lozier, T., Cober, J., and Van Eerd, L. 2021. (Invited Presentation) Phosphorus and cover crops in Ontario. Thirteenth Annual Nutrient Management Conference. Feb. 16, 2021 (Virtual).

Macrae, M.L., Jarvie, H., Baulch, H., Elliott, J., King, K., Kleinman, P., Smith, D., Williams, M., Zwonitzer, M., Liu, J., Lobb, D., Pomeroy, J., Plach, J., Pluer, W., and Wilson, H. 2020. (Invited Presentation) The Risk of “One-Size Fits All” Conservation Recommendations to Improve Agricultural Water Quality. Annual Meeting of the American Geophysical Union. Dec. 14, 2020 (Virtual).

Macrae, M.L., Brouwer, R., Gunn, G., Jarvie, H., Joosse, P., King, K., Kleinman, P., Reid, K., Smith, D., Williams, M. and Zwonitzer, M. 2020. (Keynote Presentation) Avoiding “one-size fits all” – Defining Lake Erie’s phosphorus management regions. Understanding Algal Blooms: State of the Science. Sept. 2, 2020, Cleveland, Ohio, USA (Virtual).

Macrae, M.L. 2020. (Invited Presentation) Phosphorus transport in agricultural landscapes: Avoiding “one-size fits all” solutions. Grand River Conservation Authority Stakeholder Workshop. Nov. 23, 2020.

Macrae, M.L. 2020. (Invited Presentation) Phosphorus Dynamics in Agricultural Fields and Watersheds in the Great Lakes Region. Ontario Soil and Crop Improvement Association. Guelph, ON. Feb. 24, 2020.

Macrae, M.L. 2020. (Invited Presentation) What Research is Telling Us About Phosphorus in Ontario. Annual Meeting of the Canadian Certified Crop Advisors. Guelph, ON. Feb. 10, 2020.

Macrae, M.L. 2020. (Invited Presentation) What Research is Telling Us About Phosphorus in Ontario. FarmSmart Annual Meeting. Guelph, ON. Jan. 18, 2020.

Pluer, W., and Macrae, M.L. 2019. (Invited Presentation) Edge-of-field research in Ontario: Can BMPs affect phosphorus losses? Grain Farmers of Ontario meeting. London, ON. Jan. 2019.

Macrae, M.L. 2020. Phosphorus Leaving Ontario Cropland. Soils at Guelph Workshop. Ontario Ministry of Agriculture, Food and Rural Affairs. Dec. 7, 2020 (Virtual).

Macrae, M.L. 2019 and 2020. Recent Progress on Agricultural Phosphorus Research in Ontario. Presentations to Stakeholders on three occasions: Lower Thames Valley Conservation Authority Workshop, Chatham, ON, Aug. 22, 2019; Sydenham Conservation Authority, St. Thomas, ON., Oct. 23, 2019; Land Improvement Contractors of Ontario Annual Meeting, London, ON, Jan. 22, 2020.

Macrae, M.L. 2019. Recent Progress on Agricultural Phosphorus Research in Ontario. Presentation at Ontario Phosphorus Research for Today’s Farming Workshop. London, ON. Aug. 31, 2019.

Macrae, M.L. 2019. Synthesis of Agricultural Phosphorus Research in Ontario. Meeting with Policy Branch of the Ontario Ministry of Agriculture, Food and Rural Affairs. April 6, 2019.

Macrae, M.L. 2018. Edge-of-Field Research in Ontario: Can Cover Crops Affect Phosphorus Losses? Soil Health and Cover Crop Field Day. Dresden, Ontario. Sept. 14, 2018.

Van Esbroeck, C. and Macrae, M.L. 2018. Agricultural Phosphorus Loss and Land Management Practices. Presentation to the Land Improvement Contractors Association. Sept. 12, 2018.