Landscape sensitivity to P losses: biogeochemical analysis of agricultural soils

Principal Investigator

Merrin L. Macrae

Research Institution

University of Waterloo

External Funding Partners

This project was funded in part through Growing Forward 2 (GF2), a federal-provincial-territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario.

Project Start

April 2016

Project End

May 2019


  • Determine soil stratification of plant available phosphorus (P) across different soil textures in agricultural soils.
  • Characterize soil P retention to determine how "tightly" bound P is within soil and which environmental conditions may release P.
  • Determine the soil P sorption capacity to give an idea of how much P can be held in the soil and how close to saturation the soils are and if this varies with depth.


  • Understanding the relationship between surface and subsurface soil test phosphorus (STP) and dissolved phosphorus (P) runoff losses will provide valuable insight into predicting the fate of applied P to agricultural soils, and may allow farmers to apply P more strategically, while minimizing environmental impact and maximizing profits.
  • Understanding the amount and type of P in subsoils may lead to a better understanding of the potential role of tile drains in P loss in different regions.

Scientific Summary

Phosphorus (P) losses from agricultural lands are major environmental, economic and political issues in the lower Great Lakes region because of their impacts on downstream water-quality. Predicting the efficacy of best management practices (BMPs) to minimize P loss (dissolved and particulate) has been challenging due to the fact that BMPs that appear to be effective in one region are not necessarily effective in another. Tile drains in the lower Great Lakes region of the USA exhibit different P loss patterns relative to tile drains in Ontario. For example, most Ohio studies report that most P in tile drainage is lost in a dissolved form, whereas most P is lost as particulate in Ontario. Although some of this variability has been attributed to soil cracking and preferential transport, this does not appear to be the only factor controlling P loss. It is critical that differences in soil P loss/retention potential are assessed before the efficacy of BMPs can be fully evaluated and to explain the clear differences seen between studies in the USA and Ontario. Soil texture and biogeochemical properties are important to soil P sorption capacity and thus storage of soil P within agricultural lands. As such, characterizing linkages between soil properties and P stratification is essential for understanding P movement and fate in agricultural subsoils. This is directly relevant to predicating P availability to crops and risk to downstream aquatic ecosystems, and can be used to build predictive models.

This project will provide a spatial assessment of soil P stratification of agricultural lands across the lower Great Lakes region of Southern Ontario and USA, establishing linkages with soil texture and biogeochemical properties to evaluate and explain the differences in reported P losses between these two regions. Soil cores will be collected at 8 sites between Ontario (4 sites) and USA (2 sites in Ohio and 2 sites in Indiana) to establish P stratification of soil test P (STP) concentrations (Olsen, Bray, Mechlich-3) and soil textures (sand, silt and clay). Additional soil biogeochemical analysis on P sorption capacity and retention will also be done on selected subsamples to further evaluate the nature of the P stored within these soils.

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