Ontario Grain Farmer February 2021

14 REDUCING NUTRIENT LOSS to improve aquatic ecosystems is a big task. Doing so requires an understanding of how phosphorus (P) moves through Ontario’s soils, and how management practices could be adapted to fit our province’s diverse landscape. Two recent research projects have made progress on both fronts. SOIL DIFFERENCES While P loss from agricultural land is a known issue on both sides of the border, studies from the United States often do not reflect what researchers have been finding in Ontario, say Dr. Merrin Macrae and Dr. Janina Plach, the University of Waterloo researchers tasked with investigating why this is the case. “What we’re seeing in Ontario in terms of concentrations in tile drainage, is that there is a pretty stark contrast in what we were seeing for water quality compared to what they were seeing at similar sites in the U.S.,” says Macrae. The theory was differences in soil chemistry between Ontario and the United States, as well as within the province itself, were the controlling factor. Much of Ohio, for example, is characterized by more acidic clay plains. Areas like Essex County largely fall into the same category, while central areas of Ontario are comprised of more alkaline, loamy soils. Core samples were taken at eight Lake Erie watershed sites in Ontario, Ohio, and Indiana, with Macrae and Plach analyzing how Pmoved through each. Overall, the alkaline, loam soils of central Ontario were much more effective at binding the nutrient. “The phosphorus ends up binding to the soil less strongly in acidic, fine textured clay. It ends up being more available in the soils, so the risk of loss can be higher in that area,” says Plach, though she adds there is still significant variability in phosphorus concentrations in surface runoff. They also found differences in P concentrations between the soil surface and subsurface are not as profound as that between different soil types. Both surface and the subsurface (via tile drains) are important pathways for P loss, but the relative role of each appears to differ depending on the topography and soil type of the region. “We can’t put it down to a retention factor. We’re just trying to say there is stronger buffering in some of these soils. But subsoils can’t take care of everything,” Macrae says. ON-FARM IMPACTS Froman academic standpoint, these revelations explain why there are such discrepancies between Ontario data and data published by the United States Department of Agriculture. This reduces uncertainty, meaning future Phosphorus loss FINDING CUSTOMIZABLE SOLUTIONS IN DIVERSE LANDSCAPES Matt McIntosh recommendations on mitigating P loss can be made with greater confidence. On the farm, farmers who know their soil type can take field-specific action. In Ohio, for example, some farmers are trying to increase soil pH levels via lime and calcium applications. “If you know how phosphorus is bound to the soil you can try to manage that,” says Macrae, though she reiterates more work needs to be done before definitive solutions that work for Ontario farmers can be established. ACCOUNTING FOR ONTARIO’S DIVERSE LANDSCAPE To that end, Macrae and Plach are also working on an ongoing project designed to find exactly how P is being lost from Ontario fields. The complementary study involves collecting field-edge samples from three Ontario agricultural landscapes, each with different soil textures, topography, and climate zone. Samples were taken over five years at sites in Ilderton and Londesborough, and four years at sites in Essex. Results thus far indicate most P is being lost via tile drains in areas with fine textured soils (such as in Essex County). In the hilly terrain of midwestern and south-central Ontario, however, the greatest loss come from surface runoff. Thus, priority management approaches will have to reflect these differences, although farmers are encouraged to manage P both in the surface and subsurface. Average field losses measured at these well- managed sites amount to half a kilogram of P per hectare. But regardless of how those losses are occurring, both researchers say farmers need to be concerned about surface and drainage. A major snowmelt, for example, can have an enormous impact for sites which otherwise have little surface loss. Research