Managing phosphorus for improved water quality: best practices depend on where you farm
Thanks to drier-than-normal spring seasons the past two years, harmful algae blooms in 2020 and 2021 in the western basin of Lake Erie have not been as severe as in preceding years. Phosphorus (P) loading to the lake, however, remains an important issue on both sides of the Canada-US border. Ontario farmers want to do their part to reduce phosphorus losses to water bodies. And as we learn more, it appears that the most effective best management practices (BMPs) may depend on where you farm and the type of land you manage.
Dr. Merrin Macrae, Professor at University of Waterloo, has led multiple on-farm, edge-of-field trials on phosphorus and water quality within the Lake Erie watershed over the past decade. Recently, she pored over the findings and, along with other experts from Ontario and the US, developed regionally specific guidance on phosphorus management. In this crop report we will share some of the highlights.
Phosphorus management regions
Macrae breaks up the Lake Erie watershed into four regions based on climate, topography, elevation and soil characteristics (Figure 1). In the Lake Erie watershed, they include the southwest (SW), northeast (NE) and the transition zone between the two. The SW represents a warmer region with flat, clay-based soil, less snow, and more precipitation as rain. The NE has fewer winter thaws and more snowpack, higher elevation and more topography.
While certain conservation practices can be beneficial in one region, they may have unintended consequences in another. Macrae found that on the flat clay soils of the SW Lake Erie watershed, no-till fields with tile drainage can increase losses of dissolved phosphorus. This is because no-till enhances the surface soil’s connectivity with tile – through macropores – and is associated with build-up of phosphorus at the soil surface over time.
In contrast, on medium-textured and undulating soils characteristic of the northeastern Lake Erie watershed, no-till can be beneficial. On these soil types, most phosphorus is typically lost from surface runoff and soil erosion in a small number of freeze-thaw events in the early spring. Practices that improve infiltration and reduce runoff are critical. Also, subsoil tends to do a better job filtering water and phosphorus before it reaches tile drains on these soil types.
4R nutrient management
Applying 4R principles is critical to reducing phosphorus losses. Research shows that fields with legacy P build-up and high soil tests contribute disproportionately to phosphorus loss. For growers with very high soil tests, drawing levels down and diverting farm nutrients to lower testing fields can make a big difference.
Applying phosphorus in the right place is also very important, particularly for those in the SW Lake Erie watershed. Non-incorporated surface broadcast phosphorus can easily find its way to tile drains. For farmers on flat, clay plain soils, subsurface placement of phosphorus is the best option (Figure 2). Incorporation of P with conservation tillage is another tactic that reduces risk of P loss.
Finally, timing of nutrient application matters. Phosphorus applied in the late fall is much more vulnerable to loss than that applied either at planting or following wheat harvest in summer, regardless of where you farm. Winter applications when soils are frozen and snow covered carry the highest risk.
Cover crops, when left to cover soil over winter, can significantly reduce soil erosion. This makes them a useful tool for reducing P loss in the northeastern Lake Erie watershed and on other similar landscapes (Figure 3). While cover crops release dissolved P after a freeze-thaw event, snowpack in regions such as Wellington and Dufferin counties can help insulate plants and mitigate this. Also, most dissolved P is retained by soil. In the SW Lake Erie watershed, however, less snow cover means greater P leaching from cover crop residues. Selection of frost-tolerant species can reduce this risk.
The bottom line
So, what does this mean? If you live in a cooler region, on soils with some topography and generally one major spring melt event, the traditional conservation practices – reduced tillage, crop rotation, structural erosion control – tend to be most effective to lower P losses. Focus on reducing surface runoff and erosion, as well as sound nutrient management.
On the other hand, if you farm in the deep southwest of the province (or a region with similar climate and soils), concentrate on severing the connection between phosphorus sources and tile drains. This may mean a shift to more sub-surface nutrient application or addressing P stratification.
When best management practices are combined, they have a synergistic effect. By knowing which BMPs are most likely to be effective in your region, you can make the greatest impact.
If you’re interested in evaluating options for lowering the risk of phosphorus loss on your farm, check out OMAFRA’s Phosphorus Loss Calculator, PLATO, on AgriSuite.
For quick feedback on your 4R fertilizer practices, you can use the 4R Calculator on OSCIA’s Soil Test Manager webpage, which also provides Ontario fertilizer recommendations.
To learn more about Dr. Macrae’s research and advice on managing phosphorus for improved water quality, listen to her interview on Real Agriculture’s Soil School here.
Macrae, M., H. Jarvie, R. Brouwer, G. Gunn, K. Reid, P. Joosse, K. King, P. Kleinman, D. Smith, M. Williams, M. Zwonitzer. 2021. One size does not fit all: Toward regional conservation practice guidance to reduce phosphorus loss risk in the Lake Erie watershed. Journal of Environmental Quality. 50:529-546. DOI: 10.1002/jeq2.20218.
|Location||Year||Weekly August 30 – September 5||Accumulated|
|Highest Temp (°C)||Lowest Temp (°C)||Rain (mm)||Rain (mm) April 1st||GDD 0C April 1st||GDD 5C April 1st||CHU May 1st|
|Report compiled by OMAFRA using Environment Canada data. Data quality is verified but accuracy is not guaranteed. Report supplied for general information purposes only. An expanded report is available at www.fieldcropnews.com.|