Crop Report – September 8, 2021

Figure 1. Differing phosphorus management regions in the Lake Erie watershed as proposed by Dr. Macrae and colleagues.

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.

Figure 1. Differing phosphorus management regions in the Lake Erie watershed as proposed by Dr. Macrae and colleagues.
Figure 1. Differing phosphorus management regions in the Lake Erie watershed as proposed by Dr. Macrae and colleagues.

No-till

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.

Figure 2. Subsurface application of fertilizer, along with cover crop seed, on a clay soil in the SW region of Ontario.
Figure 2. Subsurface application of fertilizer, along with cover crop seed, on a clay soil in the SW region of Ontario.

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

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.

Figure 3. Red clover on a highly erodible, sloping soil characteristic of the NE region of the Lake Erie watershed.
Figure 3. Red clover on a highly erodible, sloping soil characteristic of the NE region of the Lake Erie watershed.

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.

Resources

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.

References

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.

Weather Data

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
Harrow 2021 29 10 2 452 2845 2074 3073
2020 28 11 2 356 2741 1982 3050
2019 26 12 34 448 2643 1881 2846
Ridgetown 2021 28 7 1 437 2690 1926 2894
2020 27 9 5 388 2601 1857 2867
2019 26 7 5 551 2497 1738 2669
London 2021 28 9 8 359 2671 1914 2865
2020 27 8 52 429 2535 1793 2768
2019 24 6 5 541 2390 1656 2577
Brantford 2021 29 7 8 322 2645 1886 2805
2020 28 8 10 222 2562 1826 2750
2019 25 6 274 2468 1721 2638
Welland 2021 28 10 9 339 2664 1900 2869
2020 27 9 3 295 2608 1865 2888
2019 24 10 39 442 2541 1779 2760
Elora 2021 26 7 3 289 2454 1703 2615
2020 27 7 30 329 2354 1637 2571
2019 23 5 7 425 1976 1303 2090
Mount Forest 2021 25 7 9 295 2450 1699 2609
2020 26 8 15 443 2332 1628 2596
2019 23 4 129 2225 1496 2401
Peterborough 2021 27 6 0 279 2502 1747 2700
2020 27 5 21 242 2222 1537 2379
2019 24 3 8 359 2205 1472 2290
Kingston 2021 29 9 1 252 2541 1780 2751
2020 25 9 4 330 2533 1785 2815
2019 23 8 34 385 2453 1694 2648
Kemptville 2021 29 8 3 291 2636 1869 2766
2020 27 9 9 352 2486 1748 2697
2019 24 6 203 2342 1605 2470
Earlton 2021 23 8 24 498 2283 1541 2369
2020 24 5 21 345 2001 1376 2218
2019 21 1 34 397 1848 1200 1977
Sudbury 2021 25 4 9 397 2344 1602 2477
2020 24 6 23 441 2080 1431 2315
2019 21 5 15 417 1894 1246 2040
Thunder Bay 2021 25 4 5 287 2169 1442 2262
2020 21 5 6 235 1867 1249 2044
2019 22 7 25 347 1784 1133 1886
Fort Frances 2021 24 4 6 259 2254 1520 2385
2020 21 0 15 324 2022 1365 2203
2019 19 6 19 432 1921 1235 2023
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.