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

LocationYearWeekly August 30 – September 5Accumulated
Highest Temp (°C)Lowest Temp (°C)Rain (mm)Rain (mm) April 1stGDD 0C April 1st GDD 5C April 1stCHU May 1st
Harrow202129102452284520743073
202028112356274119823050
2019261234448264318812846
Ridgetown20212871437269019262894
20202795388260118572867
20192675551249717382669
London20212898359267119142865
202027852429253517932768
20192465541239016562577
Brantford20212978322264518862805
202028810222256218262750
2019256274246817212638
Welland202128109339266419002869
20202793295260818652888
2019241039442254117792760
Elora20212673289245417032615
202027730329235416372571
20192357425197613032090
Mount Forest20212579295245016992609
202026815443233216282596
2019234129222514962401
Peterborough20212760279250217472700
202027521242222215372379
20192438359220514722290
Kingston20212991252254117802751
20202594330253317852815
201923834385245316942648
Kemptville20212983291263618692766
20202799352248617482697
2019246203234216052470
Earlton202123824498228315412369
202024521345200113762218
201921134397184812001977
Sudbury20212549397234416022477
202024623441208014312315
201921515417189412462040
Thunder Bay20212545287216914422262
20202156235186712492044
201922725347178411331886
Fort Frances20212446259225415202385
202021015324202213652203
201919619432192112352023
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.