Ontario Field Crop Report – August 17, 2022

Evaluating Soil Structure

Many Ontario soils display a similar pattern of compaction. The secondary tillage layer – usually the top 2-4 inches – is relatively loose and friable, with mostly small, rounded aggregates, especially earlier in the season before rains reconsolidate the soil. Below this layer is usually one that is significantly denser and more poorly structured (see Figure 1.).

Figure 1. A common pattern – loose seedbeds with compacted layers beneath.
Figure 1. A common pattern – loose seedbeds with compacted layers beneath.

In the second layer, generally between 4 and 10 inches from the surface but sometimes extending deeper, aggregates are larger, blocky with sharp edges, and packed more densely together. While the rounded aggregates in the first layer can be easily crushed between finger and thumb, those in the second layer can be quite hard and don’t easily break into smaller component aggregates. This layer is compacted, usually by wheel traffic, though a tillage pan may exist above or within it as well.

By the numbers

Bulk density is the standard indicator of soil structure. It measures the weight of dry soil per cubic centimetre. If the soil has good structure with plenty of pore space, bulk density will be low. If the soil is dense and compacted, bulk density is higher. As density increases plant roots struggle and can even fail to grow through the soil. This is very clearly illustrated by the image below from Strachan and Jeschke of Pioneer (Figure 2.).

Figure 2. Root growth of corn plants (V5 growth stage) growing in soil compacted to different bulk densities before corn seeds were planted (Strachan and Jeschke 2017)

Figure 2. Root growth of corn plants (V5 growth stage) growing in soil compacted to different bulk densities before corn seeds were planted (Strachan and Jeschke 2017)

Looking at over 600 soil samples taken by OMAFRA soil specialists in 2020 from topsails across Ontario, preliminary analysis shows that average soil bulk density is close to or over the critical level for fine and medium textured soils (Figure 3).

Figure 3. Preliminary bulk density data from the Topsoil Sampling Project. Red lines and text indicate threshold bulk density values for root growth per texture group; black lines and text show the average measured values.
Figure 3. Preliminary bulk density data from the Topsoil Sampling Project. Red lines and text indicate threshold bulk density values for root growth per texture group; black lines and text show the average measured values.

Roots or Iron?

Since the tillage layer is often less dense, it begs the question: why not do deeper tillage? Deep tillage can weaken the soil and reduce its capacity to carry equipment, meaning that future passes will just recompact it even deeper. Even most subsoiling operations show no benefit after the first year for this reason. Add to that the fact that most growers have shifted to reducing tillage to save fuel, labour, and equipment, and this is not a decision to take lightly. Research has also shown that “biodrilling” with cover crop roots can sometimes re-establish enough porosity through compacted layers to maintain function. (See Figure 4.).

Figure 4. Grey images of vertical cross-sections for representative samples from each plant treatment, before and after 1 year under potential bio-subsoilers. Below each grey images are the respective 3-D renderings of the pores analysed in this study (Pulido-Moncada et al 2019).
Figure 4. Grey images of vertical cross-sections for representative samples from each plant treatment, before and after 1 year under potential bio-subsoilers. Below each grey images are the respective 3-D renderings of the pores analysed in this study (Pulido-Moncada et al 2019).

There are certainly situations where the compaction is simply too severe for even the best biodrilling roots to punch through, and where properly managed mechanical loosening can help to reset the degraded structure.

How can you measure soil structure quality?

Assessing soil structure can give a clearer picture of the path forward. The Visual Evaluation of Soil Structure (VESS) system is a popular option that provides a simple framework for quantitatively scoring soil structure quality (see Figure 5.). It requires only a shovel for extracting a block of soil and a scoring sheet for evaluating it and can be performed in about 10 minutes with a little practice.

Figure 5. Visual Evaluation of Soil Structure (VESS) system provides a simple framework for scoring soil structure quality.
Figure 5. Visual Evaluation of Soil Structure (VESS) system provides a simple framework for scoring soil structure quality.

The scoring sheet, which includes instructions for how to extract, break up, and score the soil block, as well as a video explaining the process can be found at https://fieldcropnews.com/2022/08/visual-evaluation-of-soil-structure/ on fieldcropnews.com.

VESS can be used to compare different parts of a field, to track soil structure over time, or to inform soil structure management decisions. A score less than 3 is generally acceptable and indicates soil structure is not likely to be severely limiting to crop growth. Scores greater than 3 require a change in management. Reducing disturbance through tillage, crop rotations to include more and different root systems, and organic amendments can improve soil structure over time. Soils or layers with a score greater than 4 will often be slow to improve, with severe impacts on crop yields in any season without perfect, consistent rainfall. These are the conditions in which targeted tillage can pay, but only if followed by management changes as mentioned above.

One major limitation with VESS is that the standard method only evaluates the top 6-8 inches (15-20cm). If your compacted layer extends below this, it’s worth digging deeper to extract a second block from underneath the first. Options for improving this layer are more limited – either biodrilling or subsoiling. See this article for guidance in this scenario.

Weather DataAugust 8 – 14, 2022

LocationYearHighest Temp (°C)Lowest Temp (°C)Rain (mm)Rain (mm) April 1stGDD 0C April 1stGDD 5C April 1stCHU May 1st
Harrow202229.911.320.5356233016832568
202131.013.352.6428233216702469
10 YR Avg. (2011-20)26.915.923.6413230916312549
Ridgetown202229.27.77.6238219615562367
202130.310.334.5397219815452324
10 YR Avg. (2011-20)26.313.719.7383217215002374
London202227.28.730.0293213615012298
202129.711.024.4328218415352293
10 YR Avg. (2011-20)26.314.230.3388215314842359
Brantford202228.38.918.0267214315022251
202131.49.39.3292215115012248
Welland202228.59.81.8276219615452377
202131.313.72.7295215715032278
10 YR Avg. (2011-20)27.114.417.7352216414952376
Elora202226.98.02.7223196013322061
202129.710.25.3265199013502082
10 YR Avg. (2011-20)25.211.928.1387194312872111
Mount Forest202225.86.86.7307195513332091
202129.99.08.0327199713602096
10 YR Avg. (2011-20)24.812.526.9395192512772115
Peterborough202227.85.71.2267197013272085
202131.09.37.3300199213402065
10 YR Avg. (2011-20)27.112.017.6348197413182118
Kemptville202226.38.43.6407210514432258
202130.612.323.7264214514882208
10 YR Avg. (2011-20)27.613.527.5370207414192259
Earlton202226.06.34.8295177111771931
202129.18.532.0451185312221897
10 YR Avg. (2011-20)24.310.129.3334167611041874
Sudbury202225.18.79.8289178811851971
202128.58.661.0375188312531961
10 YR Avg. (2011-20)25.112.427.6360179812032014
Thunder Bay202231.75.63.342115619951663
202127.55.17.9228174511271808
10 YR Avg. (2011-20)26.19.516.2337159110071730
Fort Frances202228.55.70.0533163710731811
202132.91.416.2199184312191934
10 YR Avg. (2011-20)25.58.515.3341171411121879
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.