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.).
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.).
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).
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.).
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.
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 Data – August 8 – 14, 2022
|Location||Year||Highest Temp (°C)||Lowest Temp (°C)||Rain (mm)||Rain (mm) April 1st||GDD 0C April 1st||GDD 5C April 1st||CHU May 1st|
|||10 YR Avg. (2011-20)||26.9||15.9||23.6||413||2309||1631||2549|
|||10 YR Avg. (2011-20)||26.3||13.7||19.7||383||2172||1500||2374|
|||10 YR Avg. (2011-20)||26.3||14.2||30.3||388||2153||1484||2359|
|||10 YR Avg. (2011-20)||27.1||14.4||17.7||352||2164||1495||2376|
|||10 YR Avg. (2011-20)||25.2||11.9||28.1||387||1943||1287||2111|
|||10 YR Avg. (2011-20)||24.8||12.5||26.9||395||1925||1277||2115|
|||10 YR Avg. (2011-20)||27.1||12.0||17.6||348||1974||1318||2118|
|||10 YR Avg. (2011-20)||27.6||13.5||27.5||370||2074||1419||2259|
|||10 YR Avg. (2011-20)||24.3||10.1||29.3||334||1676||1104||1874|
|||10 YR Avg. (2011-20)||25.1||12.4||27.6||360||1798||1203||2014|
|||10 YR Avg. (2011-20)||26.1||9.5||16.2||337||1591||1007||1730|
|||10 YR Avg. (2011-20)||25.5||8.5||15.3||341||1714||1112||1879|