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 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
Harrow	2022	29.9	11.3	20.5	356	2330	1683	2568
	2021	31.0	13.3	52.6	428	2332	1670	2469
	10 YR Avg. (2011-20)	26.9	15.9	23.6	413	2309	1631	2549
Ridgetown	2022	29.2	7.7	7.6	238	2196	1556	2367
	2021	30.3	10.3	34.5	397	2198	1545	2324
	10 YR Avg. (2011-20)	26.3	13.7	19.7	383	2172	1500	2374
London	2022	27.2	8.7	30.0	293	2136	1501	2298
	2021	29.7	11.0	24.4	328	2184	1535	2293
	10 YR Avg. (2011-20)	26.3	14.2	30.3	388	2153	1484	2359
Brantford	2022	28.3	8.9	18.0	267	2143	1502	2251
	2021	31.4	9.3	9.3	292	2151	1501	2248
Welland	2022	28.5	9.8	1.8	276	2196	1545	2377
	2021	31.3	13.7	2.7	295	2157	1503	2278
	10 YR Avg. (2011-20)	27.1	14.4	17.7	352	2164	1495	2376
Elora	2022	26.9	8.0	2.7	223	1960	1332	2061
	2021	29.7	10.2	5.3	265	1990	1350	2082
	10 YR Avg. (2011-20)	25.2	11.9	28.1	387	1943	1287	2111
Mount Forest	2022	25.8	6.8	6.7	307	1955	1333	2091
	2021	29.9	9.0	8.0	327	1997	1360	2096
	10 YR Avg. (2011-20)	24.8	12.5	26.9	395	1925	1277	2115
Peterborough	2022	27.8	5.7	1.2	267	1970	1327	2085
	2021	31.0	9.3	7.3	300	1992	1340	2065
	10 YR Avg. (2011-20)	27.1	12.0	17.6	348	1974	1318	2118
Kemptville	2022	26.3	8.4	3.6	407	2105	1443	2258
	2021	30.6	12.3	23.7	264	2145	1488	2208
	10 YR Avg. (2011-20)	27.6	13.5	27.5	370	2074	1419	2259
Earlton	2022	26.0	6.3	4.8	295	1771	1177	1931
	2021	29.1	8.5	32.0	451	1853	1222	1897
	10 YR Avg. (2011-20)	24.3	10.1	29.3	334	1676	1104	1874
Sudbury	2022	25.1	8.7	9.8	289	1788	1185	1971
	2021	28.5	8.6	61.0	375	1883	1253	1961
	10 YR Avg. (2011-20)	25.1	12.4	27.6	360	1798	1203	2014
Thunder Bay	2022	31.7	5.6	3.3	421	1561	995	1663
	2021	27.5	5.1	7.9	228	1745	1127	1808
	10 YR Avg. (2011-20)	26.1	9.5	16.2	337	1591	1007	1730
Fort Frances	2022	28.5	5.7	0.0	533	1637	1073	1811
	2021	32.9	1.4	16.2	199	1843	1219	1934
	10 YR Avg. (2011-20)	25.5	8.5	15.3	341	1714	1112	1879

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.