By Heidi Johnson

2018 will go in the books as one of the more difficult harvests for many parts of Wisconsin. Late August and early September brought untimely and unprecedented rain, topping 10-plus inches in 24-hour periods in many areas. Soggy fields proved difficult to get into, and out of necessity, many farmers harvested fields that in most years would have been considered too wet to take equipment into.

Rutted fields
Now we are left with rutted-up fields and compacted soil. But while ruts are more easily identified and fixed, compaction is a bit more complicated to diagnose. Compaction is the result of force (heavy equipment) pushing soil aggregates and particles together, increasing the soil’s density and reducing the soil pore space needed for water and air. This reduces plant root growth and function, decreases soil drainage, reduces soil biological activity and creates other conditions that increase production costs while reducing yields.

The degree to which fields can become compacted due to equipment traffic can be correlated with the soil moisture at the time of field operations. Compaction more likely occurs in fields where the soil water content is at or near field capacity, which is reached when a field has been saturated but allowed to drain freely for 24 to 48 hours. Drier soils below field capacity compact less because there is less water to allow soil particles to slide between each other when trafficked. Similarly, fully saturated soils compact less in the subsoil because the water resists compression, but under these conditions, there is a greater risk for rutting and surface compaction.

The only way to know for sure if you have caused compaction is to look for it. This can be done by digging a small soil pit and looking at the sidewall for a compaction zone or by using an instrument called a penetrometer. Penetrometers are available through many county Extension or land conservation departments and can take the guesswork out of determining if compaction is truly the issue. You will also likely see the signs and symptoms of compaction, such as water ponding, poor and uneven plant growth, and surface clods that won’t break up; but using a penetrometer or a soil pit will tell you the depth of compaction, so you can better choose a remediation plan. 

Minimize compaction
Minimizing compaction can be done in different ways. If possible, allowing fields to dry out is best, but obviously this was difficult in 2018. Optimizing tire pressure and spreading out the load with tracks or dual wheels can help reduce the chance of compaction. Minimizing field traffic by keeping heavy equipment such as dump or grain carts at the field edge are is another option to help localize compaction to specific zones.

Once it is determined that compaction has occurred, strategies can be developed to remediate it. If the compaction is shallow, cover crops can be helpful in breaking it up. Cool-season small grains such as cereal rye will develop the type of fibrous root systems that are best at alleviating compaction. Cereal rye is a winter-hardy cover crop, so it will grow late into the fall and early in the spring to optimize root production.  Natural freeze-and-thaw cycles in winter can also help break up compaction, but they are only effective on the top couple of inches of the soil profile after multiple freeze-thaw cycles. Subsurface compaction may require the use of a subsoiler to break up the deeper hardpan. Subsoilers should be run 2 to 3 inches deeper than the compacted zone.

Long-term management choices can also greatly impact how easily a soil can become compacted.  Reducing or eliminating tillage in combination with cover crops will allow soils to regain their aggregate structure due to increases in organic matter and biological activity. This also increases water infiltration, which helps soils dry out more quickly and increases the number of days for field operations. Soils with higher organic matter are more resistant to compaction due to better aggregation. 

For more information on compaction, please see University of Wisconsin-Extension publications A3367 (Soil Compaction: Causes, Concerns and Cures) and A4158 (Managing Soil Compaction at Planting and Harvest). UW-Extension also has a YouTube video on using cone penetrometers.

Johnson is the Extension crops and soils agent in Dane County, Wis.