We know producers are ready for the soil to dry out so they can start topdressing wheat with their first shot of nitrogen. This also makes us think about soil compaction, which is simply compressing a given volume of soil into a smaller volume. Compaction can occur in different places in the field and can be due to different reasons. The main reason for soil compaction in row crop production comes down to doing some operations when the soil is too wet. Soil compaction reduces the soil pore space, the amount of air and water a soil can hold, and the pore space continuity that supports air and water exchange/movement in the soil. Compacted soil also has higher densities that restrict root proliferation and water infiltration, and these can reduce crop yield. Further, if compacted areas are found on sloping fields, reduced infiltration can promote surface water runoff and thus soil erosion.

The main types of compaction we deal with in Kentucky are due to traffic, tillage, and planting (sidewall). The ideal soil moisture for a compaction event is at or near the soil’s field capacity (Figure 1). Field capacity in Figure 1 is around 0.20 to 0.25 g water per gram of soil (20 to 25 % moisture). Field capacity is when free water ceases to drain due to gravity. This is roughly when the soil first dries enough to traffic it without leaving ruts. When soil is wetter than field capacity, ruts will form. This is another issue to contend with and though definitely not desirable, is really not the same as compaction. We can see in Figure 1 that as the soil moisture levels increase the soil bulk density (another measure of compaction) also increases, up to a point, then it decreases. The reason the soil bulk density decreases after it peaks is because you can’t compact water. This is where ruts would be formed if trafficked in the field.

Figure 1. Soil compaction as influenced by soil moisture and previous tillage. NT (no-tillage) and CT (plow tillage) soil organic matter contents were 3.2 and 2.7%, respectively.

Tillage pans can form when tillage operations are conducted to the same depth year after year. The bottom edge of the tillage tool can cause dense pans to form. Other tillage compaction occurs when a tillage operation is executed when the soil is too wet. These “tillage pans” reduce water infiltration and accelerate erosion on sloping land. Soil erosion is a long-term detriment to field productivity, removing the topsoil and the soil nutrients contained there, reducing overall soil depth that plant roots can explore for water and nutrients. This loss in soil productivity can be exacerbated as even more soil is lost and even less water can infiltrate and refresh the soil profile. 

A couple of tillage compaction examples come to mind. First, using a disc when the soil is too wet can create a compacted zone at the lower operating edge of the disc blades, regardless of whether the operational element is a traditional curved blade or a less traditional vertical blade. This is one of the most effective ways to create soil compaction. Another example is from multiple passes of a shallow tillage tool used for seedbed preparation (e.g., vertical tillage) when the soil is too wet. All these tillage tools will dry out the soil above the lower depth of operation, but can effectively create compaction at that lower operating depth. 

Sidewall compaction occurs when planting into wet soil. The sidewalls of the planter furrow are smeared/compacted, usually by the row opener, and plant roots can have difficulty growing outside the furrow/through the furrow sidewall. Of course, if sidewall compaction occurs, then traffic compaction is probably also a concern. This is especially evident when tractor/and planter traffic patterns cause planted crop rows to be bounded on each side by a tire-compacted interrow area. This is often called ‘pinch-row’ compaction – crop growth in the affected row appears stunted or pinched by the compaction found on each side. 

Traffic compaction is due to field traffic when the soil is too wet. The degree of compaction is influenced by soil type, soil wetness, tire pressure, load pressure, and the number of traffic events over a given area. Most of the time, the entire field area is not compacted; rather, areas within a field that are wetter than the rest of the field and/or subject to greater traffic. Larger tractors, combines, grain carts, manure spreaders/injectors and other equipment weigh more than before and often have greater axial load, though less of the field area may be trafficked. Paths where grain carts travel or areas where trucks are parked/loaded can be confined, limiting overall compaction. With this in mind, we would like to discuss some approaches to identifying and dealing with soil compaction. 

A standard soil probe in the hands of a skilled agronomist can indicate a lot about soil structure and density in the amount of resistance encountered when collecting a soil sample. Note the amount of pressure it takes to stick the probe in the ground, and if greater pressure occurs at a similar depth across a field. This is a good preliminary diagnostic for identifying soil compaction, but a more detailed approach is done with a soil penetrometer.

A soil penetrometer is a more accurate tool for determining the extent and depth of compaction. A soil penetrometer measures penetration resistance (PR), the amount of force needed to insert the penetrometer into the soil. A soil penetrometer has a pointed tip attached to the end of a rod that is connected to a load cell showing the amount of force needed as the rod tip moves into and through the soil, proportional to the resistance encountered. Penetrometers usually have a dial facing the user so that PR values/thresholds can be viewed as the penetrometer is inserted into the soil (Figure 2).

Figure 3. A soil penetrometer being used to diagnose soil compaction in a wheat field.
Figure 2. A close-up of a penetrometer face, showing the amount/thresholds of resistance.

The caveat to properly using a soil penetrometer is that soil moisture content matters. This is the purpose of this newsletter – NOW is a great time to check for compaction. It is generally agreed that a soil over 300 psi (lb/in2 ) is considered compacted, but a non-compacted soil can easily read over 300 psi in summer when soils are dry. If PR is determined when the soil is dry, or dry at a certain depth, then the information can be misleading. You want the differences in PR to be due to differences in soil density, not differences in soil moisture. The best time to take soil PR measurements is when the soil is thoroughly wetted throughout the entire soil profile, like now and for the next few weeks.

Soil compaction can be “mapped” with a penetrometer, by location and depth. Most penetrometers have marks every 3 to 4 inches on the shaft. Insert the penetrometer into the soil at a constant speed (Figure 3). Watch the PSI as the shaft is pushed into the soil and note the depth where a high PR resistance is observed. Do this in multiple field areas to determine if corrective action is needed. Field edges and other high-traffic areas are usually the most prone to compaction. Other areas to check include areas that are/have been trafficked at greater soil moisture levels than the rest of the field, areas with stunted plants, or areas that have standing water for longer periods of time. Mapping the compacted area will allow a producer to focus on specific field areas to address, rather than treating the entire field. Remember that the entire field area is being evaluated; one PR reading > 300 psi does not mean that the entire field needs to be treated. Look for areas where there are multiple high PR readings and treat those areas appropriately. Consult ID-153 for additional information on assessing soil compaction.

There are several ways to deal with soil compaction, depending on the extent and depth encountered. The first method might be to do nothing. Freezing and thawing will help to remedy shallow soil compaction. We don’t get the same amount of freezing and thawing as more northern states, but still enough to help in some years. Also, plant roots can penetrate moderately compacted soils with adequate moisture and additional management might not be needed. The next approach is to do something when compaction is severe enough to warrant some additional management operation. 

The additional management is usually going to include some sort of tillage operation. This is where the time spent mapping the soil compaction can pay off. A chisel plow works well at breaking up shallow compaction. Make sure the depth of operation is below the lower depth of the compacted zone. A chisel plow is usually going to require less energy to pull than other tillage tools used to deal with compaction, like a subsoiler. Use a subsoiler for compacted layers deeper than a chisel plow can address. Again, set the operating depth below the depth of the compacted layer. In both instances, a focused approach can be used to target only compacted areas in the field. This will save time and fuel and reduce costs. The best time to break up soil compaction is when the soil is dry, and remember the reason that compaction occurred - likely due to trafficking soil when the soil was too wet. Make sure that the soil moisture conditions are on the dry side of ideal for compaction-breaking tillage. Don’t create additional compaction while trying to alleviate compaction. 

In summary, the best thing to do about compaction is to avoid causing it. Don’t traffic soil when the soil is too wet – wait for the soil to dry. This is not always possible, and sometimes management operations must be done in less-than-ideal soil moisture conditions, leading to compaction. When suspected, try to diagnose compaction by soil probe, plant and/or root growth, ponded water, or a soil penetrometer. Most of the time, an entire field is not compacted; certain areas can be targeted so as to save time and money. Remember that a good time to detect and identify compaction is also a really good time to create compaction, so if you think a field is too wet to traffic, then it is probably a good time to check for soil compaction. 

Additional resources: 

AGR-161, Soil Compaction in Kentucky 

AGR 197, Compaction, Tillage Method, and Subsoiling Effects on Crop Production

ID-153, Assessing and Preventing Soil Compaction in Kentucky 

Optional Citation: Ritchey E., Grove J., 2025. Too Wet to Soil Sample but Ideal to Check for Soil Compaction. Kentucky Field Crops News, Vol 1, Issue 2. University of Kentucky, February 14, 2025.