Soil Compaction: An inevitable part of modern agriculture or a symptom of poor soil health?

Author: Natalie Sturm, Soil Science Ph.D student at Washington State University

I have worked on farms with machinery ranging from small Kubota tractors to full-sized Case IH equipment – and I can guarantee that I have caused soil compaction with both types of tractors (especially that one time I got stuck on a muddy slope in the Kubota…). Most people who work in agriculture have probably also experienced this widespread occurrence of soil compaction. We can all point to the areas where the crops just don’t grow as well, where the tractor has to pull harder, or where water ponds and runs off. But how bad is the compaction problem? Is it something that will always inevitably exist in modern agriculture? Or can managing for soil health also help to manage soil compaction?

Impacts of soil compaction:

The impacts of soil compaction on crop growth and productivity can vary depending on the severity of the compaction. But in general,

Impacts of compaction on plants can include:

Reduced crop emergence
Reduced crop stand
Reduced or altered root growth
Potential for increased root diseases
Potential for reduced crop yields

Soil surface compaction can slow down crop emergence and reduce crop stands. Subsurface compaction impedes root growth, which can limit plant access to water and nutrients. This is especially important during extreme weather scenarios (heat, drought, flooding) when subsoil water and nutrient resources can be vital for crop survival. However, when soils are compacted, their ability to buffer against these extremes and capability to support growing crops is reduced.

Impacts of compaction on soils can include:

Loss of soil structure
Increased soil bulk density
Increased water runoff
Decreased water infiltration

An illustrative example of above- and below-ground crop growth in uncompacted and compacted soils. From: Sustainable Agriculture Research and Education: “Building Soils for Better Crops; Chapter 6. Soil Degradation: Erosion, Compaction, and Contamination.

The impacts on crop yield and farm profitability due to compaction are highly variable. In drought years, the impacts of compaction on crop yields tend to be much more obvious than during normal precipitation years. Some soils may be more resilient to compaction due to their texture, water content, and organic matter content, for example.

Sometimes, soil compaction may reduce crop yields, but not enough to significantly reduce farm profitability. There is no clear answer to the question, “how much will compaction reduce crop yield?” But regardless of this uncertainty, many growers are considering ways to manage soil compaction on their farms. So, what are some ways to manage soil compaction?

The best way to manage soil compaction is to avoid it in the first place!

Some amount of soil compaction is inevitable in production agriculture. Since the 1960s, the weight of farming equipment has increased dramatically. Common axle loads on tractors and other farm equipment can range from 10-20 tons per axle for tractors and up to 70 tons per axle for full grain carts.

Tractor rear wheel load weight increasing over time. The dashed line indicates the 25% increase in wheel load that occurs during plowing. From: Keller, et al. (2019) “Historical increase in agricultural machinery weights enhanced soil stress levels and adversely affected soil functioning”

While this increase in equipment size/weight has made farming operations much more efficient, it has also led to increased soil compaction. But there are ways to avoid severe compaction even while using modern equipment.

Farm machinery generally compacts soil due to:

Driving on wet soils
Heavy axle loads
Improperly inflated tires

Managing these machinery factors can help to avoid soil compaction and avoid making any current soil compaction worse.

Moist soils are most likely to become compacted when driven on compared to dry soils. Soil particles can move – and macropores can compress – when a soil is moist. Soil is likely too wet to drive on if, when balled up and dropped from about waist height, the soil does not break apart when it hits the ground. Of course, planting, harvesting, and other operations often cannot wait for the soil moisture levels to be “just right.” Therefore, other steps can be taken to minimize the risk for soil compaction.

Heavy axle loads lead to compaction deep into the soil – not just at the surface. Axle loads greater than 5-10 tons can cause compaction as deep as three feet in the soil. Utilizing dual tires and/or additional axles can reduce the weight on each axle, which can help to reduce the potential for severe subsoil compaction. Utilizing tracks instead of tires can help to reduce subsoil compaction only if axle load is also reduced. Even with tracks, equipment that has an axle load above approximately 10 tons will cause subsoil compaction down to two or three feet. Both dual tires and tracks will still cause some surface soil compaction.

General approximations of axle loads for agricultural equipment. Source: Jodi DeJong-Hughes, University of Minnesota Extension (2022)

If tires are used, the inflation of those tires plays a role in surface compaction. High inflation pressure causes greater ground pressure and increased surface soil compaction. Dual tires with lower inflation pressure can help reduce the pressure exerted onto the soil surface.

Mechanical cause of compaction in surface soil and subsoil. From: Pennsylvania State University Extension: “Avoiding Soil Compaction”

It’s not just tractor management – what about the soil?

Additionally, the properties of a soil also influence the likelihood that it will become compacted. Some soil properties cannot be changed or managed by the farmer. For example, soil texture plays a huge role in soil compaction. Sandy soils are much less likely to become compacted to levels that impact crop growth compared to silty or clay-y soils (silty soils are quite susceptible to compaction).

Some soil properties, however, can be managed and changed over time.

Improving soil health to defend against compaction:

Soil structure is a crucial factor when it comes to resisting compaction. Soils that are “well-aggregated” and have connected networks of macropores are more resistant to compaction and its negative effects. If there are macropores in a soil, roots, air, and water will move through those pores – even if the soil is compacted. Since organic matter plays an important role in promoting soil aggregation, soils higher in organic matter may also be more resistant to severe compaction.

Top image: soil with poor aggregation and low organic matter. Bottom image: soil with good aggregation and high organic matter. Source: N. Sturm.

In other words, soil compaction can be considered a symptom of poor soil health, since soils with poor structure and low levels of organic matter may be more likely to become compacted.

Improving soil organic matter levels – either through retaining crop residue on the soil, adding compost or manure, and/or adopting no-till practices – may help to alleviate soil compaction. These practices may also make soils more resistant to further compaction.

The tillage question:

Tillage is often discussed as a potential management tool to reduce soil compaction. While tilling or deep ripping a field may alleviate compaction in the short term, tillage does not solve the underlying causes of compaction. In fact, tillage is known to reduce soil organic matter levels, degrade soil structure, and disrupt pore networks – all three of which are factors necessary help to reduce the severity and negative consequences of soil compaction. In addition, tillage operations often must be repeated every year or every few years to keep alleviating the negative impacts of compaction. This is costly (via time, labor, and fuel) and can continue the soil degradation, erosion, and compaction cycle.

A compacted layer of soil (also known as a plow pan or hardpan layer). From: New Mexico State University: “Understanding and Managing Soil Compaction in Agricultural Fields.”

So you want to manage soil compaction…

Soil compaction may take many years to manage and reduce. And soil compaction will almost always be present to some degree in mechanized production agriculture. However, farm management can be focused on defending against some severe impacts of soil compaction.

How to defend against soil compaction:

Carefully consider equipment axle load and tire pressure – when possible, minimize axle load on wet soils and use low-pressure tires
As much as possible, avoid driving on soil when it is too wet – soils are less likely to become compacted when they are dry or frozen
Maintain macropore networks by reducing or eliminating tillage
Improve soil structure (aggregates) by reducing/eliminating tillage and increasing soil organic matter
Improve soil organic matter by retaining crop residues, adding manure/compost, incorporating cover crops where possible, and reducing/eliminating tillage

Ultimately, there will likely always be some amount of soil compaction present in mechanized agricultural systems. But with careful consideration of equipment use and timing and a focus on improving soil structure and organic matter, severe soil compaction, and its negative impacts may be minimized.