BU-07400     2000

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SOIL SCIENTIST

What is Compaction?

Compaction is a change in soil structure, not just an increase in soil density. Healthy soils have a diversity of pore sizes, while compacted soils have mostly small pores.

During the compaction process soil aggregates are pushed closer together. This reduces the size of pores, the continuity of pores, and the size and stability of aggregates. Only under severe compaction will aggregates break down.

Because the change in soil structure is complex, there is not a simple relationship between increased soil density and decreased crop yield. The changes in soil structure affect the movement of water, air, roots, and soil organisms through the soil, so the effect on yield depends on the weather, the amount and depth of compaction, and the crop type.

What causes compaction?

Wheel traffic is the main cause of compaction on most farms, though rain, drying, and animal traffic also contribute on certain soils. The amount of compaction depends on the size and weight of the equipment, the moisture level of the soil, and the type of soil (soils high in clay or low in organic matter compact more readily).

How does compaction occur?

A typical chain of events begins in the fall when heavy harvest equipment pushes soil aggregates closer together and reduces soil drainage. The next spring, the soil will not drain as quickly, and the short window of opportunity for spring field work becomes even shorter. Farmers are forced to work wet soils and more compaction occurs.

Spring tillage will loosen surface soil, but seeds in the tire tracks may have trouble emerging from the packed soil, and roots will not spread as easily. To compensate for the poor root growth, a farmer may increase the amount of fertilizer or use split applications. The extra fertilizer may mean more traffic, and may discourage extensive root growth deep into subsoil where they can help alleviate compaction.

The process of loosening soil might begin when soil swells from a good rainfall, and then shrinks during a dry spell. The swelling and shrinking causes cracks that can be filled by roots or soil organisms that further improve soil structure. Freezing and thawing also creates cracks, especially near the surface where soil may freeze and thaw several times in a season. But all of this is a slower process than the compaction caused by a 20-ton combine or grain cart.

Types of compaction

Not all cases of soil compaction are the same. Each type has different causes and treatments.

Surface crusting reduces seed emergence and water infiltration. It is caused by the impact of rain drops on weak soil aggregates. Soils with high organic matter, high biological activity, or high sand content are less likely to form crusts.

Surface compaction occurs from the surface down to the tillage depth. It can be loosened by normal tillage, root growth, and biological activity. The degree of surface compaction is determined by the moisture in the soil at the time of compaction and the ground contact pressure (measured in pounds per square inch or psi) of equipment or animals.

Subsoil or deep compaction lies beneath the level of tillage. Ground contact pressure and the total weight on the tire (the axle load) significantly affect the amount of subsoil compaction. Deep compaction is difficult to eliminate and may permanently change the soil structure. Prevention is important.

Plow pan or tillage pan is subsoil compaction that is only a few inches thick and lies directly beneath the normal tillage depth. It develops when the depth of tillage is the same from year to year, or when the rear wheel of the tractor rides in the moldboard plow furrow.

How Does Compaction Affect Productivity?

Compaction changes several structural characteristics and functions of soil. This makes it difficult to predict the effect of compaction on crop yield or on the need for inputs such as fertilizer and irrigation. The diagram below summarizes how compaction affects plant growth and input use.

Is compaction always bad? Subsoil compaction is probably never beneficial, but some people see benefits from moderate surface compaction (i.e. compaction created by less than five-ton axle loads on dry soil). For example, if recently-tilled soil is excessively loose, some compaction directly around the seed promotes germination. In drier climates (less than 14 inches of rain in the growing season), moderate surface compaction may improve yields by promoting root branching and phosphorus uptake, and by increasing the water holding capacity of the soil.

How much does compaction reduce yield?

The effect of compaction depends on

• the amount and depth of compaction,
• the type of soil (high sand and organic matter levels reduce the effects of compaction),
• weather (under good conditions, yields may not suffer at all), and
• the crop. (For example, corn is sensitive to deep compaction but is less affected by surface compaction. The opposite is true for soybeans.)

These are the roots of corn plants grown at the University of Minnesota Lamberton Research and Outreach Center (ROC). The plant on the left had no wheel traffic compaction between the rows. The plant on the right had wheel tracks on both sides. Wheel tracks were made by three wheel passes (planting, spraying, and cultivation) with axle loads of less than five tons. Note the lack of root growth under the wheel tracks. Plants like this cannot access much of the P and K broadcast over a field. Photos courtesy of Ward Voorhees

Researchers have reported yield losses due to compaction of greater than 40%, but they have also seen yield increases from mild surface compaction during dry years. At the Waseca Research and Outreach Center in southern Minnesota, subsoil compaction caused yield losses of more than 25%. Researchers created the compaction in 1981 by driving over the Webster clay loam (38% clay) with 10- and 20-ton axle loads. Corn yields in the first year after compaction were:

The cost of compaction

In addition to reduced yields, crops grown on compacted soils may have higher input costs and are more sensitive to inadequate nutrient and moisture levels. Researchers are just beginning to develop tools that will predict the financial cost.

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