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Soil compaction

Quick facts

Soil compaction concerns have been growing in Minnesota as both annual precipitation and farm equipment size have dramatically increased.

Wet soils are particularly susceptible to compaction. Heavy equipment and tillage implements amplify damage to the soil's structure, decreasing pore space and limiting soil and water volume even further.

Improving soil structure is the best defense against soil compaction. A well-structured soil holds and conducts the water, nutrients and air necessary for healthy plant root activity.

What is compaction?

Soil compaction occurs when soil particles are pressed together, reducing pore space between them (Figure 1). Heavily compacted soils contain few large pores, less total pore volume and, consequently, a greater density.

A compacted soil has a reduced rate of both water infiltration and drainage. This happens because large pores more effectively move water downward through the soil than smaller pores.

In addition, the exchange of gases slows down in compacted soils, causing an increase in the likelihood of aeration-related problems. Finally, while soil compaction increases soil strength – the ability of soil to resist being moved by an applied force – a compacted soil also means roots must exert greater force to penetrate the compacted layer.

Figure 1: Effects of compaction on pore space.

Soil compaction changes pore space size, distribution and soil strength. One way to quantify the change is by measuring the bulk density. As the pore space decreases within a soil, the bulk density increases. Soils with a higher percentage of clay and silt, which naturally have more pore space, have a lower bulk density than sandier soils.

Myths about soil compaction

There are two common, widespread myths about soil compaction:

  1. Freeze-thaw cycles will alleviate a majority of the soil compaction created by machinery.
  2. Whatever compaction Mother Nature doesn’t take care of, deep tillage or subsoiling will.
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Minimal crop rotation

The trend towards a limited crop rotation has had two effects:

  1. Limiting different rooting systems and their beneficial effects on breaking subsoil compaction.
  2. Increased potential for compaction early in the cropping season, due to more tillage activity and field traffic.
diagram of grain yield versus compaction.
Figure 6: Effects of weather on crop yield response to compaction level.

How soil compaction affects plant growth

Soil compaction can have both desirable and undesirable effects on plant growth. Research from North America and Europe indicates that crops respond to soil compaction as shown in Figure 6.

Dry weather

In a dry year, at very low bulk densities, yields gradually increase with a slight increase in soil compaction.

A slightly compacted soil can speed up the rate of seed germination because it promotes good seed-to-soil contact. This is why corn planters have been specifically designed to provide moderate compaction with planter-mounted packer wheels that follow seed placement.

As soil compaction increases beyond optimum, yields begin to decline. In dry years, soil compaction can lead to stunted, drought-stressed plants due to decreased root growth. Without timely rains and well-placed fertilizers, yields will reduce.

Wet weather

In wet weather, yields decrease with any increase in compaction. Soil compaction in wet years decreases soil aeration, increasing denitrification. The risk of root diseases can also increase. All of these factors add stress to the crop and, ultimately, lead to yield loss.

Consequences of compaction

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Strategies for reducing soil compaction

The best way to manage soil compaction is to prevent it from happening. The old adage of “stay off the field until it is fit to work” still applies.

However, the possible severe economic repercussions of delaying planting, harvesting or other operations may outweigh compaction damage or loss. The dilemma farmers face in a wet spring or fall isn’t easy to resolve.

Since farmers need to be in the field in less-than-ideal soil moisture conditions, minimizing or controlling compaction is the next best management option. This includes reducing axle load, proper inflation and size of tires, and band-applying nutrients to maximize availability. Inflating tires to the proper air pressure will reduce surface compaction, while reducing axle loads will reduce depth of compaction.

Your soil is your most important resource when growing a healthy and profitable crop. Preventing soil compaction will

  • Increase water infiltration and storage capacity
  • Increase timeliness of field operations
  • Decrease the stress on plant roots
  • Decrease diseases potential

How to manage soil compaction

One of the most important factors for decreasing soil compaction potential is to stay off the soil when it’s wet. Unfortunately, this isn’t always possible, as it often limits field work opportunities. While compaction may not be eliminated, it should be managed.

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Jodi DeJong-Hughes, Extension educator

Reviewed in 2018

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