an image of compacted soil showing poor plant growth

What Causes Soil Compaction?

Soil compaction arises when soil particles are pressed together, eliminating or severely limiting the air pores in the soil. Under ideal conditions, soil particles are separated by air pockets (pores) that allow water to penetrate the soil and allow roots to expand and grow. When the soil is compacted, the air pores disappear, limiting the amount of water that can penetrate the soil and inhibiting healthy root growth.

What is soil compaction?

When soil becomes compacted, water cannot seep into the soil and may pool on top of the soil or runoff after rains. Plant root growth may be severely limited as they need air pockets to probe the soil. Likewise, because water cannot penetrate the soil, plant roots cannot access the water and nutrients the plants need to thrive.

Where does soil compaction happen?

Soil compaction can happen to nearly any soil, from large fields used for farming to small garden plots. Soil compaction can happen anywhere in your yard or garden.

Home gardens may suffer from soil compaction of the top 3 to 6 inches of soil. However, the soil in large farms may experience compaction to a depth of a foot or more.

What causes soil compaction?

Soil compaction is the result of applied pressure to wet soil. Because the water fills the pores in wet soil, pressure applied from above causes the soil particles to shift and push out the water. In this case, the water acts as a lubricant, making it easy for particles to shift or move resulting in compacted soil that lacks air pockets between the soil particles.

In compacted soil, the soil particles are compressed, and the pores in the soil are eliminated or severely reduced. Without air pores, the soil becomes compacted, shutting off access to water and nutrients to the roots of plants. It also hardens, making root development difficult.

In the home garden, tilling or working the soil while it is wet, using heavy garden equipment, and even walking on the wet soil are the most common causes of soil compaction.

Using heavy farm equipment in fields before the soil dries is the most common reason for soil compaction on farms.

Why does some soil compact easier than others?

If your garden soil has a lot of clay particles, it is more susceptible to compaction. Because clay particles are small, they can become compressed with minimal pressure. These tiny soil particles are sticky and smooth and susceptible to compaction.

Soils with a lot of sand are the least likely soils to suffer from compaction because the sand particles are much larger than clay particles and are gritty in nature. Even when squeezed tightly, the spaces between sand particles remain larger, allowing water to penetrate and plant roots to grow.

footprint in wet soil causing compaction
Avoid working on your garden when the soil is wet to help reduce compaction.

How do I know if I have soil compaction?

The most obvious sign your soil is compacted in your garden is its visual appearance. It will look hard and crusty when it dries. But there are other signs to look for.

  • Hard Soil: Compacted soil is dense and hard when it dries. You can often identify areas in your yard that suffer from soil compaction by measuring the hardness of the soil. The New Jersey Agricultural Experiment Station, Rutgers, recommends assessing soil hardness with a wire flag.

Insert the wire as far as it will go without bending the wire and measure the depth.  If the wire flag can be pushed into the soil to a depth of 12 inches, your soil is not compacted.  A depth between 4 and 12 inches means your soil is fair. A depth of fewer than 4 inches means your soil is compacted.

  • Shallow or exposed roots: Because roots cannot penetrate compacted soil, you may notice your plants develop shallow roots that may show on top of the ground.
  • Areas with little or no vegetation: Compacted soil may cause sparse areas in your lawn or yard where little or no vegetation grows. This is common along footpaths or other areas where foot traffic (or machinery) may have caused soil compaction.
  • Pooling water: If water pools in areas of your lawn or yard after rains, it may be due to soil compaction. The pools may look muddy when wet and when the soil is crusty once it dries.

What can I do to fix soil compaction?

When soil is compacted, it needs to be broken up and aerated before producing healthy plant growth. How you do that depends on whether the compacted soil is in your garden or if it is part of your yard or lawn.

Correcting soil compaction in garden soil

To fix soil compaction in garden soil, you need to break up the clods of soil. This can be done with a rototiller or hand tools, depending on the size of the area.

  1. Select the smallest and lightest weight tool for the job. If the compacted area can be broken up with garden tools or a small hand tiller, try that option first. Keep in mind that heavy equipment like garden tractors can do the job, but they also exert more pressure on the soil, which can lead to further compaction.
  2. Work the soil to break up clods. The object is to break the clods apart without pulverizing them. Overworking the soil to break up soil compaction can lead to further issues.
  3. Rake the area smooth with a garden rake.

Correcting soil compaction on the lawn

To fix soil compaction in the lawn, you will likely need an aerator. Aerators come in three basic types and can typically be rented at your local equipment rental outlets or garden supply centers.

  • Core/Plug Aerators: These aerators are designed with rows of hollow tines that remove a plug of soil from your lawn. The plugs are deposited on top of the lawn and left to break down on their own. If you choose to use a core or plug aerator, you manually break apart the plugs and scatter the soil over the lawn, if you prefer. Either way, core or plug aerators open up the soil in your lawn, allowing air and water to penetrate the soil.
  • Spike Aerators: These aerators use a spike-like tine to poke holes in the soil. While not as effective as core or plug aerators, spike aerators loosen the soil and allow water and air to penetrate the soil too. They may be well-suited for areas with light soil compaction or for small sections of compacted soil.
  • Slice Aerators: These aerators use blades to slice through the grass and into the soil. This allows water and air to reach the roots of plants and loosens the soil.

How to prevent soil compaction

You can prevent soil compaction in your home garden by waiting until the soil has dried in the spring before tilling it or working the soil. Use this quick test to tell if the soil is dry enough to work. Squeeze a handful of the soil into a ball. Poke the ball of soil with your finger. If the soil crumbles and breaks apart easily, the soil has dried sufficiently to till. If the ball of the soil maintains its shape, your soil is too wet to till.

But early spring isn’t the only time that soil compaction can occur. It can happen if you walk on wet soil after heavy rains too. Avoid walking on or using garden tools when the soil is saturated after heavy rains. Let the soil dry out for a few days before you use garden tools or walk on the soil to help avoid issues with soil compaction.

Summary

Soil compaction in the garden occurs when you try to till or work the soil while it is wet. It can even happen from walking on wet soil. The key to avoiding soil compaction is to avoid working the soil or walking in the garden when the soil is wet.

further reading:

https://njaes.rutgers.edu/fs1313/

https://extension.umn.edu/soil-management-and-health/soil-compaction

https://crops.extension.iastate.edu/cropnews/2010/04/how-and-why-avoid-soil-compaction

https://extension.psu.edu/effects-of-soil-compaction