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Soil Compaction and What To Look For

You may have an enemy lurking beneath the soil that can cut crop yields by as much as 50 percent. This hard-to-detect foe isn't a disease or insect pest; it's a physical condition of the field—soil compaction.

Agricultural specialists define soil compaction as a reduction in soil porosity that occurs when the solid particles in the soil are pressed together, reducing the amount of space between particles.

Ideally, soil should consist of approximately 50 percent solid material and 50 percent pore space. In turn, that space should consist of 25 percent air and 25 percent water. In normal, healthy soil, a variety of large channels, such as those made by worms, and small pores allow for the best air and water movement as well as root growth.

In contrast, heavily compacted soils contain less space between the pores where pressure on the soil has essentially smashed the soil particles together. This dense soil retains less space, preventing air, water and roots from moving through the soil easily.

Compacted soils cause a host of problems for plants. Stunted root systems limit the plants' ability to take up water and nutrients. Decreased aeration within the soil may lead to loss of nitrogen to the atmosphere. Compacted soils may also increase risk of disease development. Finally, less water penetrates the soil, which can increase the risk of runoff, erosion and standing water.

What Is Soil Compaction Caused By?

Soil compaction can be shallow or deep. Tillage can usually correct shallow compaction in the first 20 inches of soil fairly easily. Deep compaction beyond the reach of a plow may require years to correct as the soil repairs itself naturally.

Wheel traffic from heavy equipment on the field is the primary cause of both shallow and deep compaction, but equipment traffic causes each type of compaction in different ways.

The contact pressure a wheel puts on the surface of the soil is the typical cause of shallow compaction. Contact pressure is measured in pounds per square inch (PSI), and higher pressure increases the risk of compaction.

Increasing the total tire area in contact with the ground can mitigate effects of increasing weight load with regard to shallow compaction. For example, if you double a grain cart's load, but switch from single to dual tires at the same time, the PSI exerted on the soil doesn't increase.

Deeper compaction is a more serious problem and is caused primarily by total vehicle weight (as opposed to tire contact pressure.) Operating equipment with heavy axle loads (per-axle weights of more than 10 tons) in wet field conditions can lead to compaction as deep as 2 feet below the soil surface. A large, four-wheel-drive tractor may exceed 10 tons per axle and a full grain cart or combine can weigh 20 tons per axle or more. Deep soil compaction risks have only increased over the years as the trend to increased operation size and technology improvements have led to heavier equipment.

Plowing or disking multiple times at the same depth can also lead to a shallow compaction layer, also known as a tillage pan, just below the tillage depth. Compaction can also occur naturally; rainfall, runoff and livestock can also cause soil density problems. Compaction caused by rainfall is rarely serious.

What To Look For With Soil Compaction

Visible symptoms of soil compaction can be hard to spot. You might notice decreased yields in certain fields or parts of a field. Water may also tend to pool in areas of the field such as wheel tracks.

If you suspect compaction, break out the shovel. Dig a hole and look for soil breaking off the shovel in packed clumps and a lack of worm channels and other biological activity, which may indicate compaction. Using a stake or screwdriver, see how difficult it is to penetrate the soil at different levels.

During the season, you dig up plants to look for "pancake roots," where roots spread horizontally after they encounter a layer of compaction that they cannot penetrate easily.

Another method is to drive a stake into the ground in an area of the field where you suspect compaction and perform the same test in a nearby area where you know little compaction exists, such a fencerow. Compare how difficult the stake is to drive into the earth between the two areas.

You may get a more precise diagnosis with a soil penetrometer, a device that measures the resistance in the soil. But be sure to consult instructions on using it correctly. A penetrometer may not be as accurate in rocky soil, fields without proper soil moisture, or fields that have been in sod or no-till for a number of years.

Preventing Soil Compaction

It’s probably one of the oldest adages in agriculture: "Never run when the fields are wet!" This is the most important factor in preventing soil compaction. Avoid fieldwork, especially with heavy equipment, when the soil is wet. Dry soil can support much heavier loads with less risk of compaction problems.

If you are forced to work in moist conditions, remember that total per-axle load weight is the primary cause of compaction in deeper soil layers. Axle loads lower than 5 tons generally only affect the surface; axle loads higher than 10 tons can cause compaction below the normal tillage zone. Minimizing axle loads keeps possible compaction near the surface, within the reach of tillage equipment or biological agents that can break down the soil. To minimize per-axle weight, reduce loads or use equipment with more axles.

You can also take steps to reduce shallow compaction by minimizing soil-to-tire contact pressure. Again, one way to do this is to reduce weight. Another way is to increase the size of the tire area in contact with the soil, reducing PSI. Solutions could include wider tires, taller tires, dual tire sets, and minimizing tire inflation pressure (within manufacturer recommendations). Try to keep road tires out of the field.

It may also be beneficial to minimize tracks in the field. Researchers say about 80 percent of compaction damage is done on the first pass, so it's usually better to keep using the same tire path.

Finally, note that healthy soil tends to be more compaction-resistant. To encourage biological activity in the soil, try practices such as using cover crops, minimizing erosion and reducing tillage.

What about your farm? Have you ever noticed compaction problems? How did you diagnose the problem, and what was the solution? Share your thoughts below in our comments section.

Read more about how Monty's Carbon increases yields on our blog, the End Row.

photos of corn soil compaction

Corn root growth limited by soil compaction (left) and healthy roots from noncompacted soils (right). Photo credit: Virginia Cooperative Extension.  [Source:]

Measuring soil penetration resistance with a penetrologger

Measuring soil penetration resistance with a penetrologger. Photo credit: James Foerster, USDA-ARS [Source:]

Sources and further reading:

“Avoiding Soil Compaction.” Penn State College of Agricultural Sciences.

“Deep Tillage Prior to No-Till Corn: Research and Recommendations.” Virginia Cooperative Extension.

“Diagnosing Soil Compaction Using a Penetrometer.” Penn State Extension.

“Soil Compaction: Causes, Concerns and Cures.” University of Wisconsin Extension.

“Soil Compaction: Causes, Effects and Control.” University of Minnesota Extension.

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