Soil compaction is a common problem on vegetable farms in the Northeast, but the trouble it causes is not always recognized. Trouble may be in the form of reduced yields, excessive runoff and soil erosion, or an increase in root disease. Compaction isn't something you observe directly, but you can see its indirect effects, such as standing water or stunting of plants, in some portions of a field.
If water drains slowly in some parts of a field, it may be due to excessive subsurface soil compaction.
Photos by Vern Grubinger.
Soil is a mixture of minerals, organic matter and pore space. The mineral particles are a combination of sand, silt and clay (the soil's texture). These are rather loosely arranged, surrounded by spaces that contain air and water. Soil compaction occurs when the soil's particles are pressed together, limiting the space that's available for air and water.
Water and compaction
The amount of water in a soil's pore space affects how easily it can be compacted. In a relatively dry soil, there is more friction between the soil particles, so they resist compaction. When sufficient water is present, it acts as a lubricant between the particles, and that makes the soil easier to compact. But water cannot be compressed, so once the soil pore spaces are mostly filled with water, it helps soil resist compaction. The result is that neither a dry soil nor an extremely wet soil will compact as readily as a moist soil.
Texture and structure affect compaction
Soil texture also influences the compaction potential of a soil. Particles of similar size will compact less than a mixture of particle sizes, because smaller particles can fill in the pore spaces between larger particles. Thus, soils with a diverse mix of sand, silt and clay are relatively susceptible to compaction when compared to soils that are dominated by one class of particle size.
Soil structure is another characteristic that plays a role in soil compaction or its prevention. Soils that are high in organic matter and have good aggregation tend to resist compaction better than soils with low organic matter levels and relatively few aggregates. Thus, excessive tillage can promote compaction by breaking down soil aggregates and speeding up decomposition of soil organic matter.
A penetrometer, or soil compaction meter, can be used to find out if and where compaction is a problem in your fields. Here, compaction is being measured in a strip after using deep zone tillage.
Types of compaction
Surface compaction occurs in the zone of tillage, and subsurface compaction is in the zone below the depth of normal tillage. Surface compaction, including soil crusting, may be temporarily alleviated with typical tillage operations, but subsurface compaction will remain in place unless special steps are taken to fracture or cut through the compacted soil underneath the tillage zone.
Causes of compaction
A widespread cause of soil compaction is farm vehicle traffic, especially heavy equipment with poor weight distribution. It's well known that compaction from vehicles is made worse when farm operations are conducted before the soil has adequately dried. However, it's also true that farming must go on, and sometimes that means getting work done even if soil conditions are not optimal. Limiting vehicular traffic to nongrowing areas such as alleyways can help avoid compaction.
Tillage is another cause of compaction, especially with implements that shear the soil, such as moldboard plows, discs and heavy sweeps. When operated repeatedly at the same depth, tillage implements orient soil particles in the same direction, creating a layer of compacted soil known as a tillage pan or plow pan. As with vehicle compaction, the potential to create a tillage pan is greater when the soil is moist than when it's dry.
Symptoms of compaction
The ultimate indicator of compaction is a reduction in yield, but many factors can contribute to that, from lack of root growth and nutrient uptake to crop losses from root diseases that thrive in soils with poor drainage.
More direct signs of compaction are: water that is slow to drain after rainfall or irrigation; a wheel-track pattern that is visible in crop growth; difficulty pulling tillage implements through the soil; and leaf discoloration and/or premature drought stress as a result of weak root systems that can't take up adequate nutrients or water.
These symptoms may be seen in one field but not another, or in portions of a field. That can be due to natural variability in soil characteristics that affect compaction as described above.
Many growers have seen evidence of a compaction problem, but they may not be sure it's there. The way to measure both the extent and depth of subsurface compaction is with a penetrometer, which is a soil compaction meter.
By pushing the penetrometer into the soil at a number of locations, you can assess the location and severity of compaction. Having this information will help you make management decisions related to compaction, such as crop rotation choices and whether or not to subsoil a field.
A basic penetrometer will cost about $230. They are available from several sources, including Gempler's (www.gemplers.com) and Forestry Suppliers (www.forestry-suppliers.com). You may also be able to borrow a penetrometer from your local cooperative extension or NRCS office.
The best approach to compaction is to prevent it from becoming a problem. Do not work soils before they have sufficiently dried. Prior to tillage, check the soil's moisture at the depth of the tillage operation, not at the soil surface. If the soil can be pressed into a ball that holds its shape, it is too wet for tillage.
Try to use tillage implements in a way that will reduce the formation of a tillage pan. Moldboard plows and heavy discs, especially those that are widely offset, are known to cause tillage pans. Large, rigid sweeps commonly found on some field cultivators can have a similar effect. Operating these kinds of tools at the same depth repeatedly over the years without varying tillage equipment or depth can cause a compacted layer in the soil.
Different types and depths of tillage should be rotated, just like crops themselves - and the two go together. For example, following a diverse rotation of vegetable crops may allow you to change the tools used to prepare land (chisel plowing one year, rotovating the next). A diversity of tillage is even more beneficial if land can be rotated out of vegetable production and put into forage crops or long-term cover crops in order to avoid tillage altogether for extended periods of time.
Permanent beds and alleys
Establishing parts of the field specifically for vehicle traffic is one way to avoid compaction where it matters most: where your crops are grown. With a permanent bed system, the goal is to have growing zones that never get driven on, with permanent wheel tracks in between. The tracks may be bare or sodded. This system may provide the additional benefit of reducing soil amendment costs if applications to wheel tracks or alleyways can be avoided.
This field has been taken out of vegetables and planted with red clover, which avoids tillage for a year and adds organic matter. Harvest alleys bracket the field, limiting vehicle traffic on top of growing areas. Both practices help avoid compaction problems.
A similar approach is to have wide traffic alleys in between sections of a field for harvest wagons and other large equipment in order to avoid compacting the growing areas, which may include many rows or beds maintained with conventional tillage.
To see videos of a variety of tillage practices for vegetables, go to www.extension.org/pages/18437/video:-vegetable-farm ers-and-their-sustainable-tillage-practices.
Subsoiling prior to planting vegetables is one way to break up subsurface compacted layers. This requires relatively powerful tractors and is not always effective, especially in the long term, if other practices are not put into place to manage compaction.
When subsoiling, it is important to know the exact depth of the compacted layer, so you are sure to set the subsoiler below it. A penetrometer is useful for determining the depth of a compacted layer, as well as its resistance to pressure.
Deep zone tillage is a reduced-tillage system that has been implemented on some vegetable farms. It combines strip tillage and subsoiling, and it can alleviate compaction while also conserving organic matter and reducing the number of passes needed to prepare a field for planting.
Zone tillage requires specialized equipment, such as an Unverferth Ripper-Stripper or a Yeomans Plow. Careful attention must be paid to surface residue management in order to create a killed cover between the tilled strips. For more information on reduced tillage on vegetable farms, visit www.vegetables.cornell.edu/reducedtillage.
Cover crops can help reduce compaction. Taking land out of vegetable production for two to three years in order to grow alfalfa or another deep-rooted perennial can be effective, but it may not be an economically viable option for vegetable farmers. Also, it can sometimes be difficult to establish a healthy alfalfa crop on compacted soils. Other, shorter-lived cover crops are also known to help reduce compaction, including deep-rooted sweet clover. Research has shown that sorghum-Sudan grass and, somewhat surprisingly, perennial ryegrass are also good cover crops for alleviating compaction.
The author is a vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office.