15.3: Relieving and Preventing Compaction

Preventing or reducing soil compaction generally requires a comprehensive, long-term approach to addressing soil health issues and rarely gives immediate results. Compaction on any particular field may have multiple causes, and the solutions are often dependent on the soil type, climate and cropping system. With some exceptions, let’s go over some general principles of how to solve these problems.

Reducing surface sealing and crusting. Crusting is a symptom of the breakdown of soil aggregates that occurs especially with intensively and clean-tilled soils. As a short-term solution, farmers sometimes use tools like rotary hoes to break up the crust. The best long-term approach is to reduce tillage intensity (or eliminate tillage altogether), leave residue or mulch on the surface, plant cover crops, and improve aggregate stability with organic matter additions. Even residue covers as low as 30% of the soil surface area will greatly reduce crusting and provide important pathways for water entry. A good, heavy duty conservation planter—with rugged coulter blades for in-row soil loosening, tine wheels to remove surface residue from the row, and accurate seed placement—can be a highly effective implement because it can successfully establish crops without intensive tillage (see Chapter 16). Reducing tillage and maintaining significant amounts of surface residues not only prevent crusting but also rebuild the soil by increasing aggregation. Soils with very low aggregate stability, especially those high in sodium, may sometimes benefit from surface applications of gypsum (calcium sulfate). Aggregation is promoted by the added calcium and the effect of the greater salt concentration in the soil water as the gypsum dissolves.

Reducing surface layer compaction through proper use of tillage. Tillage can either cause or lessen problems with soil compaction. Repeated intensive tillage reduces soil aggregation and compacts the soil over the long term, causes erosion and loss of topsoil, and may bring about the formation of plow pans. On the other hand, tillage can relieve compaction by loosening the soil and creating pathways for air and water movement and root growth. This relief, however, as effective as it may be, is temporary and would probably need to be repeated in the following growing seasons if poor soil management and traffic patterns are continued. Farmers frequently use more intense tillage to offset the problems of cloddiness associated with compaction of the plow layer. But this puts them in a downward cycle and it should be avoided.

Organic Matter Helps Soils Resist Compaction

Organic matter in all its forms—including living roots, soil organisms, fragments of crop residue and dead organisms, and the “very dead”—helps to resist soil compaction in a number of ways. Using cover crops, leaving residues on the surface and increasing soil organic matter levels are indicated in Table 15.1 as remedies for all forms of compaction. The ways in which organic matter help:

• Residue on the surface helps dissipate the compacting force of equipment.
• Soil organic matter is less dense than mineral particles.
• Remnants of plant residue and soil organisms (the dead) help stabilize aggregates.
• Roots and large soil organisms (the living) create channels and other spaces for air and water movement and storage. Roots and mycorrhizal fungi secrete sticky substances that promote formation of stable aggregates and help bind smaller aggregates into larger ones. Some cover crops are able to break through compacted layers, allowing roots of the following commercial crops to better penetrate the soil. And the extensive root systems of perennial forage crops in a rotation, undisturbed by tillage, work to improve soil structure.

The long-term solution to these problems is to eliminate or significantly reduce tillage and to better manage soil organic matter (see below), but not necessarily to stop tillage altogether right away. Compacted soils frequently become “addicted” to tillage, and converting to no-till “cold turkey” may result in failure. Practices that perform some soil loosening with minor disturbance at the soil surface may help in the transition from a tilled to an untilled management system. Aerators (Figure 15.4) have rotating tines that provide shallow compaction relief in dense surface layers but do minimal tillage damage and are especially useful when aeration is of concern. They are also used to incorporate manure with minimal tillage damage. Strip tillage (6–8 inches deep) employs narrow shanks that disturb the soil only where future plant rows will be located (Figure 15.4). It is especially effective at promoting root proliferation. This practice is a good transition to more pure no-till, but many farmers find strip tillage a good long-term strategy that gets them similar benefits to no-till with fewer concerns about compaction. Yeoman’s plows achieve similar results with virtually no surface disturbance.

Figure 15.4.Tools that provide compaction relief with minimal soil disturbance: aerator (left) and strip tiller (middle and right). Right photo by Georgi Mitev.

Another approach may be to combine organic matter additions (compost, manure, etc.) with reduced tillage intensity and a planter that ensures good seed placement with minimal secondary tillage. Such a soil management system builds organic matter over the long term. In general, the benefits of no-till take 2–5 years to be realized, but this timeline can be accelerated with the use of cover crops.

Relieving subsoil compaction through deep tillage. Deeper tillage may be beneficial on soils that have developed a plow pan or other deep compacted layers. Simply shattering this pan allows for deeper root exploration. To be effective, deep tillage needs to be performed when the entire depth of tillage is sufficiently dry and in the friable state, otherwise it causes smearing of the soil.

Subsoiling relates to a range of methods to alleviate compaction below the 6- to 8-inch depths of normal tillage and is often done with heavy duty rippers (Figure 15.5) and large tractors. Subsoiling is often erroneously seen as a cure for all types of soil compaction, but it does relatively little to address surface layer compaction that is caused by aggregate breakdown. Subsoiling is a rather costly and energy consuming practice, and its use is difficult to justify on a regular basis. Also, large rippers generally cause more soil disturbance than needed, and practices such as zone building and deep strip tillage (Figure 15.4) are better ways to loosen the soil below the plow layer. They have narrow shanks that disturb the soil less and leave crop residues on the surface (Figure 15.5). Deep tillage tends to be more effective on coarse-textured soils (sands, gravels), as crops on those soils respond better to deeper rooting. In fine-textured soils, the entire subsoil often has high strength values, so the effects of deep tillage are less beneficial. In some cases it may even be harmful for those soils, especially if the deep tillage was performed when the subsoil was wet and caused smearing, which may generate drainage problems. After performing deep tillage, it is important to prevent future re-compaction of the soil by keeping heavy loads off the field and not tilling the soil when inappropriate soil moisture conditions exist, because otherwise the benefits are short lived. Soils that are naturally susceptible to subsoil compaction due to the washing down of fine particles may need repeated deep tillage.

Lessening and preventing soil compaction are both important to improving soil health. The specific approaches should meet the following criteria:

• They should be selected based on where the compaction problem occurs (subsoil, surface layer, or at the very surface).
• They must fit the soil and cropping system and their physical and economic realities.
• They should be influenced by other management choices, such as the tillage system and use of organic matter amendments.

Figure 15.5.Left: Subsoiler shank provides deep compaction relief (wings at the tip provide lateral shattering). Right: Zone building provides compaction relief and better rooting with minimal surface disturbance.Right photo by George Abawi.

Better attention to working the soil and field traffic. Compaction of the plow layer or subsoil is often the result of working the soil or running equipment when it is too wet (Figure 15.6). Avoiding this may require equipment modifications and different timing of field operations. The first step is to evaluate all traffic and practices that occur on a field during the year and determine which operations are likely to be most damaging. The main criteria should be:

• the soil moisture conditions when the traffic occurs, and
• the relative compaction effects of various types of field traffic (mainly defined by equipment weight and load distribution).

For example, with a late-planted crop, soil moisture conditions during tillage and planting may generally be dry, and minimal compaction damage occurs. Likewise, mid-season cultivations usually do little damage because conditions are usually dry and the equipment tends to be light. However, if the crop is harvested under wet conditions, heavy harvesting equipment and uncontrolled traffic by trucks that transport the crop off the field will do considerable compaction damage. In this scenario, emphasis should be placed on improving the harvesting operations. In another scenario, a high-plasticity clay loam soil is often spring plowed when still too wet. Much of the compaction damage may occur at that time, and alternative approaches to tillage and timing should be a priority.

Figure 15.6.Compaction and smearing from wet (plastic) soil conditions: wheel traffic (left), plowing (middle) and zone building leaving an open and smeared slot (right).

Better load distribution. Using improved designs of field equipment may help reduce compaction problems by better distributing vehicle loads. The best example is the use of tracks (Figure 15.7), which greatly reduce the potential for subsoil compaction. But beware! Tracked vehicles may provide a temptation to traffic the land when the soil is still too wet. Tracked vehicles have better flotation and traction, but they can still cause compaction damage, especially through smearing under the tracks. Surface layer compaction may also be reduced by using flotation tires and lowering the air pressure in the tires. A rule of thumb: Cutting tire inflation pressure in half doubles the size of the tire footprint to carry an equivalent equipment load and cuts the contact pressure on the soil in half.

Figure 15.7.Reduction of soil compaction by increased distribution of equipment loads. Left: Tracks on a tractor. Middle: Dual wheels on a tractor that also increase traction. Right: Multiple axles and flotation tires on a liquid manure spreader

The use of multiple axles reduces the load carried by individual wheels and tires. Even though the soil receives more tire passes by having a larger number of tires, the resulting compaction is still reduced (most compaction occurs from the first tire and subsequent tires result in little additional damage). Using large, wide tires with low inflation pressures also helps reduce potential soil compaction by distributing the equipment load over a larger soil surface area. Use of dual wheels similarly reduces compaction by increasing the footprint, although this load distribution is less effective for reducing subsoil compaction because the pressure cones from adjacent tires (see Figure 6.12) merge at shallow depths. Dual wheels are very effective at increasing traction but, again, pose a danger because of the temptation (and ability) to do fieldwork under relatively wet conditions. Duals are not recommended on tractors for performing seeding and planting operations because of the larger footprint (see also discussion on controlled traffic below). Improved soil drainage. Fields that do not drain in a timely manner often have more severe compaction problems. Wet conditions persist in these fields, and traffic or tillage operations often are done when the soil is too wet. Sometimes, the fields are plowed when the bottom of the plow layer is still too wet, causing smearing and plow pans. Improving drainage may go a long way toward preventing and reducing compaction problems on poorly drained soils. Subsurface (tile) drainage improves timeliness of field operations, helps dry the subsoil and, thereby, reduces compaction in deeper layers. On heavy clay soils where the need for close drain spacing is very expensive, surface shaping and mole drains are effective methods. Drainage is discussed in more detail in Chapter 17.

Figure 15.8. Cover crops enhance the drying of a clay soil. Without cover crops (left), evaporation losses are low after the surface dries. With cover crops (right), water is removed from deeper in the soil because of root uptake and transpiration from plant leaves, resulting in better tillage and traffic conditions.

Clay soils often pose an additional challenge with respect to drainage and compaction because they remain in the plastic state for extended periods after wet conditions. Once the upper inch of the soil surface dries out, it becomes a barrier that greatly reduces further evaporation losses. This is often referred to as self-mulching. This barrier keeps the underlying soil in a plastic state, preventing it from being worked or trafficked without causing excessive smearing and compaction damage. For this reason, farmers often fall-till clay soils. A better approach, however, might be to use winter cover crops to dry the soil in the spring. When a crop like cereal rye grows rapidly in the spring, the roots effectively pump water from layers below the soil surface and allow the soil to transition from the plastic to the friable state (Figure 15.8). Because these soils have high moisture-holding capacity, there is normally little concern about cover crops depleting water for the following crop.

Figure 15.9. A combination of deep alfalfa roots and shallow, dense grass roots helps address compaction at different depths.

Cover and rotation crops. Cover and rotation crops can significantly reduce soil compaction by creating and stabilizing large voids in the soil that allow for better water and air movement, and by supplying food for microbes. The choice of crop should be determined by the climate, cropping system, nutrient needs and the type of soil compaction. Perennial and cool-season crops commonly have active root growth early in the growing season and can reach into the compacted layers when they are still wet and relatively soft. Grasses generally have shallow, dense, fibrous root systems that have a very beneficial effect by alleviating compaction in the surface layer, but these shallow-rooting crops don’t help ameliorate subsoil compaction. Crops with deep taproots, such as alfalfa, have fewer roots at the surface, but the taproots can penetrate into a compacted subsoil. Forage radish roots can penetrate deeply and form vertical “drill” holes in the soil (see Figure 10.6), as described and shown in Chapter 10. In many cases, a combination of cover crops with shallow and deep rooting systems is preferred (Figure 15.9). Ideally, such crops are part of the rotational cropping system, as is typically used on ruminant livestock farms.

The relative benefits of incorporating or mulching a cover or rotation crop are site specific. Incorporation through tillage loosens the soil, which may be beneficial if the soil has been heavily trafficked. This would be the case with a sod crop that was actively managed for forage production, sometimes with traffic under relatively wet conditions. Incorporation through tillage also encourages rapid nitrogen mineralization. Compared to plowing down a sod crop, cutting and mulching in a no-till or zone-till system reduces nutrient availability and does not loosen the soil. But a heavy protective mat at the soil surface provides some weed control and better water infiltration and retention. Some farmers have been successful with cut-and-mulch systems involving aggressive, tall cover or rotation crops, such as rye and sudangrass.

Addition of organic materials. Regular additions of animal manure, compost or sewage sludge benefit the surface soil layer onto which these materials are applied by providing a source of organic matter and glues for aggregation. The long-term benefits of applying these materials for addressing soil compaction may be very favorable, but in some cases the application procedure itself is a major cause of compaction. Livestock-based farms in humid regions often apply manure using heavy spreaders (sometimes with poor load distribution) on wet or marginally dry soils, which results in severe compaction of both the surface layer and the subsoil. In general, the addition of organic materials should be done with care to obtain the biological and chemical benefits while not aggravating compaction problems.

Controlled traffic and permanent beds. One of the most promising practices for reducing soil compaction is the use of controlled traffic lanes, in which all field operations are limited to the same lanes, thereby preventing compaction in all other areas. The primary benefit of controlled traffic is the lack of compaction for most of the field at the expense of narrow lanes that receive all the compaction. Because the degree of soil compaction doesn’t necessarily worsen with each equipment pass (most of the compaction occurs with the heaviest loading and does not greatly increase beyond it), damage in the traffic lanes is not much more severe than that occurring on the whole field in a system with uncontrolled traffic. Controlled traffic lanes may actually have an advantage in that the consolidated (firmer) soil better facilitates field traffic. Compaction also can be reduced significantly by guiding traffic of farm trucks along the field boundaries and by using planned access roads, rather than allowing them to randomly travel over the field.

Figure 15.10. A tractor with wide wheel spacing to fit a controlled traffic system.

Controlled traffic systems require adjustment of field equipment to ensure that all wheels travel in the same lanes, and they require discipline from equipment operators. For example, planter and combine widths need to be compatible (although not necessarily the same), and wheel spacing may need to be expanded (Figure 15.10). A controlled traffic system is most easily adopted with row crops in zone, strip or no-till systems (not requiring full-field tillage; see Chapter 16) because crop rows and traffic lanes remain recognizable year after year.

Adoption of controlled traffic has expanded in recent years with the availability of RTK (real-time kinematic) satellite navigation systems and auto-steer technology. With these advanced guidance systems, a single reference station on the farm provides the real-time corrections to high levels of accuracy, facilitating precision steering of field equipment. Controlled traffic lanes can therefore be laid out with unprecedented accuracy, and water (for example, drip irrigation) and nutrients can be applied at precise distances from the crop (Figure 15.11).

Ridge tillage dictates controlled traffic, as wheels should not cross the ridges. A permanent (raised) bed system is a variation on controlling traffic in which soil shaping is additionally applied to improve the physical conditions in the beds (Figure 15.11, right). Beds do not receive traffic after they’ve been formed. This system is especially attractive where traffic on wet soil is difficult to avoid (for example, with certain fresh-market vegetable crops, or rice production) and where it is useful to install equipment, such as irrigation lines, for multiple years.

Figure 15.11.Controlled traffic farming with precision satellite navigation. Left: twenty-row corn-soybean strips with traffic lanes between the sixth and seventh row from the strip edge (Iowa; note that corn is still standing while soybean crop has been harvested). Right: zucchini on mulched raised beds (Queensland, Australia).