Building Soils for Better Crops, Third Edition

Plow Layer and Subsoil Compaction

Figure 15.2. Large soil clods after tillage are indicative of compaction and poor aggregation.
Figure 15.2. Large soil clods after tillage are indicative of compaction and poor aggregation.

Deep wheel tracks, extended periods of saturation, or even standing water following a rain or irrigation may indicate plow layer compaction. Compacted plow layers also tend to be extremely cloddy when tilled (figure 15.2). A field penetrometer, which we will discuss in greater detail in chapter 22, is an excellent tool to assess soil compaction. A simple shovel can be used to visually evaluate soil structure and rooting, and digging can provide good insights on the quality of the soil. This is best done when the crop is in an early stage of development but after the rooting system has had a chance to get established. If you find a dense rooting system with many fine roots that protrude well into the subsoil, you probably do not have a compaction problem. Well structured soil shows good aggregation, is easy to dig, and will fall apart into granules when you throw a shovelful of soil on the ground. Compare the difference between soil and roots in wheel tracks and nearby areas to observe compaction effects on soil structure and plant growth behavior.

Figure 15.3. Corn roots from a compacted plow layer are thick, show crooked growth patterns, and lack fine laterals and root hairs.
Figure 15.3. Corn roots from a compacted plow layer are thick, show crooked growth patterns, and lack fine laterals and root hairs.

Roots in a compacted plow layer are usually stubby and have few root hairs (figure 15.3). The roots often follow crooked paths as they try to find zones of weakness in the soil. Rooting density below the plow layer is a good indicator for subsoil compaction. Roots are almost completely absent from the subsoil below severe plow pans and often move horizontally above the pan (see figure 6.6). Keep in mind, however, that shallow-rooted crops, such as spinach and some grasses, may not necessarily experience problems from subsoil compaction.

Compaction may also be recognized by observing crop growth. A poorly structured plow layer will settle into a dense mass after heavy rains, leaving few large pores for air exchange. If soil wetness persists, anaerobic conditions may occur, causing reduced growth and denitrification (exhibited by leaf yellowing), especially in areas that are imperfectly drained. In addition, these soils may “hard set” if heavy rains are followed by a drying period. Crops in their early growth stages are very susceptible to these problems (because roots are still shallow), and the plants commonly go through a noticeable period of stunted growth on compacted soils.


Some crops are particularly hard on soils:

  • Root and tuber crops like potatoes require intensive tillage and return low rates of residue to the soil.
  • Silage corn and soybeans return low rates of residue.
  • Many vegetable crops require a timely harvest, so field traffic occurs even when the soils are too wet.

Special care is needed to counter the negative effects of such crops. Counter measures may include selecting soil-improving crops to fill out the rotation, extensive use of cover crops, using controlled traffic, and adding extra organic materials such as manures and composts. In an eleven-year experiment in Vermont with continuous corn silage on a clay soil, we found that applications of dairy manure were critical to maintaining good soil structure. Applications of 0, 10, 20, and 30 tons (wet weight) of dairy manure per acre each year of the experiment resulted in pore spaces of 44, 45, 47, and 50% of the soil volume, respectively.

Reduced growth caused by compaction affects the crop’s ability to fight or compete with pathogens, insects, and weeds. These pest problems may become more apparent, therefore, simply because the crop is weakened. For example, during wet periods dense soils that are poorly aerated are more susceptible to infestations of fungal root diseases such as Fusarium, Pythium, Rhizoctonia, Thieviopsis and plant-parasitic nematodes such as northern root-knot. These problems can be identified by observing washed roots. Healthy roots are light colored, while diseased roots are black or show lesions.

In many cases, soil compaction is combined with poor sanitary practices and lack of rotations, creating a dependency on heavy chemical inputs.

Preventing or Lessening Plow Layer 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. Let’s go over some general principles of how to solve these problems.

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 may 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. The solution to these problems is not necessarily to stop tillage altogether. Compacted soils frequently become “addicted” to tillage, and going “cold turkey” by converting to no-till management 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) provide some 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 to 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 for promoting root proliferation.

Tools that provide compaction relief with minimal soil disturbance: aerator (left) and strip tiller (right). Right photo by Bob Schindelbeck

Another approach may be to combine organic matter additions (compost, manure, etc.) with reduced tillage intensity (for example, chisel plows with straight points, or chisel plows specifically designed for high-residue conditions) and a planter that ensures good seed placement with minimal secondary tillage. Such a soil management system builds organic matter over the long term.

Deep tillage (subsoiling) is a method 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 plow layer compaction. Subsoiling is a rather costly and energy-consuming practice that is difficult to justify for use on a regular basis. Practices such as zone building also loosen the soil below the plow layer, but zone builders have narrow shanks that disturb the soil less and leave crop residues on the surface (figure 15.5).

Lessening and preventing soil compaction are 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, plow layer, or 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 tillage system and use of organic matter amendments.

Deep tillage may be beneficial on soils that have developed a plow pan. 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. The practice 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.

Better attention to working and traveling on the soil.

Compaction of the plow layer or subsoil is often the result of working or traveling on a field when the soil 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 be generally 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 open and smeared slot (right).

Better load distribution.

Improving the design of field equipment may help reduce compaction problems by better distributing vehicle loads. The best example of distributing loads is through 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. Plow layer compaction may also be reduced by lowering the inflation pressure of 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.

The use of multiple axles reduces the load carried by the tires. Even though the soil receives more tire passes by having a larger number of tires, the resulting compaction is significantly reduced. 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 neighboring tires (see figure 6.10) 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).

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.

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 have to be performed when the soil is too wet. 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.

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 drying from 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 soil below 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 cover crops to dry the soil in the spring. When a crop like winter 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.

Cover and rotation crops.

Cover and rotation crops can significantly reduce soil compaction. The choice of crop should be defined by the climate, cropping system, nutrient needs, and the type of soil compaction. Perennial 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, 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. As described and shown in chapter 10, forage radish roots can penetrate deeply and form vertical “drill” holes in the soil (see figure 10.4). 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, which 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 sudan grass.

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

Addition of organic materials.

Regular additions of animal manure, compost, or sewage sludge benefit the surface soil layer to which these materials are applied by providing a source of organic matter and glues for aggregation. The long-term benefits of applying these materials relative to soil compaction may be very favorable, but in many cases the application procedure itself is a major cause of compaction. Livestock-based farms in humid regions usually apply manure using heavy spreaders (often with poor load distribution) on wet or marginally dry soils, resulting 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 limited areas 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 soil is able to bear greater loads, thereby better facilitating field traffic. Compaction also can be reduced significantly by maximizing traffic of farm trucks along the field boundaries and 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.
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; they also require some 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, ridge, or no-till systems (not requiring full-field tillage; see chapter 16), because crop rows and traffic lanes remain recognizable year after year. Ridge tillage, in fact, dictates controlled traffic, as wheels should not cross the ridges. Zone and no-till do not necessarily require controlled traffic, but they greatly benefit from it, because the soil is not regularly loosened by aggressive tillage.

Adoption of controlled traffic has been rapidly expanded in recent years with the availability of RTK (real-time kinematic) satellite navigation systems. With these advanced global positioning systems, a single reference station on the farm provides the realtime corrections to less than 1 inch level of accuracy, which facilitates 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).

A permanent (raised) bed system is a variation on controlling traffic in which soil shaping is addition ally applied to improve the physical conditions in the beds (figure 15.11, right). Beds do not receive traffic after they’ve been formed. This bed system is especially attractive where traffic on wet soil is difficult to avoid (for example, with certain fresh-market vegetable crops) 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: Twelve-row corn-soybean strips with traffic lanes between the fourth and fifth row from the strip edge (Iowa; note that both current and previous year harvested crop rows are still visible). Right: Zucchini on mulched raised beds with drip irrigation (Queensland, Australia).

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