15Preventing and Lessening

Compaction

A lasting injury is done by ploughing land too wet.

—S. L. DANA, 1842

We’ve already discussed the benefits of cover crops, rotations, reduced tillage, and organic matter additions for improving soil structure. However, these practices still may not prevent compacted soils unless specific steps are taken to reduce the impact of heavy loads from field equipment and inappro-priately timed field operations. The causes of compaction were discussed in chapter 7, and in this chapter we’ll discuss strategies to prevent and lessen soil compaction. The first step is to decide whether compaction is a problem and which type is affecting your soils. The symptoms, as well as remedies and preventive measures, are summarized in table 15.1.

CRUSTING AND SURFACE SEALINGCrusting and surface sealing may be seen at the soil surface after heavy rains in the early growing season, especially with clean-tilled soil and in the fall and spring after a summer crop (figure 15.1). Keep in mind that crusting and sur-face sealing may not happen every year, especially if heavy rains do not oc-cur before the plant canopy forms to protect the soil from direct raindrop im-pact. Certain soil types, such as sandy loams and silt loams, are particularly susceptible to crusting. Their aggregates usually aren’t very stable and, once broken down, the small particles fill in the pore space between the larger par-ticles.

The impact of surface crusting is most damaging when heavy rains occur between planting and seedling emergence. The hard surface crust may delay seedling emergence and growth until the crust mellows with the next rains. If such follow-up showers do not occur, the crop may be set back considerably. Crusting and sealing of the soil surface also reduce water infiltration capacity. This reduction in infiltration increases runoff and erosion, and lessens the

amount of available water for crops.

TABLE 15.1Types of Compaction and Their Remedies

COMPACTION TYPE INDICATIONS REMEDIES/PREVENTIONSurface crusting Breakdown of surface ag-

gregates and sealing of sur-face.

Poor seedling emergence.

Accelerated runoff and ero-sion.

Reduce tillage intensity.Leave residues on surface.Add other sources of com-posts).Grow cover crops.

Plow layer Deep wheel tracks.

Prolonged saturation or standing water.

Poor root growth.

Hard to dig and resistant to penetrometer.

Cloddy after tillage.

Plow with moldboard or chisel plow, but reduce sec-ondary tillage.Do primary tillage before winter (if no erosion danger exists).Use zone builders.Increase organic matter addi-tions.Use cover crops or rotation crops that can break up com-pact soils.Use better load distribution.Use controlled traffic.Don’t travel on soils that are wet.Improve soil drainage.

Subsoil Roots can’t penetrate sub-soil.

Resistant to penetrometer.

Don’t travel on soils that are wet.Improve soil drainage.Deep tillage with a subsoiler.

Use cover crops or rotation crops that penetrate com-pact subsoils.Use better load distribution.Use controlled traffic.No wheels in open furrows.

Figure 15.1. Rainfall energy destroys weak soil aggregates and creates a surface seal that increases runoff potential (photo from wheat soil in the Palouse region of Washington State).  When it dries, the seal turns into a hard crust that prevents seedling emergence.

Reducing Surface CrustingCrusting is a symptom of the breakdown of soil structure that develops espe-cially with intensively and clean-tilled soils. As a short-term solution, farmers sometimes use tools such as rotary hoes to break up the crust. The best long-term approach is to reduce tillage intensity, use tillage and cover cropping systems that leave residue or mulch on the surface, and improve aggregate stability with organic matter additions. Even residue covers as low as 30 per-cent 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 prevents crusting, but also rebuilds the soil by building organic matter and aggregation. Soils with very low aggregate stability may sometimes ben-efit from surface applications of gypsum (calcium sulfate). The added calcium and the effect of the greater salt concentration in the soil water as the gyp-sum dissolves promotes aggregation.

PLOW LAYER AND SUBSOIL COMPACTIONDeep 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 is an excellent tool to assess soil compaction, which we will dis-cuss in greater detail in Chapter 21. A simple shovel can be used to visually evaluate soil structure and rooting, and digging can provide good insights to 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 estab-lished. 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 dif-ference between soil and roots in wheel tracks and nearby areas to observe compaction effects on soil structure and plant growth behavior.

Figure 15.2. Large soil clods after tillage are indicative of com-paction and poor aggregation.

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 fig-ure 6.6, page XX). Keep in mind, however, that shallow rooted crops, such as spinach and some grasses, may not necessarily experience subsoil com-paction problems under those conditions.

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

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 yel-lowing), especially in areas that are imperfectly drained. In addition, these soils may “hardset” 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 pe-riod of stunted growth on compacted soils.

Reduced growth caused by compaction affects the crop’s ability to fight or

compete with pathogens, insects or weeds. These pest problems may, there-fore, become more apparent simply because the crop is weakened. For exam-ple, dense soils that are poorly aerated are more susceptible to infestations of certain soil-borne pathogens during wet periods, including root pathogens such as fungi 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, poor soil management is combined with poor sanitary practices and lack of rotations, creating a dependency on heavy chemical inputs. Improved soil and crop management can reduce such dependencies and increase farm profitability.

Preventing or Lessening Plow Layer CompactionPreventing or lessening soil compaction generally requires a comprehensive, long-term approach to addressing soil health issues and rarely gives immedi-ate results. Compaction on any particular field may have multiple causes and the solutions are often dependent on the soil type, climate and cropping sys-tem. Let’s go over some general principles of how to solve these problems.

Tillage is a problem, but sometimes can be a solution. Tillage can ei-ther cause or lessen problems with soil compaction. Repeated intensive tillage reduces soil aggregation and compacts the soil over the long term,

Some Crops are Particularly Hard on Soils

Potatoes require intensive tillage and return low rates of residue to soil. Silage corn and soybeans return low rates of residue. Many vegetable crops require 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. These 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 11-year experiment in Vermont with continuous corn silage on a clay soil, we found that applications of dairy manure were critical to main-taining 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 percent of the soil volume, respectively.

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 re-lief, as effective as it may be, is temporary. Tillage may need to be repeated in the following growing seasons if soil management and traffic patterns stay the same.

Over time, farmers frequently use more intense tillage to offset the prob-lems of cloddiness associated with compaction of the plow layer. The solution to these problems are not necessarily to stop tillage altogether. Compacted soils frequently become “addicted” to tillage and going “cold turkey” in con-verting to no-till management of a seriously compacted soil may result in fail-ure. 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 use-ful when aeration is of concern. They are also used to incorporate manure with minimal tillage damage. Strip tillage (6 inches depth) employs narrow shanks that only disturb the soil where future plant rows will be located (fig-ure 15.4). It is especially effective for promoting root proliferation.

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 condi-tions) and a planter that ensures good seed placement with minimal sec-ondary tillage. Such a soil management system reduces builds organic matter over the long term.

Figure 15.4 Tools that provide compacting relief with minimal soil disturbance: Aerator (left) and strip tiller (right; photo by Bob Schin-delbeck).

Deep tillage (subsoiling) is a method to alleviate compaction below the six- to eight- inch depths of normal tillage, and is often done with heavy-duty rip-pers (figure 15.5, left) and large tractors. Subsoiling is often erroneously seen as a cure for all types of soil compaction, but does relatively little to address

Lessening and preventing soil compaction is important to improving soil health. The specific ap-proaches:

should be selected based on where the compaction problem occurs (subsoil, plow layer, or surface);

must fit the soil and cropping system and their physical and economic realities; and

are influenced by other management choices, such as tillage system and use of

plow layer compaction. Subsoiling is a rather costly and energy-consuming practice that difficult to justify for use on a regular basis. Practices such as zone building also loosen the soil below the plow layer, but have narrow shanks that do lees disturbance and leave crop residues on the surface (fig-ure 15.5, right).

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 suffi-ciently 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. For fine-textures soils, the entire subsoil often has high soil strength values, so the effects of deep tillage are then less beneficial. In some cases it may even be harmful for these soils, especially if the deep tillage was performed when the subsoil was moist and caused smearing, which may then 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 con-ditions exist.

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.

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). This may require equipment modifica-tions 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 field 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 con-siderable 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 com-paction damage may occur at that time and alternative approaches to tillage and timing should be a priority.

Figure 15.6 Compaction and smearing from inadequately dry field condi-tions: wheel traffic (left) and plowing (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 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 con-tact 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 re-sulting compaction is significantly reduced. Using large, wide tires with low in-flation pressures also helps reduce potential soil compaction by distributing the equipment load over a larger soil surface area. Use of dual wheels simi-larly reduces compaction by increasing the footprint, although this load distri-bution is less effective for reducing subsoil compaction, because the pressure cones from neighboring tires (figure 6.10) merge at shallower depths. Dual wheels are very effective at increasing traction, but again, pose a danger be-cause of the temptation (and ability) to do field work under relatively wet con-ditions. Duals are not recommended on tractors performing seeding/planting operations because of the larger footprint (see also discussion on controlled traffic below).

Figure 15.7 Left: Tracks on farm equipment provide better load distribution and reduces compaction damage. Right: Dual wheels spread loads and in-crease traction, but also increase the tire footprint.

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.

Clay soils often pose an additional challenge with respect to com-paction, because they remain in the plastic state for extended peri-ods after a winter or wet periods. 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 be-ing worked or trafficked without causing excessive smearing and compaction damage. For this reason, farmers often fall-till clay soils. A better approach 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.

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 transpira-tion from plant leaves, resulting in better tillage and traffic conditions.

Cover and rotation crops. Cover and rotation crops can significantly re-duce soil compaction. The choice of cover/rotation 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 sea-son and can reach into the compacted layers when they are still wet and rela-tively 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 (see figure 10.4), forage radish roots can penetrate deeply and form vertical “drill” holes in the soil. In many cases, a combination of cover crops with shallow and deep rooting systems are preferred (figure 15.9). Ide-ally, such crops are part of the rotational cropping system, which is typically done on ruminant livestock-based farms.

The relative benefits of incorporating or mulching a cover/rotation crop are site-specific. Incorporation through tillage loosens the soil, which may be ben-eficial if the soil has been heavily trafficked. This incorporation would be the case with a sod crop that was actively managed for forage production, some-times 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 re-tention. Some farmers have been successful with cut-and-mulch systems in-volving aggressive, tall cover/rotation crops, such as rye and sudangrass.

Figure 15.9 A combination of deep alfalfa roots and shallow dense grass roots help 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 mate-rials are applied by providing a source of organic matter and glues for aggre-

gation. The long-term benefits of applying these materials relative to soil compaction may be very favorable, but in many cases the application proce-dure itself is a major cause of compaction. Livestock-based farms in humid re-gions usually apply manure using heavy spreaders (often with poor load dis-tribution) on wet or marginally dry soils, resulting in severe compaction of both the surface layer and the subsoil. In general, the addition of organic ma-terials should be done with care to obtain the biological and chemical bene-fits, 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 ex-pense 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 in-crease 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. Con-trolled traffic lanes may actually have an advantage in that the consolidated soil is able to bear greater loads, thereby better facilitating field traffic. Com-paction also can be reduced significantly by maximizing traffic of farm trucks along the field boundaries and using planned access roads, rather than allow-ing them to randomly travel over the field.

Controlled traffic systems require adjustment of field equipment to insure that all wheels travel in the same lanes and also requires 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 easiest 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 rec-ognizable 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 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. With

these advanced global positioning systems, a single reference station on the farm provides the real-time corrections to less than one inch level of accu-racy, 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 case soil shaping is additionally applied to improve the physical condi-tions 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 multi-ple years.

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

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). Right: Zucchini on mulched raised beds with drip irrigation (Queensland, Australia).

SUMMARYCompaction frequently goes unrecognized by farmers, but can result in de-creased yields. There are a number of ways to avoid the development of com-pacted soil, the most important of these is keeping equipment off wet soil (when it’s in a plastic state). Draining wet soils, using controlled traffic lanes, and permanent beds (that are never driven on) are ways to avoid com-paction. Also, reduced tillage and larger organic matter additions makes the surface less susceptible to aggregates breaking down and crust formation—as does maintaining a surface mulch and routine use of cover crops. Reduc-ing compaction once it occurs involves using cover crops that are able to break into subsurface compact layers and/or equipment such as subsoilers and zone builders to break up compact subsoil.

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