15
Preventing 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
inappropriately timed field operations. The causes of compaction were discussed in chapter 6
[reference ok?], 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.
[table 15.1 about here]
[H1]CRUSTING AND SURFACE SEALING
Crusting 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 surface sealing may not happen every year, especially if
heavy rains do not occur before the plant canopy forms to protect the soil from direct raindrop
impact. 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 particles.
[fig. 15.1 about here]
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 15.1
Types of Compaction and Their Remedies
COMPACTION TYPE
Surface crusting
INDICATIONS
REMEDIES/PREVENTION
Breakdown of surface aggregates
Reduce tillage intensity.
and sealing of surface
Leave residues on surface.
Poor seedling emergence
Add organic matter .
Grow cover crops.
Accelerated runoff and erosion
Plow layer
Deep wheel tracks
Prolonged saturation or standing
water
Plow with moldboard or chisel plow,
but reduce secondary tillage.
Do primary tillage before winter (if
no erosion danger exists).
Poor root growth
Use zone builders.
Hard to dig and resistant to pene-
Increase organic matter additions.
trometer
Use cover crops or rotation crops
that can break up compact soils.
Cloddy after tillage
Use better load distribution.
Use controlled traffic.
Don’t travel on soils that are wet.
Improve soil drainage.
Subsoil
Roots can’t penetrate subsoil
Resistant to penetrometer at greater
depths
Don’t travel on soils that are wet.
Improve soil drainage.
Till deeply with a subsoiler. [edit ok
to make form consistent?]
Use cover crops or rotation crops
that penetrate compact subsoils.
Use better load distribution.
Use controlled traffic.
Don’t use wheels in open furrows.
Figure 15.1. Rainfall energy destroys weak soil aggregates and creates a surface seal that increases runoff potential. Photo is of soil [“wheat soil” ok?] in the wheat growing Palouse region
of Washington State. When it dries, the seal turns into a hard crust that prevents seedling emergence. [photo credit?Photo courtesy of USDA-NRCS.]
[H2]Reducing Surface Crusting
Crusting is a symptom of the breakdown of soil structure that develops especially 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% 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 benefit from
surface applications of gypsum (calcium sulfate). The added calcium and the effect of the greater
salt concentration in the soil water as the gypsum dissolves promotes aggregation.
[H1]PLOW LAYER AND SUBSOIL COMPACTION
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, [chapter ref ok?] 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.
[fig. 15.2 about here]
Figure 15.2. Large soil clods after tillage are indicative of compaction and poor aggregation. [photo credit?mine]
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). [figure ref ok?OK] Keep in mind, however, that shallow-rooted crops, such as spinach
and some grasses, may not necessarily experience problems from subsoil compaction [“those
conditions” clear?]
[fig. 15.3 about here]
Figure 15.3. Corn roots from a compacted plow layer are thick, show crooked growth patterns,
and lack fine laterals and root hairs. [photo credit? mine]
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.
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. [The two preceding
sentences need to be connected to the topic of soil compaction, or perhaps omitted.]
[H2]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.
Tillage is a problem but sometimes can be a solution. 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 man-
Crops That Are Hard on 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.
agement 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 of a seriously compacted soil 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 fu-
ture plant rows will be located (figure 15.4). It is especially effective for promoting root proliferation.
[fig. 15.4 about here]
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 [which is correct? Builds is correct, deleted other word] organic matter over the long term.
Figure 15.4. Tools that provide compacting relief with minimal soil disturbance: aerator (left) and strip tiller (right). Right photo by Bob Schindelbeck. [credit is only for
the righthand photo?yes—left is Harold’s]
Lessening and preventing soil compaction is 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 (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 [what have narrow shanks?The
zone builder implement] that disturb the soil less and leave crop residues on the surface (figure
15.5).
[fig. 15.5 about here]
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 moist and caused smearing, which may generate drainage problems. After performing deep tillage, it is important to prevent future recompaction of the soil by keeping heavy loads off the field and not tilling the soil when inappropriate soil moisture conditions 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. Right photo by George Abawi. [photo
credits? Left is Harold’s]
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).
[fig. 15.6 about here]
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 inadequately dry field conditions: wheel traffic
(left) and plowing (right). [photo credits?mine]
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.
[fig. 15.7 about here]
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 re-
duced. 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 shallower 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).
Figure 15.7. Left: Tracks on farm equipment provide better load distribution and reduce compaction damage. Right: Dual wheels spread loads and increase traction but also increase the tire
footprint. [photo credits?mine]
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 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 barri-
er 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.
[fig. 15.8 about here]
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. [source? own]
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, as is typically used on ruminant livestock farms.
[fig. 15.9 about here]
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 incorporation 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. [photo credit?mine]
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.
[H1]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.
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.
[fig. 15.10 about here]
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 real-time 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).
[fig. 15.11 about here]
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 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 15.10. A tractor with wide wheel spacing to fit a controlled traffic system.
[photo credit? Harold’s]
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). [photo
credits?mine]
[H1]SUMMARY
Compaction frequently goes unrecognized by farmers, but it can result in decreased yields. There
are a number of ways to avoid the development of compacted soil, the most important of which is
keeping equipment off wet soil (when it’s in a plastic state). Draining wet soils, using controlled
traffic lanes, and using permanent beds (that are never driven on) are ways to avoid compaction.
Also, reduced tillage and larger organic matter additions make the surface less susceptible to the
breakdown of aggregates and to crust formation—as does maintaining a surface mulch and routine use of cover crops. Reducing compaction once it occurs involves using cover crops that are
able to break into subsurface compact layers and using equipment such as subsoilers and zone
builders to break up compact subsoil.
SOURCES
Gugino, B.K., Idowu, O.J., Schindelbeck, R.R., van Es, H.M., Wolfe, D.W., Thies, J.E., et al.
2007. Cornell Soil Health Assessment Training Manual (Version 1.2). Geneva, NY: Cornell
University.
Kok, H., R.K. Taylor, R.E. Lamond, and S. Kessen. 1996. Soil Compaction: Problems and Solutions. Cooperative Extension Service publication AF 115. Manhattan: Kansas State University.
Moebius, B.N., H.M. van Es, J.O. Idowu, R.R. Schindelbeck, D.J. Clune, D.W. Wolfe, G.S.
Abawi, J.E. Thies, B.K. Gugino, and R. Lucey. 2008. Long-term removal of maize residue for
bioenergy: Will it affect soil quality? Soil Science Society of America Journal 72: 960–969.
Ontario Ministry of Agriculture, Food, and Rural Affairs. 1997. Soil Management. Best Management Practices Series. Available from the Ontario Federation of Agriculture, Toronto, Ontario,
Canada.