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Preventing and Lessening Compaction
A lasting injury is done by ploughing land too
wet.
S. L. Dana, 1842
We've 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, 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
14.1.
Crusting and Surface Sealing
Crusting and surface sealing are easy to see at the
soil surface after heavy rains in the early growing season, especially
with clean-tilled soil. Keep in mind that it may not happen every
year, if heavy rains do not occur before the plant canopy protects
the soil from direct raindrop impact. Certain soil types, such as
sandy loams, are particularly susceptible to crusting. Their aggregates
usually aren't very stable and, once broken down, the small particles
fill in gaps between the larger particles.
The impact of surface crusting is most damaging when
heavy rains occur between planting and seedling emergence. The hard
surface that forms may delay seedling emergence and growth until
the crust mellows with the next rains. If such showers do not occur,
the crop may be set back considerably. Crusting and sealing of the
soil surface also reduce water infiltration capacity. This increases
runoff and erosion, and lessens the amount of available water for
crops.
Reducing Surface Crusting
Crusting is a symptom of poor 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 systems that leave residue or mulch on the surface,
and improve aggregate stability with organic matter additions. Even
residue covers as low as 30 percent 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 is a key implement because it can successfully
establish crops without intensive tillage (see chapter
15). Reducing tillage and maintaining significant amounts of
surface residues not only prevent crusting, but also rebuild the
soil by reducing decomposition of organic matter. Practices that
improve soil structure, such as cover cropping, rotations with perennial
crops, and adding organic materials, also help reduce crusting problems.
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
both promote aggregation.
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. A shovel can be used to visually evaluate soil
structure and rooting. This is best done when the crop is in an
early stage of development, but after the rooting system had a chance
to get established. Digging can be very educational and provide
good clues to the quality of the soil. 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. Compacted soil shows
little aggregation, is more difficult to dig, and will dig up in
large clumps rather than granules. Compare the difference between
soil and roots in wheel tracks and nearby areas.
Roots in a compacted plow layer are usually stubby
and have few root hairs. They also often follow crooked paths as
they try to find zones of weakness. Rooting density below the plow
layer is an indicator for subsoil compaction. Roots are almost completely
absent from the subsoil below severe plow pans and often move horizontally
above the pan (figure 6.6). Keep in mind, however, that shallow
rooted crops, such as spinach and some grasses, may not necessarily
experience subsoil compaction problems under those conditions.
Compaction also may 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 "hardset" if heavy rains are followed by a drying
period. Crops in their early growth are very susceptible to these
problems (because roots are still shallow) and commonly go through
a noticeable period of stunted growth on compacted soils.
Reduced growth because of compaction affects the crop's ability
to fight or compete with pathogens, insects or weeds. These pest
problems may, therefore, become more apparent simply because the
crop is weakened. For example, compacted soils that are put into
a no-till system may initially show greater weed pressure because
the crop is unable to effectively compete. Also, dense soils that
are poorly aerated are more susceptible to infestations of certain
soil-borne pathogens, such as Phytophthora during wet periods.
Preventing or Lessening Soil Compaction
Preventing or lessening 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,
as effective as it may be, is temporary. Tillage may need to be
repeated in the next growing seasons if soil management and traffic
patterns stay the same.
Lessening and preventing soil compaction
is important to improving soil health. The specific approaches:
- 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 organic matter amendments.
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Over time, farmers frequently use more intense tillage
to offset the problems of cloddiness associated with compaction
of the plow layer. The solution to this problem is not necessarily
to stop tillage altogether. Compacted soils frequently become "addicted"
to tillage and going "cold turkey" to a no-till system
with a seriously degraded soil may result in failure. Practices
that perform some soil loosening with minor disturbance at the soil
surface help in the transition from a tilled to an untilled system.
This may include a zone-building tool (figure 14.1a) with narrow
shanks that disturb soil only where future plant rows will go. Also,
paraplows (figure 14.1b) that loosen the soil by lifting it from
underneath can relieve some compaction. Another approach may be
to gradually reduce tillage intensity through the use of tillage
tools that leave residue on the surface (for example, chisels with
straight points, or specifically designed for high-residue conditions)
and a good planter that ensures good seed placement even with minimal
secondary tillage. Such a system reduces organic matter losses and
erosion over the long term and through better germination rates
may produce more crop residues.
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a) zone-builder
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b) paraplow |
Figure 14.1 Tillage implements
that loosen soil with minimum surface disturbance. |
Deep tillage (subsoiling) is a method to alleviate compaction below
the depth of normal tillage, although it is often erroneously seen
as a cure for all types of soil compaction. It is a rather costly
and energy-consuming practice that should not be done regularly.
(Practices such as "zone building" and paraplowing also
may loosen the soil below the plow layer, but are less rigorous
and leave residue on the surface.) 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. The entire subsoil of fine-textured
soils is often hard, so the effects of deep tillage are then less
beneficial and in some cases even harmful. After performing deep
tillage, it is important to prevent future recompaction of the soil
by heavy loads and plows.
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 it is too wet. The first
step when addressing compaction 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 condition under which 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 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 operation. 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 should be a priority.
Better load distribution. Improving the design
of field equipment may help reduce compaction problems by better
distributing vehicle loads. The ultimate example of this is the
use of tracks, like those on a bulldozer, which especially reduce
the potential for subsoil compaction. (Beware! Tracked vehicles
may tempt a farmer to traffic the land when it's still too wet.
Tracked vehicles have better flotation and traction, but still cause
compaction damage, especially through smearing under the tracks.)
Plow layer compaction also can 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 and cuts the contact
pressure on the soil in half.
Use of multiple axles reduces the load on tires. Even
though the soil receives more tire passes, the resulting compaction
is significantly reduced. Using large, wide tires with low inflation
pressure also helps decrease the compaction effect of loads on soil.
Use of dual wheels similarly reduces compaction by increasing the
footprint, although this is less effective for reducing subsoil
compaction, because the pressure cones from neighboring tires (figure
6.11) 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 field work under relatively wet conditions.
Duals are not recommended on tractors performing seeding/planting
operations because of the larger footprint.
Improved soil drainage. Fields that are imperfectly
drained often have more severe compaction problems, because wet
conditions persist and it is almost impossible to prevent traffic
or tillage under those conditions. 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.
Clay soils often pose the greatest challenge with
respect to compaction, because they remain in the plastic state
for extended periods in the spring. After the top inch near the
soil surface dries out, it becomes a barrier that greatly reduces
further evaporation losses (figure 14.2). This 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 might
be to use cover crops to dry the soil in the spring. When a crop
like winter rye grows rapidly in the spring, its roots effectively
pump water from layers below the soil surface and allow the soil
to transition from the plastic to the friable state (figure 14.2).
Because these soils have high moisture-holding capacity, there is
normally little concern about cover crops depleting water for the
following crop.
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Figure 14.2 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. |
Cover and rotation crops. Cover and rotation crops can significantly
reduce 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 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 on the surface layer, but don't help much with subsoil compaction.
Crops with deep taproots, such as alfalfa, have fewer roots at the
surface, but can penetrate into a compacted subsoil. In many cases,
a combination of cover crops with shallow and deep rooting systems
are preferred. Ideally, 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 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. On the other hand, 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/rotation
crops, such as rye and sorghum-sudangrass.
Addition of other organic materials. Adding
animal manure, compost or sewage sludge benefits the surface soil
layer in which they are incorporated by providing a source of organic
matter. The long-term benefits of applying these materials, relative
to soil compaction, may be very favorable, but in many cases, the
spreaders themselves are 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. The need to incorporate animal manure for efficient nitrogen
use and odor control is also often a barrier to the adoption of
no-till or zone-till systems. This problem can be overcome only
through an additional investment in manure injection tools. 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, but rarely adopted, practices for reducing
soil compaction is the use of controlled traffic lanes. In this
system, traffic associated with all field operations is limited
to the same lanes. A controlled traffic system is easiest adopted
with row crops in zone, ridge or no-till systems (not requiring
full-field tillage, see chapter 15), because
crop rows and traffic lanes remain recognizable year after year.
Ridge tillage, in fact, dictates controlled traffic, as wheels cannot
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 lanes
typically requires some adjustment of field equipment to insure
that all wheel traffic occurs in the same lanes and also requires
considerable discipline from equipment operators.
Crops 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 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 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 space of 44, 45, 47, and 50 percent of the
soil volume. |
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 load (most of the compaction
occurs with the heaviest loading and does not greatly increase with
other passes), 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.
Using a Penetrometer for
Assessing Compaction
A penetrometer is a tool that costs
about $200 and measures the resistance to soil penetration.
A penetrometer has a rod with a cone-shaped tip that is pushed
into the soil. When penetration resistance is greater than
about 300 psi, the soil is usually too hard for roots to grow
(see chapter 6). Remember that the
strength of the soil depends on the water content as well
as bulk density (also chapter 6),
so penetrometer measurements need to be repeated several times
during the growing season to make a good assessment. However,
you can sometimes get important information from a single
set of penetrometer measurements made when the soil is very
moist (for example at the beginning of the growing season
in humid regions). If penetrometer readings at that time are
near or above 300 psi, they will surely be higher when the
soil dries out later in the season. When making penetrometer
measurements, try to notice soil strength in both the plow
layer and the subsoil corrective action may be different for
each case.
When using a penetrometer,
also keep in mind that soil strength is extremely variable
and multiple penetrations throughout the field should be made
and averaged before drawing conclusions. Penetrometers do
not work well in rocky soils, as the measurement is not valid
when the tip hits a rock. The devices are not very good for
predicting rooting behavior in clayey soils. Although clays
may get very hard upon drying, they may still have enough
large pores to allow roots to proliferate.
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A permanent (raised) bed system is another way of controlling traffic.
In this case, controlled traffic is combined with soil shaping to
improve the physical conditions in the beds. Beds do not receive
traffic after they've been formed. This is especially attractive
where traffic on wet soil is unavoidable for economic reasons (for
example, with certain fresh-market vegetable crops) and where it
is useful to install equipment, such as irrigation lines, for multiple
years.
Sources
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).
Kok, H., R.K. Taylor, R.E. Lamond, and S. Kessen.
1996. Soil Compaction: Problems and Solutions. Cooperative
Extension Service publication AF 115. Kansas State University. Manhattan,
KS.
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