Soil Compaction And Fall Tillage
URBANA, ILL.
With the early start of
corn harvest, and in
some cases a short
wait before soybean harvest
revved up, fall tillage got underway
early this fall. We
don’t keep statistics on tillage
like we do for harvest
progress, but in some areas,
especially east-central Illinois, many of the harvested
corn fields have already been tilled.
Soil in good condition is, by volume, about
half mineral (and organic) material and about
half pore space. Soil at field capacity (the
amount of water held against gravity, typically
about 20 percent to 25 percent of the soil volume)
has about half the pore space filled with
water and the other half with air. Soil compaction
takes place when we compress the soil,
forcing some of the air out. Mineral matter and
water aren’t “compressible,” so they are unaffected.
Dry soils don’t compact much because
dry mineral matter is strong enough to protect
pore space. Saturated soils have a lot of water
and little air in the pore space, so they also don’t
compact much. Moist soils are the easiest to
compact.
Most soil compaction results from driving
heavy equipment over soils that are at or somewhat
above field capacity. Heavy rains can help
puddle the surface and result in some “surface
compaction” or crusting, but they don’t contribute
to deep compaction. Soils in the spring,
including at planting time, are often at or near
field capacity, and for a time after a rain they
are above field capacity.
With the removal of water by the crop and normally
dry weather in the fall, soils at harvest are
often dry enough to limit compaction. That was
not the case in 2009, when soils were wet at
harvest. Adding that compaction to spring
tillage or planting-time compaction in both
2009 and 2010 means that soils in many Illinois
fields this fall are more compacted than
they have been for some time.
Because the physical resistance to penetration
increases greatly as soils dry out, it can be difficult
to get a meaningful measure of the
amount of soil compaction. If soils are relatively
moist, a simple tile probe can often be used to
locate layers in the soil that resist the probe.
Dry soils, though, provide a lot of resistance to
penetration even when they aren’t compacted
much. This year, it may be enough to recall soil
conditions at harvest in 2009 and at planting in
2009 and 2010 to assess the likelihood that
compaction is an issue. Some of the standing
water we saw in May and June might have
stood a little longer in the more compacted
areas of fields, but with so much rainfall it was
hard to tell if compaction had a direct effect. In
general, more poorly drained areas will
have the most compaction.
Repeated cycles of freezing and thawing can
help relieve compaction – the water volume in
the pore space expands during freezing and
contracts during thawing. This works on the
surface of the soil, where soils typically freeze
and thaw often during the winter and early
spring. But at depths greater than six inches or
so, soils go through only a few freeze-thaw cycles
in most years. And below a foot deep, soils
often freeze and thaw only once or twice during
the winter. This past winter, many soils froze
and stayed frozen until they thawed, with no repeated
cycle at all. There was thus little relief
from deeper compaction through “natural”
processes.
Though heavy equipment is the main cause of
deep compaction, large tractors are usually
needed to help relieve compaction by pulling implements
that penetrate soil into the deeper,
compacted zones. Deep tillage can’t fully restore
soil pore space to its “precompacted” state, but
it can introduce space for air to enter the soil,
which over time helps increase pore space. It
also helps break up the physical barrier that
compacted soil produces as it dries out.
Several types of equipment can be run deep
enough to penetrate the compacted zones. The
most basic are “deep rippers,” which typically
consist of five or seven heavy standards on a
heavy toolbar and can rip to a depth of 12 to 18
inches. Either straight points or sweeps can be
used, with sweeps requiring more horsepower,
and typically a little shallower, but providing
more “soil shatter,” which is helpful in relieving
compaction. Ripper standards with a narrow
cut and straight points can be operated without
disturbing much surface residue; fields following
such “minimum residue disturbance” (MRD)
implements can qualify as no-till.
Because the deep-ripping operation is expensive
in terms of equipment wear, fuel, and time,
it has been common practice to use these implements
only when standing water or other signals
point to the need to use them in individual
fields or part of fields.
Other implements used to relieve compaction
include combination disk-rippers, which typically
have a front gang of straight coulters with
an adjustable angle to bury more or less
residue, a row or two of heavy ripping standards,
and a rotating cage or tine harrow for leveling.
These tend to bury more residue and to
leave soil surfaces more level than do deep rippers.
Some also use heavy disk harrows, often
with large, notched blades that they use to penetrate
to depth and with gang angles that adjust
to cover the desired amount of residue.
Whatever implement is used to relieve compaction,
there are a few basic principles to remember
during such operations:
• Running implements to depth to relieve compaction
does little good if soils are not dry
enough. Soils shatter well only if they’re somewhat
below field capacity. Running heavy equipment
over the field pulling a heavy implement
causes some compaction even when soils are
dry enough-and it causes a lot of compaction
when soils aren’t dry enough to shatter well. So
it’s not difficult to cause as much compaction
as you relieve. While it has been relatively dry
in most of Illinois for the past month, water use
by the corn crop ended early this year, and soils
without active crop roots and with corn residue
after harvest dry slowly, especially as temperatures
drop. So it pays to check to see if soils are
actually as dry as we think they are and that
they are breaking up at the depth of tillage. As
a hint, ripping to relieve compaction when soils
are dry enough takes a lot of horsepower; if the
implement pulls easier than you expected, it’s
possible that it is running in soil wetter than
was thought.
• Tillage operations can only relieve compaction
to the depth at which they’re operated.
A heavy axle load can cause compaction to a
depth of 18 inches or more, so you can cause a
net increase in compaction if it’s too wet or
you’re running too shallow. While implements
like the offset disk or disk harrow are often
blamed for causing compaction, their real drawback
is that they usually aren’t run deep
enough to relieve compaction to the desired
depth. The moldboard plow often has a similar
limitation, though the “modified mini-moldboard”
can, if operated to a depth of 12 inches
or so, effectively relieve compaction.
• Primary tillage needs to leave some residue
on the surface to minimize soil loss before next
spring. This is especially important on slopes,
where much soil loss occurs through water
runoff. On flatter soils, wind erosion can be substantial,
and having surface residue can effectively
decrease the amount of soil picked up and
blown off the field.
While we typically consider corn fields that will
go back into corn in 2011 as having the highest
priority for relieving compaction, there have also
been questions about the need for deep tillage
following soybean harvest in 2010, in preparation
for corn in 2011. While soybean fields often
have a mellower surface in the fall than corn
fields, deeper compaction that was caused during
the 2009 season is still present, and at minimum
soybean stubble fields should be checked
for compaction this fall. With so much less
residue than in corn fields, soybean fields need
as much residue as possible to be retained on
the surface during the tillage process.
Finally, many who are seeing high soybean
yields in 2010 are wondering why, if compaction
was so widespread, soybean yields don’t seem
to have suffered. We don’t have a simple explanation,
but it is clear that corn and soybean responded
to weather and soil conditions very
differently in 2010 and that soybean plants
somehow got enough water to keep them going
in August, as corn was coming to an end.
This also reminds us that soil compaction, as
much as we work to prevent and relieve it, does
not always have a negative effect on yield; in
fact, in some cases it might even increase the
water available to a crop by improving the connection
of root systems with water that can
move up from deeper layers in the soil. Compaction
is more often negative than positive,
though, so we need to pay attention to it, preventing
it to the extent possible and relieving it
when we can’t prevent it. Δ
DR. EMERSON NAFZIGER: Extension Specialist/
Crop Production, University of Illinois