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The physics of soil compaction

This yield thief can largely be avoided with patience.

November 12, 2007
By Peter Darbishire


6aThe reduction of soil pore spaces resulting from compaction is a yield robber
of the greatest proportions. Often undetected and usually ignored, it restricts
the movement of air, water and crop roots to impact yields. It is often the
consequence of the time crunch imposed on crop producers who are under pressure
to start and complete field work in a short window of time. Sometimes they must
get this done even when soil and weather conditions are less than ideal on at
least part of a field.

Soil is made up of a matrix of soil particles and aggregates which are interspersed
with voids, explains Ridgetown College soils specialist Doug Young. The susceptibility
of any soil to compaction from field traffic is dependent on many factors. Axle
weight, tire pressure, tire slippage are some of the more controllable factors;
soil texture, of course, is unchangeable; soil moisture content is a function
of recent rainfall and the condition of the soil; and the condition of the soil
is largely governed by the practices to which it has been subjected.

The state of a soil has a greater influence on compaction than the weight of
the machinery or the way it is loaded. Generally, a dry soil can support tremendous
loads without undue effects. At 'field capacity', the term used to define how
much water held by surface tension on soil particles while excess moisture has
been drained away, soil moisture is held tightly to soil particles and aggregates
by surface tension forces. While this moisture may be accessed by plant roots,
it will not freely drain away. However, soil at field capacity is very susceptible
to compaction. As a soil's moisture content rises, it becomes more 'plastic':
in effect, the soil particles can slide relative to each other as the moisture
between them acts as a lubricant.


So long as the soil is not saturated, machinery and implement loads will cause
the particles to move more closely together, becoming more compact. This effect
is greatest when soil moisture fills 80 percent of the available pore space
between soil particles. The higher the machinery load, the greater the movement;
the poorer the soil structure and strength, the greater the compacting action
will be. In extreme cases, as can be seen in poorly structured, soft soils,
or in areas of a field where there has been repeated traffic, the soil will
begin to 'flow', pushing up beside tires, causing ruts. This is especially evident
if soil beneath is either very firm or is so saturated that the voids are completely
filled with water and cannot be compressed. The same happens if underlying soil
is frozen.

The solutions to compaction are not simple. Ontario Ministry of Agriculture
and Food agricultural engineer, Helmut Speiser says it takes years to remedy.
“Plant roots take the path of least resistance,” he says. If their
intended path is impeded by compacted soil, they will look for another, but
only to a certain point: beyond this the plant begins to suffer.

6bIt is an error to believe that the frost action of Ontario winters will relieve
a soil of compaction: “Frost breaks up soil lumps at the surface where
it is subjected to several freeze-thaw cycles in a season,” he says. “At
a depth of six inches, we get one freeze-thaw cycle and the heave is all vertical,
not horizontal, so it cannot break up compacted layers at this depth or deeper.”
His colleague Anne Verhallen, soil management specialist for horticultural crops
for OMAF at Ridgetown, adds that the first freeze-thaw cycle does the most for
improving aggregate structure, while subsequent cycles have no further impact.

If a field is suffering from severe compaction, deep ripping tools can shatter
compacted layers if the operation is done when the soil is dry. But if the practices
that caused the compaction continue, the problem will return. The real solution
is to avoid the causes.

Speiser, Young and their colleagues have used many ways to illustrate how radial
tires, proper ballasting and low tire pressures can reduce compaction. “You
want to keep tire pressure as low as possible and certainly less than 15psi,”
he says, “but if you don't reduce axle load, there will still be some compaction
at depth.” A fully loaded combine, grain buggy or manure tank have huge
axle loads. Even if flotation tires spread the load over a greater area, to
reduce sinkage and surface compaction, they can still compact deeper layers
of soil.

“Anytime we roll machinery across the surface, we have the potential to
cause a problem,” says Speiser. He advises that common sense can reduce
the effects: “A grain buggy does not always have to travel with a full
load and it doesn't always need to travel the full length of the field,”
he adds.

An observant eye will show a good deal about the condition of the soil in a
field. If a single cultivator pass produces a good seedbed, it can be accepted
that the soil is in good condition. “If you need a second pass to break
up the clods, your soil 'wants' some care and attention,” says Young. “Organic
matter level can be gathered from a soil test and it is a good indicator that
the soil is in good condition. Relating this to soil structure is more involved.
We are working on a compact disc to help educate growers about this,” he

Other visual signs are:

  • Good root formation: if plant roots are restricted, they will tell the tale
    if the plant is dug up and washed off in a pail of water.
  • A handful of moist soil can be formed into a ball: if it then crumbles easily,
    this is a sign of good condition; if it does not crumble, this indicates it
    has poorer structure.

Making a practice of checking soil conditions is worth it, say these experts.
The more you do it, the better you become at diagnosing problems and finding
the solutions or avoiding the causes. Ignoring the symptoms, though, is not
wise. As Speiser puts it: “If you compact your soil, you will suffer the
consequences. Even after nine years without further damaging a soil, we can
still see the structural impact.”

Compaction facts

  • Combines can load axles up to 35,000lb, grain carts 16,500lb and tractors
    to pull them another 30,000lb. That totals tons of compaction action!
  • The worst offenders are highway tractor-trailer trains with highway tires.
    Keep them out of the field!
  • Tires and tracks both compact wet soils.
  • Moderate compaction of top layers of a seedbed are acceptable: it is alright
    to plant into a tire track as long as it is not a deep rut!
  • Roots that are skewed to one side or kinked, or that follow cracks or layers
    in the soil indicate compaction.
  • Standing water and stunted plant growth indicate compaction.
  • When a soil is barely 'dry enough to work', it is most susceptible to being
    compacted. Allowing it to dry one more day is of major benefit.
  • Use the largest diameter and width tires possible at the lowest practical
  • If your neighbour's field does not show the same symptoms: maybe you have
    the problem!

The Bottom Line
Another little known fact: Your tire pressure, no matter what
the axle weight, will directly reflect the 'tire to soil' pressure. For
example, if you have 15lb of air in your tire, then you have 15lb 'tire
to soil' pressure. If you increase the axle weight, the tire will get
flatter, but your 'tire to soil' pressure will remain the same.

If you want to carry more axle weight, then increase the size and capacity
of your tires, but leave the inflation pressure below 15psi in the field.
Andy van Niekerk, Stayner, Ontario.

I keep tillage to a minimum. We use a disc harrow only on dry soils and
only as a primary tillage tool. The cultivator, with finger harrows, is
used only for levelling prior to planting.

At planting we run all tires and duals between the rows with low air

When harvesting, I have found that grain buggies are a major cause of
compaction on wet to moderately wet soils but they are necessary to speed
harvest operations. By only running the buggy half-way up the field we
can reduce some compaction. Also, when possible, the grain buggy is not
filled to maximum capacity. In late fall harvest, when fields likely will
not be dry, I wait for the ground to freeze. Tony
Pynenburg, Bright, Ontario