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Let's Get Physical: Soil Tilth,
Aeration, and Water
Moisture, warmth, and aeration; soil texture;
soil fitness; soil organisms;
its tillage, drainage and irrigation; all these are quite as important
factors
in the make up and maintenance of the fertility of the soil as are
manures, fertilizers, and soil amendments.
J.L. Hills, C.H. Jones, and C. Cutler, 1908
A
soil's physical condition has a lot to do with its ability to produce
crops. A degraded soil usually has reduced water infiltration and
percolation (drainage into the subsoil), aeration, and root growth.
This reduces the ability of the soil to supply nutrients, render harmless
many hazardous compounds (such as pesticides), and to maintain a wide
diversity of soil organisms. Small changes in a soil's physical conditions
can have a large impact on these essential processes. Creating a good
physical environment, which is a critical part of building and maintaining
a healthy soil, requires a certain amount of attention and care.
Let's first
look at the physical nature of a typical agricultural soil. It usually
contains about 50 percent solid particles and 50 percent pores on
a volume basis (figure 6.1). We discussed earlier how organic matter
is only a small, but very important component of the soil. The rest
of the soil particles are a mixture of various size minerals, ranging
from fine-grained microscopic clay to easily visible large sand grains.
The relative amounts of the various particle sizes defines the texture
of a soil, such as a clay, clay loam, loam, sandy loam, or sand. Although
management practices don't change this basic soil physical property,
they may modify the effects of texture on other properties.
The sizes
of the spaces (pores) between the particles and between aggregates
are much more important than the sizes of the particles themselves.
The total amount of pore space and the relative quantity of various
size pores (large, medium, small, very small) govern water movement
and availability for sustaining soil organisms and plants. We are
interested mostly in the pores, because that's where all the important
processes, such as water and air movement, take place. Soil organisms
live and function in the pores, which is also where plant roots grow.
Most pores in a clay loam are small (generally less than 0.0004 inch),
whereas most pores in a loamy sand are large (generally still smaller
than 0.1 inch). Although soil texture doesn't change over time, the
total amount of pore space and the relative amount of various size
pores (called the pore size distribution) are strongly affected by
management.
WATER
AND AERATION
The soil pore
space can be filled with either water or air, and their relative amounts
change as the soil wets and dries (figure 6.1). When all pores are
filled with water, the soil is saturated and soil gases can't
exchange with the atmospheric gases. This means that carbon dioxide
from respiring roots and soil organisms can't escape from the soil
and oxygen can't enter, leading to undesirable anaerobic (no
oxygen) conditions. On the other extreme, a soil with little water
may have good gas exchange, but it can't supply sufficient water to
plants and soil organisms.
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Figure 6.1 Distribution
of solids and pores in soil. |
The way in which a soil
holds and releases water is pretty similar to the way it works with
a sponge (figure 6.2). When it's fully saturated (you take the sponge
out of a bucket of water), a sponge loses water by gravity, but will
stop dripping within about 30 seconds. It's only the largest pores
that lose water during that rapid drainage because they are unable
to hold the water against gravity. The sponge still contains a lot
of water when it stops dripping. The remaining water is in the smaller
pores, which hold it more tightly. The sponge's condition following
drainage is almost the same as a soil reaching field capacity
water content, which occurs after about two days of free drainage
following saturation by a lot of rain or irrigation. If a soil contains
mainly large pores, like a coarse sand, it loses a lot of water through
gravitational drainage. This is good because these pores are now open
for aeration, but it's also bad because little water remains for plants
to use, leading to frequent drought stress. Coarse sandy soils have
very small amounts of water available to plants before they reach
their wilting point (figure 6.2a). On the other hand, a dense,
fine-textured soil, such as a compacted clay loam, has mainly small
pores, which hold on tightly to water and don't release it as free
drainage (it has little gravitational water, figure 6.2b). In this
case, the soil will have long periods of poor aeration, but more plant-available
water than a coarse sand. Leaching of pesticides and nitrates to groundwater
is also controlled by the relative amounts of different size pores.
The rapidly draining sands lose these chemicals along with the percolating
water, but this is much less of a problem with clays.
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Figure 6.2 Water storage for three soils. (Shaded
area represents water stored in soil that is available for plant
use.) |
The ideal soil is somewhere
between the two extremes. This can be found in a well aggregated medium-textured
loam soil (figure 6.2c, figure 6.3). Such a soil has enough large
pore spaces between the aggregates to provide adequate drainage and
aeration during wet periods, but also has adequate amounts of small
pores and water-holding capacity to provide sufficient water to plants
and soil organisms between rainfall or irrigation events. Besides
holding and releasing water well, such soils also allow for good water
infiltration, thereby increasing plant water availability and reducing
runoff and erosion. This ideal soil condition is indicated by crumb-like
aggregates, which are common in good topsoil.
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Figure 6.3 A well aggregated soil has a range
of pore sizes. This medium size soil crumb is made up of many
smaller ones. Very large pores occur between the medium size
aggregates. |
GOOD SOIL TILTH IS GOOD
AGGREGATION
Good aggregation,
or structure, helps to make a high quality soil. Aggregation
in the surface soil is favored by organic matter and surface residue.
As pointed out earlier, a continuous supply of organic materials,
roots of living plants, and mycorrhizal fungi hyphae are needed to
maintain good soil aggregation. Surface residues protect the soil
from wind and raindrops and moderate the temperature and moisture
extremes at the soil surface. An unprotected soil may reach very high
soil temperatures at the surface and become very dry. Worms and insects
will move deeper into the bare soil, developing a surface zone containing
few active organisms. Many small microorganisms, such as bacteria
and fungi that live in thin films of water, will die or become inactive,
slowing the natural process of organic matter cycling. Large and small
organisms function better in a soil that is well protected by crop
residue cover, a mulch, or a sod, which helps maintain good soil aggregation.
An absence of both erosion and the forces that cause compaction helps
maintain good surface aggregation.
WATER
INFILTRATION, RUNOFF, AND EROSION
As rainfall
reaches the ground, most water either infiltrates into the soil or
runs off the surface (some may stand in ruts or depressions before
infiltrating or evaporating). The maximum amount of rainwater that
can enter a soil in a given time, called infiltration capacity,
is influenced by the soil type, soil structure, and the soil moisture
at the start of the rain. Early in a storm, water usually enters a
soil readily. When the soil becomes more moist, it can soak up less
water. If rain continues, runoff is produced due to the soil's reduced
infiltration rate. When an intense storm hits an already saturated
soil, runoff occurs very rapidly, because the soil's ability to absorb
water is low. Rainfall or snowmelt on frozen ground generally poses
even greater runoff concerns, as pores are blocked with ice. Runoff
happens more readily with poorly structured soils, because they have
fewer large pores to quickly conduct water downward. As soil tilth
degrades, water infiltration decreases, producing more runoff.
Runoff water
concentrates into tiny streams, which loosen soil particles and take
them downhill. As runoff water gains more energy, it scours away more
soil. Runoff also carries agricultural chemicals and nutrients, which
end up in streams, lakes, and estuaries. Soil degradation in many
of our agricultural and urban watersheds has resulted in increased
runoff during intense rainfall and increased problems with flooding.
Also, the lower infiltration capacity of degraded soils reduces the
amount of water that is available to plants, as well as the amount
that percolates through the soil into underground aquifers. This underground
water feeds streams slowly. Watersheds with degraded soils experience
lower stream flow during dry seasons due to low groundwater recharge
and increased flooding during times of high rainfall due to high runoff.
Soil erosion is the result of exposing the soil directly
to the destructive energy of raindrops and wind. Soil erosion has
long been known to decrease soil quality, and at the same time cause
sedimentation in downstream or downwind areas. Soil is degraded
when the best soil material -- the surface layer -- is removed by
erosion. Erosion also selectively removes the more easily transported
finer soil particles. Severely eroded soils, therefore, become low
in organic matter and have less favorable physical properties, leading
to a reduced ability to sustain crops and increased potential for
harmful environmental impacts.
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Figures 6.4 Effects of tillage erosion on soils.
Photo by NRCS. |
Some ancient farming civilizations recognized soil
erosion as a problem and developed effective methods for runoff
and erosion control. Evidence of ancient terracing methods are apparent
in various parts of the world, notably in the Andean region of South
America and in Southeast Asia. Other cultures effectively controlled
erosion using mulching and intercropping, thereby protecting the
soil surface from the elements. (Some ancient desert civilizations,
such as the Anasazi in the Southwestern U.S. and the Nabateans in
the Middle East, took advantage of surface runoff to harvest water
to grow crops in downhill depressions. Their methods, however, were
specific to very dry conditions.) For most agricultural areas of
the world today, water and wind erosion still cause extensive damage
(including the spread of deserts) and remain the greatest threat
to agricultural sustainability.
Tillage degrades land even beyond promoting water and wind erosion
by exposing soil to the elements. It can also cause erosion by directly
moving soil down the slope to lower areas of the field. In complex
topographies such as seen in figure 6.4 this ultimately results
in the removal of surface soil from the knolls and its deposition
in depressions (swales) at the bottom of the slopes.
What causes this type of tillage erosion? Gravity causes more soil
to be moved by the plow or harrow down slope than up slope. Soil
is thrown farther down slope when tilling in the down slope direction
than is thrown uphill when tilling in the up slope direction (figure
6.5a). Down slope tillage typically occurs at greater speed than
when traveling uphill, making the situation even worse. Tillage
along the contour also results in down slope soil movement. Soil
lifted by a tillage tool comes to rest at a slightly lower position
on the slope (figure 6.5b). A more serious situation occurs when
using a moldboard plow along the contour. This is typically performed
by throwing the soil down the slope, as better inversion is obtained,
than by trying to turn the furrow up the slope (figure 6.5c).
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Figure 6.5 Three causes of erosion resulting
from tilling soils on slopes. |
Soil loss from slopes due
to tillage erosion can far exceed losses from water or wind erosion.
On the other hand, tillage erosion does not generally result in off-site
damage, because the soil is merely moved from higher to lower positions
within a field. However, it is another reason to reduce tillage on
sloping fields!
Soil
Compaction
A soil becomes
more compact, or dense, when aggregates or individual particles of
soil are forced closer together. Soil compaction has various causes
and different visible effects. Three types of soil compaction may
occur (figure 6.6):
- surface crusting
- plow layer compaction
- subsoil compaction
Surface crusting
occurs when soil is unprotected by surface residue or a plant canopy
and the energy of raindrops disperses wet aggregates, pounding them
together into a thin, but dense, surface layer. The sealing of the
soil reduces water infiltration and the surface forms a hard crust
when dried. If the crusting occurs soon after planting, it may delay
or, in some cases, prevent seedling emergence. Even when the crust
is not severe enough to limit germination, it can reduce water infiltration.
Soils with surface crusts are prone to high rates of runoff and erosion
(see figure 4.4 in chapter 4). You can reduce surface crusting by
leaving more residue on the surface and maintaining strong soil aggregation.
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Figure 6.6 Plants growing in a) soil with good
tilth and b) soil with all three types of compaction. |
Plow layer compaction compaction
of the entire surface layer has probably occurred to some extent in
all intensively worked agricultural soils. There are three primary
causes for such compaction erosion, reduced organic matter levels,
and forces exerted by field equipment. The first two result in a reduced
supply of sticky binding materials and a subsequent loss of aggregation.
Compaction
of soils by heavy equipment and tillage tools is especially damaging
when soils are wet. To understand this, we need to know a little about
soil consistence, or how soil reacts to external forces. At
very high water content, a soil may behave like a liquid (figure 6.7)
and simply flow as a result of the force of gravity as with mudslides
during excessively wet periods. At slightly lower water contents,
soil can be easily molded and is said to be plastic. Upon further
drying, the soil will become friable it will break apart rather
than mold under pressure.
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Figure 6.7 Soil consistency states for a sand
and a clay soil (friable soil is best for tillage). |
The point between plastic
and friable soil, the plastic limit, has important agricultural
implications. When a soil is wetter than the plastic limit, it is
seriously compacted if tilled or traveled on, because soil aggregates
are pushed together into a smeared, dense mass. This is why you often
see smeared cloddy furrows or deep tire ruts in a field (figure 6.8).
When the soil is friable (the water content is below the plastic
limit) it breaks apart when tilled and aggregates resist compaction
by field traffic. This is why the potential for compaction is so strongly
influenced by timing of field operations.
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Figure 6.8 Deep tire ruts in a hay field after
liquid manure was applied when soil was wet and plastic. |
A soil's consistency is
strongly affected by its texture (figure 6.7). For example, as coarse-textured
sandy soils drain, they rapidly change from being plastic to friable.
Fine-textured loams and clays need longer drying periods to lose enough
water to become friable. This extra drying time delays field operations.
Surface crusting
and plow layer compaction are especially common with intensively tilled
soils. This is often part of a vicious cycle in which a compacted
soil tills up very cloddy (figure 6.9a), and then requires extensive
secondary tillage and packing trips to create a satisfactory seedbed
(figure 6.9b). Natural aggregates break down and organic matter decomposes
in the process contributing to more compaction in the future. Although
the final seedbed may be ideal at the time of planting, rainfall shortly
after planting may cause surface sealing and further settling (figure
6.9c), because few sturdy aggregates are present to prevent the soil
from dispersing. The result is a dense plow layer with a crust at
the surface. Some soils may hardset like cement, even after the slightest
drying, slowing plant growth. Although the soil becomes softer when
it re-wets, this provides only temporary relief to plants.
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Figure 6.9a ) Stage 1: Cloddy soil after tillage
makes for a poor seedbed. |
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Figure 6.9b ) Stage 2: Soil is packed and pulverized
to make a fine seedbed. |
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Figure 6.9c ) Stage 3: Raindrops disperse soil
aggregates, forming a surface crust. |
Figure 6.9 Three tilth stages for a compacted
soil. |
Subsoil compaction compacted
soil below the normally tilled surface layer is usually called a plow
pan, although it's commonly caused by more than just plowing.
Subsoil is easily compacted, because it is usually wetter and tends
to be naturally dense, higher in clay content, lower in organic matter,
and have naturally lower aggregation than topsoil. Also, subsoil is
not loosened by regular tillage and cannot easily be amended with
additions of organic materials, so its compaction is more difficult
to manage.
Check Before Tilling
To be sure that a soil is ready for equipment
use, you can do the simple "ball test" by taking
a handful of soil from the lower part of the plow layer and
trying to make a ball out of it. If it molds easily and sticks
together, the soil is too wet. If it crumbles readily, it
is sufficiently dry for tillage or heavy traffic.
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Figure 6.10 Tractor wheels in open furrow during
plowing compacts subsoil. |
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Figure 6.11 Forces of heavy loads are transferred
deep into the soil, especially when wet. |
Subsoil compaction is the
result of either direct loading or the transfer of forces of compaction
from the surface. Direct loading occurs by the pressure of a tillage
implement, especially a plow or disk, pressing on the subsoil. It
also occurs when a field is moldboard plowed and a set of tractor
wheels is placed in the open furrow, thereby directly compacting the
soil below the plow layer (figure 6.10). Subsoil compaction also occurs
when farmers run heavy vehicles with poor weight distribution. The
load exerted on the surface is transferred into the soil along a cone-shaped
pattern (figure 6.11). With increasing depth, the force of compaction
is distributed over a larger area, thereby reducing the pressure in
deeper layers. When the loading force at the surface is small, say
through foot traffic or a light tractor, the pressure exerted below
the plow layer is minimal. But when the load is high, the pressures
at depth are sufficient to cause considerable soil compaction. When
the soil is wet, compaction forces near the surface are more easily
transferred to the subsoil. Clearly, the most severe compaction damage
to subsoils occurs by heavy vehicle traffic during wet conditions.
CONSEQUENCES
OF COMPACTION
As compaction
pushes soil particles closer together, the soil becomes more dense
and pore space is lost. When the bulk density increases during compaction,
mainly larger pores are eliminated. Loss of aggregation from compaction
is particularly harmful for fine and medium-textured soils that depend
on these pores for good infiltration and percolation of water, as
well as air exchange with the atmosphere. Although compaction can
damage coarse-textured soils, these soils depend less on aggregation,
because pores between many of their particles are sufficiently large
to allow good water and air movement. Compacted soil becomes hard
when dried and can restrict root growth and the activity of soil organisms.
The resistance to penetration, called soil strength, for a
moist, high-quality soil is well-below the critical level (300 pounds
per square inch (psi)), when root growth ceases for most crops. As
the soil dries, its strength increases, but may not exceed the critical
level for most (or all) of the moisture range. A compacted soil has
a very narrow water content range for good root growth. It's harder
in the wet range where it may even be above the critical level, depending
on the severity of compaction. When it dries, a compacted soil hardens
quicker than a well-structured soil, rapidly reaching a hardness well
above the 300 psi level that restricts root growth.
Actively growing
roots need pores with diameters greater than about 0.005 inch, the
size of most root tips. Roots must enter the pore and anchor themselves
before continuing growth. Compacted soils that have few or no large
pores don't allow plants to be effectively rooted limiting growth
and water and nutrient uptake.
Some Crops More Sensitive Than
Others
Compaction doesn't affect all crops to the same
extent. An experiment in New York found that direct-seeded
cabbage and snap beans were more harmed by compaction than
cucumbers, table beets, sweet corn, and transplanted cabbage.
Much of the compaction damage was caused by secondary effects,
such as prolonged soil saturation after rain, reduced nutrient
availability or uptake, and greater pest problems.
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What happens when root growth is limited? The root system will probably
have short thick roots and few fines ones or root hairs (see figure
6.6). The few existing thick roots are able to find some weak zones
in the soil, often by following crooked patterns. These roots have
thickened tissue and are not efficient at taking up water and nutrients.
In many cases, roots in degraded soils do not grow below the tilled
layer into the subsoil (see figure 6.6) it's just too dense and
hard for them to grow. Deeper root penetration is especially critical
under rain-fed agriculture. The limitation on deep root growth by
subsoil compaction increases the probability of yield losses from
drought stress.
There is a more direct effect on plant growth beyond
reduced root growth that limits the volume of the soil that is used
for water and nutrient supply. A root system that's up against mechanical
barriers sends a chemical signal to the plant shoot, which then
slows down respiration and growth. This seems to be a natural survival
mechanism similar to when plants experience water stress. In fact,
because some of the same hormones are involved and mechanical resistance
increases when the soil dries you usually can't separate the effects
of compaction from those of drought.
THE WATER RANGE FOR BEST PLANT GROWTH
The limitations to plant growth caused by compaction
and water extremes can be combined into the concept of the optimum
water range for plant growth the range of water contents for
which neither drought, mechanical stress, nor lack of aeration reduces
plant growth (figure 6.12). This range, referred to by scientists
as the least-limiting water range, is bounded on two
sides when the soil is too wet and when it's too dry.
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Figure 6.12 The optimum water range for crop
growth for two different soils. |
The optimum water range in a well-structured soil has its field
capacity on the wet end, as water above this water content readily
drains out by gravity. On the dry end is the wilting point beyond
which the soil holds water too tightly to be used by plants. However,
the soil water range for best growth in a compacted soil is much
narrower. A severely compacted soil at field capacity is still too
wet because it lacks large pores and is poorly aerated even after
the soil drains. Good aeration requires about 20 percent of the
pore space (about 10 percent of the volume of the whole soil) to
be air-filled. On the dry end, plant growth in a compacted soil
is commonly limited by soil hardness rather than by lack of available
water. Plants in compacted soils experience more stress during both
wet and dry periods than plants in soils with good tilth. The effects
of compaction on crop yields usually depend on the length and severity
of excessive wet or dry periods and when those periods occur relative
to critical times for plant growth.
Sources
Hillel, D. 1991. Out of the Earth: Civilization and the
Life of the Soil. University of California Press. Berkeley,
CA.
Letey, J. 1985. Relationship between soil physical
properties and crop production. Advances in Soil Science
1:277294.
Ontario Ministry of Agriculture, Food, and Rural
Affairs (OMFARA). 1997. Soil Management. Best Management
Practices Series. Available from the Ontario Federation of Agriculture,
Toronto, Ontario (Canada).
da Silva, A.P., B.D. Kay, and E. Perfect. 1994. Characterization
of the least limiting water range of soils. Soil Science Society
of America Journal 58:17751781.
Unger, P.W., and T.C. Kaspar. 1994. Soil compaction
and root growth: a review. Agronomy Journal 86:759766.
Soehue, W. 1958. Fundamentals of pressure distribution
and soil compaction under tractor tires. Agricultural Engineering
39:276290.
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