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How Good Are Your Soils? Assessing
Soil Health
the Garden of Eden, almost literally, lies under our feet almost
anywhere on the earth
we care to step. We have not begun to tap the actual potentialities
of the soil for producing crops.
E.H. Faulkner, 1943
By now, you should have some ideas about
practices for increasing soil health on your farm. Most farmers
know that soil health is important. However, when you work to improve
your soils, how can you tell that they're actually getting any better?
We're all used to taking soil samples and having them analyzed for
available nutrients, pH, lime requirement, and total organic matter.
Those tests are important pieces of information when used to adjust
nurient management practices. But the real issue is not how well
a soil does in a lab test, but how well it does in the field. Does
it do all the things we expect from a healthy soil? Of course, we
want a soil to supply nutrients in adequate amounts but not in such
excess that might cause plant health or environmental problems.
Does your soil also
Allow water to infiltrate easily during a downpour
and drain afterward?
Provide sufficient water to plants during dry spells?
Allow crops to fully develop healthy root systems?
Suppress root diseases and parasitic nematodes?
We can evaluate the quality of our soils in many
different ways, just as we do with human health. We can assess
human health with a variety of diagnoses or procedures, ranging
from "you look a little pale today" to taking a person's
temperature to doing a simple blood pressure test to computerized
body imaging. For soils, we are limited in our ability to diagnose
problems because we do not have the equivalent of the extensive
medical knowledge base that is available for humans. We also have
some additional challenges with soils. For example, a single blood
sample will assess the entire human body, because the blood circulates
rapidly through the entire vascular system. Soils do not function
as a single organism, but as part of an ecosystem. Therefore,
to obtain a good assessment of the soil's health, we need to make
multiple observations at different locations in a field and over
a period of time.
A simple but very good place to start assessing
a soil's health is to look at its general performance as you go
about your normal practices. It's something like wondering about
your own performance during the course of a day: Do you feel more
tired than usual? Are you concentrating less well on tasks than
you commonly do? Do you look paler than normal? These are indications
that something isn't quite right. Likewise, there are signs of
poor soil health you might notice as part of the normal process
of growing crops:
Are yields declining?
Do crops perform as well as those on neighboring farms with similar
soils?
Do your crops quickly show signs of stress or stunted growth during
wet or dry periods?
Does the soil plow up cloddy, and is it difficult to prepare a
good seedbed?
Does the soil crust over easily?
If you are no-tilling, is it difficult to get the planter to penetrate?
The next step should be a little more quantitative. In a few
states, farmers and researchers have developed "soil health
score cards that you fill out for each field. Card choices may
vary from state to state. The differences in soils and climates
suggest that there is no uniform soil health card that can be
used everywhere. Nor is there a magic number or index value for
soil health. The goal of any evaluation is to help you make changes
and improve your soil's health over time by identifying key limitations
or problems. Perfect soil can't be created everywhere, but you
want to help the land reach its fullest potential.
Whenever you try to become more quantitative in assessing your
soils, you should be aware that measurements may naturally vary
considerably within a field, or may change over the course of
the year. For example, if you decide to evaluate soil hardness
with a penetrometer, or by using a thin metal rod, your results
depend on the soil moisture conditions at the time of measurement.
If you use a penetrometer in June of a year with low early-season
rainfall, you may find the soil quite hard. If you go back the
next year following a wet spring, the soil may be much softer.
You shouldn't conclude that your soil's health has dramatically
improved because you really measured the effect of variable soil
moisture on soil strength. Similarly, earthworms will be abundant
in the plow layer when it's moist, but tend to go deeper into
the soil during dry periods. This type of variability with time
of year or climatic conditions should not discourage you from
starting to evaluate your soil's health just keep in mind the
limitations of certain measurements. Also, you can take advantage
of the fact that soil health problems tend to be more obvious
during extreme conditions. It's a good idea to spend some extra
time walking your fields and digging in the soil after extended
wet or dry periods.
In the next section, we use ideas and expressions
developed for soil health or soil quality cards in Maryland, Oregon,
and Wisconsin.
The indicators are not discussed below in any special order all
are important to help you assess soil health as it relates to
growing crops.
Soil testing is a very common way to assess your soil's
health from a chemical perspective (see detailed discussion in
chapter 19). This provides information
on potential nutrient and pH imbalances. To get the most benefit
from soil tests, sample soils frequently and keep good records.
If you change soil management practices, look at soil test trends
and evaluate whether your soil is improving. Evaluate whether
your soil test values are remaining in the optimal range, without
adding large amounts of fertilizers. Also, make sure that you
do not end up with excessive nutrient levels, especially phosphorus
and potassium, due to over-application of organic materials. If
your soil test report includes information on cation exchange
capacity (CEC), you should expect it to increase with increasing
organic matter levels.
Soil color is an indicator of soil organic matter content,
especially within the same general textural class. The darkness
is an indicator of the amount of humus (see chapter
2) in the soil. We generally associate black soils with high
quality. The Illinois color chart, relating color to organic matter
content, is proving useful in other parts of the country. However,
don't expect dramatic color change when you add organic matter;
it may take years to notice a difference.
Soil organisms such as ants, termites, and earthworms
are "ecosystem engineers" that aid the initial organic
matter breakdown that allows other species to thrive. They are
easily recognized, and their general abundance is strongly affected
by temperature and moisture levels in the soil. Their presence
is best assessed in mid-spring, after considerable soil warming,
and in mid-fall, before the soils become cold. Spring and fall
assessment are practical during moist, but not excessively wet,
conditions. Just take a spadeful of soil from the surface layer
and sift through it looking for bugs and worms. If the soil is
teeming with life, this suggests that the soil is healthy. If
few invertebrates are observed, then the soil may be a poor environment
for soil life and organic matter processing is probably low. Earthworms
are often used as an indicator species of soil biological activity
(see table 20.1). The most common worm types, such as the garden
and red worms, live in the surface layer when soils are warm and
moist and feed on organic materials in the soil. The long nightcrawlers
dig almost vertical holes that extend well into the subsoil, but
they feed on residue at the surface. Look for the worms themselves
as well as their casts (on the surface, for nightcrawlers) and
holes to assess their presence. If you dig out a square foot of
soil down to 1 foot depth and find 10 worms, the soil has a lot
of earthworm activity.
Soil tilth and hardness can be assessed with an inexpensive
penetrometer (the best tool), a tile finder, a spade, or a stiff
wire (like those that come with wire flags). Tilth characteristics
vary greatly during the growing season due to tillage, packing,
settling (dependent on rainfall), crop canopy closure, and travel
over the field to cultivate, apply pesticides, and harvest. It
is, therefore, best to assess soil hardness several times during
the growing season. If you do it only once, the best time is when
the soil is moist, but not too wet it should be in the friable
state. Soil is generally considered too hard for root growth if
the penetrometer resistance is greater that 300 psi. Note also
whether the soil is harder below the plow layer. We cannot be
very quantitative with tile finders and wire, but the soil is
generally too hard when you cannot easily push them in. If you
use a spade when soil is not too wet, evaluate how hard the soil
is and also pay attention to the structure of the soil. Is the
plow layer fluffy and does it mostly consist of granules of about
quarter-inch size? Or does the soil dig up in large clumps? A
good way to evaluate that is by lifting a spade full of soil and
slowly turning it over. Does the soil break apart into granules
or does it drop in large clumps? When you dig below the plow layer,
take a spade full of soil and pull the soil clumps apart. They
should generally come apart easily in well-defined aggregates
of several inches in size. If the soil is compacted, it does not
easily come apart in distinct units.
Root development also can be evaluated by digging and
is best done when the crop is in its rapid phase of growth generally
during late spring. Have the roots properly branched and extend
in all directions to their fullest potential for the particular
crop? Look for obvious signs of problems: short stubby roots,
abrupt changes in direction when hitting hard layers, signs of
rot or other diseases. Make sure to dig deep enough to get a full
picture of the rooting environment.
Crusting, ponding, runoff, and erosion can be observed
from the soil surface. However, the extent of their occurrence
depends on whether there was an intense rainstorm. The presence
of these symptoms are a sign of poor soil health, but the lack
of visible signs doesn't necessarily mean that the soil is in
good health it must rain hard for signs to occur. Try to get out
into the field after heavy rainstorms, especially in the early
growing season. Crusting is recognized by a dense layer at the
surface, which may become hard after it dries. Ponding is recognized
either directly when the water is still in a field depression,
or afterwards by small areas where the soil has slaked (aggregates
have disintegrated ). Areas that were ponded often show cracks
after drying. Slaked areas going down the slope are an indication
that runoff and early erosion have occurred. When rills and gullies
are present, a severe erosion problem is at hand. Another idea:
put on your raingear and go out during a rainstorm (not during
lightning, of course) and actually see runoff and erosion in action.
Compare fields with different crops, management, or soil types.
This might give you ideas about changes you can make to reduce
runoff and erosion.
You also can easily get an idea about stability of soil aggregates,
especially those near the surface. If the soil crusts readily,
you already know the answer the aggregates are not very stable
and break down completely when wet. If the soil doesn't usually
form a crust, you might take a sample of aggregates from the top
3 or 4 inches of soil from a number of different fields that seem
to have different soil quality. Gently drop a number of aggregates
from each field into separate cups that are half-filled with water
the aggregates should be covered with water. See if they hold
up or if they break apart. You can swirl the water in the cups
to see if that helps to break up the aggregates. Very turbid water
indicates that the aggregates have broken down. If the water stays
fairly clear, the aggregates are very stable.
Click here to view Table
20.1 Qualitative Soil Health Indicators.
The effects of soil health problems on general crop performance
are most obvious during extreme conditions. That's why it is worthwhile
to occasionally walk your fields during a wet period (when a number
of rains have fallen or just after a long, heavy rain) or during
an extended drought. During prolonged wet periods, poor soils
often remain saturated for extended periods. The lack of aeration
stunts the growth of the crop, and leaf yellowing indicates loss
of available N by denitrification. This may even happen with high-quality
soils if the rainfall is very excessive, but it is certainly aggravated
by poor soil conditions. Dense, no-tilled soil may also show greater
effects. Purple leaves indicate a phosphorus deficiency and are
also often an indirect sign of stress on the crop. This may be
related to soil health, but also can be brought on by other causes,
such as cold temperatures.
Watch for stunted crop growth during dry periods and also look
for the onset of drought stress leaf curling or sagging leaves
(depending on the crop type). Crops on soils that are in good
health generally have delayed occurrence of drought stress. Poor
soils, especially, may show problems when heavy rainfall, causing
soil settling after tillage, is followed by a long drying period.
Soils may hardset and completely stop crop growth under these
circumstances. Extreme conditions are good times to look at crop
performance and, at the same time, evaluate soil hardness and
root growth.
Using the simple tools and observations suggested above, you
can evaluate your soil's health. Soil health cards or soil quality
books provide a place to record field notes and assessment information
to allow you to compare changes that occur over the years. You
also can make up your own assessment sheets.
More "scientific" measurements can be made: infiltration
capacity, bulk density, the volume of large pores, soil strength,
etc. However, making these measurements in a meaningful way is
challenging and you should get a soil scientist or extension agent
involved if you want to pursue more sophisticated measurements.
Soils also can be tested for their biological characteristics
for potentially harmful organisms (usually for specific species
of nematodes) or, more broadly, for large organisms and microbiology.
Laboratories that do these types of tests are listed in Resources.
Source
U.S. Department of Agriculture (USDA). 1997. Maryland Soil Quality
Assessment Book.
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