hoW good are your soils?
Field and laboratory evaluation oF 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
Most farmers know that it is important to increase
or surface compaction, and target your management
soil health. And by now, you should have some ideas
practices. A second reason might be to monitor the
about ways to increase soil health on your farm, but
health of your soils over time. Is your soil improving
how can you identify the specific problems with your
after you start planting cover crops, beginning a new
soil, and how can you tell if your soil’s health is actually
rotation, or switching to reduced tillage? While the
getting better?
goal of building soil health is to prevent problems from
Does your soil . . .
developing, it also helps to correct previous problems
• allow water to infiltrate easily during a downpour
you might have had. A good soil health assessment done
and drain afterward to let air in?
over a number of years allows you to see whether you
• provide sufficient water to plants during dry spells?
are going in the right direction. Another reason might
• allow crops to fully develop healthy root systems?
be to better valuate your soils. If they are in excellent
• suppress root diseases and parasitic nematodes?
health due to many years of good management, your
• have beneficial organisms like mycorrhizal fungi that
land should be worth more when sold or rented than
promote healthy crops?
fields that have been worn out. After all, a healthy soil
• supply nutrients from organic sources that reduce
produces more and allows for reduced purchased inputs.
the need for fertilizer?
Being able to effectively appraise soil health may be an
First ask yourself why you would do a soil health
additional incentive for farmers to invest in good man-
assessment. The most obvious reason is that it allows
agement and build equity in their land.
you to identify specific constraints, such as P deficiency
We can generally approach soil health assessment
Photo courtesy Harold van Es
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chAPter 22 hoW good are your soils?
at four levels of detail: (1) general field observations; (2)
field assessments using qualitative indicators; (3) com-
prehensive soil health tests; and (4) other targeted soil
analyses. We’ll discuss them each in some detail.
gENERAl FIElD ObSERVATIONS
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 wonder-
ing about your own performance during the course of a
day: Do you have less energy than usual? This might be
an indication 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?
Figure 22.1. A soil penetrometer is a useful tool to assess soil compaction.
• Do your crops perform less well than those on neigh-
boring farms with similar soils?
for soil health. The goal of these scorecards is to help
• Do your crops quickly show signs of stress or stunted
you make changes and improve your soil’s health over
growth during wet or dry periods?
time by identifying key limitations or problems.
• Do you notice any symptoms of nutrient deficiencies?
Whenever you try to become more quantitative, you
• Is the soil obviously compacted, or does it plow up
should be aware that measurements naturally vary within
cloddy and take a lot of secondary tillage to prepare
a field or may change over the course of a year. For exam-
a fine seedbed?
ple, if you decide to evaluate soil hardness with a pen-
• Does the soil crust over easily, or do you observe
etrometer (figure 22.1) or metal rod, you should perform
signs of runoff and erosion?
at least ten penetrations in different parts of the field and
• Does it take more power than it used to to run tillage
be aware that your results also depend on the soil mois-
or planting equipment through the soil?
ture conditions at the time of measurement. If you do
• Do you notice increased problems with diseases or
this in June after a dry spring, you may find the soil quite
nutrient stress?
hard. If you go back the next year following a wet spring,
These questions are all indicators of soil health, and
the soil may be much softer. You shouldn’t then conclude
any affirmative answers should prompt you to consider
that your soil’s health has dramatically improved, because
further action.
what you mostly would have measured was the effect of
variable soil moisture on soil strength. Similarly, earth-
FIElD INDIcATORS
worms will be abundant in the plow layer when it’s moist
The next approach is more specific. In several states,
but tend to go deeper into the soil during dry periods.
farmers and researchers have developed “soil health
Make sure you select your locations well. Avoid unusual
scorecards.” The differences in soils and climates sug-
areas (e.g., where machinery turns) and aim to include
gest that there is no uniform scorecard that can be used
areas with higher and lower yields.
everywhere. Nor is there a magic number or index value
This type of variability with time of year or climatic
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chAPter 22 hoW good are your soils?
Table 22.1
Qualitative Soil Health Indicators
Indicator
Best Assessed
Poor
Medium
good
0–1 worms in shovelful of
10+ in top foot of soil. Lots of
Earthworms
Spring/fal .
2–10 in shovelful. Few casts,
Good soil moisture.
top foot of soil. No casts or
casts and holes in tilled clods.
holes.
holes, or worms.
Birds behind tillage.
Organic Matter
Surface color closer to
Topsoil clearly defined,
color
Moist soil.
Topsoil color similar to
subsoil color.
subsoil color.
darker than subsoil.
Organic Matter
Residues
Anytime.
No visible residues.
Some residues.
Noticeable residues.
Few, thick roots. No subsoil
Roots well branched. A few
Roots fully branched and
Root Health
Late spring (rapid growth
extended, reaching into
stage).
penetration. Off color
roots grow through cracks
(staining) inside root.
and reach into subsoil.
subsoil. Root exterior and
interior are white.
Subsurface
Best pre-tillage or post
Wire breaks or bends when
Have to push hard, need fist
Flag goes in easily with
Compaction
harvest.
fingers to twice the depth of
Good soil moisture.
inserting flag.
to push flag in.
plow layer.
Soil Tilth
Looks dead. Like brick or
Somewhat cloddy, balls up,
Soil crumbles well, can slice
Mellowness
Good soil moisture.
concrete, cloddy. Either
requires multiple secondary
through, like cutting butter.
Friability
blows apart or is hard to pull tillage passes for good
drill through.
seedbed.
Spongy when you walk on it.
Large gullies over 2 inches
Few rills or gullies, gullies up
Erosion
After heavy rainfal .
deep joined to others, thin or
No gullies or rills, clear or no
no topsoil, rapid run-off the
to 2 inches deep. Some swift runoff.
color of soil.
runoff, colored water.
Water-Holding
After rainfal .
Plant stress two days after a
Water runs out after a week
Holds water for a long period
Capacity
During growing season.
good rain.
or so.
of time without puddling.
Drainage,
Water sits for a long time,
Water sits for short period of No ponding, no runoff, water
Infiltration
After rainfal .
evaporates more than drains,
moves through soil steadily.
always very wet ground.
time, eventual y drains.
Soil not too wet, not too dry.
Crop Condition
Growing season.
Problem growing throughout Fair growth, spots in field
Normal healthy dark green
(how well it grows) Good soil moisture.
season, poor growth, yellow
different, medium green
color, excellent growth all
or purple color.
color.
season, across field.
pH
Anytime, but at same time of Hard to correct for desired
year each time.
crop.
Easily correctable.
Proper pH for crop.
Nutrient-Holding
Over a five-year period,
Soil tests dropping with more Little change or slow
Soil tests trending up in
Capacity
always at same time of year.
fertilizer applied than crops
relation to fertilizer applied
used.
downward trend.
and crop harvested.
Source: Modified from USDA (1997).
conditions should not discourage you from starting
be more obvious during extreme conditions. It’s a good
to evaluate your soil’s health—just keep in mind the
idea to spend some extra time in your fields during
limitations of certain measurements. Also, you can take
extended wet or dry periods.
advantage of the fact that soil health problems tend to
The following paragraphs present some soil health
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chAPter 22 hoW good are your soils?
indicators developed for scorecards in Maryland,
the soil crusts readily, the aggregates are not very stable
Oregon, and Wisconsin. They are not discussed in any
and break down completely when wet. If the soil doesn’t
special order—all are important to help you assess soil
usually form a crust, you might take a sample of aggre-
health as it relates to growing crops. Table 22.1 provides
gates from the top 3–4 inches of soil from fields that seem
further guidance on good sampling times and interpre-
to have different soil quality. Gently drop a number of
tation of the measurements.
aggregates from each field into separate jars that are half
Soil color is an indicator of soil organic matter con-
filled with water—the aggregates should be covered with
tent, especially within the same general textural class.
water. See whether they hold up or break apart (slake).
Darkness indicates the amount of organic matter (see
You can swirl the water in the cups to see if that breaks up
chapter 2) in the soil. We generally associate black soils
the aggregates. If the broken-up aggregates also disperse
with high quality. However, don’t expect a dramatic
and stay in suspension, you may have an additional prob-
color change when you add organic matter; it may take
lem with high sodium content (a problem that usually
years to notice a difference.
occurs only in arid and semiarid regions).
Crusting, ponding, runoff, and erosion can be
Soil tilth and hardness can be assessed with an
observed from the soil surface. However, their extent
inexpensive penetrometer (the best tool), a tile finder,
depends on whether an intense rainstorm has occurred,
a spade, or a stiff wire (like those that come with wire
and whether a crop canopy or mulch protects the soil.
flags). Tilth characteristics vary greatly during the grow-
These symptoms are a sign of poor soil health, but
ing season due to tillage, packing, settling (dependent
the lack of visible signs doesn’t necessarily mean that
on rainfall), crop canopy closure, and field traffic. It is
the soil is in good health—it must rain hard for these
therefore best to assess soil hardness several times during
signs to occur. Try to get out into the field after heavy
the growing season. If you do it only once, the best time is
rainstorms, especially in the early growing season.
when the soil is moist but not too wet—it should be in the
Crusting can be recognized by a dense layer at the
friable state. Make sure the penetrometer is pushed very
surface that becomes hard after it dries. Ponding can
slowly into the soil (figure 22.1). Also, keep in mind that
be recognized either directly when the water is still in a
stony soils may give you inaccurate results; the soil may
field depression, or afterward by small areas where the
appear hard, but in fact your tool may be hitting a rock.
soil has slaked (that is, aggregates have disintegrated).
Soil is generally considered too hard for root growth
Areas that were ponded often show cracks after drying.
if penetrometer resistance is greater than 300 psi. Note
Slaked areas going down the slope are an indication that
also whether the soil is harder beneath the plow layer. It
runoff and early erosion have occurred. When rills and
is common to measure a dramatic increase in resistance
gullies are present, a severe erosion problem is at hand.
when the bottom of the plow layer is reached. This
Another idea: Put on your raingear and go out during a
indicates subsoil compaction, or a plow pan, which may
rainstorm (not during lightning, of course), and you may
limit deep root growth. It’s difficult to be quantitative
actually see runoff and erosion in action. Compare fields
with tile finders and wire, but the soil is generally too
with different crops, management, and soil types. This
hard when you cannot easily push them in. If you use
might give you ideas about changes you can make to
a spade when the soil is not too wet, evaluate how hard
reduce runoff and erosion.
the soil is and also pay attention to the structure of the
You also can easily get an idea about the stability
soil. Is the plow layer fluffy, and does it mostly consist
of soil aggregates, especially those near the surface. If
of granules of about a quarter inch in size? Or does the
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chAPter 22 hoW good are your soils?
soil dig up in large clumps? A good way to evaluate that
is by lifting a spade full of soil and slowly dropping it
from about waist height. 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.
Soil organisms can be divided into six groups:
bacteria, fungi, protozoa, nematodes, arthropods, and
earthworms. Most are too small to see with the naked eye,
but some larger ones like ants, termites, and earthworms
are easily recognized. They are also important “ecosystem
Figure 22.2. A healthy corn root system with many fine laterals. Compare engineers” that assist the initial organic matter break-with figure 15.3, p. 163.
down that allows other species to thrive, but their general
observed. Since these soil organisms like their environ-
abundance is strongly affected by temperature and
ment to be cool, dark, and moist, they will crawl away
moisture levels in the soil. Their presence is best assessed
when we add heat and light. With a simple desk lamp
in mid-spring, after considerable soil warming, and in
shining on soil in an inverted cut-off plastic soda bottle
mid-fall during moist, but not excessively wet, conditions.
(called a Berlese funnel), you will see the organisms
Just take a full spade of soil from the surface layer and
escape down the funnel, where they can be captured on
sift through it looking for bugs and worms. If the soil is
an alcohol-soaked paper towel (the alcohol keeps them
teeming with life, this suggests that the soil is healthy.
from escaping; see a description of the procedure at
If few invertebrates are observed, the soil may be a poor
http://pnwsteep.wsu.edu/edsteep/SoilInvertebrates/
environment for soil life, and organic matter processing is
Berlese.doc).
probably low. Earthworms are often used as an indicator
Root development can be evaluated by digging
species of soil biological activity (see table 22.1). The most
anytime after the crop has entered its rapid phase of
common worm types, such as the garden and red worms,
growth. Have the roots properly branched, and are they
live in the surface layer when soils are warm and moist
extending in all directions to their fullest potential for
and feed on organic materials in the soil. The long night-
the particular crop? Do they show many fine laterals
crawlers dig near-vertical holes that extend well into the
and mycorrhizal fungal filaments, and will they hold on
subsoil, but they feed on residue at the surface. Look for
to the aggregates when you try to shake them off (figure
the worms themselves as well as their casts (on the sur-
22.2)? Look for obvious signs of problems: short stubby
face, for nightcrawlers) and holes to assess their presence,
roots, abrupt changes in direction when hitting hard
which is typically greatly enhanced in no-till systems. If
layers, signs of rot or other diseases (dark-colored roots,
you dig out a square foot of soil and find ten worms, the
fewer fine roots). Make sure to dig deep enough to get a
soil has a lot of earthworm activity.
full picture of the rooting environment, because many
With a little more effort, nematodes, arthropods,
times there is a hard pan present.
and earthworms can be removed from a soil sample and
The effects of soil health problems on general crop
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chAPter 22 hoW good are your soils?
performance are most obvious during extreme condi-
after tillage, is followed by a long drying period. Soils
tions. That’s why it is worthwhile to occasionally walk
may hardset and completely stop crop growth under
your fields during a wet period (when a number of rains
these circumstances. Extreme conditions are good times
have fallen or just after a long, heavy rain) or during an
to look at crop performance and, at the same time,
extended drought. During prolonged wet periods, poor
evaluate soil hardness and root growth.
soils often remain saturated for an extended time. The
Nutrient deficiency symptoms can appear on
lack of aeration stunts the growth of the crop, and leaf
plant leaves when soils are low in a particular nutrient
yellowing indicates loss of available N by denitrifica-
(table 22.2). However, many nutrient deficiency symp-
tion. This may even happen with high-quality soils if
toms look similar, and they also may vary from crop to
the rainfall is excessive, but it is certainly aggravated by
crop. In addition, typical symptoms may not occur if the
poor soil conditions. Dense, no-tilled soil may also show
plant is suffering from other stresses, including more
greater effects. Purple leaves indicate a phosphorus defi-
than one nutrient deficiency. However, some symp-
ciency and are also often an indirect sign of stress on the
toms on some crops are easy to pick out. For example,
crop. This may be related to soil health but also can be
N-deficient plants are frequently a lighter shade of
brought on by other causes, such as cold temperatures.
green than plants with sufficient N. Nitrogen deficiency
Watch for stunted crop growth during dry peri-
on corn and other grasses appears on the lower leaves
ods and also look for the onset of drought stress—leaf
first as a yellowing around the central rib of the leaf.
curling or sagging (depending on the crop type). Crops
Later, the entire leaf yellows, and leaves further up the
on soils that are in good health generally have delayed
stem may be yellow. However, yellowing of the lower
occurrence of drought stress. Poor soils, especially, may
leaves near maturity is common with some plants. If the
show problems when heavy rainfall, causing soil settling
lower leaves of your corn plant are all nice and green
Table 22.2
Examples of Nutrient Deficiency Symptoms
Nutrient
Deficiency Symptoms
Calcium (Ca)
New leaves (at top of plant) are distorted or irregularly shaped. Causes blossom-end rot.
Nitrogen (N)
General yellowing of older leaves (at bottom of plant). The rest of the plant is often light green.
Magnesium (Mg)
Older leaves turn yellow at edge, leaving a green arrowhead shape in the center of the leaf.
Phosphorus (P)
Leaf tips look burnt, followed by older leaves turning a dark green or reddish purple.
Potassium (K)
Older leaves may wilt and look scorched. Loss of chlorophyll between veins begins at the base, scorching inward from leaf margins.