FroM routine soil tests
. . . the popular mind is still fixed on the idea that a fertilizer is the panacea.
—J.l. hills, C.h. Jones, and C. Cutler, 1908
Although fertilizers and other amendments pur-
recommendations. In this chapter, we’ll go over sources
chased from off the farm are not a panacea to cure all
of confusion about soil tests, discuss N and P soil tests,
soil problems, they play an important role in maintain-
and then examine a number of soil tests to see how the
ing soil productivity. Soil testing is the farmer’s best
information they provide can help you make decisions
means for determining which amendments or fertilizers
about fertilizer application.
are needed and how much should be used.
The soil test report provides the soil’s nutrient and
TAKINg SOIl SAMPlES
pH levels and, in arid climates, the salt and sodium
The usual time to take soil samples for general fertility
levels. Recommendations for application of nutrients
evaluation is in the fall or the spring, before the growing
and amendments accompany most reports. They are
season has begun. These samples are analyzed for pH
based on soil nutrient levels, past cropping, and manure
and lime requirement as well as phosphorus, potas-
management and should be a customized recommenda-
sium, calcium, and magnesium. Some labs also routinely
tion based on the crop you plan to grow.
analyze for organic matter and other selected nutrients,
Soil tests—and proper interpretation of results—are
such as boron, zinc, sulfur, and manganese. Whether
an important tool for developing a farm nutrient man-
you sample a particular field in the fall or in the early
agement program. However, deciding how much fertil-
spring, stay consistent and repeat samples at approxi-
izer to apply—or the total amount of nutrients needed
mately the same time of the year and use the same
from various sources—is part science, part philosophy,
laboratory for analysis. As you will see below, this allows
and part art. Understanding soil tests and how to inter-
you to make better year-to-year comparisons.
pret them can help farmers better customize the test’s
Photo by Dena Leibman
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GuIdeLInes for tAKInG soIL sAMPLes
1. Don’t wait until the last minute. The best time to sample for a general soil test is usual y in the fal . Spring samples should be taken early enough to have the results in time to properly plan nutrient management for the crop season.
2. Take cores from at least fifteen to twenty spots randomly over the field to obtain a representative sample. One sample should not represent more than 10 to 20 acres.
3. Sample between rows. Avoid old fence rows, dead furrows, and other spots that are not representative of the whole field.
4. Take separate samples from problem areas if they can be treated separately.
5. Soils are not homogeneous—nutrient levels can vary widely with different crop histories or topographic settings. Sometimes different colors are a clue to different nutrient contents. Consider sampling some areas separately, even if yields are not noticeably different from the rest of the field.
6. In cultivated fields, sample to plow depth.
7. Take two samples from no-till fields: one to a 6-inch depth for lime and fertilizer recommendations, and one to a 2-inch depth to monitor surface acidity.
8. Sample permanent pastures to a 3- or 4-inch depth.
9. Collect the samples in a clean container.
10. Mix the core samplings, remove roots and stones, and al ow mixed sample to air dry.
11. Fill the soil-test mailing container.
12. Complete the information sheet, giving all of the information requested. Remember, the recommendations are only as good as the information supplied.
13. Sample fields at least every three years and at the same season of the year each time. On higher-value crops annual soil tests will al ow you to fine-tune nutrient management and may al ow you to cut down on fertilizer use.
Note: For a discussion of how to sample to assess the extent of nutrient variability across a large field, see the section “Managing Field Nutrient Variability,” p. 251.
—MODIFIED FROM THE PENN STATE AGRONOMY GUIDE (2007–2008)
AccURAcY OF REcOMMENDATIONS
you will probably increase yield by adding the nutrient.
bASED ON SOIl TESTS
However, adding fertilizer may not increase crop yields
Soil tests and their recommendations, although a criti-
in a field with a low soil test level. The higher yields may
cal component of fertility management, are not 100%
be prevented because the soil test is not calibrated for
accurate. Soil tests are an important tool, but they need
that particular soil (and because the soil had sufficient
to be used by farmers and farm advisors along with
potassium for the crop despite the low test level) or
other information to make the best decision regarding
because of harm caused by poor drainage or compac-
amounts of fertilizers or amendments to apply.
tion. Occasionally, using extra nutrients on a high-test-
Soil tests are an estimate of a limited number of
ing soil increases crop yields. Weather conditions may
plant nutrients based on a small sample, which is
have made the nutrient less available than indicated
supposed to represent many acres in a field. With soil
by the soil test. So it’s important to use common sense
testing, the answers aren’t as certain as we might like
when interpreting soil test results.
them to be. A low-potassium soil test indicates that
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SOURcES OF cONFUSION AbOUT SOIl TESTS
index (for example, all nutrients are expressed on a scale
People may be easily confused about the details of soil
of 1 to 100). In addition, some labs report phosphorus
tests, especially if they have seen results from more
and potassium in the elemental form, while others use
than one soil testing laboratory. There are a number of
the oxide forms, P2O5 and K2O.
reasons for this, including the following:
Most testing labs report results as both a number
• laboratories use a variety of procedures;
and a category such as low, medium, optimum, high,
• labs report results differently; and
and very high. However, although most labs consider
• different approaches are used to make recommenda-
high to be above the amount needed (the optimum),
tions based on soil test results.
some labs use optimum and high interchangeably. If the
significance of the various categories is not clear on your
Varied Lab Procedures
report, be sure to ask. Labs should be able to furnish you
One of the complications with using soil tests to help
with the probability of getting a response to added fertil-
determine nutrient needs is that testing labs across the
izer for each soil test category.
country use a wide range of procedures. The main differ-
ence among labs is the solutions they use to extract the
Different Recommendation Systems
soil nutrients. Some use one solution for all nutrients,
Even when labs use the same procedures, as is the case
while others will use one solution to extract potassium,
in most of the Midwest, different approaches to making
magnesium, and calcium; another for phosphorus; and
recommendations lead to different amounts of recom-
yet another for micronutrients. The various extracting
mended fertilizer. Three different systems are used to
solutions have different chemical compositions, so the
make fertilizer recommendations based on soil tests: (1)
amount of a particular nutrient that lab A extracts may be
the sufficiency-level system; (2) the buildup and main-
different from the amount extracted by lab B. Labs fre-
tenance system, and (3) the basic cation saturation ratio
quently have a good reason for using a particular solution,
system (only used for Ca, Mg, and K).
however. For example, the Olsen test for phosphorus (see
The sufficiency-level system suggests that there is
table 21.1, p. 242) is more accurate for high-pH soils in
a point, the sufficiency or critical soil test value, above
arid and semiarid regions than the various acid-extract-
which there is little likelihood of crop response to an
ing solutions commonly used in more humid regions.
added nutrient. Its goal is not to produce the highest
Whatever procedure the lab uses, soil test levels must be
yield every year, but rather to produce the highest aver-
calibrated with the crop response to added nutrients. For
age return over time from using fertilizers. Experiments
example, do yields increase when you add phosphorus to
that relate yield increases with added fertilizer to soil
a soil that tested low in P? In general, university and state
test level provide much of the evidence supporting this
labs in a given region use the same or similar procedures
approach. As the soil test level increases from optimum
that have been calibrated for local soils and climate.
(or medium) to high, yields without adding fertilizer are
close to the maximum obtained by adding more fertilizer
Reporting Soil Test Levels Differently
(figure 21.1). Of course, farmers should be aiming for the
Different labs may report their results in different ways.
maximum economic yields, which are slightly below the
Some use parts per million (10,000 ppm = 1%); some
highest possible yields, as indicated in figure 21.1.
use pounds per acre (usually by using parts per two
The buildup and maintenance system calls for build-
million, which is twice the ppm level); and some use an
ing up soils to high levels of fertility and then keeping
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chAPter 21 getting the Most FroM routine soil tests
magnesium, and potassium—usually
K soil test
the dominant cations on the CEC—
are in a particular balance. This
system was developed out of work
100
by Firman E. Bear in New Jersey
and William A. Albrecht in Missouri
90
and has become accepted by many
farmers despite a lack of modern
research supporting the system (see
80
“The Basic Cation Saturation Ratio
System,” p. 251). Few university test-
% of maximum relative yield
70
ing laboratories use this system, but a
number of private labs use it because
60
many “alternative” and organic farm-
ers believe that it is valuable. This
very low
low
optimum
high
(medium)
system calls for calcium to occupy
about 60–80% of the CEC, magne-
Figure 21.1. Percent of maximum yield with different K soil test levels.
sium to be 10–20%, and potassium
them there by applying enough fertilizer to replace
2–5%. This is based on the notion that if the percent
nutrients removed in harvested crops. This approach
saturation of the CEC is good, there will be enough of
usually recommends more fertilizer than the sufficiency
each of these nutrients to support optimum crop growth.
system. It is used mainly for phosphorus, potassium, and
When using the BCSR, it is important to recognize its
magnesium recommendations; it can also be used for cal-
practical as well as theoretical flaws. For one, even when
cium when high-value vegetables are being grown on low-
the ratios of the nutrients are within the recommended
CEC soils. However, there may be a justification for using
crop guidelines, there may be such a low CEC (such as in
the buildup and maintenance approach for phosphorus
a sandy soil that is very low in organic matter) that the
and potassium—in addition to using it for calcium—on
amounts present are insufficient for crops. If the soil has
high-value crops because: (1) the extra costs are such a
a CEC of only 2 milliequivalents per 100 grams of soil,
small percent of total costs; and (2) when weather is sub-
for example, it can have a “perfect” balance of Ca (70%),
optimal (cool and damp, for example), this approach may
Mg (12.5%), and K (3.5%) but contain only 560 pounds
occasionally produce a higher yield that would more than
Ca, 60 pounds of Mg, and 53 pounds of K per acre to a
cover the extra expense of the fertilizer. If you use this
depth of 6 inches. Thus, while these elements are in a
approach, you should pay attention to levels of phospho-
supposedly good ratio to one another, there isn’t enough
rus; adding more P when levels are already optimum can
of any of them. The main problem with this soil is a low
pose an environmental risk.
CEC; the remedy is to add a lot of organic matter over a
The basic cation saturation ratio system (BCSR;
period of years, and, if the pH is low, it should be limed.
also called the base ratio system), a method for esti-
The opposite situation also needs attention. When
mating calcium, magnesium, and potassium needs, is
there is a high CEC and satisfactory pH for the crops
based on the belief that crops yield best when calcium,
being grown, even though there is plenty of a particular
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chAPter 21 getting the Most FroM routine soil tests
nutrient, the cation ratio system may call for adding
potassium levels are needed as the soil CEC increases.
more. This can be a problem with soils that are naturally
These are really hybrids of the sufficiency and cation
moderately high in magnesium, because the recommen-
ratio systems. At least one state university lab uses the
dations may call for high amounts of calcium and potas-
sufficiency system for potassium and a cation ratio
sium to be added when none are really needed—wasting
system for calcium and magnesium. Also, some labs
the farmer’s time and money.
assume that soils will not be tested annually. The recom-
Research indicates that plants do well over a broad
mendation that they give is, therefore, produced by the
range of cation ratios, as long as there are sufficient sup-
sufficiency system (what is needed for the crop) with a
plies of potassium, calcium, and magnesium. However,
certain amount added for maintenance. This is done to
the ratios are sometimes out of balance. For example,
be sure there is enough fertility in the following year.
when magnesium occupies more than 50% of the CEC in
soils with low organic matter and low aggregate stabil-
Plant Tissue Tests
ity, using gypsum (calcium sulfate) may help restore
Soil tests are the most common means of assessing fer-
aggregation because of the extra calcium as well as the
tility needs of crops, but plant tissue tests are especially
higher level of dissolved salts. As mentioned previously,
useful for nutrient management of perennial crops,
liming very acidic soils sometimes results in decreased
such as apples, blueberries, citrus and peach orchards,
potassium availability, and this would be apparent when
and vineyards. For most annuals, including agronomic
using the cation ratio system. The sufficiency system
and vegetable crops, tissue testing, though not widely
would also call for adding potassium, because of the low
used, can help diagnose problems. The small sampling
potassium levels in these limed soils.
window available for most annuals and an inability to
The sufficiency-level approach is used by most fertil-
effectively fertilize them once they are well established,
ity recommendation systems for potassium, magnesium,
and calcium, as well as phosphorus and nitrogen (where
N tests are available). It generally calls for lower applica-
To estimate the percentages of the various cations on
tion rates for potassium, magnesium, and calcium and is
the CEC, the amounts need to be expressed in terms
more consistent with the scientific data than the cation
of quantity of charge. Some labs give concentration
ratio system. The cation ratio system can be used to
by both weight (ppm) and charge (me/100g). If you
reduce the chance of nutrient deficiencies, if interpreted
want to convert from ppm to me/100g, you can do it
with care and common sense—not ignoring the total
as follows:
amounts present and paying attention to the implica-
tions of a soil’s pH. Using this system, however, will
(Ca in ppm)/200 = Ca in me/100g
usually mean applying more nutrients than suggested by
(Mg in ppm)/120 = Mg in me/100g
the sufficiency system—with a low probability of actually
(K in ppm)/390 = K in me/100g
getting a higher yield or better crop quality.
Labs sometimes use a combination of these systems,
As discussed in chapter 20, adding up the amount of
something like a hybrid approach. Some laboratories
charge due to calcium, magnesium, and potassium
that use the sufficiency system will have a target for
gives a very good estimate of the CEC for most soils
magnesium but then suggest adding more if the potas-
above pH 5.5.
sium level is high. Others may suggest that higher
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chAPter 21 getting the Most FroM routine soil tests
except for N during early growth stages, limit the useful-
ness of tissue analysis for annual crops. However, leaf
recoMMendAtIon sYsteM coMPArIson
petiole nitrate tests are sometimes done on potato and
Most university testing laboratories use the suffi-
sugar beets to help fine-tune in-season N fertilization.
ciency-level system, but some make potassium or
Petiole nitrate is also helpful for N management of
magnesium recommendations by modifying the
cotton and for help managing irrigated vegetables, espe-
sufficiency system to take into account the portion
cially during the transition from vegetative to reproduc-
of the CEC occupied by the nutrient. The buildup and
tive growth. With irrigated crops, particularly when the
maintenance system is used by some state university
drip system is used, fertilizer can be effectively delivered
labs and many commercial labs. An extensive evalua-
to the rooting zone during crop growth.
tion of different approaches to fertilizer recommenda-
tions for agronomic crops in Nebraska found that the
What Should You Do?
sufficiency-level system resulted in using less fertilizer
After reading the discussion above you may be some-
and gave higher economic returns than the buildup
what bewildered by the different procedures and ways
and maintenance system. Studies in Kentucky, Ohio,
of expressing results, as well as the different recom-
and Wisconsin have indicated that the sufficiency sys-
mendation approaches. It is bewildering. Our general
tem is superior to both the buildup and maintenance
suggestions of how to deal with these complex issues are
and cation ratio systems.
as follows:
1. Send your soil samples to a lab that uses tests evalu-
ated for the soils and crops of your state or region.
Continue using the same lab or another that uses the
with one going to your state-testing laboratory. In
same system.
general, the recommendations from state labs call for
2. If you’re growing low value-per-acre crops (wheat,
less, but enough, fertilizer. If you are growing crops
corn, soybeans, etc.), be sure that the recommenda-
over a large acreage, set up a demonstration or experi-
tion system used is based on the sufficiency ap-
ment in one field by applying the fertilizer recom-
proach. This system usually results in lower fertilizer
mended by each lab over long strips and see if there
rates and higher economic returns for low-value
is any yield difference. A yield monitor for grain crops
crops. (It is not easy to find out what system a lab
would be very useful for this purpose. If you’ve never
uses. Be persistent, and you will get to a person who
set up a field experiment before, you should ask your
can answer your question.)
extension agent for help. You might also find SARE’s
3. Dividing a sample in two and sending it to two labs
brochure How to Conduct Research on Your Farm or
may result in confusion. You will probably get differ-
Ranch of use (see “Sources” at the end of the chapter).
ent recommendations, and it won’t be easy to figure
4. Keep a record of the soil tests for each field, so that
out which is better for you, unless you are willing to
you can track changes over the years (figure 21.2). If
do a comparison of the recommendations. In most
records show a buildup of nutrients to high levels,
cases you are better off staying with the same lab and
reduce nutrient applications. If you’re drawing nutri-
learning how to fine-tune the recommendations for
ent levels down too low, start applying fertilizers or
your farm. If you are willing to experiment, however,
off-farm organic nutrient sources. In some rotations,
you can send duplicate samples to two different labs,
such as the corn–corn–four years of hay shown at
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chAPter 21 getting the Most FroM routine soil tests
the bottom of figure 21.2, it makes sense to build up
proper management
nutrient levels during the corn phase and draw them
high
down during the hay phase.
SOIl TESTINg FOR NITROgEN
Soil samples for nitrogen tests are usually taken at a dif-
low
ferent time and using a different method than samples
for the other nutrients (which are typically sampled to
over fertilization
plow depth in the fall or spring).
high
In the humid regions of the U.S. there was no reli-
able soil test for N availability before the mid-1980s. The
optimal soil test range
nitrate test commonly used for corn in humid regions
low
was developed during the 1980s in Vermont. It is usually
called the pre-sidedress nitrate test (PSNT) but is also
called the late spring nitrate test (LSNT) in parts of