|
Getting the Most From 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 purchased
from off the farm are not a panacea to cure all soil problems, they
play an important role in maintaining soil productivity. Soil testing
is the farmer's best means for determining which amendments or fertilizers
are needed and how much should be used.
The soil test report provides the soil's nutrient
and pH levels and, in arid climates, the salt and sodium levels.
Recommendations for application of nutrients and amendments accompany
most reports. They are based on soil nutrient levels, past cropping
and manure management, and should be a customized recommendation
based on the crop you plan to grow.
Soil tests and proper interpretation of results are
a very important management tool for developing a farm nutrient
management program. However, deciding how much fertilizer to apply
or the total amount of nutrients needed from various sources is
part science, part philosophy, and part art. Understanding soil
tests and how to interpret them can help farmers better customize
the test's recommendations. In this chapter, we'll go over sources
of confusion about soil tests, discuss N and P soil tests, and then
examine a number of soil tests to see how the information they provide
can help you make decisions about fertilizer application.
Taking Soil Samples
The usual time to take soil samples for general fertility
evaluation is in the fall or in the spring, before the growing season
has begun. These samples are analyzed for pH and lime requirement
as well as phosphorus, potassium, and magnesium. Some labs also
routinely analyze for selected micronutrients, such as boron, zinc,
and manganese.
Accuracy of Recommendations Based on Soil Tests
Soil tests and their recommendations, although a critical
component of fertility management, are not 100 percent accurate.
Soil tests are an important tool, but need to be used by farmers
and farm-advisors along with other information to make the best
decision regarding amounts of fertilizers or amendments to apply.
Soil tests are an estimate of a limited number of
plant nutrients, based on a small sample, which is supposed to represent
many acres in a field. With soil testing, the answers aren't quite
as certain as we might like them. A low potassium soil test indicates
that you will probably increase yield by adding the nutrient.
However, adding fertilizer may not increase crop yields in a field
with a low soil test level. The higher yields may be prevented because
the soil test is not calibrated for that particular soil (and the
soil had sufficient potassium for the crop despite the low test
level) or because of harm caused by poor drainage or compaction.
Occasionally, using extra nutrients on a high-testing soil increases
crop yields. Weather conditions may have made the nutrient less
available than indicated by the soil test. So, it's important to
use common sense when interpreting soil test results.
Guidelines for Taking Soil
Samples
1. Don't wait until the last minute. The best
time to sample for a general soil test is usually in the fall.
Spring samples should be taken early enough to have results
in time to properly plan nutrient management for the crop
season.
2. Take cores from at least 15 to 20 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. In cultivated fields, sample to plow depth.
6. Take two samples from no-till fields: one to a 6-inch depth
for lime and fertilizer recom mendations, and one to a 2-inch
depth to monitor surface acidity.
7. Sample permanent pastures to a 3- to 4-inch depth.
8. Collect the samples in a clean container.
9. Mix the core samplings, remove roots and stones, and allow
to air dry.
10. Fill the soil-test mailing container.
11. Complete the information sheet, giving all of the information
requested. Remember, the recommendations are only as good
as the information supplied.
12. Sample fields at least every three years. Annual soil
tests will allow you to fine-tune nutrient management and
may allow you to cut down on fertilizer use.
Modified from The PennState Agronomy Guide, 1999.
|
Sources of Confusion About Soil Tests
People may be easily confused about the details of
soil tests, especially if they have seen results from more than
one soil testing laboratory. There are a number of reasons for this,
including:
- laboratories use a variety of procedures;
- labs report results differently; and
- different approaches are used to make recommendations based
on soil test results.
Labs Use Varied Procedures
One of the complications with using soil tests to help determine
nutrient needs is that testing labs across the country use a wide
range of procedures. The main difference among labs is the solutions
they use to extract the soil nutrients. Some use one solution for
all nutrients, while others will use one solution to extract potassium,
magnesium and calcium; another for P; and yet another for micronutrients.
The various extracting solutions have different chemical compositions,
so the amount of a particular nutrient that lab A extracts may be
very different from the amount extracted by lab B. However, there
are frequently good reasons to use a particular solution. For example,
the Olsen test for phosphorus (see below) is more accurate for high-pH
soils in arid and semi-arid regions than are the various acid-extracting
solutions commonly used in more humid regions. Whatever procedure
the lab uses, soil test levels must be calibrated with crop yield
response to added nutrients. For example, do the yields really increase
when you add phosphorus to a soil that tests low in P? In general,
university or state labs in a given region use the same or similar
procedures that have been calibrated for local soils and climate.
Labs Report Soil Test Levels Differently
Different labs may report their results in different ways. Some
use part per million (10,000 ppm = 1 percent); others use lbs./acre
(they do this usually by using part per two million, which
is twice the part per million level); and others use an index (for
example, all nutrients are expressed on a scale of 1 to 100). In
addition, some labs report phosphorus and potassium in the elemental
form, while others use the oxide forms, P2O5
and K2O.
Most testing labs report results as both a number
and a category, such as low, medium, optimum, high, very high. However,
although most labs consider high to be above the amount needed (the
amount needed is called optimum), some labs use optimum and
high interchangeably. If the significance of the various
categories is not clear on your report, be sure to ask. Labs should
be able to furnish you with the probability of getting a response
to added fertilizer for each soil test category.
Different Recommendation Systems
Even when labs use the same procedures, as is the case in most of
the Midwest, different approaches to making recommendations lead
to different amounts of recommended fertilizer. Three different
philosophies are used to make fertilizer recommendations based on
soil tests. One approach the sufficiency level system
suggests there is a point, the sufficiency or critical soil test
value, above which there is little likelihood of response to an
added nutrient. Its goal is not to produce the highest yield every
year, but, rather, to produce the highest average return over time
from using fertilizers. Experiments that relate yield increases
with added fertilizer to soil test level provide much of the evidence
supporting this approach. As the soil test level increases from
optimum to high, yields without adding fertilizer are closer to
the maximum obtained by adding fertilizer (figure 19.1). Of course,
farmers should be shooting for the maximum economic yields,
which are slightly below the highest possible yields.
Another approach used by soil test labs the build-up
and maintenance system calls for building up soils to
high levels of fertility and then keeping them there by applying
enough fertilizer to replace nutrients removed in harvested crops.
It is used mainly for phosphorus, potassium, and magnesium recommendations.
|
Figure 19.1 Percent of maximum
yield with different soil test levels. |
The basic cation saturation ratio
system, a method of estimating calcium, magnesium, and potassium
needs, is based on the belief that crops yield best when calcium
(Ca++), magnesium (Mg++), and potassium (K+)
usually the dominant cations on the CEC are in a particular balance.
Although there are different versions of this system, most call
for calcium to occupy about 60 to 80 percent of the CEC, whereas
magnesium should be from 10 to 20 percent and potassium from 2 to
5 percent of the CEC. Great care is needed when using the base cation
saturation ratio system. For example, the ratios of the nutrients
can be within the guidelines, but there may be such a low CEC (such
as with a sandy soil that is very low in organic matter), that the
amounts present are insufficient for crops. In addition, when there
is a high CEC, there may be plenty of the nutrient, but the cation
ratio system will call for adding more. This can be a problem with
soils that are naturally high in magnesium, because the recommendations
may call for high amounts of calcium and potassium to be added when
none are really needed.
Research indicates that plants do well over a broad
range of cation ratios, as long as there are sufficient supplies
of potassium, calcium, and magnesium. However, there are occasions
when the calcium-magnesium-potassium ratios are very out of balance.
For example, when magnesium occupies more than 50 percent of the
CEC in soils with low aggregate stability, using calcium sulfate
may help restore aggregation. As mentioned previously, liming very
acidic soils sometimes results in decreased potassium availability
and this would be apparent when using the cation ratio system.
The sufficiency system would also call for
adding potassium because of the low potassium levels in these very
acid soils. The sufficiency level approach is used by most
fertility recommendation systems for potassium, magnesium, and calcium.
It generally calls for lower application rates and is more consistent
with the scientific data than the cation ratio system. The cation
ratio system can be used successfully, if interpreted with care
and common sense not ignoring the total amounts present.
Labs sometimes use a combination of these systems,
something like a hybrid approach. Some laboratories that use the
sufficiency system will have a target for magnesium, but then suggest
adding more if the potassium level is high. Others suggest that
higher potassium levels are needed as the soil CEC increases. These
are really hybrids of the sufficiency and cation ratio systems.
At least one lab uses the sufficiency system for potassium and a
cation ratio system for calcium and magnesium. Also, some labs assume
that soils will not be tested annually. The recommendation
that they give is, therefore, a combination of the sufficiency system
(what is needed for this crop) with a certain amount added for maintenance.
This is done to be sure there is enough fertility in the following
year.
Crop Value, Fertilizer Rates, and Recommendation
System
The value of your crop can have a major impact on the economics
of over-applying fertilizer. As a general rule, the lower the per
acre value of your crop, the greater the economic penalty for applying
extra fertilizer (see box on p. 183). Farmers growing agronomic
crops should take special care not to over-apply fertilizer.
To estimate the percentages of the various cations
on the CEC, the amounts need to be expressed in terms of quantity
of charge. Some labs give both concentration by weight (ppm)
and by charge (me/100g). If you want to convert from ppm to
milliequivalent per 100 grams (me/100g), you can do it as follows:
(Ca in ppm)/200 = Ca in me/100g
(Mg in ppm)/120 = Mg in me/100g
(K in ppm)/390 = K in me/100g
As discussed previously (chapter 18),
adding up the amount of charge due to calcium, magnesium, and
potassium gives a very good estimate of the CEC for most soils
for most soils above pH 5.5. |
As the soil test level of a particular nutrient increases,
there is less chance that adding the nutrient will result in a greater
yield. However, it may be worth adding fertilizer to high-value
crops grown on soils with the same test levels that call for no
fertilizer use low-value crops (figure 19.2).
This difference should be reflected in the recommendations provided
by soil testing laboratories.
Recommendation System Comparison
Most university testing laboratories use the
sufficiency level system, but some make potassium or magnesium
recommendations by modifying the sufficiency system to take
into account the portion of the CEC occupied by the nutrient.
The build-up and maintenance system is used by some state
university labs and many commercial labs. An extensive evaluation
of different approaches to fertilizer recommendations for
agronomic crops in Nebraska found that using the sufficiency
level system resulted in using less fertilizer and gave higher
economic returns than the build-up and maintenance system.
Other studies in Kentucky, Ohio, and Wisconsin indicate that
the sufficiency system is superior to the build-up and maintenance
or cation ratio systems. |
|
Figure 19.2 The chance of an economic return
at different soil test levels. |
Plant Tissue Tests
Soil tests are the most common means of assessing fertility needs
of crops, but plant tissue tests are especially useful for nutrient
management of perennial crops, such as apple, citrus and peach orchards,
and vineyards. For most annuals, including agronomic and vegetable
crops, tissue testing is not widely used, but can help diagnose
problems. The small sampling window available for most annuals and
an inability to effectively fertilize them once they are well established,
except for N during early growth stages, limits the usefulness of
tissue analysis for annual crops. However, leaf petiole nitrate
tests are sometimes done on potato and sugar beets to help fine-tune
in-season N fertilization. Petiole nitrate is also helpful for N
management of cotton and for help managing irrigated vegetables,
especially during the transition from vegetative to reproductive
growth. With irrigated crops, in particular when the drip system
is used, fertilizer can be effectively delivered to the rooting
zone during crop growth.
What Should You Do?
After reading the discussion above you may be somewhat bewildered
by the different procedures and ways of expressing results, as well
as the different recommendation approaches. The fact is that it
is bewildering! Our general suggestions of how to deal with these
complex issues are: 1. Send your soil samples to a lab that uses
tests evaluated for the soils and crops of your state or region.
Continue using the same lab or another that uses the same procedures
and recommendation system.
2. If you're growing low value-per-acre crops (wheat,
corn, soybeans, etc.), be sure that the recommendation system used
is based on the sufficiency approach. This system usually results
in lower fertilizer rates and higher economic returns for low value
crops. [It is not easy to find out what system a lab uses. Be persistent
and you will get to a person that can answer your question.]
3. Dividing the same sample in two and sending it
to two labs may result in confusion. You will probably get different
recommendations and it won't be easy to figure out which is better
for you, unless you are willing to do a comparison of recommendations.
In most cases you are better off staying with the same lab and learning
how to fine-tune recommendations for your farm. However, if you
are willing to experiment, you can send duplicate samples to two
different labs, with one going to your state-testing laboratory.
In general, the recommendations from these labs call for less, but
enough, fertilizer. If growing crops over large acreage, set up
a demonstration or experiment in one field where you apply the fertilizer
recommended by each lab over long strips and see if there is any
yield difference. A yield monitor for grain crops would be very
useful for this purpose. If you've never set up a field experiment
before, you should ask your extension agent for help, and might
find the brochure How to Conduct Research on Your Farm or Ranch
of use (see Sources at end of chapter).
4. Keep a record of soil tests for each field, so
that you can track changes over the years (figure
19.3). If records show a build up of nutrients to high levels,
reduce nutrient applications. If you're drawing nutrient levels
down too low, start applying fertilizers or off-farm organic nutrient
sources. In some rotations, such as the corn-corn-4 years of hay
shown at the bottom of figure 19.3, it makes sense to build up nutrient
levels during the corn phase and draw them down during the hay phase.
Crop Value
and Care with Fertilizer Rates
Most agronomic crops grown on large acreages
are worth around $200 to $400 per acre and the fertilizer
used may represent 30 to 40 percent of out-of-pocket growing
costs. So, if you use 100 lbs. of N you don't need, that's
about $30/acre and represents a major economic loss. Some
years ago, one of the authors worked with two brothers who
operated a dairy farm in northern Vermont that had high soil
test levels of N, P, and K. Despite his recommendation that
no fertilizer was needed, the normal practice was followed
and $70 per acre worth of fertilizer N, P, and K was applied
to their 200 acres of corn. The yields on 40 feet wide, no-fertilizer
strips that they left in each field were the same as where
fertilizer had been applied, so the $14,000 for fertilizer
was wasted.
When growing fruit or vegetable crops worth thousands of dollars
per acre fertilizers represent about 1 percent of the value
of the crop and 2 percent of the costs. But when growing specialty
crops (medicinal herbs, certain organic vegetables for direct
marketing) worth over $10,000 per acre, the cost of fertilizer
is dwarfed by other costs, such as hand labor. A waste of
$30/acre in unneeded nutrients for these crops would cause
a minimal economic penalty assuming you maintain a reasonable
balance between nutrients but there may be environmental reasons
against applying too much fertilizer. However, there may be
a justification for using the build-up and maintenance approach
for phosphorus and potassium on high-value crops because:
a) the extra costs are such a small percent of total costs
and b) there may occasionally be a higher yield because of
this approach that would more than cover the extra expense
of the fertilizer. |
Soil Testing for N
Soil samples for nitrogen tests are usually taken
at a different time and using a different method than for the other
nutrients (which are typically sampled to plow depth in the fall
or spring). Before the mid-1980s, there was no reliable soil test
for N availability in the humid regions of the US.
|
Figure 19.3 Soil test phosphorus and
potassium trends under different fertility management regimes.
Modified from The PennState Agronomy Guide, 1999. |
The nitrate test commonly used for corn in humid regions
was developed during the 1980s in Vermont. It is usually called
the Pre-Sidedress Nitrate Test (PSNT), but also goes under other
names (see box). All of these names refer to the same test a soil
sample is taken to 1 foot depth, when corn is between 6 inches and
1 foot tall. The original idea behind the test was to wait as long
as possible before sampling, because soil and weather conditions
in the early growing season may reduce or increase N availability
for the crop later in the season. After the corn is 1 foot tall,
it is difficult to get samples to a lab and back in time to apply
any needed sidedress N fertilizer. The PSNT is now used on both
field corn and sweet corn and research is underway in the northeastern
U.S. to extend its use to pumpkins and cabbage. Although the PSNT
is widely used, there are some situations, such as the sandy coastal
plains soils of the deep south, where it is not very accurate.
Different approaches to using the PSNT work for different
farms. In general, using the soil test allows a farmer to avoid
adding excess amounts of "insurance fertilizer." Two contrasting
examples follow:
- For farms using rotations with legume forages and applying
animal manures regularly (so there's a lot of active soil organic
matter), the best way to use the test to your advantage is to
apply only the amount of manure necessary to provide sufficient
N to the plant. The PSNT will indicate whether or not you need
to side-dress any additional N fertilizer. It will also show
you whether you've done a good job of estimating N availability
from manures.
- For farms growing cash grains without using legume cover
crops, it's best to apply a conservative amount of fertilizer
N before planting and then use the test to see if more is needed.
This is especially important in regions where rainfall cannot
always be relied upon to quickly bring fertilizer into contact
with roots. The PSNT test provides a backup and allows the farmer
to be more conservative with preplant applications, knowing
that there is a way to make up any possible deficit.
Other nitrogen soil tests. In the drier parts
of the country, a nitrate soil test, requiring samples to 2 feet
or more, has been used with success since the 1960s. The deep-soil
samples can be taken in the fall or early spring, before the growing
season, because of low leaching and denitrification losses and low
levels of active organic matter (so hardly any nitrate is mineralized
from organic matter). Soil samples can also be taken at the same
time for analysis for other nutrients and pH. A few states in the
upper Midwest offer a preplant nitrate test, which calls for sampling
to 2 feet in the spring.
Names Used For The PSNT
Pre-Sidedress Nitrate Test (PSNT)
Magdoff PSNT
Late Spring Nitrate Test (LSNT)
June Nitrate Test
Vermont N Soil Test |
Soil Testing for P
Soil test procedures for phosphorus are different
than for nitrogen. When testing for phosphorus, the soil is usually
sampled to plow depth at a different time in the fall or in the
early spring before tillage and the sample is usually analyzed for
phosphorus, potassium, sometimes other nutrients (such as calcium,
magnesium, and micronutrients) and pH. The methods used to estimate
available P vary from region to region, and sometimes, from state
to state within a region (Table 19.1). Although the relative test
value for a given soil is usually similar when using different soil
tests (for example, a high P-testing soil by one procedure is generally
also high by another procedure), the actual numbers can be different
(table 19.2).
The various soil tests for P take into account a large
portion of the available P contained in recently applied manures
and the amount that will become available from the soil minerals.
However, if there is a large amount of active organic matter in
your soils from crop residues or manure additions made in previous
years, there may well be more available P for plants than indicated
by soil test. (On the other hand, the PSNT reflects the amount of
N that may become available from decomposing organic matter.)
Testing Soils for Organic Matter
A word of caution when comparing your soil test organic
matter levels with those discussed in this book. If your laboratory
reports organic matter as "weight loss" at high temperature,
the numbers may be higher than if the lab uses the traditional wet
chemistry method. A soil with 3 percent organic matter by wet chemistry
might have a weight-loss value of between 4 and 5 percent. Most
labs use a correction factor to approximate the value you would
get by using the wet chemistry procedure. Although either method
can be used to follow changes in your soil, when you compare soil
organic matter of samples run in different laboratories, it's best
to make sure the same methods were used.
There is now a laboratory that will determine various
forms of living organisms in your soil. Although it costs quite
a bit more than traditional testing for nutrients or organic matter,
you can find out the amount (weight) of fungi and bacteria in a
soil, as well as analysis for other organisms. (See the Resources
section at the back of the book for laboratories that run tests
in addition to basic soil fertility analysis.)
Unusual Soil Tests?
From time to time we've come across unusual
soil test results. A few examples and their typical causes
are given below.
Very high phosphorus levels. High poultry
or other manure application over many years.
Very high salt concentration in humid region.
Recent application of large amounts of poultry manure or immediately
adjacent to road where de-icing salt was used.
Very high pH and high calcium levels, relative
to potassium and magnesium. Large amounts of lime-stabilized
sewage sludge were used.
Very high calcium levels given the soil's
texture and organic matter content. Using an acid solution,
such as the Morgan, Mehlich 1, or Mehlich 3, to extract soils
containing free limestone causes some of the lime to dissolve,
giving artificially high calcium test levels. |
Interpreting Soil Test Results
Below are five soil test examples, including discussion
about what they tell us and the types of practices that should be
followed. Suggestions are provided for conventional farmers and
organic producers. These are just suggestions there are other satisfactory
ways to meet the needs for crops growing on these soils. The soil
tests were run by different procedures, to give examples from around
the US. Interpretations for a number of commonly used soil tests
relating test levels to general fertility categories are given later
in the chapter (tables 19.3 and 19.4). Many labs estimate the cation
exchange capacity that would exist at pH 7 (or even higher). Because
we feel that the soil's current CEC is of most interest (see chapter
18), the CEC is estimated by summing the exchangeable bases.
The more acidic a soil, the greater the difference between its current
CEC and the CEC it would have near pH 7.
Following the five soil tests is a section on modifying
recommendations for particular situations.
Soil Test #1 (New England)
Field name: North
Sample date: September (PSNT sample taken the following June)
Soil type: loamy sand
Manure added: none
Cropping history: mixed vegetables
Crop to be grown: mixed vegetables
What can we tell about soil #1 based on the soil
test?
- It is too acidic for most agricultural crops, so lime is
needed.
- Phosphorus is low, as are potassium, magnesium, and calcium.
All should be applied.
- This low organic matter soil is probably also low in active
organic matter (indicated by the low PSNT test, see table 19.4a)
and will need an application of nitrogen. (The PSNT is done
during the growth of the crop, so it is difficult to use manure
to supply extra N needs indicated by the test.)
- The coarse texture of the soil is indicated by the combination
of low organic matter and low CEC.
Soil Test #1 Report Summary* |
|
lbs/Acre |
PPM |
Soil Test Category |
Recommendation Summary |
P |
4 |
2 |
low |
5070 lbs. P2O5/acre |
K |
100 |
50 |
low |
150200 lbs. K2O/acre |
Mg |
60 |
30 |
low |
lime (see below) |
Ca |
400 |
200 |
low |
lime (see below) |
|
pH |
5.4 |
|
|
2 tons dolomitic limestone/acre |
CEC ** |
1.4 me/100 g |
|
|
|
OM |
1% |
|
|
add organic matter: compost, cover crops,
animal manures |
|
PSNT |
|
5 |
low |
sidedress 80 100 lbs. N/acre |
* Nutrients extracted by modified
Morgan's Solution (see table 19.3a for interpretations). |
** CEC by sum of bases. The
estimated CEC would probably double if "exchange acidity"
were determined and added to the sum of bases. |
|
|
|
|
|
General recommendations:
1. Apply dolomitic limestone, if available, in the fall at about
2 tons/acre (and work it into the soil and establish a cover crop
if possible). This will take care of the calcium and magnesium needs
at the same time the soil's pH is increased.
2. Because no manure is to be used after the test was taken, broadcast
significant amounts of phosphate (probably around 50 to 70 lbs.
phosphate (P2O5)/acre) and potash (around
150 to 200 lbs. potash (K2O)/acre). Some phosphate and
potash can also be applied in starter fertilizer (band applied at
planting). Usually N is also included in starter fertilizer, so
it might be reasonable to use about 300 lbs. of a 10-10-10 fertilizer,
which will apply 30 lbs. of N, 30 lbs. of phosphate, and 30 lbs.
of potash per acre. If that rate of starter is to be used, then
broadcast 400 lbs. per acre of a 0-10-30 bulk blended fertilizer.
The broadcast plus the starter will supply 30 lbs. of N, 70 lbs.
of phosphate, and 150 lbs. of potash per acre.
3. If only calcitic (low magnesium) limestone is available, use
Sul-Po-Mag as the potassium source in the bulk blend to help supply
magnesium.
4. Nitrogen should be sidedressed at around 80 to 100 (or more)
lbs./acre for N-demanding crops, such as corn or tomatoes. About
300 lbs. of ammonium nitrate or 220 lbs. of urea per acre will supply
100 lbs. of N.
5. Use various medium-to-long-term strategies to build up soil organic
matter, including use of cover crops and animal manures.
Most of the nutrient needs of crops on this soil could
have been met by using about 20 tons wet weight of solid cow manure/acre
or its equivalent. It is best to apply it in the spring, before
planting. If the manure had been applied, the PSNT test would probably
have been quite a bit higher, perhaps around 25 ppm.
Recommendations for organic producers:
1. Use dolomitic limestone to increase the pH (as
recommended for the conventional farmer above).
2. Apply 2 tons/acre of rock phosphate, or about 5 tons of poultry
manure for phosphorus, or better yet a combination of 1 ton rock
phosphate and 2 ½ tons of poultry manure. If the high level
of rock phosphate is applied, it should supply some phosphorus for
a long time, perhaps a decade.
3. If the poultry manure is used to raise the phosphorus level,
add 2 tons of compost per acre to add some longer lasting nutrients
and humus. If rock phosphate is used to supply phosphorus, then
use livestock manure and compost (to add N, potassium, magnesium,
and some humus).
4. Establish a good rotation with soil-building crops and cover
crops.
5. Care is needed with manure use. Although the application of uncomposted
manure is allowed by organic certifying organizations, there are
restrictions. For example, three to four months may be needed between
application of uncomposted manure and either harvest of root crops
or planting of crops that accumulate nitrate, such as leafy greens
or beets. A two-month period may be needed between uncomposted manure
application and harvest of other food crops.
Soil Test #2
(Pennsylvania, New York)
Field name: Smith upper
Sample date: November (no sample for PSNT will be taken)
Soil type: silt loam
Manure added: none this year (some last year)
Cropping history: legume cover crops used routinely
Crop to be grown: corn
What can we tell about soil #2 based on the soil test?
- The high pH indicates that this soil does not need any lime.
- Phosphorus is high, as are potassium, magnesium, and calcium
(see table 19.3d).
- The organic matter is very good for a silt loam.
- There was no test done for nitrogen, but this soil probably
supplies a reasonable amount of N for crops, because the farmer
uses legume cover crops and allows them to produce a large amount
of dry matter.
General recommendations:
1. Continue building soil organic matter.
2. No phosphate, potash, or magnesium needs to be applied. The lab
that ran this soil test recommended using 38 lbs. potash (K2O)
and 150 lbs. of magnesium (MgO) per acre. However, with a high K
level, 180 ppm (about 8 percent of the CEC) and a high Mg, 137 ppm
(about 11 percent of the CEC), there is a very low likelihood of
any increase in yield or crop quality from adding either element.
3. Nitrogen fertilizer is probably needed in only small to moderate
amounts (if at all), but we need to know more about the details
of the cropping system or run a nitrogen soil test to make a more
accurate recommendation.
Soil Test #2 Report Summary* |
|
lbs/Acre |
PPM |
Soil Test Category |
Recommendation Summary |
P |
174 |
87 |
high |
none |
K |
360 |
180 |
high |
none |
Mg |
274 |
137 |
high |
none |
Ca |
3880 |
1940 |
high |
none |
|
pH |
7.2 |
|
|
no lime needed |
CEC ** |
11.7 me/100g |
|
|
|
OM |
3% |
|
|
add organic matter: compost, cover crops,
animal manures |
|
N |
No N soil test |
little to no N needed |
* Soil sent to a commercial
lab. P using Bray-1 solution. This is probably the equivalent
of over 20 ppm by using Morgan or Olsen procedures. Other
nutrients extracted with pH 7 ammonium acetate (see table
19.3d). |
Recommendations for organic producers:
1. A good rotation with legumes will provide nitrogen for other
crops.
Soil Test #3
(Humid Midwest)
Field name: #12
Sample date: December (no sample for PSNT will be taken)
Soil type: clay (somewhat poorly drained)
Manure added: none
Cropping history: continuous corn
Crop to be grown: corn
What can we tell about soil #3 based on the soil test?
- The high pH indicates that this soil does not need any lime.
- Phosphorus and potassium are low.
- The organic matter is relatively high. However, considering
that this is a somewhat poorly drained clay, it probably should
be even higher.
- About half of the CEC is probably due to the organic matter
with the rest probably due to the clay.
- Low potassium indicates that this soil has probably not received
high levels of manures recently.
- There was no test done for nitrogen, but given the field's
history of continuous corn and little manure, there is probably
a need for nitrogen. A low amount of active organic matter that
could have supplied nitrogen for crops is indicated by past
history (the lack of rotation to perennial legume forages and
lack of manure use) and the moderate percent organic matter
(considering that it is a clay soil).
General recommendations:
1. This field should probably be rotated to a perennial forage crop.
2. Phosphorus and potassium are needed. Probably around 30 lbs.
of phosphate (P2O5) and 200 or more lbs. of
potash (K2O) applied broadcast, preplant, if a forage
crop is to be grown. If corn will be grown again, all of
the phosphate and 30 to 40 lbs. of the potash can be applied as
starter fertilizer at planting. Although magnesium, at about 3 percent
of the effective CEC, would be considered low by relying exclusively
on a basic cation ratio saturation recommendation system, there
is little likelihood of an increase in crop yield or quality by
adding magnesium.
3. Nitrogen fertilizer is probably needed in large amounts (100
to 130 lbs./acre) for high N-demanding crops, such as corn. If no
in-season soil test (like the PSNT) is done, some preplant N should
be applied (around 50 lbs./acre), some in the starter band at planting
(about 15 lbs./acre) and some side-dressed (about 50 lbs.).
4. One way to meet the needs of the crop is as follows:
a) broadcast 500 lbs. per acre of an 11-0-44 bulk blended fertilizer;
b) use 300 lbs. per acre of a 5-10-10 starter; and
c) sidedress with 150 lbs. per acre of ammonium nitrate.
This will supply approximately 120 lbs. of N, 30 lbs.
of phosphate and 210 lbs. of potash.
Recommendations for organic producers:
1. 2 tons/acre of rock phosphate (to meet P needs) or about
5 to 8 tons of poultry manure (which would meet both phosphorus
and nitrogen needs), or a combination of the two (1 ton rock phosphate
and 3 to 4 tons of poultry manure).
2. 400 lbs. of potassium sulfate per acre broadcast preplant. (If
poultry manure is used to meet phosphorus and nitrogen needs, use
only 200 to 300 lbs. of potassium sulfate per acre.)
3. Care is needed with manure use. Although the application of uncomposted
manure is allowed by organic certifying organizations, there are
restrictions. For example, three to four months may be needed between
application of uncomposted manure and either harvest of root crops
or planting of crops that accumulate nitrate, such as leafy greens
or beets. A two-month period may be needed between uncomposted manure
application and harvest of other food crops.
Soil Test #3 Report Summary* |
|
lbs/Acre |
PPM |
Soil Test Category |
Recommendation Summary |
P |
20 |
10 |
very low |
P2O5/acre |
K |
58 |
29 |
very low |
200 lbs. K2O/acre |
Mg |
138 |
69 |
high |
none |
Ca |
3168 |
1584 |
high |
none |
|
pH |
6.8 |
|
|
no lime needed |
CEC ** |
21.1 me/100g |
|
|
|
OM |
4.3% |
|
|
rotate to forage legume crop |
|
N |
No N soil test |
100-130 lbs. N/acre |
*all nutrients determined using
Mehlich 3 solution (see table 19.3c). |
Soil Test #4
(Alabama)
Field name: River A
Sample date: October
Soil type: sandy loam
Manure added: none
Cropping history: continuous cotton
Crop to be grown: cotton
What can we tell about soil #4 based on the soil
test?
- With a pH of 6.5, this soil does not need any lime.
- Phosphorus is very high, and potassium and magnesium are
sufficient.
- Magnesium is high, compared with calcium (Mg occupies over
26 percent of the CEC).
- The low CEC at pH 6.5 indicates that the organic matter content
is probably around 1 to 1.5 percent.
General recommendations:
1. No phosphate, potash, magnesium, or lime is needed.
2. Nitrogen should be applied, probably in a split application totaling
about 70 to 100 lbs. N/acre.
3. This field should be rotated to other crops and cover crops used
regularly.
Recommendations for organic producers:
1. Although poultry or dairy manure can meet the crop's needs, that
means applying phosphorus on an already high-P soil. If there is
no possibility of growing an overwinter legume cover crop (see below),
then about 15 to 20 tons of bedded dairy manure (wet weight) should
be sufficient.
2. If time permits, this soil can use a high-N producing legume
cover crop, such as hairy vetch or crimson clover, to provide nitrogen
to cash crops.
3. Develop a good rotation so that all the needed nitrogen will
be supplied to non-legumes between the rotation crops and cover
crops.
4. Although the application of uncomposted manure is allowed by
organic certifying organizations, there are restrictions when growing
food crops. Check with the person doing your certification to find
out what restrictions apply to cotton.
Soil Test #4 Report Summary* |
|
lbs/Acre |
PPM |
Soil Test Category |
Recommendation Summary |
P |
102 |
51 |
very high |
none |
K |
166 |
83 |
high |
none |
Mg |
264 |
132 |
high |
none |
Ca |
1158 |
579 |
|
none |
|
pH |
6.5 |
|
moderate |
no lime needed |
CEC ** |
4.2 me/100g |
|
|
|
OM |
not requested |
|
|
use legume cover crops, consider crop
rotation |
|
N |
No N soil test |
70-100 lbs. N/acre |
*all nutrients determined using
Mehlich 1 solution (see table 19.3b). |
Soil Test #5
(Semi-arid Great Plains)
Field name: Hill
Sample date: April
Soil type: silt loam
Manure added: none indicated
Cropping history: not indicated
Crop to be grown: corn
What can we tell about soil #5 based on the soil
test?
- The pH of 8.1 indicates that this soil is most likely calcareous.
- Phosphorus is low, there is sufficient magnesium, and potassium
is very high.
- Although calcium was not determined, there will be plenty
in a calcareous soil.
- The organic matter at 1.8 percent is low for a silt loam
soil.
- The nitrogen test indicates a low amount of residual nitrate
(table 19.4b) and, given the low organic matter level, a low
amount of N mineralization is expected.
Soil Test #5 Report Summary* |
|
lbs/Acre |
PPM |
Soil Test Category |
Recommendation Summary |
P |
14 |
7 |
low |
20-40 lbs. P2O5 |
K |
716 |
358 |
very high |
none |
Mg |
340 |
170 |
high |
none |
Ca |
not determined |
none |
|
pH |
8.1 |
|
|
no lime needed |
CEC ** |
not determined |
|
|
|
OM |
1.8% |
|
|
use legume cover crops, consider rotation
to other crops that produce large amounts of residues |
|
N |
5.8 ppm |
170 lbs. N/acre |
*K and Mg extracted by neutral
ammonium acetate, P by Olsen solution (see table 19.3d). |
General recommendations:
1. No potash, magnesium, or lime is needed.
2. About 170 lbs. of N/acre should be applied. Because of the low
amount of leaching in this region, most can be applied pre-plant,
with perhaps 30 lbs. as starter (applied at planting). Using 300
lbs. per acre of a 10-10-0 starter would supply all P needs (see
below) as well as give some N near the developing seedling. Broadcasting
and incorporating 300 lbs. of urea or 420 lbs. of ammonium nitrate
will provide 140 lbs. of N.
3. About 20 to 40 lbs. of phosphate (P2O5)
is needed per acre. Apply the lower rate as starter, because localized
placement results in more efficient use by the plant. If phosphate
is broadcast, apply at the 40 lb rate.
4. The organic matter level of this soil should be increased. This
field should be rotated to other crops and cover crops used regularly.
Recommendations for organic producers:
1. Because rock phosphate is so insoluble in high pH soils, it would
be a poor choice for adding P. Poultry (about 6 tons per acre) or
dairy (about 25 tons wet weight per acre) manure can be used to
meet the crop's needs for both N and P. However, that means applying
more P than is needed, plus a lot of potash (which is already at
very high levels).
2. A long-term strategy needs to be developed to build soil organic
matter better rotations, use of cover crops, and importing organic
residues onto the farm.
3. Care is needed with manure use. Although the application of uncomposted
manure is allowed by organic certifying organizations, there are
restrictions. For example, three months may be needed between application
of uncomposted manure and either harvest of root crops or planting
of crops that accumulate nitrate, such as leafy greens or beets.
A two-month period may be needed between uncomposted manure application
and harvest of other food crops.
Adjusting a Soil Test Recommendation
Specific recommendations must be tailored to the crops
you want to grow, as well as other characteristics of the particular
soil, climate, and cropping system. Most soil test reports use information
that you supply about manure use and previous crop to adapt a general
recommendation for your situation. However, once you feel comfortable
with interpreting soil tests, you may also want to adjust the recommendations
for a particular need. What happens if you decide to apply manure
after you sent in the form along with the soil sample? Also, you
usually don't get credit for the nitrogen produced by legume cover
crops because most forms don't even ask about their use. The amount
of available nutrients from legume cover crops and from manures
is indicated in table 19.5. Another common situation occurs because
most farmers don't test their soil annually and the recommendations
they receive are only for the current year. Under these circumstances,
you need to figure out what to apply the next year or two, until
the soil is tested again.
No single recommendation, based only on the soil test,
makes sense for all situations. For example, your gut feeling
might tell you that a test is too low (and fertilizer recommendations
are too high). Let's say that you broadcast 100 lbs. N/acre before
planting, but a high rate of N fertilizer is still recommended by
the in-season nitrate test (PSNT), even though there wasn't enough
rainfall to leach out nitrate or cause much loss by denitrification.
In this case, you may not want to apply the full amount recommended.
Another example: a low potassium level in a soil test
(let's say around 40 ppm) will certainly mean that you should apply
potassium. But, how much should you use? When/how should you apply
it? The answer to these two questions might be quite different on
a low-organic matter, sandy soil where high amounts of rainfall
normally occur during the growing season (in which case potassium
may leach out if applied the previous fall, or early spring) versus
a high-organic matter clay loam soil that has a higher CEC and will
hold onto potassium added in the fall. This is the type of situation
that dictates using labs whose recommendations are developed for
soils and cropping systems in your home state or region. It also
is an indication that you may need to modify a recommendation for
your specific situation.
Making Adjustments to
Fertilizer Application Rates |
If information about cropping
history, cover crops, or manure use is not provided to
the soil testing laboratory, the report containing the
fertilizer recommendation cannot take these factors into
account. Below is an example of how you can modify the
report's recommendations. |
Past crop = corn Cover crop
= crimson clover, but small to medium amount of growth.
Manure = 10 tons of dairy manure that tested at
10 lbs. N, 3 lbs. of P2O5, and 9
lbs. of K2O per ton. (A decision to apply manure
was made after the soil sample was sent, so the recommendation
could not take those nutrients into account.) |
|
Making Adjustments
to Fertilizer Application Rates |
|
N |
P2O5 |
K2O |
Soil Test Recommendation
Accounts for contributions from the soil. Accounts for
nutrients contributed from manure and previous crop
only if information is included on form sent with
soil sample. |
120 |
40 |
140 |
Credits
(Use only if not taken into account in recommendation
received from lab.) |
|
|
|
Previous crop (already taken into
account) |
-0 |
|
|
Manure (10 tons @ 6 lbs. N2.4
lbs. P2O59 lbs. K2O
per ton,
assuming that 60% of the nitrogen, 80% of the phosphorus
and 100% of the potassium in the manure will be available
this year.) |
-60 |
-24 |
-90 |
Cover Crop (medium growth crimson
clover) |
-50 |
|
|
TOTAL NUTRIENTS NEEDED FROM FERTILIZER |
10 |
16 |
50 |
|
Sources
Allen, E.R., G.V. Johnson, and L.G. Unruh. 1994. Current approaches
to soil testing methods: Problems and solutions. pp. 203220.
In Soil Testing: Prospects for Improving Nutrient Recommendations
(J.L. Havlin et al., eds). Soil Science Society of America. Madison,
WI.
Cornell Cooperative Extension. 2000. Cornell
Recommendations for Integrated Field Crop Production. Cornell
Cooperative Extension, Ithaca, NY.
Hanlon, E. (ed.). 1998. Procedures Used by State
Soil Testing Laboratories in the Southern Region of the United States.
Southern Cooperative Series Bulletin No. 190Revision B. University
of Florida. Immokalee, FL.
Herget, G.W., and E. J. Penas. 1993. New Nitrogen
Recommendations for Corn. NebFacts NF 93111, University
of Nebraska Extension. Lincoln, NE.
How to Conduct Research on Your Farm or Ranch.
1999. Available from SARE regional offices. Also available at: www.sare.org/publications
Jokela, B., F. Magdoff, R. Bartlett, S. Bosworth,
and D. Ross. 1998. Nutrient Recommendations for Field
Crops in Vermont. University of Vermont Extension. Brochure
1390. Burlington, VT.
Penas, E.J., and R.A. Wiese. 1987. Fertilizer Suggestions
for Soybeans. NebGuide G87-859-A. University of Nebraska Cooperative
Extension. Lincoln, NE.
Recommended Chemical Soil Test Procedures for the North Central
Region. 1998. North Central Regional Research Publication No.
221 (revised). Missouri Agricultural Experiment Station SB1001.
Columbia, MO.
Rehm, G., 1994. Soil Cation Ratios for Crop Production.
North Central Regional Extension Publication 533. University of
Minnesota Extension. St. Paul, MN.
Rehm, G., M. Schmitt, and R. Munter. 1994. Fertilizer
Recommendations for Agronomic Crops in Minnesota. University
of Minnesota Extension. BU-6240-E. St Paul, MN.
The PennState Agronomy Guide. 1999. The Pennsylvania
State University. University Station, PA.
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