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As
the harvest of summer crops ends, many growers in Alabama begin the
annual ritual of soil sampling. While this can be time consuming during
a part of the year when other operations on the farm have slowed down,
the information obtained and the decisions made based on this sampling
are vital to the success of next year’s crop and the productivity of
fields for years to come. The ultimate result of sampling a field is the
precise application of plant nutrients and lime to produce maximum
profits.
To
achieve this, a soil sampling program must include four components: 1) a
representative sample collection; 2) laboratory analysis of samples; 3)
interpretation of laboratory results; and 4) lime and fertilizer
recommendations based on the crop to be grown and yield goals. The
grower is responsible for taking the soil sample, choosing the lab and
understanding the analysis results and lab recommendations. Co-op and
Agri-AFC locations throughout the state are available to help with
sampling and interpretation.
Soil
Sampling
The
old cliché "a soil test is only as good as the sample taken"
is absolutely true. Sampling is usually the weakest link in soil testing
programs. The reason for this is because the soil being tested is only a
small fraction of the soil it represents. To overcome this, divide the
fields being sampled into as many uniform areas as practical for
sampling and management. These uniform areas or "management
zones" should share common characteristics which are different from
adjacent areas and be large enough to be fertilized or limed separately.
Decisions
on management zones should be based on yield history, soil type and
slope. To maximize profit from a field, the factors that differentiate
these management zones need to be addressed. Yield history can now be
captured using yield monitors or through years of personal experience
with a field. Soil differences within a field can be found in the soil
surveys of each county or by accessing the USDA Web Soil Survey on the
Internet at http://websoilsurvey.nrcs.usda.gov/app/. This site
is easy to use and provides soils maps superimposed over aerial
photographs.
Once
management zones have been outlined and identified, a composite sample
which includes 10-20 sub-samples should be taken for each zone. The
sub-samples need to be equal in volume and pulled randomly across the
entire zone. To pull a sample, clean residue from the surface and sample
to six inches or to the depth of the plow layer. In no-till fields with
rows still visible, pull samples randomly from the middles and in the
rows. Mix the sub samples thoroughly in a plastic bucket before placing
the sample in the container provided by the laboratory.
Interpreting
Soil Tests
The
results sent from a lab after soil analysis come with a wealth of
information about a field or management zone. To take full advantage of
this information, the grower needs to understand each area of a soil
test report. Soils labs provide fertilizer and lime recommendations
based on pH, nutrient levels, crop to be produced and yield expected.
Reports from each lab may differ, but most also include pH, buffer pH,
percent organic matter, CEC, base saturation and levels of primary,
secondary and micronutrients. The information on nutrient levels and
recommendations is usually straightforward, but some of the other items
on the report may require explanation.
Soil
pH is a measure of the active acidity in a soil. The pH scale ranges
from 0 to 14 (extremely acidic to extremely basic) with a pH of 7 being
neutral. Corn, cotton, peanuts, soybeans and wheat perform best at a
slightly acidic pH of 6.0 to 6.5. Each unit change on the pH scale
represents a tenfold difference in acidity; a pH of 5.0 is ten times
more acidic than a pH of 6.0 and 4.0 is 100 times more acidic than 6.0.
As a result, the amount of lime needed to raise pH increases
exponentially as pH drops.
The
buffer pH of a soil also affects the amount of lime needed to bring a
field or management zone to a target pH. Buffer pH represents the level
of exchangeable and residual acidity in a soil; the lower the buffer pH,
the greater the lime requirement.
Percent
organic matter represents the amount of plant and animal residue present
in the soil. Generally, soil productivity increases as percent organic
matter increases. Most soils in Alabama have less than 1% organic
matter, but many fields have increased dramatically with the adoption of
no-till and residue management.
CEC
stands for Cation Exchange Capacity. It is the measure of a soil’s
ability to retain cations or positively charged ions and molecules such
as potassium, calcium or magnesium. Generally, soil productivity
increases as CEC increases and CEC increases as the amount of clay and
organic matter increases in soils. Typically in Alabama, sandy soils low
in organic matter will have a CEC less than 4.6, silt loam soils will be
4.6 to 9.0, and soils with a large amount of clay and/or organic matter
will be greater than 9.0.
Base
saturation or cation saturation is a measure of the percent of the CEC
that is occupied by a specific cation. Ideally, calcium (Ca) should be
65%, magnesium (Mg) should be 10% and potassium (K) should be 5%, but
these percentages can vary widely without yield effects as long as a
sufficient amount of each nutrient is present to meet crop needs. In
some situations, extreme ratios between potassium and magnesium may
cause reduced uptake of one of the other nutrients. |
