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October 2007
Figure
liming
needs
Expert View
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By NATALIA ROGOVSKA
D
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MARKET CHOICES: Grain harvested from products that
bear this mark is fully approved for food and feed use in
the United States and Japan, but is not approved in the
European Union. You must find a market for this crop
that will not ship this grain or its processed products to
Europe. Appropriate markets for this grain include:
domestic feed use or grain handlers that specifically
agree to accept this grain and handle it appropriately. For
more information on your grain market options, go
to the American Seed Trade Association’s website
at www.amseed.org or call your seed supplier.
MARKET CHOICES is a certification mark used under
license from ASTA. Know Before You GrowSM, an
information service provided by National Corn Growers
Association at www.ncga.com.
To protect the usefulness and availability of these
technologies for the future, growers must implement
an Insect Resistance Management (IRM) program
as specified in product use guides for the following
traits available in Pioneer corn hybrids: Herculex® I,
Herculex RW, Herculex XTRA and YieldGard® Corn
Borer.
For detailed IRM requirements for hybrids with
in-plant insect resistance, refer to the appropriate
product use guide, available from your
Pioneer sales professional or on the web at:
www.pioneer.com/IRM.
O your soils need lime? How
much lime should you apply per
acre? What type of liming material should you use?
Those are common soil fertility management questions farmers face each
fall, and recent Iowa State University
research provides new answers.
The first step is to figure out whether
lime is needed on a particular field or
farm. You do that by collecting soil
samples and having them tested to determine soil pH.
Sampling techniques commonly
used for pH mapping are grid sampling
(point or cell) and zone sampling. For
zone sampling, to establish the zones,
you can use remote sensing, topography maps, yield maps and/or soil
survey maps.
Farmers ask which is better: grid or
zone sampling? Each of these soil sampling techniques has positive and negative aspects.
Because soil pH can vary by going
just a few feet, common 2.5-acre grid
sampling can introduce great errors in
pH estimations when several cores with
different soil pH are mixed together to
produce a composite sample. If you do
more intensive grid sampling, that can
be quite costly and might not improve
resolution by much.
Zone sampling can reduce the
number of samples and sampling cost
and, at the same time, provide acceptable resolution in pH mapping.
Combining two or more methods for delineating zones is always better than to
rely on just one method.
VARIABLE: “Using GPS to collect soil samples, you can make an accurate map of
soil pH differences by location in a field and use the map to apply lime either at a
fixed rate or a variable rate,” says Natalia Rogovska.
Key Points
■ Best way to determine whether you need
to apply lime is to sample and test soil.
■ You can use either grid or zone sampling;
both have advantages and disadvantages.
■ Variable-rate liming can call for more or
less lime compared to the fixed rate.
6.5, based on ISU guidelines.
Recent studies suggest, however,
that for corn and soybean crops, no
lime is needed in the Clarion-NicolletWebster soil association when topsoil
has a pH as low as 5.4 and subsoils are
calcareous. There is a need to conduct
more studies on this and other soil associations.
The exceptions
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The main drawback of using zone sampling is it requires collection of additional data such as yield, soil survey
or electrical conductivity maps and
remote sensing imagery — all of which
adds up to additional cost. Moreover,
it isn’t always clear which method
would give better results in delineating
zones.
ISU lime recommendations, which
were developed in 1980, state that lime
should be applied to soils if pH is lower
than 6.5 when corn and soybeans are
grown. ISU recommends you apply
enough lime to raise the soil pH to 6.5
in this situation.
The exceptions are the soil association areas of the Des Moines Lobe,
Northwest Iowa Plains and the Loess
Hills of western Iowa. These soils often
have surface soil or subsoil that is high
in pH with a considerable amount of
free lime reaching 30% by weight. For
such soils, a pH of 6.0 is considered sufficient. If these soils are limed, then the
soil pH is recommended to be raised to
Calculate liming rate
Once you determine you need to apply
lime, the application rate of liming material is calculated based on buffer pH,
the depth of soil to be neutralized and
the effective calcium carbonate equivalent of the liming material.
Buffer pH is an important index in determining liming rates, as it shows the
capacity of the soil to resist changes in
pH. Soils with the same pH might have
different buffer pH values and, therefore, different lime requirements.
The effective calcium carbonate
equivalent is determined for all sources
of liming material in Iowa. Liming material is any material that contains calcium
or magnesium and has acid neutralizing power. For example, agricultural
lime or calcium carbonate has calcium,
which displaces hydrogen ions from
clay particles in the soil. The hydrogen
ions make the soil acidic. Ag lime also
has carbonate that neutralizes the displaced hydrogen, thus releasing carbon
dioxide and water.
Different lime materials have different neutralizing power, expressed as
a percentage of pure calcium carbonate.
Quick lime, for example, has a neutralizing value of about 178%; but wood ash
has only 30% to 70%.
Not all materials that contain calcium
are considered to be liming material.
Application of gypsum has little or no
effect on the pH of Iowa soils, although
it can be used to remediate salt-affected
soils and make them more tolerable.
Varied vs. fixed application
Lime requirements can vary within a
field, as soil pH can range from moderately acid to highly alkaline. Thus, a
variable-rate lime application can be a
better alternative to a fixed rate across
the whole field. However, it is necessary
to determine whether there is a sufficient variation in the lime requirement
of the field to justify a variable rate and
the cost associated with it.
There is a misconception that the
variable rate will always apply less lime.
Depending on the variability and extent
of the area of the field that is low in pH, a
variable rate can call for applying more
or less lime compared to the fixed rate.
Preferably, lime should be applied in
the fall when soils are dry to avoid compaction problems. Application on snow,
or frozen soil isn’t recommended. Some
of the lime may be washed away with
snowmelt, especially on slopes.
It takes time to neutralize soil acidity.
Depending on the lime particle size and
incorporation, it may take six months to
three years before soil pH can change
substantially. Therefore, if lime is
needed, application should be considered as a long-term investment.
Rogovska earned her doctorate in soil
science at ISU in 2006 and is a research
agronomist.