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Corn Route 4: LIME TO WIN
From
Farm Journal Media |
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Soil Acidity Affect on Plant Nutrient Availability
Click here to view illustration.
By
Darrell Smith, Farm Journal Conservation & Machinery Editor |
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Trying to maximize corn profits without understanding acidity is
like building a skyscraper without pouring a foundation
first—the entire structure will probably come crashing down.
Managing pH levels becomes even more important if you’re growing
continuous corn.
“I’ve looked at many yield comparisons trying to figure out why one
farmer raises 220-bu. corn and another, with similar soils, gets
only 180 bu.,” says Farm Journal Field Agronomist Ken Ferrie.
“There could be many reasons, but pH is right up there.
High-yielding growers hold soil pH constant, and don’t let it
swing up and down. Yield losses because of acid soil occur
gradually. But, when you add lime, it takes two to three years
to bring yields back up.” |
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Microbial Management |
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If you have the proper pH, you don’t fight residue because
microbes decompose it. Yields show smaller peaks and valleys
because yields in acid soils fall apart in dry years. Managing
pH is a major part of weatherproofing soil.
“Corn plants can actually grow in soil with a pH of 5.8 to 6.0
without too much trouble,” Ferrie explains. “However, besides
simply growing plants, we need to maintain a healthy population
of soil organisms. They decompose residue, and they cycle
nutrients through the soil so they become available to growing
plants.”
Microbes are less active in acid soil. “You can have a high
Illinois Soil Nitrate Test (ISNT) reading, but if you have acid
soil, that nitrogen won’t be available to plants,” Ferrie says.
“The microbes that make nitrogen available are the same ones
that mineralize phosphorus, sulfur and other nutrients from the
soil so plants can use them.”
Lack of microbial activity also means you’ll have more herbicide
carryover problems on acid soil. To keep microbes numerous and
active, Ferrie advises farmers to shoot for a pH level of 6.4 in
either continuous corn or a corn–soybean rotation.
Agronomists speak of “balancing” the pH of an acid soil because
you are moving toward 7.0, the midpoint of the 0 to 14 pH scale.
The ideal pH for most crops is a bit short of that—a slightly
acid reading of 6.3 or 6.4, Ferrie says. Getting there is sort
of a balancing act, also, because you have to take many factors
into account. |
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Hydrogen Overload |
You know that, over time, soils naturally become acid. But why?
It’s because hydrogen ions build up, Ferrie explains. The ions
come from various sources, such as nitrogen fertilizers and
manure, crop roots and the microorganisms that cycle nutrients
and decompose crop residue and herbicides.
Some hydrogen is needed to solubilize nutrients so they can be
processed by plants. But, too much of a good thing causes
problems. “If you have too much hydrogen, it solubolizes
nutrients, especially iron, aluminum and manganese, too rapidly
so you get toxicity,” Ferrie says. “Iron plugs the plants’
passageways and restricts uptake of other nutrients.”
At the same time, acid soil conditions will likely reduce microbial
activity, which will leave fewer nutrients in plant-available
forms, and less residue will be decomposed. “The soil’s ability
to feed the plant really is reduced,” Ferrie says.
When Ferrie finds residue accumulating for two or three years in a
no-till field, it is most often due to low soil pH.
To make soil less acid, you apply lime, which reduces the number of
hydrogen ions. It sounds simple, but—like so many factors in
crop production—there are numerous opportunities to make
mistakes. |
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How much? How often? |
Today’s style of farming argues for more frequent applications
of smaller amounts of lime. “If you look at pH on a four- or
five-year basis, you are likely to make a heavy lime application
and send your pH above the optimum zone,” Ferrie explains.
“Eventually, pH will come down. It may be in the optimum range
only one year out of four.”
Reduced tillage and no-till also argue for more frequent, smaller
lime applications. There’s less room for error than there was
when farmers used a moldboard plow. Plowing diluted the lime
among the 2 million pounds of soil contained in an acre-slice 6"
deep.
“With reduced tillage, if you drop four tons of lime on the surface
to neutralize 6" of soil, and only work it in 3" deep, it’s like
an eight-ton application to the top few inches of soil,” Ferrie
explains. “That could cause herbicide carryover and volatization
of surface-applied urea nitrogen fertilizer. You need to apply
lower amounts more frequently—probably every two years.”
To apply the right amount of lime, you need to understand your
soil test results. “Most soil tests report a water pH—a measure
of soil acidity as a plant sees it,” Ferrie says. “Often,
farmers and dealers look at that number and use it to calculate
how many tons of lime to apply.”
But, a water pH reading can’t tell you how much lime to apply
because the amount needed to neutralize acidity varies with
different soil types. The number that tells you how much lime is
required for a given soil type is called the buffer pH.
“For example, a heavy Drummer clay loam might have a water pH
reading of 5.7 and a buffer pH of 6.6,” Ferrie says. “It needs
about three tons of lime per acre to neutralize the acidity. A
sandy knob in the same field has a pH of 4.7, which is more
acid. But, it might have a buffer pH of 6.8 and need only 1,500
lb. of lime per acre.”
Liming the entire field at the same rate would be a big mistake.
“Light soil can’t stand high lime applications,” Ferrie says.
“Applying 6 tons of lime to a sandy soil could almost sterilize
it.”
Consult your Extension adviser or land grant university for
information about the buffering capacity of the soils you farm.
The soil types in your state will probably be broken down into
classes based on their buffering ability.
To avoid over- or under-liming knobs and pockets of various soil
types—and reducing yield—you need to know from which soil type
each sample was taken. That requires overlaying your soil test
map on a soils map. Unfortunately, this often is not taken into
account, so you probably will have to do it yourself. |
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Timing and Tillage
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Once you’ve determined your buffer pH and used it to calculate
how much lime to apply, tillage enters into the equation again.
“If you take a 6" soil sample, but won’t incorporate your lime,
you must reduce the rate by a half to one-third and apply the
total amount over several years,” Ferrie says. “A three-ton
application in a no-till field would be like a 10-ton
application on the surface of the soil.”
Along with tillage, soil type influences how often you lime a
field. “Light soils become acid more quickly,” Ferrie says. “The
lighter your soil, the closer and smaller your lime applications
need to be.”
If you’re converting to no-till, it’s extremely important to
balance the pH in the top 6" of soil before you quit tilling.
“In most soils, lime moves downward at a speed of only ½" per
year,” Ferrie says. “So, if the top 6" of soil are acid, it will
take six to 12 years to neutralize it by applying lime only to
the surface. If you try to catch up, you probably will wind up
with the first few inches of soil at a very high pH and the 3"
to 6" level still acid.”
However, if soil pH is balanced when you begin no-tilling, you can
keep it that way by applying small, frequent lime
applications—“salt-shakering it on,” Ferrie calls it. “Some of
my clients’ fields haven’t been tilled for 20 years, but they
have remained in balance using that approach,” he says.
If you’re in a no-till situation and think you have stratified
pH—high in the top few inches and low in the bottom
portion—don’t take one 6" soil sample. “It will average the two
readings and read about neutral,” Ferrie says. “Instead, take
two samples, at the 1" to 3" depth and the 3" to 6" depth.”
To deal with stratification, Ferrie recommends taking the field out
of no-till for awhile and incorporating lime until the acid
condition is corrected throughout the soil profile. |
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Correction Factors |
Having taken all this into account, you’re almost ready to call
your dealer and order a lime application. First, check whether
your lime recommendation is stated in terms of calcium carbonate
equivalent (CCE) or tons of lime product. (Tons of lime
recommendations are based on a typical CCE for your area.) If
you mistake CCE for tons of lime, you’ll probably under-apply.
Finally, be sure to check the quality of the lime that you’re about
to buy. Quality depends on particle size—small particles
neutralize acidity faster than large ones. The smaller the
particles, the higher the quality, the lower the correction
factor and the fewer tons of lime will be required.
The only way to decide which lime source is the best buy is to take
the correction factor into account. For example: One quarry
sells higher-quality lime with a correction factor of 0.53 for
$8/ton. Another sells lower-quality lime with a correction
factor of 1.52 for $5/ton. If your recommendation calls for
three tons of lime per acre, you’ll need to apply 4.56 tons of
the lower-quality lime, costing $22.80/acre. However, you will
need only 1.59 tons of the higher-quality material, costing
$12.72/acre. The higher-priced lime is now the better buy.
Most states have their own formulas for calculating correction
factors, so be sure to discuss this with your Extension adviser
or crop consultant.
It would be a shame to do all your homework, find the best buy
on lime and then wind up with a streaky application. Calibrate
your spreader, taking the coarseness of the product into
account—the finer it is, the harder it will be to spread evenly.
“If you’re in a no-till or strip-till situation, try to spread
your lime in the fall to avoid creating soil compaction,” Ferrie
says. “Spring applications can leave nasty tracks.”
Be sure to lay the foundation for maximum corn yields—and
profits.
Editor’s Note: The recommendations in this story are for mineral
soils, which are what most producers farm. One exception is
organic soils—peat and muck. Those soils naturally have a lower
pH. You still lime them, but only to the 5.4 to 5.6 range. |
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What About High-pH Soils? |
Some farmers, especially in the northern Corn Belt, have to deal
with high soil pH, typically from 7.2 to 8.1. “High pH can
result from over application of poultry layer manure or
limestone,” says Farm Journal Field Agronomist Ken Ferrie. “But
typically, it is just a characteristic of certain soil types.”
In those calcareous soils, if you dig below the 4" to 5" tillage
zone, you will usually find shells or pieces of shells. That
indicates aquatic creatures once lived in the area when it was a
swamp or lakebed.
Soil organisms don’t like high pH any better than low pH. But, in
high-pH soils, rather than becoming toxic, nutrients, such as
iron and aluminum, become deficient, causing iron and aluminum
chlorosis. Phosphorus is another nutrient that becomes
deficient.
If you get high pH results from over liming, simply quit liming for
awhile. But, there isn’t much you can do to change a calcareous
soil, Ferrie says. “Your best bet is to plant varieties bred to
resist iron chlorosis,” he says. “Be careful about applying urea
on the surface; it could be lost to volatilization. Band your
phosphorus to keep it from becoming tied up. And, manage your
herbicides because there may be carryover.” |
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For More Information
To
obtain a free fact sheet about how lime quality is interpreted
in Indiana, Illinois, Michigan, Ohio and Wisconsin, log on to
www.aglime.org (for Indiana) or
http://www.algreatlakes.com/pdf/factsheets/ALGLFS30_Making_Lime_Recommendations.pdf.
Farmers in those states can also get a spreadsheet for
calculating other limestone correction factors by e-mailing
their request to
lab@algreatlakes.com.
Editor’s Note: Check the Aglime Quality Report at
www.aglime.org for Indiana’s ratings of lime quality from
various quarries and other soil tips that you should be aware
of. |
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Illustration
of Soil Acidity Affect on Plant Nutrient Availability: |
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“Corn Route 4: Lime to Win” © 2008
AgWeb.com All Rights Reserved. |
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