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Soil Acidity and Lime
Sven Johnson, Mycogen Research
Soil pH is an important but often overlooked factor affecting crop
growth and performance. Soil pH is a measure of how acidic or alkaline
a soil is. Soil with a pH of 7.0 is neutral, neither alkaline or acidic.
A pH of less than 7.0 is acidic. A pH more than 7.0 is alkaline. The
farther away from 7.0 the pH is, the more extreme the acidity or alkalinity.
In general, most crops perform best in a soil that is between neutral
and slightly acidic. A soil with a pH of between 6.5 and 7.0 will
provide a favorable growing environment for most all crops. However,
problems can arise when the soil pH drops much below this range.
Acid soils are relatively common across the corn belt. Low pH can
cause plant growth and development problems. Many essential elements
will begin decreasing in availability at a pH of 6.0. As pH decreases
below 5.5, aluminum becomes available in such amounts that it may
be toxic to crops. If soil pH drops to 5.0 or lower, severe problems
occur with soil nutrients. At a pH of this level and lower, every
essential crop nutrient except for manganese, iron and zinc is decreasing
in availability.
Low pH will decrease the activity of soil bacteria. This will result
in fewer nutrients being released from the soil organic matter, fewer
nodules on legume roots, and even a slower breakdown of crop residue.
Correcting soil acidity can be an expensive and lengthy proposition.
By adding alkaline minerals to the soil, acidity can be neutralized.
The most cost-effective element for accomplishing this task is lime.
Chemical properties within the soil make it resistant to rapid changes
in pH. It may take several years and as many lime applications to
achieve even a small change in pH.
Lime recommendations are usually given as tons of "ag lime"
required to correct the pH. "Ag lime" is a ground limestone
which is 60% effective in reducing soil acidity. Lime will vary in
the percent effectiveness from supplier to supplier and even within
batches from a single supplier. A reputable supplier will have tested
the product for percent effectiveness and will adjust the actual tonnage
applied to be equal to that of 60% effective "ag lime".
Different crops vary in their tolerance of soil acidity. Corn is
less sensitive to acidic soil than the legumes are. University of
Nebraska guidelines suggest the application of lime to corn only when
the pH is 5.5 or lower. Guidelines for alfalfa, clover and other legumes
suggest that the producer consider applying lime if the pH is less
than 6.2.
In addition to crop species varying in tolerance to acidity, hybrids
and varieties within crop species will also vary. Your Mycogen Seeds
sales representative may have suggestions for overcoming mild pH problems
through hybrid and variety selection, or help you find a good ag lime
supplier for those tougher situations.
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