SoilFacts
Soil Acidity and Liming : Basic
Information for
Farmers and Gardeners
Situation in North Carolina
Nearly all North Carolina soils are naturally
acidic and need lime, which neutralizes the
acidity, for optimum growth of crops,
forages, turf, trees, and many ornamentals.
Even though most of these soils have been
limed in the past, periodic additions of lime
based on soil tests are still needed. Soil-test
summaries and field records compiled by the
North Carolina Department of Agriculture
and Consumer Services (NCDA&CS)
emphasize that poor management of soil pH
accounts for a high percentage of the “crop
problems” in North Carolina.
Nature and Cause of Soil Acidity
residues are added to the soil. Lime will
neutralize this acidity by dissolving, where¬
upon it releases a base into the soil solution
that reacts with the acidic components,
hydrogen and aluminum.
Soil pH is an indicator of “soil acidity"
(Figure 1). A pH of 7.0 is defined as neutral.
Values below 7.0 are acidic, and values above
7.0 are basic or alkaline. Small changes in
numbers indicate large changes in soil acidity.
A soil with a pH of 5 is 10 times more acidic
than a soil with a pH of 6 and 100 times more
acidic than a soil with a pH of 7. Most plants
can grow in slightly acidic soils, so the goal of
liming is not to raise the pH to neutral (7.0),
but to avoid crop problems related to exces¬
sive acidity.
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June 30, 1914. North Carolina
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University, U.S. Department of
Agriculture, and local
governments cooperating.
“Soil acidity” is the term used to express the
quantity of hydrogen (H) and aluminum (Al)
cations (positively charged ions) in soils.
When levels of hydrogen or aluminum
become too high — and the soil becomes too
acid — the soil's negatively charged cation
exchange capacity (CEC) becomes “clogged”
with the positively charged hydrogen and
aluminum, and the nutrients needed for plant
growth are pushed out. This is why root
growth and plant development suffer when
soils become too acid.
Over time, soils also become acidic because
calcium and magnesium leach out, because
hydrogen is added to soils by decomposition
of plant residues and organic matter, or
because nitrification of ammonium occurs
when fertilizer (UAN solutions, urea,
ammonium nitrate, ammonium sulfate,
anhydrous ammonia), manure, or plant
Soil pH
4 5 6 7 8
I _ I _ I _ I _ I _ I _ I _ I _ L
ACIDIC NEUTRAL BASIC
l I I I I I I I r
Figure 1. General relationship between soil
pH and acidity.
Benefits of Proper Lime Use
Proper liming provides a number of benefits:
■ Plants develop healthier roots because they
are exposed to less potentially toxic
aluminum. Better root growth may enhance
drought tolerance.
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