Page 148 - Soil Degradation, Conservation and Remediation

5.3 Acidification 137

5.3.3.1 Liming Materials
Liming materials or lime are any materials that contain Ca or Mg and are able to
neutralize soil acidity. Liming materials are generally carbonates and bicarbonates,
sometimes oxides and hydroxides of calcium and magnesium.
Carbonates: The most widely used liming materials are the carbonates. They are
less expensive and easier to handle than other lime materials. Ground high-grade
limestone or calcite is concentrated calcium carbonate (CaCO 3 ). Dolomite
(MgCO 3 + CaCO 3 ) is also a common source of agricultural lime. Dolomite
usually costs a little more than calcite and changes the soil pH more slowly, but
it has the advantage of containing Mg in addition to Ca. Together, calcite and
dolomite account for more than 90 % of the lime used in agricultural fields. Both
materials are naturally occurring rocks that are mined and ground for agricultural
use. Marl and oyster shells are also carbonate materials. Marl is a naturally
occurring mixture of clays, carbonates of Ca and Mg, and shell remnants. Oyster
shells also contain high proportion of calcium carbonate but are important only
in some coastal regions.
Oxides: Oxide liming materials include burned lime, unslaked lime, and quicklime.
Oxides are made by baking crushed calcite limestone or dolomite limestone in
a furnace, thereby driving off carbon dioxide (CO 2 ) to form a concentrated
oxide (CaO or MgO). This material is of low molecular weight and reacts rapidly
in the soil to raise the pH. Oxides are the most efficient of all liming materials
on a pound-for-pound basis. But oxides are caustic; that is, they react with
moisture and are difficult to handle. In addition, their cost is high relative to
carbonate materials. One ton of calcium oxide has the neutralizing power of
1.8 tons of calcite.
Hydroxides: Hydroxides are simply oxide materials with water added. They are
also known as hydrated lime, slacked lime, or builders lime. These materials
are similar to oxides because they are powdery, quick acting, and unpleasant to
handle. Hydroxides are also more expensive than carbonate materials.
Fluid Lime: Lime can be applied in liquid form as a suspension. Lime is not
dissolved but is suspended in water. Liquid lime may be prepared by pouring
50 % water to 48 % lime solids and 2 % clay to maintain a suspension. The material
used in this suspension should pass a 100-mesh sieve. The advantages of using
liquid lime include (1) good application uniformity, (2) high-quality lime material
(mesh size), and (3) quick soil pH change.
Miscellaneous Lime: Many byproducts of mining, refining, processing, and manufac-
turing processes are used as liming materials. Slags from blast furnaces and
electric furnaces as well as fly ash and bottom ash from coal-burning plants are
often applied as lime. Lime sludges from sugar beet processing plants (sugar
lime), paper mills, ore processing, and water-softening plants are sometimes
used to raise soil pH. Wood ashes from wood stoves or fireplaces also may be
used. Active gradients in these materials are quite variable and their requirement
cannot be precisely assessed. Their contaminants also restrict their use.