Types of liming materials
Agricultural lime (calcium carbonate)
This is the most commonly used liming material on the North Coast.
It consists of limestone crushed to a fine powder and is usually
the cheapest material for correcting soil acidity. Good quality
lime has 37–40% calcium.
Burnt lime (calcium oxide)
Also known as quicklime, burnt lime is derived by heating limestone
to drive off carbon dioxide. It is more concentrated and caustic
than agricultural lime and unpleasant to handle, so is rarely used
in agriculture.
Hydrated lime (calcium hydroxide)
This is made by treating burnt lime with water, and is used mainly
in mortar and concrete. It is more expensive than agricultural
lime.
Dolomite
Widely but often incorrectly used on the North Coast, particularly
in horticulture, dolomite is a naturally occurring rock containing
calcium carbonate and magnesium carbonate. Good quality dolomite
has an NV of 95–98, and contains 22% calcium and 12% magnesium. It
is good for acid soils where supplies of calcium and magnesium are
low, but if used constantly may cause a nutrient imbalance, because
the mix is two parts calcium to one part magnesium (2:1), whereas
the soil ratio should be around 5:1.
There is a blend of lime and dolomite available with a 5:1 ratio.
Magnesite (magnesium carbonate)
Made from crushed magnesium carbonate rock, good quality magnesite
has 25–28% magnesium, virtually no calcium, and NV of 95–105.
Burnt magnesite (magnesium oxide)
This is derived by heating magnesite, and contains about 50%
magnesium. It is sold in granulated form and its NV is 180–220. Use
magnesite and burnt magnesite if your acid soil already has enough
calcium.
Wet lime
Wet liming materials are sometimes available at low prices. Their
usefulness is determined by the NV and water content. If the water
content is 10%, then the lime will only be 90% as effective as dry
lime. You need to consider the extra costs of handling, freight and
spreading.
Lime and cement kiln dusts
These dusts can be good value but you need to check the analysis
before you buy. Their NV varies from 70–150 and calcium from
25–54%. Magnesium is usually less than 1%.
Crushed shells
Shells of oysters and other shellfish are mainly calcium carbonate,
but the shell tends to be contaminated with sand and organic
material and is usually too coarse to be effective in soil.
Gypsum (calcium sulfate)
Gypsum is classified by the Fertilizer Act as a liming material,
but is not considered one in farming as it does not reduce soil
acidity. It is used mainly to improve the structure of sodic clay
soils, and these are not common on the North Coast.
