If you have acid soil, adding lime will make it less acid, because lime is alkaline. There are several liming materials available, so you need to know which one will do the best job for you and give you value for money.
Before you buy any liming material, check these details.
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.
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.
This is made by treating burnt lime with water, and is used mainly in mortar and concrete. It is more expensive than agricultural lime.
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.
Made from crushed magnesium carbonate rock, good quality magnesite has 25–28% magnesium, virtually no calcium, and NV of 95–105.
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 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.
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%.
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 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.
You can compare the value of different liming materials by checking NV and fineness against spread cost.
(Fineness × NV) ÷ 100 = efficiency
(Spread cost × 100) ÷ efficiency = comparative cost
(50 × 95) ÷ 100 = 47.50
($70 × 100) ÷ 47.50 = $147.37/tonne
(100 × 99) ÷ 100 = 99
($90 × 100) ÷ 99 = $90.91/tonne
Lime B is comparatively cheaper because its fineness makes it more efficient at neutralising acidity.
The NSW Fertilizers Act requires liming materials to be labelled. Labelling has to include
If this information is not on the lime bag, or on the invoice if you buy bulk lime, consider buying another product.