| (5) SOURCES OF LIMING MATERIALS |
| Ground limestone is generally a mixture of calcium carbonate (CaCO3)
and dolomite (CaCO3 - Mg CO3) that has been crushed sufficiently
finely that it will react with acid soils. The quality of limestone is measured by its
purity, its fineness of grinding and sometimes the proportion of dolomite or magnesium in
the material. Pure calcium carbonate (calcite) contain 60% of carbonate and pure dolomite
i.e. 40% MgCO3 + 60% CaCO3 contains 64.5% carbonate. Thus, in its
pure state dolomite has more neutralising value than calcite. Although it is slower to
react than limestone it is rarely used unless magnesium deficiency is encountered. |
| All producers of ground limestone must hold a licence from the Department
of Agriculture. Before issuing such a licence the Department must be satisfied that the
rock deposit being quarried is of the required standard and that the machinery being used
is capable of producing ground limestone conforming to statutory requirements. Samples are
drawn regularly at each licensed quarry and analysed to ensure that standards are complied
with. |
| The standards are: |
| - |
Product must have a Total Neutralising Value of not less than 90 per cent. |
| - |
All the product must pass through a 3.35 mm sieve. |
| - |
Not less than 35% must pass through a 150 m m sieve. |
| - |
The moisture content must be less than 3.0%. |
|
| There are many forms of lime: (1) Ground limestone
(CaCO3). A trial by Myer and Volk (1952) on the fineness of grinding showed the
effectiveness of particle size on the yield of Lucerne and Soya bean (Table 7). The work
suggested that limestone sized from 60 to 100 mesh (i.e. 250-150 m n) was most effective
in increasing yield. Work from ADAS (1992) showed a similar result with winter wheat Total
Neutralising Value (TNV) of pure limestone is 100. |
| Some limestone deposits contain a percentage of Magnesium carbonate. The
ground limestone from such deposits may be brownish in colour, and its neutralising value
is slightly higher than that from deposits of pure ground limestone. |
Table 7: Effects of particle size of limestone on yield of
Lucerne and Soya beans
(Meyer and Volk, 1952)
Particle Size |
Plant Yield (oven dry wt per
pot mg) |
(mesh) |
Lucerne |
Soya Beans |
5-8
8-20
20-30
30-40
40-50
50-60
60-800
80-100
<100
No Lime |
5.3
5.6
7.5
8.0
8.2
8.8
9.5
9.5
9.5
4.6 |
7.8
-
10.8
-
12.2
-
13.3
-
11.9
- |
|
Calcium Oxide Burnt Lime or Quicklime (CaO): This can act somewhat more
rapidly than ground limestone. The TNV is 178 and 560 kg of CaO is equivalent to 1 ton of
ground limestone. |
| Hydrated lime, Slaked lime Ca (OH)2. This reacts very rapidly
and has a TNV of 135, thus 740 kg of slacked lime is equivalent to one ton of ground
limestone i.e. the TNV = 135. |
| Other materials can be used for liming soils. Lime rich sea sand has been
used on the west coast. The TNV is very variable and the particle size is generally large
and therefore it is slow acting. |
| Calcified sea weed is also used. The TNV is somewhat similar to ground
limestone and is often finely ground before use. Grinding helps the distribution in the
soil. |
| Sugar factory sludge is usually has large amounts of water (up to half its
weight) and about 30-40% calcium carbonate. It is an effective liming material and
contains useful amounts of nitrogen and phosphorus as well. It can be difficult to spread
evenly. |
|
