| (3) LIME LOSSES |
| Lime is constantly lost from soil through a series of mechanisms |
| 1. |
Crop losses |
| 2. |
Drainage losses |
| 3. |
Air pollution |
| 4. |
Fertiliser/manures |
|
| A. Crop Losses |
| Since Ca is an essential plant nutrient, it enters into the composition of
all crops and the removal of these crops, either directly, or through the feeding of
livestock, leads to depletion of the soils’ lime reserves. With crops like cereals,
very little, if any of this Ca is returned to the soil. With crops utilised for stock
feeding much of it is returned. Johnston and Whinham (1980) reported offtakes of 3 kg/ha
of calcium for potatoes, while Wild (1988) quotes Johnston and Whinham (1980) give
removals of 65 kg/ha of Ca in grass, although much of this is returned either directly or
via the slurry. Gately (unpublished data) calculated that the losses via crop or animal
are probably less than 100 kg limestone equivalent per hectare per year. |
| B. Drainage Losses |
| Rain and soil water are charged with carbon dioxide (Carbonic acid), which
dissolves the lime contained in the soil. Some of this dissolved lime finds its way down
into the drainage system thus being lost from the soil. The rate of loss depends on the
rainfall, the texture of the soil and the amount of calcium in the soil. In the U.K.
losses equivalent to 400 kg limestone equivalent per ha per annum have been measured in
drainage water. Long term studies in Ireland indicated losses in the range 250-500
kg/ha/annum (Gately, unpublished data). |
| C. Air Pollution |
| Air pollution contributes to losses via the drainage mechanism. The lime
losses in drainage are even greater when rain water contains other acids besides carbonic
acid e.g. sulphur and nitrogen dioxide arising from combustion of fuels can be washed into
the soil by rain. This can greatly increased the lime loss. SO2 is the main
ingredient of ‘acid rain’. |
| D. Fertilisers |
| One of the causes of soil acidification on agricultural land is the
application of ammonia N fertiliser. Kennedy (1992) calculated that in the order of
400-1000 kg of limestone is required annually in grassland systems based on using ammonium
nitrate. Ammonium salts create acidic conditions. In the nitrification process H+
ions are created and this leads to acidity. The most common form of N fertiliser used in
Ireland is CAN. This contains 20% of ground limestone as filler and this neutralises at
least some of the acid generated. |
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