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  Liming in forestry.  

Agricultural lime - the natural solution

Trees, particularly conifer species, prefer acid soil conditions and can tolerate very low soil pH (pH<4.0). There is therefore no requirement to apply lime to forest soils for the purpose of improving tree growth. In those situations where soil acidity may be limiting to certain broad-leaved species, alternative 'tolerant' species are planted.

Liming is used in UK forestry solely as a treatment to ameliorate stream water acidity. Many forests are located within regions where there is a serious problem of acid waters e.g. parts of central and south-west Scotland, Cumbria, the Pennines and central and north Wales. These areas are underlain by acid, poorly buffered soils and rock and receive large inputs of acid pollutants from the atmosphere-the primary cause of surface water acidification. A century and a half of air pollution has resulted in the decline or complete loss of fish populations within affected waters.

Trees are able to filter out and capture more acid pollutants from the atmosphere than shorter crops due to the greater size and height of their canopies. Because this 'scavenging' effect could contribute to the further acidification of stream waters, the Forestry Authority recommend liming for new planting proposals within those areas at risk (see Forestry Commission's Forests and Water Guidelines (1991) published by The Stationery Office, London).

Liming in Forestry Picture

The benefits of lime applications to forest landscapes are two fold;

1. To dramatically increase water quality where acidification has become a problem or maintain water quality.

2. To promote biodiversity and wildlife.


Although soil liming in combination with fertiliser is being used on a widespread scale in central Europe for ameliorating soil and groundwater acidification and revitalising forests damaged by air pollution, this treatment is not recommended in the UK. This is because UK forests have not been directly affected by the lower air pollutant concentrations in this country and the fact that acidified areas are drained mainly by surface waters rather than groundwaters. Research has also shown that liming of our generally nutrient poor forest soils can result in a decrease in tree growth, which may last for between 5 and 20 years. This growth reduction is believed to be due to the lime reducing the soil nitrogen supply to the forest crop.

Research studies have shown that the most effective way of ameliorating the acidity of surface waters is to apply a large amount of fine powdered limestone (50%< 10 micron) to the boggy, usually unplanted, source areas in the headwater regions of sensitive catchments. These are the areas through which much of the run-off passes from the surrounding land on route to the stream, particularly after sustained rainfall when most drainage is near to the soil surface and acidity levels are at their greatest. Fine powdered limestone must be used in order to achieve the rapid rate of neutralisation that is required under high flow conditions. The current recommendation is to treat these areas by hand or all-terrain vehicle at a rate of 15 tonnes per hectare. A helicopter will usually be required to deliver the lime to such remote sites.

A headwater liming treatment is expected to remain effective at ameliorating stream water acidity for a period of between 5 and 10 years. This is the time it takes for the surface applied limestone either to be exhausted or to move down the soil and become unavailable to neutralise surface run-off. Repeat applications will be required to be made until air pollutant emissions from industry and cars are sufficiently reduced to protect a given site from acidification.

The need for liming and the exact treatment details will be addressed by the Forestry Authority in consultation with an applicant and the water regulatory authority when a grant application is received for new planting. Because liming may adversely affect the conservation value of treated sites the appropriate national conservation agency will also be consulted. Liming will only be allowed in areas where there will be no detriment to the existing flora and fauna.

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