LIME LOSSES

Unless steps are taken to redress the balance of soils by applying a liming material there will be a natural reduction in the lime status of most soils. This results in a natural increase in acidity and in many cases a reduction in soil fertility.
Losses occur as a result of:

- LEACHING
- CROPPING
- FERTILISING
- POLLUTION

AVERAGE LIME LOSSES CAN AMOUNT TO:

1250 Kg CALCIUM CARBONATE PER HECTARE PER YEAR

LOSS MECHANISMS

The diagram below will help to explain the following points on lime loss. Plants make the soil more acid by

Organic and inorganic acids formed during decomposition of crop residues (acid forming)
Plants remove calcium and magnesium from the soil (acid forming)
Deep tillage may bring acid subsoil into the root zone (acid forming)
Plants release hydrogen ions (H⁺) to the soil (acid forming)

Acid precipitation
The products of fossil fuel combustion from power plants, industry and motor vehicles are returned to earth in rainfall as nitric and sulphuric acid.

Leaching
Calcium and magnesium can be carried out of the root zone by the downward movement of soil water, frequently in association with nitrate (NO₃^-)

Ammonium rich fertilisers
Ammonium nitrate rich fertilisers contribute to soil acidity by nitrification of ammoniacal (NH₄) nitrogen to nitrate which releases hydrogen ions.

SOIL ACIDIFICATION

The increased acidification of the natural environment by various pollutants including 'Acid Rain' raises issues of ecological significance not only for the Agricultural Industry, but also for Forestry and the Water Industry alike.

Acid soils in large areas of coniferous woodland give rise to acidic moorland streams and water courses which are unable to support fish and various algal species. This in turn has accelerated the increased acidification of fresh water lakes and reservoirs with the consequent change in their water chemistry.

The richness of our plant and animal ecology, the diversity in our fauna and flora, the health of our woodlands, and the quality and balanced chemistry of our water are all influenced by lime losses and soil acidity.

There are almost no soils in the United Kingdom where the lime status is naturally retained without loss from the factors previously outlined. The exception arises where a significant movement of liming material takes place by ploughing - such as on some of the thin soils over chalk where the chalk is ploughed up in quantity.

Soils over limestone do not, by and large, achieve any benefit from the liming material lying below

pH VALUE AND LIME REQUIREMENTS

The degree of soil acidity or alkalinity is measured by what is known as the pH scale.

A figure of pH 7 represents a materials relationship to the neutral position of pure water at pH 7.0.

Figures below 7 indicate increasing acidity and above 7 increasing alkalinity.

The optimum for general cropping is between pH 6.8 and 7.0

For permanent grassland the optimum pH is slightly lower.

There are several methods of testing the pH of soils and with some knowledge of the relationship between pH and lime requirement for various soil types the pH can be used as a guide to the lime requirement.

*multiply by 0.4047 to obtain tonnes per acre

Notes:

Recommendations based on soil depths of 200mm/8in (arable) and 150mm /6in (grassland). Greater depths of soil will require more lime.
Maximum surface application for grass is 7 tonne/ha (3 tonne/acre). Minimum application rate: 2 tonne/ha (1 tonne/acre). Where application rates of over 10 tonnes/ha (4 tonnes/acre) are necessary on arable land the ALA recommends that lime should be applied as two dressings (first dressing ploughed in).
The application rates shown are based on material having a neutralising value of 54 and fineness of 40% passing 150 microns. When using other materials permitted under the Fertiliser Regulations the dressing should be adjusted accordingly. Thus coarser materials and those having a lower neutralising value will also require a heavier dressing.

The above table is a guide to the lime requirement but the actual assessment should be undertaken professionally.

SAMPLING FOR ASSESSMENT OF LIME REQUIREMENTS

Soil samples should be taken methodically from a number of places in the field and tested individually since acidity frequently occurs in patches in the field. Test results should be plotted on a field map so that any lime required may be applied in the right place and at the correct and most economic rate. Although poor and patchy crop performance and the presence of acid loving weed species are rough indications of lime deficiency, the acid reaction to indicator solution or pH meter is the only reliable method of assessing lime requirements.

Reports based on bulk field samples are rarely reliable for lime status as they average out variations across fields and fail to identify deficient the areas.

SOIL pH, NUTRIENT AVAILABILITY AND PLANT GROWTH

click on image to enlarge it

CROP REQUIREMENTS

The replacement of calcium lost from the soil by leaching and crop uptake is essential to maximise production and profits from cereal crops. The growth of cereals of high protein content depends on stability of pH during the growing cycle; barley is an example where sensitivity to soil pH is particularly apparent. Sugar beet also takes up nutrients most effectively in soils with a pH 6.5-7.0. Increasing acidity results in stunted plants and fangy roots.

The diagram below provides a guide to the optimum pH levels for some important crops. If soil pH is lower than the bottom of the indicated range, then crop yields will begin to suffer severely due to the crops' inability to tolerate that level of acidity.

Those crops which are tolerant to acidity would be more profitable at higher pH values. At a pH of 4.9 one is not getting as good a response from the fertilisers applied to maximise the potato crop as at the optimum pH. Lime also aids soil fertility in grassland and ensures that added fertilisers are utilised to maximum effectiveness and helps to increase crop yield either as hay, silage, or grazing. It is particularly important to adjust soil pH well in advance for sensitive crops such as oilseed rape, sugar beet, barley and peas. Spreading should be even, accurate, and cause little disruption to the soil structure.

OPTIMUM pH FOR CROP GROWTH

phrange.jpg (70421 bytes)

pH MANAGEMENT + NATURAL LIME = SUCCESSFUL CROPS

NATURAL LIMING MATERIALS

All ALA members supply agricultural lime in accordance with the current Fertiliser regulations and the 'Statutory Statement' is provided to the initial purchaser.

The UK has a good spread of deposits of natural chalk and limestone suitable for this purpose, though Scotland is short of limestone and relies on imports to a large extent from the North of England.

The Fertiliser Regulations control the sale of agricultural liming materials. Quarry-produced materials can be sold under a number of permitted names (the term Agricultural Lime' alone is insufficient), each name has a precise meaning and associated declarations, which are required by law.

Farmers should ensure that such information is available to them at the time of delivery (as required by law) and that the lime quality is clearly identified at the time of quotation.

NAME, MEANING AND DECLARATIONS OF SOME QUARRY-PRODUCED LIMING MATERIALS COMPLYING WITH THE CURRENT FERTILISER REGULATIONS

Name of material	Meaning	Declarations (see Note)
Ground limestone	Sedimentary rock consisting largely of calcium carbonate and not more than 15% of magnesium expressed as MgO and of which 100% will pass through a sieve of 5 mm, not less than 95% will pass through a sieve of 3.35 mm and not less than 40% will pass through a 150 micron sieve	Neutralising value Any amount of material as a percentage by weight that will pass through a 150 micron sieve
Screened limestone	Sedimentary rock consisting largely of calcium carbonate and not more than 15% of magnesium expressed as MgO and of which 100% will pass through a sieve of 5 mm, not less than 95% will pass through a sieve of 3.35 mm and not less than 20% will pass through a 150 micron sieve	Neutralising value Any amount of material as a percentage by weight that will pass through a 150 micron sieve
Limestone dust
Coarse screened limestone	Sedimentary rock consisting largely of calcium carbonate and not more than 15% of magnesium expressed as MgO and of which 100% will pass through a sieve of 5 mm, not less than 90% will pass through a sieve of 3.35 mm and not less than 15% will pass through a 150 micron sieve	Neutralising value Amount of material as a percentage by weight that will pass through a 150 micron sieve
Coarse limestone dust
Magnesian ground limestone	Sedimentary rock consisting largely of calcium and magnesium carbonates and not more than 15% of magnesium expressed as MgO and of which 100% will pass through a sieve of 5 mm, not less than 95% will pass through a sieve of 3.35 mm and not less than 40% will pass through a 150 micron sieve	Neutralising value Amount of material as a percentage by weight that will pass through a 150 micron sieve
Magnesian screened limestone	Sedimentary rock consisting largely of calcium and magnesium carbonates and not more than 15% of magnesium expressed as MgO and of which 100% will pass through a sieve of 5 mm, not less than 95% will pass through a sieve of 3.35 mm and not less than 20% will pass through a 150 micron sieve	Neutralising value Amount of material as a percentage by weight that will pass through a 150 micron sieve.
Course magnesian screened limestone Course magnesian screened limestone dust	Sedimentary rock consisting largely of calcium and magnesium carbonates and not more than 15% of magnesium expressed as MgO and of which 100% will pass through a sieve of 5 mm, not less than 90% will pass through a sieve of 3.35 mm and not less than 15%	Neutralising value Amount of material as a percentage by weight that will pass through a 150 micron sieve.
Chalk	Cretaceous limestone	Neutralising value
Ground chalk	Cretaceous limestone of which 98% will pass through a sieve of 6.3 mm	Neutralising value
Screened chalk	Cretaceous limestone of which 98% will pass through a sieve of 45 mm	Neutralising value
Ground burnt lime	Commercial calcium oxide containing not more than 27% magnesium as MgO and of which 100% will pass through a sieve of 6.3 mm	Neutralising value
Kibbled burnt lime	Commercial calcium oxide containing not more than 27% magnesium as MgO and of which 100% will pass through a sieve of 45 mm	Neutralising value
Burnt lime	Commercial calcium oxide containing not more than 27% magnesium as MgO	Neutralising value
Magnesian ground burnt lime	Commercial oxide obtained from magnesian limestone containing more than 27% magnesium as MgO and of which 100% will pass through a sieve of 6.3mm	Neutralising value
Magnesian kibbled burnt lime	Commercial oxide obtained from magnesian limestone containing more than 27% magnesium as MgO and of which 100% will pass through a sieve of 45mm	Neutralising value
Magnesian burnt lime	Commercial oxide obtained from magnesian limestone containing more than 27% magnesium as MgO	Neutralising value
Hydrated lime	Product obtained by slaking burnt lime or magnesian burnt lime of which not less than 95% will pass through a 150 micron sieve	Neutralising value
Mixed lime	A product resulting from mixing two or more forms of liming material specified in this schedule not being materials for which there is no minimum standard laid down in column 3 of this schedule, or material produced during the manufacture of commercial burnt lime or hydrated lime.	Neutralising value Amount of material as a percentage by weight that will pass through a 150 micron sieve

LIME EFFECTIVENESS

The effectiveness of a liming material is dependent upon its neutralising value, the fineness of grinding, reactivity and the relative hardness of the parent rock.

Neutralising values
The neutralising value of a liming material is expressed in terms of the percentage of calcium oxide equivalent. Thus, 100 kg of a liming material with a neutralising value of 52% will have the same neutralising value as 52 kg of pure calcium oxide (CaO). Neutralising value is determined in the laboratory and is calculated from the results of the chemical reaction with known strength hydrochloric acid, and always refers to the sample 'as received' rather than on a dry matter basis.

Reactivity
The effectiveness and speed of reaction of a liming material can be quantified in the laboratory using the "Reactivity Test". The results obtained from this test may be used to estimate the behaviour of a liming material in the soil. These results bear a good correlation with results obtained from long term pot trials.

The Reactivity Test involves the decomposition of the liming material in hydrochloric acid under stable pH conditions. The acid consumption during a given time is a direct criterion for the reaction time of the liming material being tested. The results of the test are expressed as a percentage, and they compare the speed and effectiveness of the sample with pure precipitated calcium carbonate.

ADAS FINENESS OF GRINDING TRIALS

COARSER MATERIAL REQUIRES A HEAVIER APPLICATION

For maximum effectiveness, the harder and less porous the parent rock, the finer the liming material must be ground. An indication of the importance of the fineness of grinding can be seen from the field trial results shown on the previous page.

These field trials were carried out over a number of years. In the short term, the effect of the finer liming materials was even more marked.

Where lime is applied to an acid soil, there is a well-proven relationship between the fineness of grinding and the crop yield response.

There is a considerable reduction in the effectiveness of liming materials containing particles above 600 microns (0.60mm, 60 mesh) unless the material is easily broken down.

Consequently, fineness is slightly less important in respect of:

a. Burnt lime which breaks down to a fine powder as a result of chemical reaction with water.

b. Soft, porous chalks which break down easily in the soil from the action of frost or the passage of cultivating implements.

The particle sizes of many waste limes such as sugar beet sludge are very fine, the moisture content can be high and the effectiveness of these materials can be impaired by the difficulty of obtaining an accurate and even spread. Uneven spreading may result in over or under liming.

Effective, economic liming will rely, not only upon the quality and suitability of the liming materials, but also upon having a soundly managed liming policy. The aim should be to maintain the lime status of the soil on a rotational basis. The cycle of the rotation will depend upon the locality, the type of soil, and the style of management. The rate of lime loss will need to be established from regular soil pH testing by ADAS or a reputable lime supplier. Annual lime losses vary considerably over the British Isles and a liming rotation cannot be established by guesswork.

The ultimate objective is to maintain the soil for arable crops at pH 7 and pH 6.5 for grassland. (Peaty soils should be maintained at pH 6.5 and pH 6 respectively).

Neutralising value, fineness and reactivity are the three factors which influence lime quality.

The answer to the question 'How often should I lime?' is, in general, 'as often as necessary to maintain a pH as near enough 7.0 on arable land, with permanent grassland at 6.5'. To achieve these soil pH values during the growing cycle, ALA recommends liming to Target pH values shown:

Liming may be carried out at any time when ground conditions are suitable. For arable land, lime is usually applied when the land is available after cropping. It is important to allow sufficient time for the soil to adjust to the correct pH before sowing. This can take several weeks, depending on the quality of lime used. It is particularly important to adjust soil pH well in advance of sensitive crops such as sugar beet, barley and peas.

Depth of Penetration
When agricultural lime is applied as a top dressing it will affect the surface layers. With time, rain will gradually wash the material to lower soil layers. When a soil is very acid throughout its depth, or when the lower soil layers are significantly more acid than the surface layers, it is advisable to plough in half the lime dressing and spread the remaining half on the ploughed surface. If such soils are only surface dressed there is a likelihood that the roots of acid sensitive crops will reach the acid layers of the soil and result in the crops exhibiting stress symptoms due to calcium deficiency.

FERTILISER UTILISATION

LIME IS A FERTILISER

The availability of plant nutrients is affected by the pH of soil. The major plant nutrients nitrogen (N), phosphorus (P) and potash (K), as well as calcium and magnesium, show a marked reduction in availability in acid conditions.

This diagram shows the scale of availability. The diagram illustrates the risk of shortage of iron, manganese and boron in alkaline conditions which, if not monitored, can give rise to specific problems in fruit and some root crops. The most important and significant illustration, however, is the increasing unavailability of the major and most commonly applied plant nutrients, nitrogen, phosphate and potash (N, P, and K) with increasing acidity. Maintaining an adequate balance annually requires constant attention and necessitates regular crop inspection and field-walking practice.

The addition of lime helps to release soil nutrients. Fertilisers and manure cannot be fully effective if the land is short of lime. In addition water that leaches from acid soils may contain undesirable materials that can adversely affect the quality of surface and groundwaters.

Heightened environmental controls and regulations on the disposal of sewage and other industrial wastes to landfill or sea outfalls have led to an annually increasing volume of application to agricultural land. These products do bring beneficial residual fertiliser and organic matter to the soil. However, problems do arise as these wastes also contain a number of metallic and other inorganic Potentially Toxic Elements (PTEs). With repeated applications these contaminants accumulate in the soil and can remain indefinitely, causing restrictions on plant growth, increased uptake of metals by animals and man via the food chain and reductions in soil microbial activity. Heavy metals become more available in acid soils and adverse effects will then increase. When sludge or waste is applied there will be a need to maintain alkaline pH values for an indefinite period thereby inhibiting the release of heavy metals, whilst gaining the manurial values of the material.

PLANT FOOD AVAILABILITY CHART

The coloured bands show how liming makes essential plant nutrients more available and toxic aluminium less available. A pH of 6.5-7.0 (just on the acid side of neutral) is the best level.

When straw is incorporated there is a need to encourage the activity of aerobic bacteria to accelerate decomposition. Bacteria can only flourish when the lime status is maintained.

With the fixed and other variable costs to add to the cost of fertilisers, to say nothing of rent, and return on capital, it is essential that the correct pH level for the crop to be grown is looked upon as good agricultural practice in the efficient management of any profitable operation.

Regular liming in order to maintain appropriate pH levels also helps in achieving the right balance between profitable farming and environmental protection.

Certainly there are few improvements so easily and cheaply carried out which can have so fundamental an effect on the success or failure of crops and farming.

LIMING FOR PROFIT - YIELD BENEFITS

LIMING FOR PROFIT

The trial shown on the right shows a liming trial.

The results from this trial showed us that four years after the initial lime treatments, further lime dressings were needed on plots receiving the 11t/ha (4.5t/acre) treatment. This is typical of the normal liming interval for light-textured soils in low rainfall areas.

The effects of the lime treatments on the growth and yield of sugar beet, spring barley and spring wheat were monitored for three years. Acidity symptoms were clearly visible in crops on the control plots which received no lime, but there was no root or leaf damage on any of the limed plots. Crop growth was, however, more vigorous on the plots receiving the highest rate of lime, showing low soil pH can have an adverse effect on growth, even when no acidity symptoms are visible.

Crop growth was also closely related to soil pH value and consistent yield increases came with increasing soil pH in all four years of the experiment. The yield responses resulting from an increase of 1 pH unit were 9.2t/ha (3.7t/acre) of sugar beet in the first year; 0.7t/ha (0.3t/acre) of spring barley in second year; 3.6t/ha (1.5t/acre) of sugar beet in third year; and 0.2t/ha (0.8t/acre) of spring wheat in fourth year.

The economic benefit from liming was assessed by calculating the cumulative value of the crop yield increases and comparing that with the cost of the liming. Returns were adjusted to first year values.

The benefits of liming vary but the greatest returns will be achieved by maintaining the soil pH in a narrow range within about 0.2 units of the optimum pH value.

When the cost of lime is related to the long-term benefits, it is one of the best investments you can make.

This is shown in the ALA liming yield benefit diagram above.

click to enlarge
The benefits of lime applications to arable crops were investigated by ADAS in an experiment sponsored by the Agricultural Lime Association. The trial site near Wetherby, North Yorkshire was on a sandy loam soil and had an initial pH value of 5.1. Lime requirement was 11t/ha (4.5t/acre).

The experiment assessed the effect of 0, 5.5, 11 and 22 t/ha (0, 2.2, 4.5, and 9t/acre) of screened limestone applied during first winter.

Soil analysis results confirmed progressive increases in soil pH values with increasing rates of lime application. The highest soil pH values were achieved in the first or second year after lime application. After that the pH level began to decline.

At no stage did the recommended dressing of 11 t/ha (4.5t/acre) achieve the target pH value of 6.7.

The results confirmed the reduced effectiveness of screened limestone compared with ground limestone.

Natural Agricultural lime is used to correct acidity and provide the right conditions in which plants and aquatic life can develop.
Natural lime in the form of calcium carbonate or magnesian limestone has a major influence on:

Plant development ●Bacterial activity in the soil ● Efficient use of fertilisers ● Water quality in rivers and streams ● Animal health Development of woodland areas ● Soil structure ● Efficient use of some herbicides

SOIL STRUCTURE

All plant and animal life has the best potential to grow and reach full maturity when in an environment best suited to its specific needs; similarly any restrictive condition will ultimately reduce its potential. Soils are the living environment, which provide the essential plant necessities of water, air and nutrients.

Good soil management requires three 'golden' basics;

Adequate drainage to prevent water-logging
The correct level of compaction for root anchorage and the movement of air
The correct pH level to allow the plant to grow and utilise optimally the available soil nutrients

Any imbalance of these basics will reduce a plant's ability to optimise its potential, irrespective of any further 'cosmetic' treatments.

Soil is a medium that enables worms and micro-organisms to live and carry out the function of breaking animal and plant residues. This assists the cycle of further plant development, but in doing so produces acid soils which display significantly reduced bacteriological activity.

This build up reduces pH levels, creates 'stale' soils and places plants under stress which reduces their ability to make effective use of the nutrients and organic matter available to them.

Heavy soils, particularly clays, although benefiting from regular liming tend to maintain pH levels since they have greater ability to retain calcium/magnesium ions and displace hydrogen ions. In clay soils the soil particles are of denser construction and do not allow so easily the free movement of bacteria and micro-organisms. The addition of lime modifies the characteristics of the clay particles so that they flocculate resulting in improved drainage and easier movement of all major ingredients in plant and life development.

Sandy soils have reduced capacity for holding liming materials and as a result of their free draining nature they require more frequent liming at lower dose rates. It is on these soils that troubles from acidity are most common and most acute, but easily remedied.

STOCK WELLBEING AND HEALTHY GRASSLAND

The benefits to stock of limed pastures are due mainly to increased intake from the more palatable and nutritious grasses (e.g. perennial rye grass) and clovers which, being more lime sensitive, are encouraged to replace the more acid loving, low calcium content, meadow foxtail agrostis and other weed grasses.

Trials conducted by ADAS over the years show liming will increase the percentage of calcium within a plant and will also encourage and help to maintain a sward with the highest nutritional value.

Where the correct soil pH status is maintained the activity of micro-organisms on clover roots will fix atmospheric nitrogen to the benefit of them and associated grasses. When the pH is allowed to fall to a critical level the micro-organisms can die and may take many years to re-establish even if the lime status is corrected. Magnesian limestone is a helpful addition to the soils where their ADAS nutrient index is below one (25 mg/ 1 Mg) and they are being used to establish grasslands.

AGRO-CHEMICAL EFFECTIVENESS

It has been shown that the efficiency of some pesticides, insecticides and residual herbicides applied to the soil can be affected by the degree of acidity of the soil. Soils deficient in lime absorb chemicals more strongly than well-limed soils and so the effectiveness of chemicals is reduced.

In neglecting to lime the correct pH level, usually 6.8 or above for arable crops, farmers may be risking inadequate weed pest control in addition to the adverse effects of soil acidity on the growing crop. At a soil pH of 6, which may be satisfactory for a crop like maize, it should be remembered that the activity of some herbicides is one half or less of the activity on a well-limed soil.

AGRICULTURAL LIME BENEFITS THE ENVIRONMENT

LIME & UPLAND CATCHMENTS by Dr. W A. Adams, The University of Wales, Aberystwyth

Agricultural soils in the lowlands of the UK are limed on a regular basis to maintain a pH of 6 or higher depending upon the cropping system. This general situation of neutral or near neutral soils does not apply to extensive areas of hill and upland. Here the benefits to herbage production enabled by liming may only be economic on small areas of the farm. Historically the practice of liming has moved up and down the hills depending upon the profitability of the livestock sector.

Soils in most of the uplands have low natural reserves of calcium. High rainfall and 'acid rain' have resulted in the loss of bases by leaching and a fall in soil pH to values of 4 or lower. The upland areas of Wales originating in unlimed rough grazing or forest pass are typical and in the very acidic soils, aluminium accounts for over 70% of the exchangeable cations and calcium a few per cent only. In limed soils, even when the pH is no higher than 5.5, there is no exchangeable aluminium, and calcium is dominant. Exchangeable cations are the immediate source of these. There is no need on agricultural grounds to increase ions in the soil solution which are available to plants and liable to loss by leaching. Aluminium is negligible in the soil solution of limed soils but is toxic to many plants in acid soils. It is necessary to lime soils as part of upland pasture improvement to prevent aluminium toxicity in ryegrass and clover.

The way pasture improvement has progressed in the uplands has resulted in a mosaic of limed and unlimed areas on many catchments. The patchwork of improved and unimproved pasture has important consequences for the area as a whole and the impact of liming should not be viewed as affecting agricultural production alone.

Limed soils support not only the sown herbage species, typically perennial ryegrass and white clover, but also bent grasses, fine fescues and herbaceous species that are intolerant of extreme acidity. Thus botanical diversity as a whole is increased. Earthworms, which are a food source not only for many birds but also for animals including badgers, cannot survive in unlimed soils.

Beneficial effects of liming are not restricted to the land itself, because the water draining from limed soils, especially those in coniferous woodland, are very acidic, high in aluminium and often low in bases. Biological diversity is restricted over a wide range of species ranging from algae to insects to fish and the birds and animals that depend upon them as a food source. A dramatic improvement in stream water quality is usually observed as streams originating in unlimed rough grazing or forest pass through improved pasture.

There is no need on agricultural grounds to increase the area of improved land in the uplands but the substantial contribution made to environmental quality by the current mosaic of limed and unlimed areas should be recognised. A strategy for liming in the uplands is needed to exploit its potential contribution to catchment management.

LIMING IN FORESTRY by Dr. T. R. Nisbet. Forestry Authority Research Division

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).

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, rather than ground water. 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.