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Cutover and Cutaway bogs

There are three main types of peatland occurring in Ireland, namely fens, raised bogs and blanket bogs. Fens can be described as mineral-rich or "minerotrophic", while raised and blanket bogs can be described as mineral-poor or "ombrotrophic". This broad classification scheme is related to the source of the nutrients available to the growing plants that eventually form the peat. The minerotrophic fens get their nutrients from the mineral-rich groundwater. On the other hand, the ombrotrophic peatlands get their nutrients from the rainfall, which is a very poor source of minerals.

Peatland classification schemes have been drawn up, to accurately describe and differentiate the different peat types, and the different peat residues which remain after harvesting or reclamation. Table 1 describes the classifications that refer to cutover and cutaway bogs (after Hammond, 1981).

Table1. Peat classification relating to cutover and cutaway bog.

Cutover bog		Cutaway bog
Turbary complex	Ombrotrophic peat remaining after peat has been removed for fuel, by hand cutting or using tractor-mounted harvesting machines	Boora complex	Mainly minerotrophic peats remaining after milled peat harvesting has ceased
		Clonsast complex	Mainly ombrotrophic peats remaining after sod peat harvesting has ceased

The Peat Enquiry Committee in 1917 (Anon., 1921) observed that large areas of cutaway and cutover bog had already been reclaimed for agriculture. The reclamation method usually included 'marling' or mixing of sea sand or domestic fuel ash with the top 15-20 cm peat. An additional classification system has been devised to describe the outcome of various different reclamation procedures on different cutaway or cutover bogs (Table 2).

Table 2. Peat classification relating to peatlands reclaimed for agriculture.

Reclaimed Fen		Reclaimed Raised bog		Reclaimed Blanket bog
Banagher	Drained and reclaimed minerotrophic fen peatland	Gortnamona	Drained and reclaimed farmer cutover peatland	Gweesalia	Low level blanket peatland reclaimed by sea-sand addition
		Garrymona	Drained and reclaimed ombrotrophic peatland, which has had little or no peat harvested

Distribution
A survey in 1810-1814 by the Bog Commissioners, estimated that there was almost 1.2 million hectares of peatlands in the Republic of Ireland. Table 3 gives a breakdown of how much of the estimated original areas of the different peatland types, have been modified by peat harvesting and reclamation (from Hammond, 1981). The blanket bog areas are predominantly located in the west of Ireland and on mountain tops throughout the country. Most of the peat harvesting from blanket bogs, was by hand or tractor-mounted machine. The raised bogs are generally located in the midlands and have deep peat deposits, which makes them most suitable for industrial peat harvesting. Fens are located at the edges of raised bogs, and along river valleys throughout the country. A must larger proportion of fens have been reclaimed for agriculture, as the fen peat is more nutrient rich than the peat in raised or blanket bogs, and can be very successfully turned into grassland.

Table 3. Approximate areas of peatland modified by peat harvesting or reclamation.

Peatland type	Original area (ha)	Modified by turf cutting or agricultural reclamation (ha)	Modified by industrial harvesting (ha)
Fen	92,500	90,000	0
Raised bog	311,300	180,000	88,000
Blanket bog	775,000	200,000	8,000

Peat types, physical and chemical properties
There are a number of different peat types found in the peatlands of Ireland. The differences between the peat types are based on (1) the plant remains that make up the peat and (2) the degree of decomposition or humification. Different peat types, also have widely different chemical and physical characteristics.

Blanket bog:
The peat in a blanket bog is generally consistent throughout the peat profile. The peat is derived from the plant remains of mainly grasses, sedges and heathers. The peat is very dense and very highly decomposed. This means that there is a very slow downward movement of water through this peat. In general, lying just above the sub-peat mineral soil, there is a thin layer of very highly decomposed peat, which has the consistency of shoe-polish. This acts as an impermeable barrier, which prevents the downward movement of water to the soil, and creates the waterlogged conditions favourable for peat production. Blanket bog peat is very low in nutrients, as its only source of nutrients is from the rainfall. However, it is a dense peat, which makes it very good for burning, after harvesting and drying.

Raised bog:
Despite its appearance, a raised bog comprises a number of different peat types. In a typical raised bog, there is a stratigraphy of peat types which generally occurs in the following order from the top of a raised bog dome: Poorly humified Sphagnum peat, Highly humified Sphagnum peat, Woody Fen peat, Phragmites peat. Woody fen peat and Phragmites peats are minerotrophic fen peats, which occur beneath the ombrotrophic Sphagnum peats.

Poorly and Highly humified Sphagnum peat
The upper layers of a raised bog comprise ombrotrophic peat types, which are highly acidic and nutrient poor, as they derive all their nutrients from the rainfall. The dominant plant remains are Sphagnum mosses, bog cotton (Eriophorum) and heathers (Calluna). This peat type can be highly humified (decomposed), or poorly humified. The poorly humified Sphagnum peat is most in demand for horticultural use, as it has a high water-holding capacity and is generally free draining, with a pH of between 3.3 and 4.2. On the other hand, it is a very poor peat type for burning, either for domestic or industrial use. This is sometimes referred to as "white peat" by turf-cutters.

Highly humified Sphagnum peat is often referred to as "red bog" by turf cutters. It is a very dense peat type, and has a high calorific value for burning. Downward water movement in this peat type is very slow, and it has a pH of approximately 3.7.

In cutaway bogs, it would be rare to find either of these peat types, as it would be expected that these peat types would have been removed by industrial peat harvesting.

Fen
Fen peats occur beneath raised bogs, and also at the edges of raised bogs, and, to a limited extent, in mineral-rich "flushed" areas of blanket bogs. They are minerotrophic, as they derive their nutrients from the mineral-rich groundwater and are often described as "black bog". There are two main fen peat types, Woody fen peat and Phragmites peat.

Woody-Fen peat
This peat type is made up largely of tree remains, with grasses and sedges. The peat is relatively free draining, and has a pH of 5.3. It is not as acidic as the Sphagnum peat types described above. This peat type is very desirable for agricultural reclamation, and is also a very good peat for burning. Its main disadvantage for reclamation, is the amount of fossil timber which is found in the peat.

Phragmites peat
This peat type mainly comprises the remains of the common reed, Phragmites australis. It is a very dense peat type, and is less free draining than the woody fen peat, and mineral rich with a pH of 5.9. However, it is an inherently difficult peat type for reclamation, as it has a tendency to form deep cracks or fissures on drying out. This peat type also tends to occur in the most depressed parts of the landscape, which further limits its ability to drain, as it may be located below the level of the outfalls. Phragmites peat is a good peat for burning, as it is very dense and highly humified. However, because of drainage difficulties, it is also the most difficult to harvest, especially by hand.

Sub-peat mineral soil types, physical and chemical properties
The soils that we are familiar with, have developed over thousands of years. After the last Ice Age, approximately 12,000 years ago, the country was covered with a layer of glacial till left after the retreating glaciers. The influences of the weather have worked on the glacial till, breaking down stones into fine sand, and leading to the downward movement of some elements such as iron and aluminium. Soils are aerobic (rich in oxygen) and most of the iron that is present in the surface layers of the soil is in the ferric form, which means it is brown in colour. In addition, due to the growth of plants on the soil surface and the biological life within the soil, there is a build up of organic matter, especially in the upper soil horizons, leading to the rich earthy colour with which we are familiar.

On the other hand, sub-peat mineral soils are generally grey in colour. They are often described as "relict" soils. Some have been exposed to weathering influences for a limited period, and have started the process of soil formation. Others have remained isolated from weathering influences since the end of the last Ice Age. These soils have been subjected to several thousand years of saturation beneath the bog, and are devoid of aerobic biological life and lack structure. Because they are lacking in oxygen (ie are anaerobic) the iron which is present is in the ferrous form, so the soils are generally grey/blue in colour. There is little or no organic matter incorporated into the mineral soils, as they have no aerobic biological life. The soils are generally very low in nutrients.

There are a number of different mineral soils which can be found beneath the peat deposits. There is a strong correlation between the type of sub-peat mineral soil and the topography of the bog floor, as well as a good correlation between the type of sub-peat mineral soil and the peat type overlying it. Sub-peat mineral soils can be divided into three broad groups: S1/S2 (weathered soils), S3(unweathered soils), S5 (lake marl and chalk mud) (Carey and Hammond, 1970).

S1/S2: Weathered sub-peat mineral soils
These soils generally occur on the uplifts in the sub-peat mineral soil topography. They remained uncovered with peat or water for a period of time after the retreat of the glaciers, and were exposed to weathering influences which began the process of breaking down the stones into fine sand or loam. Some plant roots also penetrated into the weathering soil. Some of the limestone material began to decalcify, which led to the downward leaching of carbonates in the soil, to a depth of 30 to 40 cm. The pH of the soil ranges from 5 in the upper decalcified layer, to 8 in the lower layers.

S3: Unweathered sub-peat mineral soils
These soils remained covered with peat or water from the end of the last Ice Age. They are found on the middle to lower slopes of the underlying bog floor topography. They comprise alluvial soils, consisting of gravels, sands, silts and silty clays. They have not been exposed to weathering influences, thus do not exhibit any soil profile development. These soils are highly dense, have very limited downward movement of water and are generally high in free carbonates. The pH is approximately 7 in the surface horizon, and 8 in the lower layers.

S5: Lake marl and chalk muds
There are extensive areas of lake marl beneath the bogs in the Shannon catchment. Lake or shell marl is formed when the bodies of gastropods and fresh-water molluscs which dwell in the calcium rich lakes, accumulate on the bottom of the lake, and form a thick layer of lake marl. If shells are present in the deposit, it is often called shell marl. This marl is very high in calcium, with a very high pH and is creamy-white in colour.

As stated earlier, there is a strong correlation between the sub-peat mineral soil type and the peat type overlying it. Phragmites peat is generally found overlying the unweathered sub-peat mineral soils and the lake marl and chalk muds. This is because both the peat and sub-peat mineral soils are generally found on the most low-lying parts of the landscape. On the other hand, woody fen peat is often found overlying a more weathered sub-peat mineral soil. The ombrotrophic peat types (poorly and highly humified Sphagnum peats) do not directly overlie any sub-peat mineral soils, as they are found on top of the fen peats.

The depth of the peat residue remaining after peat harvesting has ceased is crucial to the possible future uses. However, the actual depth required as been the subject of much debate. It depends on which future use the areas is being put to. And it also depends on the type of peat and the type of sub-peat mineral soils - and on the type of harvesting that has been carried out. All in all, there are too many unknowns to be able to categorically say what depth of peat should remain - and the undulations in the sub-peat mineral soil may be too variable to allow a stated depth of peat to be left. ....

References
Anon, 1921. Commission of Enquiry into the Industries and Resources of Ireland, Report on Peat. Stationery Office, Dublin.
Carey, M.L. and R.F. Hammond, 1970. The Soils beneath the Midland Peats. Irish Forestry, Volume 27: 23-36.
Hammond. R.F., 1981. The Peatlands of Ireland. An Foras Taluntais, Dublin. 60 pp.
Hammond, R.F., 1988. The Characteristics of Irish Midland Peatlands which can influence future land use programmes. In: C. Mollan (Ed.). The Utilisation of Irish Midland Peatlands, RDS. Pages 49 - 62.

Copyright Gillian Boyle
Research Administrator,
Office of Funded Research Support Services,
University College Dublin

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