Forestry Network Newsletter

// The definition of peat soils

An anomaly has arisen in relation to the definition of ‘peat' within the Department of Agriculture.

The ‘Nitrates' Regulations 2006 [European Communities (Good Agriculture Practice for the Protection of Waters) Regulations, 2006 (S.I. No. of 2006)] Schedule 1 has been amended to clarify the definition of peat soils. This is given as ‘soils with an organic matter content exceeding 20%'. [Department of Environment, Press Release 19 July, 2006.]

However the definition of peat soils used by the Forest Service was given by the Minister for Agriculture in a written reply to a Parliamentary Question this April. The soils must constitute ‘a peat layer greater than 30 cm on drained peats and greater than 45 cm on undrained peats.' [13955/06]. No reference was made to the organic matter in the soil. For her authority, she referred the Deputy to the ‘Peatlands of Ireland, Soil Survey Bulletin' by R.F. Hammond, 1979.

However, in Chapter 3 of Peatlands of Ireland, ‘Classification of Peat Soils' [R.F. Hammond, An Foras Taluntais, 1979], the author explains that a ‘knowledge of the different peat types, their related internal soil factors and fabric arrangement are important in considering the production and adaptability of these soils to various crops, and productivity under defined sets of management conditions can only be quantified when soil properties are known.'

He gives three definitions of peat soils. At the ‘highest level', the classification is based on ‘phytosociology and genetical concepts' - essentially the plant communities. This definition is useful for the classification of bog types. It was the difficulties the European Environmental Agency [EEA] experienced in interpreting the satellite mapping of Irish vegetation that led to the exchange between Ireland and the EEA over the amount of ‘peatlands' afforested between 1990 - 2000.

Hammond comments that this method of classification is of limited value for peatlands which are being used or are potential areas for agriculture, horticulture, or forestry as ‘such peatlands require a classification scheme which defines the special soil characteristics essential for evaluating their ultimate use.'

The definition given by the Minister is explained by Hammond as ‘from the early days of soil survey a depth of 30cm was taken as constituting a peat soil. This has in part conditioned the acceptance of a definition based on depth.' However, Hammond is adamant that ‘productivity under defined sets of management condition can only be quantified when soil properties are known.'

He therefore gives the third definition which is reflected in Schedule 1 of the ‘Nitrates' Regulations 2006:

‘Organic soil materials that are saturated with water for prolonged periods, or are artificially drained, and have 30% or more organic matter if the mineral fraction is 50% or more clay, or 20% or more organic matter if the mineral fraction has no clay, or proportionally intermediate organic matter contents if the clay fraction is intermediate'

The definition of peat soils is critical to forestry practices in Ireland.

In initial afforestation, the soil type governs the suitability of a site for afforestation and the degree of restrictions necessary for environmental protection. Applications for felling licenses on peat soils must also be treated differently.

This is because there are three specific concerns associated with forestry on peat soils.

Acidification

Peat soils are naturally acid. Acidification is accelerated on poorly buffered soils by afforestation with conifers. In extreme situations, acidification of surface waters can cause mobility of aluminium from the soils and rocks which can be fatal to many forms of invertebrates and aquatic organisms - e.g. salmon and trout.

Siltation

Siltation is likely to occur when fragile peat soils are disturbed because of the lack of soil structure. Siltation in itself can result in mortalities and is responsible for smothering the gravel beds where fish lay their eggs and juvenile mussels live exclusively within the gravel. In cases of extreme siltation (or pollution) the innate response of the fresh water pearl mussel is to ‘clam up' and remain so until the detrimental conditions have passed.  If these conditions last for more than a few hours, the mussel becomes stressed and may die from lack of oxygen.

Phosphate enrichment

Phosphorous is generally immobile in mineral soils where leaching is unlikely under forestry, although surface runoff is possible. However, on peat soils recent research [Farrell 2004] has confirmed that phosphorous is leached at clearfell from these soils. The availability of phosphorous to the trees begins to decline at a pH of 5 to be virtually unavailable below a pH of 4. This is the case in acid soils including peat, peaty podsols and peaty gleys, where phosphorous is locked up as insoluble iron or aluminium phosphates and is unavailable as a nutrient for plants. Low pH tends to mobilise ions bound to soil particles and they are then readily leached into watercourses, potentially causing eutrophication which can be fatal to many forms of aquatic life. Phosphorus is used extensively to raise otherwise poor yield classes on peat soils where it is applied at the rate of 350 kg/hectare per application [Forestry Standards, 2001]. This practice can cause significant and long term environmental damage to fragile ecosystems.

The current application form for initial afforestation shows only two soil types, mineral and peat. With the definition based solely on depth, many sites which are peat soils in terms of their soil properties and pose threats to the environment when fertilised remain unconsidered.

At the felling stage, clearfelling on peat soils is especially destructive, creating as it does a pulse of both siltation and nutrients - phosphates and nitrates - which are potentially lethal to vulnerable species, including the endangered fresh water pearl mussel. No environmental information is normally required with a Felling Licence although in these cases the application for afforestation was made 35 or more years ago, when our understanding of soil sciences and forestry was poorly developed.

It would, therefore, be prudent to seek to have the Forest Service apply the same definition of peat soils as that agreed with the Commission for the ‘Nitrates' Regulations 2006 and to ensure that this definition forms part of all considerations of applications for afforestation and felling, particularly in catchments that have been designated for protection for listed species under the Habitats Regulations 1997.

[This document was submitted by registered post to David Byrne, Assistant Secretary of the Department of the Environment in charge of Forestry on 7 August, 2006. Reminders were issued on 1 September, 2006 and again on 21 September 2006. No acknowledgement or reply has yet been received.]