2 Soil classification

Soil classification

Soil types are defined by their profile, or sequence of layers (horizons). The nature of horizons is due to the action of the main soil-forming process(es) operating in that soil type. Most soils have A, B and C horizons; podsol are typified by changes in the B horizon, lithomorphic soils do not have a B horizon; peats only have O horizons. Many horizons have subtypes indicating more detail about the nature of that horizon. The site descriptions that follow include a description of each soil profile, so a brief definition of the main horizon types is given below.

Soil Horizons

L litter layer above soil profile.

O surface accumulation of organic matter (peat) overlying the mineral soil.

A surface layer of mineral soil mixed with well-humified organic matter by organisms or cultivation.

Ah topsoil with more organic matter than A.

E mineral layer underlying an O or A horizon with less organic matter, iron or clay than the layer beneath, due to downward movement. Usually light in colour, often grey or white.

B subsoil layer underlying an O, A or E horizon and showing obvious alteration of parent material, or deposition of organic matter, iron or clay from overlying layers.

Bh with an accumulation of humus; typical of some members of the podsolic soil group.

Bt with an accumulation of translocated clay.

Bf with an accumulation of iron and aluminium oxides, often cemented; typical of some members of the podsolic soil group.

Bs subsoil layer characterised by bright orange-brown colour indicating enrichment with extractable iron and aluminium oxides; typical of the podsolic soil group.

C subsurface layer, excluding bedrock, showing very little evidence of alteration but may contain accumulation of soluble salts, or show evidence of gleying.

CG intensely reduced (gleyed) layer characterised by greyish, greenish or bluish colours.

R hard bedrock showing little sign of disturbance or alteration.

Soils are classified using a hierarchical system for England and Wales

There are ten major soil groups all of which are present in Wales, many of these major groups are subdivided into groups and subgroups.  For example the brown soils include brown earths, brown calcareous earths, brown sands, brown alluvial soils and argillic brown earths.  These groups reflect variants on the basic definition and add detail such as texture or parentage.  Each subgroup is further divided into soil series, defined as “similar profiles…under similar conditions .. on one parent material” , which usually have a geographical name suggesting where they were first mapped. Sadly, many of the original (Welsh) series names have been lost due to rationalisation (the Ynys has been subsumed by the Wilcocks series), though many important soils retain their Welsh names.

Terrestrial Raw Soils

Well drained sediments on land that as yet have no soil formation, e.g. the blown sand and dunes found along the Cardigan Bay coast and the west coast of Anglesey.

Raw Gley Soils

Sediments that have remained waterlogged since deposition and as yet have no soil formation, e.g. the mudflats an salt marshes found along many river estuaries and especially behind the shingle spits and sand bars caused by longshore drift.

Lithomorphic Soils

Soils which are less than 30cm deep and consist only of a topsoil over parent material which may be solid rock or loose sediment, e.g.

Rankers – non calcareous soils, such as on mountain tops etc.

Sand Rankers – non calcareous soils over loose sand, e.g. vegetated sand dunes on the west coast of Britain.

Rendzina – calcareous soil over chalk or limestone.

Brown Soils

Soils with well drained brownish subsoils, and most easily defined as lacking all the features of gleys, podsols etc.  They are the best all round agricultural soils, being deep, fertile, loamy, neutral acidity.  They aer characterised by broad transition zones, rather than horizon boundaries, due to the mixing action of earthworms across the topsoil/subsoil boundary.  Originally these soils would have developed under broadleaf woodland (or possibly natural grassland).

Brown Calcareous Soil – as above by pH>7 and containing calcium carbonate, therefore better structure and no chance of acidity, but with the risk of trace element problems.

Argillic Brown Soils  as above, but moderately acid and characterised by the downward migration of clay particles into the lower subsoil, producing E (eluviated) and Bt (textural change) horizons.  The important consequence of this process is the gradual blocking of the drainage pores in the soil, by definition about 30% minimum reduction in water movement.

Podsolic Soils

Defined by the presence of a bright orange coloured subsoil due to the weathering out of iron (and aluminium) by organic molecules released by either the vegetation growing on the soil or the surface organic layer.  Conifers, heathers and to a lesser extent beech produce organic acids known variously as chelates, tannins or polyphenols which react with iron and aluminium (and other metals) weathering them out of rock fragments, and assisting with downward leaching.  Podsolic soils are typically very acid (pH 4 – 5) with peaty surface horizons, but by definition very well drained, sandy or loamy textures and therefore potentially productive land.  The formation of posolic soils is favoured by a combination of high conifer/heath vegetation.

Brown Podsolic – characterised by the podsolising weathering process, but no vertical movement of iron, i.e. a peaty topsoil directly overlies the bright orange subsoil.  These soils have the potential for very productive hill grassland, or in favourable positions (south facing slopes) for horticultural crops.  Improvement consists of high rates of lime potassium and magnesium.  The most extensive example is the Manod Series on rolling countryside throughout Mid-Wales.

Podsols – characterised by a grey horizon between the peat and the orange subsoil produced by the downward leaching of iron and aluminium which may or may not accumulate at depth to form an iron pan which can become completely impenetrable to roots and sometimes to water.  These soils are typically used for conifer plantations or heather moorland (grouse shoots).

Gley Soils

These soils are named after the gleying process, a series of chemical reactions that occur when oxygen is in short supply, typically because the soil is waterlogged.  The most noticeable effect is the chemical reduction of iron from Fe3+ to Fe2+ which is accompanied by a colour change from yellow/brown/red to bluish/greenish/grey, but other reactions include the reduction of nitrate to nitrogen gas, and of organic compounds to sulphides.  There are two soil groups with this process:

Surface Water Gley

These soils are formed in slowly permeable materials (i.e. usually clay textures, or with clay subsoils) where these mottled colours are present within 40 cm. below the surface, indicating seasonal waterlogging.  There are two main types.

Stagnogley – The typical heavy clay soil with poor drainage found extensively in lowland Britain, requiring artificial under drainage (pipes at 90 cm depth, permeable backfill to within 30 cm depth, mole channels at 45 cm redrawn every 3 – 5 years, and possibly also subsoiling).  Typical examples are the Denchworth Series around Cricklade and at Harnhill

Stagnohumic gley – very common in upland areas, consisting of a peaty surface horizon over a typical stragnogley type profile.  Commonly encountered in upland agriculture or forestry where they are most economically drained by ditching.  The most extensive example is the Wilcocks (formerly Ynys) Series in Wales.

Ground Water Gley

These are soils whose drainage problems stem not from the soil material (they are usually sandy in texture) but from their topographic position close to the water table;  typical examples are coastal marshes, river floodplains etc.  The profile and general appearance of these soils are very similar to stagnogleys, but without the blue clay as parent material.

Peat Soils

Peat is simply undecomposed organic material, accumulating either because conditions are too acid, or more usually because they are too wet.  Defined as deeper than 40 cm. with more than 50% organic matter.

Raw Peat – acid, infertile, very fibrous material, usually in upland areas, includes raised bog and blanket bog.  Usually colonised by sphagnum, eriophorum and molinia.

Earthy Peat – normally lowland peat in “flushed” sites, i.e. formed in depressions receiving drainage water from surrounding areas.  Most of these originated as open water, gradually infilled and colonised by reed beds, sedges (“Fen”) etc., followed by woodland (alder, willow etc.) (“Carr”) and other trees as the bog gradually became drier.

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