Soil development

The nature of a new soil (regosols) is largely determined by the rock minerals from which it is formed, but it continues to undergo changes under the influence of climate, vegetation, topography and drainage. These interact over time to give rise to characteristic soil profiles in different parts of the world. The soils that develop can be described in terms of the characteristics of the different horizons (layers) that make up the soil profile.

The 'O' or 'L' horizon is the organic matter found on top of the mineral soil and commonly referred to as the litter layer. The upper layer of the soil, from which components are normally washed downwards, is the 'A' horizon. This is usually recognized by its darker colouring, which is a result of the significant levels of humus present. The lighter layer below it, where finer materials tend to accumulate, is the ' B ' or illuvial horizon. Under cultivation, the 'A' horizon broadly aligns with the 'topsoil' and the 'B' with the 'subsoil'. The parent material below these is the 'C' horizon and where there is an underlying unweathered rock layer it is often known as bedrock.

Soils of the British Isles

In the British Isles four main types of mineral soil are found; brown earths, gleys, rendzinas and podsols (see Figure 17.6 ). Peats develop in waterlogged conditions (see organic soils p328).

Brown earth soils develop in the well-drained medium to heavy soils in the lowlands of the British Isles. They are associated with a climax vegetation (p52) of broad-leaved woodland especially oak, ash and sycamore, the roots of which have ensured that nutrients moving down the soil profile are captured and returned to the soil via the leaf fall. Surplus water does not accumulate and the soil remains aerobic for most of the year. The plentiful earthworms incorporate the deep litter layers. The resultant dark A horizon ('topsoil') rich in organic matter merges gradually into a bright brown and deep B horizon ('subsoil'). The soil structures that develop in the surface layers are granular and rounded fine blocky in which there is an excellent balance of air and water and into which roots can readily penetrate.

Brown earths are usually mildly acid (pH 5.5 to 6.5), but acid brown earths (pH 5.5 to 4.5) can develop on lighter textured soils in wetter areas

A soil horizon is a specific layer in the soil seen by digging a soil pit. The layers revealed make up the soil profile.

Agricultural Soil Profiles
Figure 17.6 Soil types: (a) Podsol; (b) Gley; (c) Brown earth; (d) Rendzina.
Brown Earth Soil Profile
Figure 17.7 Soil colours. Brown (top), Mottled, Gley (bottom).

(800-1000 mm rain per year) especially under beech or birch woodland. There is less earthworm activity, with a resultant reduced incorporation of organic matter down the profile. The soil structure is usually less satisfactory and clay particles that work their way down can form clay pans (see p313) in the B horizon. These can be productive soils if ameliorated with lime and fertilizer (see Chapter 21).

Gleys occur in poorly drained soils (see p343). Surface water gleys are found where the percolation of water is restricted by the poor structure in the A or B horizon to produce a perched water table (see p340). This is typically where the subsoil is heavy and impervious, especially in wetter regions. Oxygen in the waterlogged soil is depleted and, in these anaerobic conditions below the water table, the iron oxides that colour the soil become dull grey or bluey (in aerobic conditions the iron oxides are rust coloured). The extent of the waterlogging that the soil has been subjected to as the water table fluctuates can be judged from the degree to which it has become completely grey; usually there is a rusty mottle present, indicating that aerobic conditions exist in the soil for part of the year (see Figure 17.7). Plants growing in them are often shallow rooted and suffer from drought in dry periods. These soils are only productive after they have been drained, limed and fertilized.

Ground water gleys develop where there is a permanent water table that is very near the surface of the soil, so that to lower the water table drainage has to be undertaken on a regional basis, e.g. Romney Marsh. Drainage pipes can only be used when the water can be run to a ditch with a water level below that desired in the field (see p344); for some areas this can only be achieved by maintaining an artificially low level by the use of pumps (powered in former times by windmills).

Podsols (from the Russian 'under-ash') are strongly leached, very acid soils that develop on freely draining soils, such as coarse sands and gravels, commonly under heather or pine or spruce forest in high rainfall areas. Because of the high acidity levels, earthworms are absent so there is a build-up of the litter layer. Poorly decomposed organic matter that is not incorporated (a 'mor' humus) is characteristic of this soil type. Some of the organic matter combines with the iron in the top layers to form soluble compounds which are leached ('podsolization') to leave a grey ('ash-like') A horizon (all that remains are bleached sand grains). These compounds become insoluble again in the conditions that prevail in the B horizon, where organic matter accumulates to create a dark or black horizon below which is an iron rich red layer. The iron compounds that accumulate can form a strongly cemented 'iron pan'. As a continuous pan that water (and roots) cannot penetrate is formed, a waterlogged area develops and peat can form at the surface. Podsols are prone to drought and are 'hungry' soils that require considerable ongoing inputs of lime and manure to make them productive. They are of little use in horticulture except for the growing of acid loving trees and shrubs.

Rendzinas are very thin dark-brown, sometimes black, soils with a strong granular structure sitting directly on chalk or limestone. They are typical of the soils on the steeper slopes of chalk or limestone hills under grass. Shallow soils develop because of the continued erosion on the slopes, which also keeps these soils heavily charged with lime. Where the soils become deeper on the less severe slopes it is common for the A horizons to become acid, as the lime is leached downwards. They are well drained because of the slope and because of the porous nature of the underlying rock. Rendzinas are not suitable for most horticultural purposes because the high lime content causes induced nutrient deficiencies (see p372). Roots are severely restricted by the shallow soils and vulnerable to drought.

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