Where does soil come from?

Although we tend to refer to ‘the soil’, there are in fact many different types of soils found in Scotland, just like the numerous plants and animals that surround us. Unlike plants and animals, however, soils do not exist as distinct types but form a continuous pattern over the land surface. Frequently, one soil type gradually merges into another. The soil found at any given location reflects how the five soil forming factors interact there. The importance of each factor varies between different areas and it is this variation that produces different soil types. In most areas, the influence of human activities such as agriculture, forestry and building have modified the natural processes.

Only a very limited amount of information can be gained from looking at the soil surface. To find out more, it is necessary to dig a pit and expose a vertical cross-section of the soil. This is known as the soil profile – each soil type is distinguished by its profile, almost like a fingerprint. A soil profile is divided into a number of distinct layers, which are called horizons. Although we can use these horizons to identify many complicated subdivisions within the soil, a much simpler approach is to split the profile into topsoil, subsoil and parent material.

Soil Structure

Well structured soils have a good balance of clay, silt and sand particles, organic matter and pore spaces to ensure good drainage and aeration. Poorly structured soils either have too much sand, which does not hold together well, or too much clay. The clay binds together tightly, becoming hard and cloddy when dry, and sticky when wet.

Despite the wide variety of soil types which exist in Scotland, soil scientists have developed a classification system to make description and interpretation much easier. Using this system, we can identify four basic soil types in Scotland – these are termed peats, gleys, podzols, and brown forest soils. The characteristics of these soils are described (right).

Peat

There are two main types of peat.

Basin Peat develops in damp, low-lying areas such as marshes and bogs. Slowly decaying vegetation gradually builds up a series of layers in the open water, eventually forming solid ground.

Blanket peat, which is by far the most common form in Scotland, as its name implies ‘blankets’ the landscape.

Blanket peat forms in areas of high rainfall, often with low temperatures.

Dead plant material builds up faster than it can be broken down by soil organisms. Their activity is reduced under such harsh conditions.

Pure peats contain relatively little inorganic material.

Peatlands are very important habitats for nature conservation.

Brown forest soil

These are fertile, often deep soils that are favoured for agriculture.

The natural vegetation cover is deciduous woodland.

This produces a litter rich in nutrients and organic matter. It is rapidly broken down and incorporated by soil organisms.

These soils are not very distinctive visually. The main feature is a gradual lightening in colour as the organic content decreases with depth.

Most brown forest soils are now used for agricultural purposes. They require artificial additions of organic matter and nutrients to maintain fertility.

Brown forest soils are mainly restricted to the area around Aberdeen, Fife and the Lothians and parts of south west Scotland.

Gley

Gley soils are characterised by waterlogging, either permanent or temporary.

Waterlogging may result from the soil being poorly drained, perhaps through a high clay content or an impermeable layer. It may also be because the soil is at the bottom of a slope or in a hollow and water collects faster than it can drain away. Gardeners who have to deal with heavy clay soils will be familiar with this situation.

When a soil is waterlogged for a long period of time it may become anaerobic – oxygen is either absent or present at very low levels. If this happens, iron compounds in the soil are changed chemically from their normal red and brown forms, to forms which are grey and green in colour.

Scientists can use the the pattern of these different colours in a gley soil to interpret the various processes which are taking place.

Podzols

Podzols are probably the most visibly distinctive of the main soil types in Scotland. They form in acid, coarse textured, well drained materials.

At the surface, a layer of dead vegetation builds up which is only slowly broken down. This is because soil organism activity is reduced under acid conditions.

The surface vegetation is usually coniferous woodland or heather moorland. Both of these produce leaves fairly resistant to breakdown.

Below, organic matter and nutrients, along with iron and aluminium compounds, have been leached out. It is mainly these substances that gives soils their brown colouring, so here the soil takes on a bleached appearance.

Organic matter is often redeposited in a thin black layer. Finally, the iron and aluminium compounds are redeposited in an ochreous (orange-brown) layer.