The formation and properties of soil, the soil-forming factors, the soil profile and horizons, and the characteristics of the podzol, brown earth and gley soils.

What this key area is asking

The SQA wants you to explain how soil forms, name and explain the factors that control it, describe a soil profile and its horizons, and compare the three soils on the specification: the podzol, the brown earth and the gley. You should be able to draw and annotate a profile for each and link each characteristic back to a soil-forming factor or process.

How soil forms

The soil-forming factors control how a soil develops:

• Parent material: the underlying rock or drift, which sets texture, mineral content and acidity. Acidic granite gives infertile soils; basalt or limestone gives richer ones.
• Climate: temperature and rainfall control the rate of weathering, the strength of leaching and the speed at which organic matter breaks down. Cold, wet climates slow decomposition and encourage leaching.
• Relief: slope affects drainage and depth. Steep slopes give thin, dry soils; flat or hollow ground stays wetter and deeper.
• Organisms: plants add litter; bacteria, fungi and earthworms decompose it, release nutrients and mix the horizons.
• Time: mature soils with clear horizons take hundreds to thousands of years to develop.

The soil profile

The three soil types

Podzol

Found in the cool, wet uplands under coniferous forest and heather, for example the Cairngorms. Heavy rainfall causes strong leaching, washing iron, aluminium and humus from the upper soil and leaving an ash-grey leached layer in the A horizon. These minerals accumulate lower down, often forming a hard iron pan that blocks drainage and waterlogs the surface. Acidic pine and heather litter decays slowly in the cold into an acid mor humus, so the soil is sharply layered, acidic and infertile.

Brown earth

Found under deciduous woodland in warmer, lowland Britain such as central Scotland and the Borders. Leaf litter decays quickly into a mild, nutrient-rich mull humus, and abundant earthworms and soil organisms mix the horizons so the boundaries are blurred. The soil is well drained, slightly acidic, deep and fertile, which is why much of it has long been cleared for farmland.

Gley

Found on flat, poorly drained ground where water cannot escape. Waterlogging removes oxygen, so the soil is anaerobic; iron is chemically reduced and turns the soil a blue-grey colour, with orange mottling where some air reaches it. Gleys are wet, cold and difficult to farm without artificial drainage.

Examples in context

Example 1. Cairngorms podzols and land use. Across the Cairngorms plateau, on acidic granite parent material under heather moorland and pine, podzols dominate. Their ash-grey leached horizon, iron pan and acid mor humus make them infertile and poorly drained, which is why the land supports rough grazing, grouse moor and forestry rather than arable farming. The soil characteristics directly explain the limited agricultural land use, a link the SQA rewards.

Example 2. Central Scotland brown earths. In the warmer, lower-relief Central Lowlands and Border valleys, brown earths developed under former deciduous woodland. Their mull humus, earthworm mixing and free drainage make them fertile and easy to work, so they were cleared early and now carry mixed and arable farming. Comparing the upland podzol with the lowland brown earth in the same country shows how parent material, climate and relief together produce very different soils and land uses.

Try this

Q1. Describe and explain the main features of a podzol soil profile. [4 marks]

• Cue. Heavy rainfall and leaching produce an ash-grey leached upper layer; iron accumulates as an iron pan in the B horizon; acid mor humus from conifers and heather decays slowly, leaving an infertile, acidic soil.

Q2. Explain why a gley soil is poorly suited to farming. [3 marks]

• Cue. Poor drainage and waterlogging make it anaerobic; the cold, wet, blue-grey soil with mottling starves roots of oxygen and needs artificial drainage before crops will grow well.