Soil profile and formation
Example of soil horizons
As soil develops over time horizons (layers) form and collectively constitute a soil profile. Most soil profiles cover the earth as two main layers—topsoil and subsoil.
The properties of horizons are used to distinguish between soils and are critical for determining land-use potential.
Most soils exhibit three main horizons including:
- A horizon—humus-rich topsoil (where most plant roots, earthworms, insects and micro-organisms are active)
- B horizon—clay-rich subsoil
- C horizon—underlying weathered rock (from which the A and B horizons form).
Many soils also have an O horizon which mainly consists of plant litter which has accumulated on the soil surface.
Soil forms continuously, but slowly, from the gradual break-up of rocks through physical, chemical and biological processes—known as weathering. The accumulation of material through the action of water, wind and gravity also contributes to soil formation.
These processes can be very slow, taking many thousands of years. Five main interacting factors affect the formation of soil including:
- parent material —minerals forming the basis of soil
- living organisms—influencing soil formation
- climate —affecting the rate of weathering and organic decomposition
- topography —grade of slope affecting drainage, erosion and deposition
- time —influencing soil properties.
Interactions between these factors cause an infinite variety of soils across the earth’s land surface.
Soil minerals form the basis of soil. They are produced from rocks (parent material) through the processes of weathering and natural erosion. Water, wind, temperature change, gravity, chemical interaction, living organisms and pressure differences all help break down parent material.
The types of parent materials and the conditions under which they break down will influence the properties of the soil formed. For example, soils formed from granite are often sandy and infertile. On the other hand, basalt under moist conditions breaks down to form fertile, clay soils.
Soil formation is influenced by organisms (e.g. plants), micro-organisms (e.g. bacteria or fungi), burrowing insects, animals and humans.
As soil forms, plants begin to grow in it; they mature, die and regrow. Their leaves and roots are added to the soil. Animals eat plants; their wastes and eventually their bodies are added to the soil. This begins to change the soil. Bacteria, fungi, worms and other burrowers break down plant litter and animal wastes and remains, to eventually become organic matter. This may take the form of peat, humus or charcoal.
Climate (rainfall, temperature and wind) influences the rate of weathering and also affects plant growth. Temperature affects the rate of weathering and organic decomposition. With a colder and drier climate, these processes can be slow, but with heat and moisture they are relatively rapid.
Rainfall dissolves some of the soil materials and holds others in suspension. The water carries these materials down through the soil. This is known as leaching. Over time this process can change the soil, making it less fertile.
The shape, length and grade of slope affects drainage. Aspect determines the type of vegetation on a slope and the amount of rainfall received. These factors cause variation in soil formation.
The length of time that soil materials have been weathered, influences soil properties. Minerals weathered from rocks are further weathered to form materials such as clays and oxides of iron and aluminium.
Soil materials are progressively moved within the natural landscape by the action of water, gravity and wind eg. heavy rains erode soils from the hills and deposit it in lower areas, forming deep soils. The soils left on steep hills are usually shallower. Transported soils include alluvial (water transported), colluvial (gravity transported) and aeolian (wind transported) soils.
More information is available on soil erosion.
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Last reviewed 10 April 2012
Last updated 6 January 2010