4. Microorganisms decompose the remains of plants, forming humus. This further enriches the soil and helps to bind it together. The humus improves the fertility of the soil, allowing a greater variety of plants to grow.
The cycle continues until the soil reaches its maximum fertility, given the climate it is in.
Rain Organic matter
Mineral fragments and organic matter
Humus
Organic matter
B horizon A horizon
Disintegrating rock
Parent material
Parent material C horizon Bedrock Stage 1:
Parent rock is broken down by weathering
Stage 2:
Organic matter forms above weathered parent rock
Bedrock Stage 3:
Mineral matter from weathered parent rock mixes with organic matter to form soil. Sparse vegetation begins to grow
Bedrock Stage 4:
As a soil develops further over time, horizons develop and vegetation is more widespread
27.3 Soil Profile
A vertical cross-section of soil shows that it is made up of several different layers known as horizons. These horizons are collectively known as a soil profile. From the top down, these horizons are as follows:
• The O (organic) horizon is located on the surface of the soil profile and consists mainly of plant litter. Fresh litter is found at the top, while the litter underneath is at different stages of decomposition.
• The A horizon is referred to as topsoil as it is the top layer of soil in the profile. It is characterised by organic matter/humus mixed with soft particles of soil. The A horizon is typically darker in colour due to the presence of humus. The A horizon can be altered by burrowing animals and the use of artificial fertilisers. It is the most biologically active layer, with most plant roots and soil organisms found here. While the A horizon is the most fertile layer, it can suffer from leaching in climates with heavy rainfall.
• The B horizon is referred to as subsoil and is lighter in colour than the A horizon. The B horizon is a ‘zone of accumulation’, meaning that
GEOECOLOGY 523
: Fig. 27.4 The processes of soil formation
GEO DICTIONARY
Soil profile: a vertical cross-section of a soil
Horizons: different layers within soil
Biologically active: a biologically active layer in soil is one which has living organisms within it
