Soil Formation and Structure

Where does soil come from? (3)

• manure

• decomposed remains of plants and animals

• humus

manure

a mixture of animal dung and urine, feed residue, and other on-farm wastes

humus

A dark brown or black substance lying beneath the leaf litter; it has a loose, crumbly texture formed by the partial decay of dead plant material.

Which process creates soil? How?

Succession: The process of change over time in an ecosystem. Over time the weathering of rock and the decomposition of organic matter work together to form soils. The leaf litter that sits on the surface of the soil today, will eventually be buried below new layers of organic matter as leaves fall, manure accumulates, animals die and all are broken down by decomposers

Why does soil in lower layers have different properties? 

As more and more layers accumulate and compact, there are fewer spaces between the soil particles for gases and water to flow. This means that the soil in lower layers has different properties, such as its ability to hold water and its soil pH, than the soil layers above

How do environments affect Soil Composition? (4 environments)

• Tropical rainforest: heavily leached, thin humus 

• Desert: low organic matter, sandy 

• Temperate forest: richer soil, well-developed O-A horizons 

• Grassland: deep A horizon, high organic content

Soil horizon 

A horizontal layer of soil that is distinctive in its chemical and physical properties. Together, they are called a soil profile

Soil profile

a vertical section through a soil, from the surface down to the parent material, revealing the soil layers or horizons.

3 layers of soil

• organic layer

• topsoil

• subsoil

subsoil 

The layer of soil immediately beneath the topsoil, often containing minerals leached from above.

topsoil

the upper layer of soil. It has the highest concentration of organic matter and microorganisms

What happens to organic matter as you move from the upper layers to the lower? 

 As you move from the upper layers of the soil profile to the lower layers, the soil horizons change from more to less organic in their composition. The lowest layers of the soil profile include parent material/bedrock and will contain very little organic matter

Characteristics of soil horizons (4) 

• O-horizon

• A-horizon

• B-horizon

• C-horizon 

O-horizon (3)

• The uppermost layer of soil consists of organic matter such as decaying leaves and humus. 

• The O horizon is present on the surface of most soils, but in regions of low biodiversity, or where the soil has been farmed intensively, the O horizon may be missing. 

• It is very vulnerable to erosion.

A-horizon (3)

• This mixed layer, also known as topsoil, contains the fully decomposed organic matter from the O horizon. 

• The dark-coloured A horizon is the most fertile because it is nutrient-rich and has lots of air spaces where plants can take root. 

• Like the O horizon, the A horizon is vulnerable to erosion and intensive farming

B-horizon

• sometimes called subsoil, is usually more compact than the O and A horizons. 

• It contains higher percentages of clay soil particles and minerals which have been deposited in the B horizon through soil flows such as leaching.

•  The B horizon also mixes with eroded minerals from the parent rock below. 

C-horizon

the layer of parent rock, broken into smaller pieces by weathering.

Flows of matter and energy that contribute to the different soil horizons (4)

• littering

• decomposition

• leeching

• solar energy

Littering

Plant material (leaves, dead organisms) falls onto the soil surface and contributes to the formation of the O horizon (organic layer)

Decomposition

Microorganisms break down organic matter into humus. This occurs in the A horizon (topsoil), enriching it with nutrients. This is a flow of matter from the O horizon downward.

Leaching 

Water percolates through soil, dissolving and transporting minerals from upper to lower horizons. Leads to loss of material from the A horizon accumulation in the B horizon. This vertical flow of minerals and nutrients differentiates A and B horizons.

Solar energy

Drives evaporation, photosynthesis (supporting plant growth and littering), and temperature-dependent decomposition. Influences the rate of organic matter breakdown and microbial activity, especially in upper layers.