Soil structure

Learn more about this precious resource vital to the environment and even to preserving our history

(Image source: Pixabay)

The function of soil is far more important than just the dirt beneath your feet. Not only is good quality soil essential to agriculture and our environment today, but soil is also a major source of information used by archaeologists to learn about our past. For example, depending on the soil type and acidity, artefacts such as bone, shell, and pottery can be preserved for thousands of years in certain soils.

Soil formation is a slow process; a single inch of topsoil can take 500 years to form. There are five key soil-forming factors: the parent material, climate, organisms, topography and time. The process begins with the weathering of primary soil material, or parent rock, which can be bedrock, an older soil surface, or sediments transported and deposited by winds, glacier movement or other water flows. The climate – in the form of heat, ice, wind, water and many other environmental forces – has a strong impact on the weathering of this parent material and the formation of soil. Once a thin layer of soil has developed, organic life and other plants help bind the material by spreading their roots through the soil to bind it, moving through the earth to aerate it and producing waste materials to enrich it.

The shape of the landscape also affects how the parent rock is weathered. For example, if the soil is forming on a slope, factors such as the steepness of the slope, the direction in which the slope faces, and the way the water drains all affect how the soil develops. And perhaps the most important factor in the formation of soil is time. The soil profile is continually changing and developing with time and well developed horizons (see ‘soil profile’
illustration) can take thousands of years.

The soil profile

(Image credit: Future PLC)

1 - Surface horizon

The surface horizon is the dark brown or black upper layer of rotting plant and leaf litter at the upper side of the soil.

2 - A horizon

The surface horizon features organic material called humus as well as other living organisms to make up the ‘topsoil’ in which plants can grow.

3 - B horizon

As you travel down through the layers, the ‘sub soil’ of the B horizon contains organic material washed down from above.

4 - Minerals

Dead and decaying plants and organisms contain minerals, which get washed downwards through the layers of soil.

5 - Downward pressure

Water in the soil pushes minerals down and out of the soil.

6 - C horizon

There is little-to-no organic material at this depth, instead you will find broken up bits of weathered rock fragments.

7 - Upward movement

Soil is formed from tiny weathered rock particles that bring minerals up from beneath.

8 - D horizon

Also known as the ‘bedrock’, this layer is the solid rock from which the soil is created.

The life cycle in soil

(Image source: Pixabay)

Organisms known as decomposers – including fungi, bacteria and insects – live in the soil and feed on the dead plant and animal matter within it. These decomposers break down organic remains into chemicals that provide nutrients essential for plant growth. Earthworms are also necessary for soil health and fertility as they eat dead plant material, which helps it to decompose. They also wriggle around in the soil, helping to keep it aerated, aiding drainage and also spread the organic matter around. Moles, mice and other burrowing creatures can also help enrich the topsoil.


This article was originally published in How It Works issue 16


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