Soil is a wonderful and valuable thing, but by most is taken for granted. Perhaps we are not taught enough about soil to pay much attention to it, but my interest has developed and continues to grow. So for those who, like me, have ever wondered what soil is made of, here you are…
Solids, liquids and gases
Soil consists of solids, liquids and gases. The solid part of soil is a complex mixture of organic matter and minerals, which come from weathered parent material (rock).
Air and Water
The solid components of soil only account for approximately half of the soil, the remainder is made up of pore space between the soil particles. Water and air fill these pore spaces, move around and consequently support life in the soil.
The pores additionally provide a habitat which hosts organisms ranging in size from microbes to worms and other invertebrates.
Organic matter and minerals
Organic matter is also found in soil, in various stages of decomposition. This is formed from decomposing organisms that once lived in/on the soil. Highly degraded organic matter is called ‘humus’.
So that is what soil is made of, but how is it formed? There is a brilliant mnemonic used to remember the factors that contribute to the formation of soils: CLORPT. This formula was made famous by soil scientist, Hans Jenny and tells us the five soil forming factors.
CL – Climate
Climate that generates the weathering of the basic parent material should be thought of in terms of ‘extremes’. Rainfall can be high or low, resulting in tropical or desert soils. Temperature can cause freezing or thawing, heating and drying, all of which add to the weathering of soil. Wind can erode rock, which when weathered into smaller particles, eventually becomes soil.
O – Organisms
As we know, plants grow in soil – their roots help to break the soil up and also move nutrients and chemicals around. Animals burrow and dig the soil which ultimately creates and alters the structure. Smaller organisms such as earthworms ingest, excrete and move the soil around with the help of microscopic bacteria and fungi.
R – Relief
The relief or topography relates to the shape of the landscape. Soil on steep slopes will be thin due to gravity and water run-off, whereas soil on flat land will be much deeper, making a difference to the formation.
P – Parent materials
The general parent material that soil is formed from is rock, yet some parent materials can be less obvious. Some examples are glacial deposits and silt and sediment from rivers.
T – Time
Time if very important and highlights how precious soil is – generally 2-3cm of soil takes about 500 years to form!
Soil profiles are very useful for understanding what is beneath our feet. They show and describe the various layers (horizons) of soil under the surface of the earth. I have produced my own soil profile and included a description of the horizons below.
O Horizon – Surface litter: Partially decomposed organic matter
A Horizon – Topsoil: Humus, living creatures, inorganic minerals
E Horizon – Eluviated Horizon: Zone of leaching (materials move downwards)
B Horizon – Subsoil: An accumulation of compounds leached down from A and E Horizons
C Horizon – Parent Material: Partially broken down inorganic minerals
R Horizon – Bedrock: Un-weathered parent material