Soil Composition

Soil is a combination of many things, such as small pieces of broken rock and stones, organic matter (for example: fallen leaves, dead invertebrates, decomposed plant material and wood and one gram of soil could hold as much as 5,000-7,000 bacteria species) and sand, silt, clay and other mineral particles.

Soil comes in different colours and textures. It could be lively colours such as red, yellow and white but most of the time it will be black, brown or gray due to the sand, silt, clay and other mineral particles in it. The combination of these components in the soil can give the soil a smooth, creamy, rough, crumbly and sticky texture.

The composition of a healthy soil sample contains the following:

· 45% minerals
· 25% water (soil stores 0.01% of the total water on Earth)
· 25% air
· 5% organic matter

Soil Formation
Soil formation, or pedogenesis, is the combined effect of physical (weathering), chemical (erosion), biological (primary succession and plant root development), and anthropogenic (human) processes on the soil parent material. It takes about 500 years to create just an inch of topsoil to break down rocks into little pieces forming the new soil. 

Soil Layers

Like the atmosphere, soil also has layers called horizons. Tere are four layers with the unexciting but unforgettable names: Horizon O, Horizon A, Horizon B and Horizon C.

Horizon O is the topsoil that we walk on. It’s one-inch (3cm) thick and made up of decayed, organic stuff that feeds the soil and keeps it healthy. Horizon O is the most fertile, productive layer because it contains the humus and busy microorganisms that make nutrients available to plants.

Horizon A is the second layer from the top and is also part of the topsoil, composed of roots and beneficial microorganisms like mycorrhizae and fungi that feed on the waste materials from the roots. Of course, this horizon is also home to much of the wildlife in the soil such as earthworms and centipedes.

Horizon B the subsoil is relatively dead and starved and is a very tough layer. It can be recognised by its lighter colour due to the lack of humus. When digging the subsoil should not be bought up to the surface as mixing this with the topsoil will reduce the availability of nutrients to the plants.
Horizon C is below the subsoil and is the parent material—rocks and old soil that form all the horizons above it. This layer contains primary bedrock, secondary materials from other places, old soil formations and the like.