Soils

What is soil?

The material that we call soil is a complex mixture of eroded rock, mineral nutrients, decaying organic matter, water, air and living organisms, mostly micro organisms.  Although soil is considered a renewable resource, it is produced very slowly by the weathering of rock, deposits of sediments and the decomposition of organic matter in dead organisms.

While soil contains living organisms, it is considered part of the non-living environment (abiotic).  However, its composition will dramatically affect the types of organisms in an area.  Other abiotic factors that will affect the area’s flora and fauna include; temperature, rainfall, light availability and so on.

The best way to examine soil is from side on.  This is known as soil profiling and is usually done by digging a hole and examining the features of the walls.

These horizons collectively are known as a soil profile. The thickness varies with location, and under disturbed conditions: heavy agriculture, building sites or severe erosion for example, not all horizons will be present.

The uppermost is called the organic horizon or O horizon. It consists of detritus, leaf litter and other organic material lying on the surface of the soil. This layer is dark because of the decomposition that is occurring. This layer is not present in cultivated fields.

Below is the A horizon or topsoil. Usually it is darker than lower layers, loose and crumbly with varying amounts of organic matter.  The more fertile the soil, the thicker the topsoil layer.  The topsoil layer is where most of the complex organic matter is broken down into simpler substances.  Some of these will leach downwards into the B layer.  In cultivated fields the ploughed layer is topsoil. This is generally the most productive layer of the soil. This is the layer that soil conservation efforts are focused.

As water moves down through the topsoil, many soluble minerals and nutrients dissolve. The dissolved materials leach downward into lower horizons.

The next layer is the B horizon or subsoil. Subsoils are usually lighter in colour, dense and low in organic matter. Most of the materials leached from the A horizon stops in this zone.  As a result this layer contains a significant amount of dissolved ions. 

Still deeper is the C horizon. It is a transition area between soil and parent material. Partially disintegrated parent material and mineral particles may be found in this horizon.

At some point the C horizon will give up to the final horizon, bedrock.