Soil Types (Horizon)

A soil is made up of layers of weathered rock. A soil will always form on top of a rock, which is often being weathered and so added to the soil. We call this the "parent rock". On top of the parent rock are layers of soil, we call these layers "Horizons". We describe the pattern of the horizons and the type of parent rock in an area it's "Soil Profile". So a soils type is determined by it's profile. Lets take a look at a soil profile and the processes that are occurring in it:

A General Profile

The above profile is a general one that is not any particular type. It shows the horizons and processes that occur in most soil profiles. It also graphically shows how hard it is to spot the different horizons in real life! If you look closely at the bottom, you will see a parent rock, it is labelled "Bedrock/Regolith"(These are other names for the parent rock, if you are taking a qualification try to remember "Regolith").
You will a letter on the right of the diagram. This refers to the horizon, as each horizon is given a letter instead of a name.

Going up from the Regolith you will see the "Weathered Bedrock", this is where the impermeable Regolith has been broken down. Water will not flow through the Regolith, it will flow across the surface downhill. Going up again you will see the "Subsoil". This is made from two horizons which are formed due to differing levels of ions. This causes a change in colour. The ions are leached (travel in solution with the water) through the soil from the surface. Some material is Eluviated from the E horizon to the B horizon, this is where particles are washed downwards in suspension instead of in solution. All the material being Leached or Eluviated is Illuviated (deposited) at the joining point of the B and C horizons.

Above the Subsoil is an area called the "Topsoil", this is made up from up to 4 horizons. These include layers of "Leaf Litter" (dead organic material)-L, "Humus" (Decomposed organic material)-H, "Fermentation Layer" ( Layer of part decomposed organic mater)-F and the real topsoil-A. Horizon A is the soil where most of the plants roots are found.

Other processes may occur in a soil profile but they don not occur in every one. It depends on the Regolith and the Biota and A biota that is found in the area.

Please click here to see the profiles of three soil types.
Then click here to find out why you should look after a soil...