Soil is made up of distinct horizontal layers. These layers are called horizons. They range from rich, organic upper layers (humus and topsoil) to underlying rocky layers (subsoil, regolith and bedrock).
1. O Horizon – The top, organic layer of soil, made up mostly of leaf litter and humus (decomposed organic matter).
2. A horizon – The layer called top soil. It is found below the O horizon and above the E horizon. Seeds germinate and plant roots grow in this dark-colored layer. It is made up of humus (decomposed organic matter) mixed with mineral particles.
3. E Horizon – This eluviation (leaching) layer is light in colour. This layer is beneath the A horizon and above the B Horizon. It is made up mostly of sand and silt, having lost most of its minerals and clay as water drips through the soil (in the process of eluviation).
4. B Horizon – Also called the subsoil – this layer is beneath the E Horizon and above the C Horizon. It contains clay and mineral deposits (like iron, aluminum oxides and calcium carbonate) that it receives from layers above it when mineralized water drips from the soil above.
5. C Horizon – Also called regolith. The layer beneath the B Horizon and above the R Horizon. It consists of slightly broken-up bedrock. Plant roots do not penetrate into this layer. Very little organic material is found in this layer.
6. R Horizon – The unweathered rock (bedrock) layer that is beneath all the other layers. These different layers of soil are called soil profiles.
