Soil is made up of many materials: rock, sand, salt, silt, clay, air, water and organic materials. Horizons are the different layers of soil materials in a particular area. These horizons may be loose, friable, firm or extremely firm, but all of it is soil.
The "O" horizon is the top layer of soil, which plants are placed into for growing. The "O" horizon is organic matter just above the topsoil layer, and is made up of humus (decaying leaves and plants), according to Enchanted Learning.
The "A" layer, says Utah State University, is the topsoil layer. It appears darker than the lower levels of dirt, usually due to moisture and higher concentrations of organic matter.
The "E" layer (or eluviation layer) is lighter in color than topsoil, and is mostly made of sand or silt. Also called the leaching layer, the "E" horizon has lost most of its minerals due to water seepage, claims Enchanted Learning.
The "B" horizon is the subsoil layer. Subsoil regions, meaning the "B" horizon and lower, contain higher concentration of clay, says Utah State University. Mineral deposits of calcium, iron and aluminum oxides exist due to the leaching of minerals from higher layers.
"C" and "R" Horizons
The "C" horizon is called the regolith, and consists of mainly broken-up bedrock material. Plants can not exist in this layer as there is little to no organic material available. The "R" horizon is strictly dense bedrock.