When examining a soil profile, scientists look at the different soil horizons, or the layers of soil or the soil's materials which, to some degree, parallel the land surface. There are differences in the different horizons like color, structure, texture and consistency, as well as in chemical, biological and mineralogical make-up. The different horizons are labeled O, A, B, C, or E.
This horizon is also called the mineral horizon, according to the United States Department of Agriculture. People sometimes call this layer topsoil. Its main components are minerals, such as sand, silt and clay, and organic matter. At times this layer may be covered due to events such as landslides, flooding or volcanic eruptions. When this is the case it is a sign to scientists that the area underwent changes in the past. The color of this horizon is dark due to the predominantly present organic matter. This layer also has less clay and more coarse materials, due to the finer particles leaching to lower horizons.
The B horizon accumulates material from layers which are located above it. When minerals leach from higher layers, some of them will be found here. Some of them are minerals, along with some other particles. The B horizon is also called the zone of accumulation. Generally, this is the area where chemical alterations of clay along with other changes occur.
This is the horizon that experiences the least weathering. It is a subsurface horizon which contains unconsolidated, loose parent material. In this level there may be a presence of carbonates and soluble salts. In the C horizon there may be some movements of particles or there may be oxidation or reduction of iron.
The O horizon was created primarily from mosses, rushes and woody material. It is also generally developed under poorly drained conditions. This condition would cause saturated, anaerobic areas, such as would be seen in wetlands or in an area of high leaf litter. Organic material accumulates as these conditions impede the decomposition of these items.
According to Soil Web, there are different stages of decomposition. Of horizon is the fibric layer. In this horizon there will recognizable parent material. Om is also called the mesic or sapric layer. Here the material is slightly more decomposed than an Of horizon. Then there is Oh horizon, or the humic layer. This is the location of the most decomposition in this horizon. Different O horizons may be found one on top of another, so the stages of decomposition blend into one another. This horizon is generally black or dark brown.
The zone of a lot of eluviation is the E horizon. This is most often found in forested areas due to the higher precipitation in these areas and the acid that is produced from the litter of the trees. Clay, organic material and minerals are gone from this zone, so the soil particles are not pigmented.
By being aware of what constitutes the different levels, scientists are able to tell the history of the area of the soil sample.