Soil Basics

Soil, by volume, on the average, consists of 45% mineral, 25% water, 25% air, and 5% organic matter (both living and dead organisms.) There are thousands of different soils throughout the world.
Five important factors influence the specific soil that develops.
The result of all of these forces is soil that develops into layers known as horizons. The first or top 48 inches of these horizons and it's unique set of characteristics is used by soil scientists to classify and name a soil. Just as an oak tree is named due to it's unique characteristics, so is a soil. The USDA NRCS Web Soil Survey has maps and descriptions of all of the soils in Shawnee County and the United States.
These horizons collectively are known as a soil profile. The thickness varies with location and, in disturbed locations such as heavy agriculture, building sites, or severe erosion, not all horizons will be present.
Five important factors influence the specific soil that develops.
- Parent Material
- Climate
- Living Organisms
- Topography
- Time
The result of all of these forces is soil that develops into layers known as horizons. The first or top 48 inches of these horizons and it's unique set of characteristics is used by soil scientists to classify and name a soil. Just as an oak tree is named due to it's unique characteristics, so is a soil. The USDA NRCS Web Soil Survey has maps and descriptions of all of the soils in Shawnee County and the United States.
These horizons collectively are known as a soil profile. The thickness varies with location and, in disturbed locations such as heavy agriculture, building sites, or severe erosion, not all horizons will be present.
Soil ProfileThe uppermost part 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 that is the A horizon or topsoil. Usually, it is darker than lower layers, loose and crumbly with varying amounts of organic matter. In cultivated fields, the plowed layer is topsoil This is generally the most productive layer of the soil. This is the layer on which 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 color, dense, and low in organic matter. Most of the materials leached from the A horizon stops in this zone.
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.
The next layer is the B horizon or subsoil. Subsoils are usually lighter in color, dense, and low in organic matter. Most of the materials leached from the A horizon stops in this zone.
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.
Note: This page was developed from information found in a USDA, Soil Conservation Service (now, Natural Resources Conservation Service) publication, "Conserving Soil".