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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 influences the specific soil that develops.
Minerals and organic materials present during it's formation. Materials from volcanos, sediment transported by wind, water or glaciers or minerals left behind by drying lakes are good examples of parent materials.
Parent material is broken down into smaller pieces by a process called weathering. Cycles of freezing and thawing, wetting and drying, and the frequency of these occurrences coupled with average temperature and moisture levels of region play an important role in soil formation. These smaller pieces are known as (sand, silt and clay), clay being the smallest size.
Both plants and animals help to create a soil. As they die, organic matter incorporates with the weathered parent material and becomes part of the soil. Living animals such as moles, earthworms, bacteria, fungi and nematodes are all busy moving through or digesting food found in the soil. All of these actions mix and enrich the soil.
Topography is the hilliness, flatness, or amount of slope of the land. Soils vary with topography primarily because of the influence of moisture and erosion. In many areas, moist, poorly drained soils are located in low areas, and depressions of the land. In contract, soils in sloping areas can be drier and well drained. These soils tend to be moderately and well developed. Erosion can remove all or part of the topsoil and subsoil, leaving weakly developed soil.
It may take hundreds of years to form one inch of soil from parent material. Only the top few inches are productive in the sense of being able to sustain plant growth. This is why soil conservation is so important.
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 its' unique set of characteristic is used by soil scientist to classify and name a soil. Just as an oak tree is named due to its' unique characteristics, so is a soil.
These horizons collectively are known as a soil profile. The thickness varies with location, and under disturbed conditions: heavy agriculture, building sites or severe erosion for example, not all horizons will be present.
|The uppermost 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 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 ploughed layer is topsoil. This is generally the most productive layer of the soil. This is the layer that 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 colour, 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