In looking closer at soils, it is important to understand that there are general regions or horizons of soil development that are usually obvious in an undisturbed area. For example, if we found an undisturbed area, and dug down 3 to 6 feet (1 or 2 meters), we would likely find a soil profile (cross-section of the horizons) of three integrated horizons. The composition and stage of development will obviously vary widely depending on where the soil profile is taken. The top horizon is called the A horizon or topsoil. This horizon is usually 4 to 8 inches on average, again depending on where your sample is from. The A horizon is further subdivided into a darker upper portion, called the A1 horizon and a lighter lower portion A2 horizon. The A1 horizon contains the majority of the organic material out of the three integrated horizons. The next horizon is the B horizon or the subsoil. This is usually 1 or 2 feet deep on average. The subsoil usually contains more clay, so fewer pore spaces, and is lighter in color than the topsoil. The lowest horizon is called the C horizon and it could be 4 inches to 10 feet deep or it may not be present. The C horizon is called the soil parent material and it extends down to bedrock.
Where a plant grows and what resources are available to it is of vital importance to the life of a plant. The soil type and quality can be the difference between survival and termination of a plant. Soil is a very dynamic and complex portion of the earth’s crust. In some places, the soil is only a few centimeters thick, while in others it is hundreds of feet deep. In the grand scheme of things, the soil is vital not only to us as humans, but also to the existence of nearly all living organisms. Soil contributes to the plants that grow in it, just as the growing plants give back to the soil. It is a point of dynamic exchange between the living and non-living components of the earth.
The soil as what it is today is a result of many factors coming together: climate, parent material, local topography, vegetation, living organisms, and, of course, time. All of the factors can be involved in various degrees, which is why there are many thousands of soil types. The solid bulk of soil consists of minerals and organic matter. In between the solid particles are pore spaces, which are filled with varying amounts of air and water. The pore sizes and how they are connected within the soil bulk, determine the quality of soil aeration. Aeration refers to how water and air are held within a soil sample.