Soil depth worldwide averages a mere six inches (fifteen centimeters). Soil and its underlying layers are classified into layers known as "horizons." From the surface to the bedrock, these layers are:
- O Horizon - The top layer of soil, composed primarily of organic material, such as the litter of leaves and plants, insects, and microorganisms.
- A Horizon - Also known as the topsoil, where seeds germinate and plants' roots thrive.
- E Horizon - Composed of sand and silt. Minerals and clay have been removed in a process known as eluviation.
- B Horizon - Also known as the subsoil, this layer contains mineral deposits that have settled down from upper layers.
- C Horizon - This layer is called the regolith and consists of rocks and little organic material (even roots don't penetrate this layer).
- R Horizon - The "R" in R horizon stands for rock and it refers to the unconsolidated rock or solid bedrock of this layer.
Various factors contribute to the production of soil, which averages out to be approximately half minerals and half water and air. Overall, only a tiny proportion of soil consists of organic materials. The minerals that compose soil are formed by the erosion of rocks.
Soil is more prevalent in regions of high moisture and high temperatures than in cold, dry regions as the increased moisture contributes to erosion and increased temperature contributes to a more rapid breakdown of organic material. When organic material breaks down, it forms "humus" - a dark tarry substance that is very fertile for plant growth.
In addition to climate, topography can have an effect on the amount of soil present in an area. For example, steep slopes can't hold soil but soil can accumulate in flat areas.
Color is a trait of soil that allows for its classification. Soil scientists (pedologists, as pedology is the study of soil classification and description) define 175 different gradations of color in soil, ranging in the blacks, browns, reds, yellows, greys, and whites. Soil color usually results from mineral stains on the surface of soil particles.
Soil contains a bounty of animal life. Earthworms, which make soil very fertile, can often be found in abundance - approximately one million earthworms can be found in an acre of soil. Microorganisms call soil their home as well, a single ounce of soil can contain over 100 million microorganisms.
There are twelve key orders of soil in soil taxonomy. Most common around the world are Aridisols (desert soils), Inceptisols (weakly developed, infertile soil), and Alfisols (reasonably fertile clayish soils). Mollisols (humus-rich) are best for agriculture and occupy approximately one quarter of the United States.