Fundamentals of Soil Science

Soils are a complex interface between inorganic mineral material, organic matter, water, air and organisms. This complex system changes in response to five different factors called the Soil Forming Factors (SFF).

The five factors that influence the development of soils are:

Climate, Organisms, Relief, Parent Materials, and Time

Climate includes both the regional and microclimatic effects. Organisms include all organisms affecting the soil--those that live in the soil, above the soil, and animals including humans. Relief refers to smaller scale changes in topography which can affect such things as drainage or stability. Parent Materials include any geologic materials that the soil is developing from, such as granite, volcanic ash or glacial outwash. Time refers to the length of time since the parent materials the soil are developing from were first exposed to the other soil forming factors--this can range from days to millions of years.

These five factors operate simultaneously to create a wide diversity in soil properties and processes. For example, laterization is a process that occurs in soils with high temperatures (sub-tropical to tropical), and high precipitation (the climate factor) and good drainage (parent materials and relief) on old stable landforms (time). In soils that are laterized, extreme leaching has occurred over a very long time and the soil has lost most of its primary minerals, and only Fe and Al oxides and some clays remain in the soil. These soils typically look very red because of all the Fe oxides.

Overall, the SFF alter different soil properties and processes in systematic ways that cause the development of different soil layers called horizons. In the example below you can see 3 differently colored layers: these color changes are due to different quantities of organic matter and Fe oxides in the soil. So for example, where the soil is very dark is an A horizon, the narrow slightly orangish/brown layer is a B horizon, and the unaltered rocks, gravel and sand at the bottom are the C horizon.

Vastly different horizons such as the dark upper horizon in the soil above and the 2 horizons below it are examples of Master horizons. There are 5 different master horizons, and modifiers called subordinate horizon designations can be used with them to indicate significant properties of the horizons. You might want to look at a description of different soil horizons.

Soils are classified according to the properties they have developed due to the dominant processes operating in the soil as a result of the soil forming factors. Classification is based on diagnostic horizons, specific physical or chemical properties. Environmental factors can be used to simplify the 'Keys to Soil Taxonomy', but for an accurate assessment, the full 'Key' should be used.

A total of six levels of classification exist within US Soil Taxonomy. These are:

Order

Suborder

Great Group

Subgroup

Family

Series

The Order is the level equivalent to the plant vs. animal kingdoms, and the Series is the equivalent of a species.

Currently 12 soil orders are recognized by the US Department of Agriculture Soil Classification system.

A simplified key to the soil orders is given below.