The Critical Zone

State Factors of Soil Formation


Soil formation, or pedogenesis, typically happens over long periods of time. The so-called "mineral" component of soil is formed from the weathering, or decomposition, of rocks and minerals. There are two types of weathering: physical and chemical. Physical weathering involves the breakdown of rocks into smaller particles through direct contact with atmospheric heat, moisture, and pressure. The effects of various chemical processes (e.g., atmospheric or biologic) on the size and composition of minerals and rock is known as chemical weathering. Weathering processes will be further explored in our landforms unit.

The rates of soil formation have not been as well characterized as soil erosion rates. This creates an interesting conundrum for scientists and land-use planners interested in the impacts of soil erosion. Though we know there are sites on Earth where soil is eroding at an unsustainable rate, do sites exist where soil is forming at a rate that equals or exceeds the rate of soil erosion? If so, soil erosion may not be an important consideration for these sites when considering land-use issues. Therefore, accurate determinations of soil formation rates under varying conditions are a key question in Critical Zone science.

Soil formation is controlled by five variables in nature. Those variables are:

  1. parent material
  2. climate
  3. topography
  4. biota
  5. time

In addition, most scientists also recognize that humanity, having influenced much of the Critical Zone, represents a sixth factor in soil formation. The varying distribution of these variables in soil formation (also known as the state factors) on Earth's surface, and therefore the differing distribution of physical, chemical, and biological processes dependent upon the distribution and interplay of the state factors, plays a role in soil formation and the development of different types of soil.

Reading Assignment

Before we progress to learn about the different soil types and their distribution on Earth, I want you to first understand the state factors of soil formation. Please read the following text chapter, which is available through Library Resources:

  • Brady, N. C., and R. R. Weil (2002). Formation of Soils from Parent Materials. In The Nature and Properties of Soils (13th ed., pp. 39–64). Upper Saddle River, N.J.: Prentice-Hall.

As you read this, keep in mind that I'd like you to leave this reading assignment able to list the state factors of soil formation. You should also understand the basics of the role each state factor plays in soil formation. Begin to think about the varying distribution of the state factors on a landscape-versus-global scale. Do you think patterns exist in the distribution of the state factors? If so, do these patterns carry over to observable patterns in the distribution of the soil orders?