Soil Composition and Color

When determining the structure and composition of soil using Appendix A of subpart P, one of the most amazing aspects is the simplicity of the soil testing in a most complex subject. Of course, this is the only way a person with little exposure to geology could determine soil type and corresponding properties to an extent that the results could be used to determine an adequate protective system.

Here's a question I was asked recently: “What does the color of the soil have to do with the composition of the soil and classifying soil?”

The answer is “Nothing”. No soil colors are mentioned in Appendix A because rock, granular, cohesive, loams, and cemented soils could be practically any color based on a variety of factors such as the presence of metals, minerals, and water, and does not help us determine Types A, B, or C.

The following is taken from “Soil Composition and Color” and the entire article can be found at color-of-soil.pdf (usda.gov). I recommend reading the entire article, because even though it's quite lengthy, it is informative and interesting. Especially to excavation safety nerds. You know who you are.

“Soil color and other properties including texture, structure, and consistence are used to distinguish and identify soil horizons (layers) and to group soils according to the soil classification system called Soil Taxonomy. Color development and distribution of color within a soil profile are part of weathering. As rocks containing iron or manganese weather, the elements oxidize. Iron forms small crystals with a yellow or red color, organic matter decomposes into black humus, and manganese forms black mineral deposits. These pigments paint the soil. Color is also affected by the environment: aerobic environments produce sweeping vistas of uniform or subtly changing color, and anaerobic (lacking oxygen), wet environments disrupt color flow with complex, often intriguing patterns and points of accent. With depth below the soil surface, colors usually become lighter, yellower, or redder.”