Have you ever paid attention to the different colors of soil? Well, they are colored by the materials and minerals that are found in them. Many of the colors we see are because of the oxides of iron. As an example, in the southern United States, there is a red tint to the soil in a lot of places because of hematite, an iron oxide mineral. Some soils will have a more brown color because of maghemite, an iron mineral; some are more yellow because of goethite, an iron oxide; some others will be a grayish green color because of hydromagnetite, an iron mineral.
With every color of the soil, the kind of iron oxide that is present is based on the soil’s conditions when the oxide was formed, such as potential organic matter, water content, and the temperature. Hot soils help hematite to form when there is a lot of oxygen present. When small amounts of oxygen are present because the soil is often saturated, hydromagnetite is formed.
There are other materials within soil that can cause colors that we see. Different salts in the soil can cause it to have a white crust. These salts are found naturally in some soil environments. High levels of organic materials that are partially decomposed can cause some top soils to have a black color.
Color is a powerful tool that is used by the soil scientists to help them understand the formation of different types of soil since the color is determined by the conditions the soil is formed under. It gives very good hints to the scientists that are studying it.
Many people wonder how iron effects our ability to grow crops and other things in the soil. The iron is actually a very small part of what makes up the soil, so it doesn’t influence it a lot. You can think of it like the paint in a home. The paint makes up a very small part of the home itself, but it can influence how the home appears. This is the same way iron oxide is in the soil. It pretty much just determines how it looks.