The shades of soil

soil-with-hematite

The reddish colors in this soil from North Carolina are from the iron-oxide hematite. All photos are from the Soil Science Society of America Marbut slide collection.

Q: Why do some soils have different colors, for instance the red soils in the South?

A: Soils are colored by the minerals or other materials found in them. Oxides of iron are responsible for many of the colors we see. For example, the red color in many soils in the southern United States is caused by the iron oxide mineral, hematite. Some soils have a yellowish color caused by the iron oxide, goethite; others have a brownish color caused by the iron mineral, maghemite; and still others a gray-green color caused by the iron mineral, hydromagnetite.

In each case, the type of iron oxide present depends on the conditions in the soils in which the oxide formed, including the temperature, water content, and presence or absence of organic matter. Hematite is formed in hot soils with plenty of oxygen present, while hydromagnetite is formed in soils that are frequently saturated with water and thus deficient in oxygen.

soil-with-accumulated-salts

The white color at the surface of this soil is due to salt accumulation.

Other materials can cause the colors we see in soils, too. Soils that are white or that have a white crust on them show the influence of various salts that are found in the soil environment. The black color found in some topsoils is caused by high levels of partially decomposed organic materials.

Because each of these colors is created based on the conditions under which the soils formed, color is a very powerful tool that soil scientists can use to help decipher the major factors that influenced the formation of any given soil.

You might wonder what effect iron has on our ability to use the soil, such as for growing crops. High iron content can influence how we use soils, but in most soils iron is actually a very minor part of the total soil.

high-organic-matter-in-soil

The dark black colors in the surface layer of this soil are due to accumulations of organic matter.

You can think of the iron oxides that color many of our soils as being analogous to the paint in a house. The paint is a very thin layer, and doesn’t actually make up much of the total house. But that thin layer really influences how the house looks. The same is true of iron oxides in soil. In most cases the iron oxides are less than 1% of the total soil mass, but because of their colors they have a profound impact on what the soil looks like.

–Answered by Eric Brevik, Dickinson State University

Have a question for Soils Matter? Post it as a comment below, or email us at soils-matter@soils.org.

4 responses to “The shades of soil

  1. A new Fraction of Black;
    The Demonstration that Pyrolitic Carbon, Using quantitative 13C nuclear magnetic resonance (NMR) spectroscopy measurements, concluds that both Terra Preta Soils and Midwest dark soils contain 40% to 50%+ of their organic carbon (SOC) as pyrolytic carbon char, that this pyrolytic carbon can account for all CEC

    Abundant and Stable Char Residues in Soils: Implications for Soil Fertility and Carbon Sequestration
    J.-D. Mao, R. L. Johnson, J. Lehmann, D. C. Olk, E. G. Neves, M. L. Thompson and K. Schmidt-Rohr
    Environ. Sci. Technol., Article ASAP
    DOI: 10.1021/es301107c
    Publication Date (Web): August 20, 2012
    Copyright © 2012, American Chemical Society
    http://pubs.acs.org/doi/abs/10.1021/es301107c

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  3. Pingback: What do scientists see in soil pits? | Soils Matter, Get the Scoop!·

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