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.
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.
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
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9 thoughts on “The shades of soil”
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
Publication Date (Web): August 20, 2012
Copyright © 2012, American Chemical Society
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I come across some information on using magnetite in veggie gardening. The reliability of the sources are questionable. The clams are also suspect. Do you have any info on this or a reliable source for such info?
i acquired a large quantity of magnetite from a friend who harvested it in an Arizona desert.
We’ll try to get you information about this, but I’m unsure if this is inside the scope of soil science! We’ll find out!
Let me know why soil become green colour