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The secret of red soil: What chemical reactions lie behind it?
Soil color is one of the most obvious properties of soil. While color itself does not directly affect the soil's properties or practical uses, it can reveal important information about the soil's organic matter content, mineral composition, moisture, and drainage. Soil colors range widely, including brown, red, yellow, black, gray, and even white, blue, or green, and can vary dramatically across different landscapes, between various upper levels of the soil profile, and even within a single soil patch.
The development and distribution of soil color results from chemical and biological weathering, specifically oxidation-reduction reactions. As the primary minerals in the soil's parent material weather, these elements combine to form new color compounds. Soil conditions can produce uniform or gradual color changes, while reducing environments can cause interruptions in the flow of color, creating complex mottled patterns and concentrated spots of color.
Changes in soil parent material or mineral origin can be indicated by color changes in the soil profile.
Causes of color
Dark brown or black
Dark brown or black color usually indicates high levels of organic matter in the soil. Organic matter covers mineral soil particles, masking or deepening the natural mineral colors. Sodium content also affects the depth of organic matter and therefore soil color. Sodium allows particles of organic matter, such as humus, to be easily dispersed into more minerals. So soil where charcoal has accumulated will appear black.
Red
Red color usually indicates the accumulation or oxidation of iron in well-oxygenated, well-aerated soils. Iron in crystallized and metamorphic rocks diffuses due to changes in iron concentration caused by redox reactions, and a red color may appear in regularly saturated soils, especially near root channels or pores.
Gray or blue
In anaerobic, saturated environments, soils may appear gray or blue due to redox reactions and/or iron depletion. In anaerobic soil, microorganisms reduce iron from ferric iron (Fe3+) to ferrous iron (Fe2+) form. Manganese may also be reduced from tetravalent manganese (Mn4+) to the divalent manganese (Mn2+) form, although reduction of iron in soil is more common.
Prolonged saturation will cause the soil to appear gray or blue due to soluble reduced iron compounds and may expose bare grayish silicate minerals.
Green
The reduction of iron may give soil a greenish-gray color, but certain minerals such as chlorite, parent ore, and green clay can also appear green. Chlorite soils are derived from specific marine sedimentary rocks, while parent ore soils are produced in acidic pyrite soils. Further examples include the weathering of green clay bentonite observed in hydrothermally modified basalts in the Mojave Desert.
Yellow
Yellow soil may also indicate iron accumulation, but its oxygen environment is worse than red soil. Jarosite accumulation may also cause the soil to appear yellow and is common in salt marshes, sulfide minerals, acid mine tailings, and other acidic soils.
White
Soils that contain salts, carbonates, or calcite often appear white, and these often occur in arid environments.
Description of color
Most soil survey organizations use the Munsell color system to reduce subjectivity in evaluating color. This system was developed by painter Albert Mansell in the early 20th century to describe the full color spectrum. Although the Munsell Soil Color Book, commonly used by soil scientists, includes only the most important colors related to soil, it contains three components: hue, value, and chroma.
Together, these components help soil experts accurately describe soil color changes, especially when the soil's moisture content changes.
Soil color (especially lightness) changes with moisture and can be described in wet and dry conditions. The soil is considered moist when the addition of water does not change its color; the soil is said to be "dry" when it dries out in the air.
As recent research reveals potential links between soil color and environmental change, we have to ask, what can changes in soil color reveal about future environmental changes?
