Why Is Some Dirt Red in Color?

Red dirt is common throughout the southeastern United States.

Red dirt is common throughout the southeastern United States.

Some soils are distinctively red in color while others are brown or black. Soil color is determined by several factors, including organic content, drainage conditions, weathering and mineral composition. Many red soils, such as Georgia’s famous red clay, obtain their color from the presence of iron oxides, a substance often used to color paint. Gain useful insight into your soil’s characteristics by understanding what its color means.

Soil Color and Drainage

Soil color and patterns found within the subsoil offer important clues to a soil’s drainage conditions. Generally speaking, well-drained soil is brighter in color while poorly drained soil tends to be dark and dull. Bright red color results from oxidized iron, similar to rust. Waterlogged, anaerobic conditions retard oxidation, resulting in dull gray or yellowish-colored soil. Bright red or brownish-red subsoil typically indicates good movement of air and water. Soils that drain slowly or experience periods of standing water often develop mottling that may include both brightly colored red and brown spots mixed with dull gray spots and streaks.

Organic Content

Soil color is a good indicator of organic content—the amount of decomposed animal and plant material found within the soil. Dark brown or black topsoil typically contains a high percentage of organic matter. Excessively moist, organic soils tend to retard the formation of red colors, but don’t necessarily indicate a lack of iron. Light-colored soil of any color indicates a low percentage of organic matter. Wind, sun and water erosion reduce organic content, resulting in light-colored soil.

Parent Material

Soil color is associated with the parent material from which it was formed. Red soil may be derived from reddish-colored rock, such as the red sandstone common to the desert regions of Arizona, California and Nevada. Hematite -- the mineral from which iron ore is obtained -- is a common source of red color for many soils, particularly those in hot, dry regions or tropical zones. However, red soil can come from rock that is not red in color but contains iron and manganese that become oxidized as the rock is broken down through weathering. For example, the red soil of Georgia’s Piedmont region was formed from highly weathered deposits of gray, black and white granite and gneiss. Over time, the original rock material was reduced to dust, containing oxidized iron that colored the soil red.


Sometimes called “red clay soils,” ultisols are one of the 12 orders of soil, identified by the U. S. Department of Agriculture’s Universal Soil Classification System. Often red in color, ultisols are found in warm, humid regions such as the Southeastern United States and parts of Asia, Africa and South America. These soils form in older, extremely weathered geologic areas and are often highly acidic. Ultisols often contain high amounts of clay and are low in nutrients but can be used for agriculture if properly amended with fertilizer and lime.

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