What Is Red Clay?

Composition

• Red clay primarily gets its color from non-hydrated iron oxide. The granite and gneiss that provide the majority of red clay's source material have been weathered by warm climates and heavy rainfall, leaving mostly iron, aluminum and silica remaining in the soil. Bases and organic materials have washed out of red clay, leaving its namesake color behind. This results in a soil made mostly of weathered rock with a low pH and little organic matter.

Location

• Ultisols, including red clay, are found worldwide. Most of the southeastern U.S. is primarily red clay, and the soil type also exists in northern South America, central Africa and southeast Asia, along with much of Oceania. The common features of these locations are warm, humid climates with occasional drought conditions.

Formation

• Soils form over tens of thousands of years, and red clay is a very old soil type, hence the meaning of its name in Latin. According to the USDA's soil taxonomy, red clay and other ultisols developed during or prior to the Pleistocene epoch. This epoch began 1.8 million years ago and lasted until around 11,500 years ago, when the most recent period of worldwide glacial activity, the Ice Age, began. The red clay that resulted from the weathering and subsequent leeching is closely related to the yellow and gray clays that are also part of the ultisol group.

Uses and Limitations

• Red clay is not highly prized by gardeners and landscape enthusiasts because it tends to drain poorly and does not contain high levels of organic material. The poor drainage can drown young plants, but it's also indicative of clay's ability to hold water, which is beneficial during times of low precipitation. However, red clay can be improved as a gardening soil by the addition of lime to decrease acidity and compost to increase organic content. Grass clippings, manure and composted kitchen scraps all add rich organic material.