Red clay or red mud, is the common name for a variety of ultisol found in the SE United States. Residents who live in the area often speak poorly of the soil, although in fact it isn’t as poor as all that. Some dislike it because it is hard to keep their clothes and homes clean. The USDA’s “Soil Taxonomy – 2nd Edition” includes ultisols among its twelve soil orders. Red clay is rich in weathered minerals. Over time, rain leaches out soil calcium, magnesium and potassium. This makes a soil ‘old.’ Particles in such soils are smaller than 2.0 microns. The red color is due to iron oxide.
General Clay Composition
Clays include the kaolinite of red ultisol and are usually made up of complex silicate minerals. Tulane University explains they are 2D 6-member ring sheet silicates called phyllosilicates. Due to weathering, the sheets are fragmented. They readily slide across each other. This imparts slipperiness. Moisture is held in the ring structures.
Red Clay Horizons
Soils, including red mud, are divided into layers. There are organic, topsoil, subsoil, parent material and bedrock layers. Junctures between layers are horizons. The layers are visible in cutaway images. These images may include a measuring stick. See this cutaway of red Georgia clay.
In Construction, Forestry and Agriculture
Red clay has low organic matter. It is stable for construction purposes. Forests thrive in it. When forests are cleared by slash and burn methods, fertility quickly goes. Although some clays are not suitable for farming, the red variety is workable. Calcium and fertilizer amendments are necessary for repeat farming. So while red soil may present a cleaning problem, it can be an asset if properly used.
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- U of Idaho: The Twelve Orders of Soil Taxonomy: Ultisols
- NCSU: Soil Science Courseware: Ultisols
- USDA: What are Soil Horizons?