Georgia Red Clay
Why are Georgia Soils Red?
Not all Georgia soils are red, but many of them are. The State is well known for its abundance of "Georgia Red Clay". People often ask why the soils are red.
The red color that is so evident in Georgia soils is due primarily to iron oxides.
Soils form from the interaction of climate, organisms, parent materials, relief, and time. Red soils in Georgia result from the affect of a warm, humid climate weathering acid crystalline rocks on rolling hills over a long period of time.
Soils in much of Georgia, particularly the Southern Piedmont region of North Georgia, developed in materials weathered primarily from granite and gneiss. These rocks are black, white, and gray, and are relatively low in bases, or natural fertility. Georgia has a climate of warm (hot) temperatures and high rainfall, so there has been intense weathering of the rocks over the years. High rainfall has leached out most of the bases that were present.
Materials that remain are composed mostly of iron, aluminum, and silica, and it is the iron that gives the soils the red color. The red color is not just from iron, but more specifically from unhydrated iron oxides. The red soils are generally on convex landforms that are well drained.
Organic matter can mask soil colors in some areas. However, organic matter content is low in most of Georgia's soils because of the warm climate, so the red color is not affected by it.
From Soil Taxonomy (1975):
"The red color of soils is generally related to unhydrated iron oxides, although manganese dioxide and partially hydrated iron oxides may also contribute red colors. Since unhydrated iron oxide is relatively unstable under moist conditions, red color generally indicates good drainage and good aeration. Strongly red soils are expected on convex surfaces underlain by pervious rocks. ...
In regions where the normal soils have red color, the well developed red color is one indication that the soils are relatively old or at least that the soil material has been subjected to relatively intense weathering for a considerable time."