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Column: Why so much clay in Georgia soil?
Rudi Kiefer

People who travel from Georgia for the first time to Iowa or Indiana are surprised how different the topsoil looks. Natural history gives the Midwest an advantage over our Southern States. Four Ice Ages, the latest one peaking 18,000 years ago, sent continental ice sheets through the center of the continent. Bodies of ice, thousands of feet thick, moved slowly across the landscape, plowing and grinding the topsoil and bedrock underneath. This mixed the minerals and organic matter and made for very fertile soils.

Georgia wasn’t so lucky. No glaciers came here. Our topsoil, the all-important layer that produces our food, has been exposed to sun and rain for thousands of years. Minerals that nourish plants have been washed away. In many places, erosion removed much of the most fertile top layer. What’s left behind in North Georgia is a mix consisting largely of clay. The red color comes from iron oxides. It’s the same as rust on the car and the patio furniture.

North Georgia’s clay is a challenge to plants because it absorbs a lot of water but doesn’t want to let it go. Like bread slices holding the ingredients of a sandwich, clay minerals grab molecules of water between them and hold them tight. When the water evaporates, the whole thing shrinks, and cracks appear in the ground. It’s difficult to make this kind of soil highly productive, let alone compete with the rich growth that Midwestern soils produce. It requires lime because all that washing-out of minerals has made North Georgia’s soils acidic. Organic matter is needed to restore the missing nutrients. And because clay forms such a gooey layer that won’t let water penetrate, flowing water erodes the topsoil instead and carries it away.

South of the Piedmont lies the Coastal Plain, where the clay is replaced by sand. This is another challenge. Sand is well drained, and it doesn’t hold water for very long. That’s a good thing for residential housing in the Coastal Plain. Even after heavy rain, the yard won’t be slick and muddy. But for plants, whether it’s a food crop or a suburban lawn, it produces a climate that’s wet (from frequent rain) and dry at the same time. Rainstorms visit the areas around Valdosta, Albany and Savannah with regularity. But still, the huge irrigation systems you see at crop fields, and sprinklers keeping residential lawns alive, are a necessity.

Rudi Kiefer, Ph.D., is a professor emeritus of physical science at Brenau University His column appears Sundays and at