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The clay goes on forever. My house sits on a soil profile known as Brazoria Clay, area 53 on the map above. Geologically, it's a floodplain of the Brazos River and its parent material is the clayey alluvium of the Holocene Age. The clay here is more than 80 inches deep. Ponder that a moment. Clay, more than 80 inches down. And it's been here for more than 12,000 years. Sigh.
On the one hand, clay usually has a higher organic content than, say, sand. Generally speaking, the Brazoria Clay soil profile is about 4% organic matter at the surface. It drops substantially the deeper you dig. I can see this when I dig a trench: the surface layer is black or very dark brown, an indication of rotted organic matter. But just below the surface is that deep blue-gray river of clay. That's the other hand, the sticky, heavy clay that lurks just beneath the arable soil.
If you remember your soil science, clay particles are the smallest of the soil mineral components (sand is the largest). Because the particles are so small, they can pack together very tightly. People sometimes advise gardeners to mix in sand to break up the clay and improve soil texture. But clay and sand particles are so different in size that they don't mix well. In fact, around here, what you get when you mix sand and clay, then add water and vegetable matter, is adobe. Better idea: add organic matter. In warm climates like mine, organic matter decomposes rapidly and is depleted. It's a good idea to try to incorporate organic matter at least once a year to maintain soil tilth and fertility.
In case you're wondering, I'm writing about clay because I'm too tired to dig any more of it up. If you're interested, a wonderfully geeky place to poke around is the USDA Natural Resources Conservation Service's Web Soil Survey. You can zoom in on your own soil profile, right down to the address level! It doesn't hurt as much as actual digging does.
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