Some gardeners take extraordinary and unnecessary steps to avoid dealing with red clay soil.
I have seen people buy truckloads of topsoil. The trouble with this is that anything in the world can be labeled topsoil. Some products are good. Other products are worse than the soil the gardener had to begin with. I wouldn't buy topsoil just to replace soil already in place. If you don't need more soil, your money would be better spend purchasing organic matter and using it as an amendment. You may have to buy topsoil to fill beds or add soil. In these cases I suggest you go look at the product and avoid anything that is mostly clay.
At the other extreme, I once overhear some garden center customers requesting a variety of unusual fertilizers. It turns out they were attempting a pseudo hydroponics system. Some people find hydroponics highly fascinating for no apparent reason, but it turned out these people were more concerned about their clay soil. They were replacing a simple garden with a technically difficult and expensive version. Eventually, they would have had less success.
An ideal soil would contain 45% mineral, 5% organic matter and 50% pore space. As long as you garden in Cabarrus County, you won't be bothered by ideal soil. I don't know where you would have to go to find ideal soil. Typical soil around here is closer to 16% pore space, 2% organic matter and the rest is mineral.
While I would like to have more pore space and more organic matter, I have seen soils that were even worse. For example, it's a wonder that the Jamestown and Lost Colony settlers didn't get back on the boat for England. The sandy soil down there won't hold nutrients. For proof you can look at the distribution of dairies. You will find none in the coastal plains. A 1000 pound milk producing animal can't get enough nutrients out of sandy soil to stay healthy. Texas has soil even worse. It can hold enough nutrients to raise cows but you would have a hard time plowing it. I have seen mud there build up in wheel wells to the point it would bust heavy duty military tires. I've seen good soil in Kentucky but parts of the state have creeks that always stay muddy. The clay soil is so fine it never drops out of solution. Some places in the world may have better soil but we can definitely work with what we have.
Undisturbed soil in Cabarrus County does have topsoil. It may only be a couple of inches thick but it is there and it is supporting plant life. Unfortunately, topsoil evaporates when the bulldozers appear. The new homeowner finds soil with almost no biological activity, no organic matter and occasionally compacted by heavy machinery. You can rebuild this soil. A little patience works better than store bought miracles.
You need to do about four things.
First adjust the pH. Our typical soils have a low pH while most ornamental plants need a pH above 6. The exceptions are members of the Ericaceae family. The most common members of this family are blueberries, azaleas and rhododendron. Our best lawn grasses need a pH of 6.5. Fruits and vegetables also need a pH of 6.5 except potatoes, beets and blueberries.
You increase the soil pH by adding lime. A soil test can tell you how much although you won't go wrong at 90 lbs. per 1000 square feet.
Next you need to add organic matter. While the average soil has less than 2% organic matter, I have seen numerous gardens and flower beds where the organic matter % is closer to 5 or 8. Compost or manure is the best choice. If you can get grass to grow, you can gradually build organic matter by returning the clippings. Homeowners typically use this for lawns because it is so much cheaper than buying amendments.
If your soil is compacted to less than 10% you will need to till or aerate. Tilling our clay soil will immediately raise the pore space to 18 or 20%. The soil will gradually pack back down but will not go under 10% pore space unless there are some external forces. Plants can grow in soil with more than 10% pore space.
Measuring pore space isn't simple. Most gardeners will till the place they are planting annuals every year. With vegetables, tillage is almost a necessity since you walk in there while harvesting. The only way you can get around tillage with vegetables is with a permanent bed that is never stepped on. On the other hand, once you have established roots on trees and shrubs, you shouldn't aerate because you can damage the roots. If the soil isn't compacted, it will be okay.
Finally you need to add nutrients. Our soil is usually short in phosphorus and potassium. You can usually see a response with nitrogen fertilizer. A soil test will give you accurate rates.
When you do all these things to the soil, it won't respond immediately. The biological life has to build up and stabilize. For proper plant growth, you need thousands of bacteria per tablespoon. I have seen people do everything right and still have a spotty lawn the first year. A year later they had a wonderful lawn. When you added the organic matter, you provided a food source for these bacteria and fungi. Once they have a food supply, they will build up over time. On lawns, you can usually see results in two or three years. On a smaller area where you can afford organic matter, you should see good results this growing season if you start now.
So work with the soil. If you try to avoid the problem you might make things worse. Soil is the foundation of a great garden. You can apply these principles to flowers, vegetables, perennials, lawns, and established woody ornamentals.
By the way, red clay is one of our better soils. When you have gray, brown, purple, white or yellow clay, the soil is worse but you deal with these soils the same way.
Article was written by David Goforth Agriculture Extension agent North Carolina Cooperative Extension Cabarrus County Center. Visit my homepage http://cabarrus.ces.ncsu.edu/ or http://cabarrus.ces.ncsu.edu/index.php?page=lawngarden or my blog http://gardeninggurugoforth.blogspot.com/
Contact me at David_Goforth@ncsu.edu. Reviewed 2007.