The mineral particles that make up soil are gravel, sand, silt and clay. By far the smallest, clay particles cling together, do not drain well, and are unsuitable for growing most garden plants. To get an idea of the relative sizes of soil particles, imagine for a moment that a single grain of sand is ½ inch across. By comparison, a grain of clay would be about 1/64th of an inch across.
A soil testing laboratory can give you an exact reading on how much clay is in your soil (find a good lab through your county Cooperative Extension Service, listed in the phone book.) While you are waiting for that, you can also do a quick and easy test yourself.
Use a clean half-gallon or gallon glass jar with a screw lid. With a garden trowel, dig a hole about six inches deep. Cut one side of the hole straight down, like you would cut a cake. Now cut a one-inch-thick slice of soil, from the surface down to about five inches. Place this in the jar. Fill the jar with water until it is a couple of inches below the top. Shake the jar thoroughly to mix everything together, then place it somewhere it will not be disturbed. The sand will quickly settle to the bottom. A layer of silt will slowly settle on top of that. Next---and it might take a day or two---the clay particles will form the top layer.
If 50 percent or more of your sample is clay, then you have what is called a heavy clay soil.
Gardening in Clay
As anyone who has tried it will tell you, not much will grow in clay. Some trees, such as apples and willows, will tolerate it, as will lilac and forsythia. But everything will do better if the soil is amended so that clay is no more than 25 or 30 percent of the total.
Despite its disadvantages, one good thing about clay (particularly red clay) is that it holds nutrients well. Once it has been amended it can become good soil.
The pH Balance
Soil pH is a reading of the acid-alkaline balance. It is a scale from 0 to 14, with 0 being acid and 14 being alkaline. Neutral is pH 7. Clay soils tend to be alkaline, so you will have to correct this with sulfur. A laboratory soil test will tell you how much sulfur to add, per 100 square feet and/or per acre.
The pH level should be 6.5 or 6.8, slightly on the acidic side of neutral, for most garden plants. If the pH is wrong, your soil can be full of nutrients and your plants will be unable to use them.
Amendments to Use
If you have a heavy clay soil, you will need to cover the whole area with a four-inch-thick layer of coarse compost and a four-inch-thick layer of coarse sand. Less than this will make the situation worse, so don't skimp on it. These additions can be dug into the top six to eight inches of soil by hand, but using a large and powerful rototiller is better and easier. These machines can be rented by the day. Never work with clay soil when it is wet.
As a rule of thumb, one cubic yard of compost or sand will cover about 100 square feet with three inches of material. While it sounds like a large area, 100 square feet is only 10 feet by 10 feet. You might need several truckloads of material for even a small garden.
Things Not to Add
Since clay soil is mostly very fine particles, avoid adding more of any kind of small particles. Get coarse compost, not finely-screened material. Use really coarse sand, sometimes called builders sand. Do not add peat moss, which is milled into tiny pieces and will turn into something like concrete if it is mixed with clay. For the same reason, do not use sawdust.