While clay is beset with problems initially, a little work and sound management can produce excellent results. It is certainly true that, in the early stages of cultivation, clay is not nearly as convenient to work as a light soil such as sand. When it is wet, it rapidly becomes a soggy mess of mud and, when it dries out, it sets like concrete.
Clay is a badly drained, cold, and heavy soil because the spaces between each particle are too small to allow free passage of water and air, so the soil is always in danger of settling down to form a solid, airless mass. Improving the soil structure can take a few years, but a good clay soil is capable of growing far better crops than a sandy soil ever could.
Digging
It is best, if you can, to dig a clay soil during the fall, either at a time when there has been a little rain to soften the hardbaked soil, or when it is drying out after being soaked, but before it is hard again. In temperate climates, these times are fairly frequent, but in drier climates it calls for good organization and rapid action when the weather changes.
When heavy soil gets wet and dries out again, it expands and contracts, causing the mass of soil to crack into innumerable small clods. If water then gets into these cracks and freezes, it will force them further apart, breaking the soil down to a sowable tillage.
So dig clay soils in the fall, leaving the surface rough and uneven through the winter to expose the maximum amount of soil surface to the elements. At the same time, you can work the organic material into the upper levels.
Drainage
Because clay was broken down chemically, it’s also possible to combine the particles chemically by a process known as "flocculation". If sufficient lime is added to the soil, the tiny particles of clay will bind together to form much larger crumbs, through which air, water, and plant roots can freely pass.
Check the requirements of the plants you want to grow and use as much lime as you can without making conditions intolerable for them. In addition, if your soil is very heavy, dig coarse sand into the soil—approximately one to two bucketfuls every square yard/meter. Raising a section of soil above its immediate surroundings will improve drainage considerably, helping the soil dry out and warm up; raise your ornamental beds slightly and grow your vegetables on the deep bed system.
The important thing with clay is that you should never walk on it when it is wet or you’ll destroy years of work. If you have to walk on it, lay boards down first.
Organic Matter
Adding plenty of bulky organic matter to a clay soil will hold the particles apart so that roots and water can pass through. After a few years, when the level of organic matter is satisfactory and the soil is filled with the roots of previous crops, clay becomes much easier to work. In fact, every plant you grow plays its part in improving the soil for the next plant generation.