Clay soil is tightly packed dirt that clumps into dense clots of soil when wet. When dry, clay soil is hard and difficult to penetrate and leads to poor drainage. Plant roots, then, are unable to pierce the soil and take up nutrients if the soil is dried out, and the roots tend to become waterlogged once the soil is dampened. You can loosen clay soil by aerating the soil, adding amendments and growing cover crops.
You can use a roto-tiller to aerate soil. A roto-tiller has circular blades with teeth that tear up the top of the soil as you move the machine along the surface. Breaking up the surface allows oxygen to enter the soil, thereby breaking it down and loosening it. You can then further break down the soil using a shovel and pitchfork to turn the soil. Aerating, however, is not a long-term solution to turning clay soil into a viable home for plants. It should be used in conjunction with amendments added to the soil.
Organic compost added to clay soil will encourage necessary bacteria, fungi, humus and earthworms to thrive within the clay soil. The presence of these elements will naturally loosen the clay soil, resulting in better drainage and easier penetration by plant roots.
Gypsum can also be added to loosen clay soil. Gypsum is made up of calcium and sulpher, both organic materials. Adding gypsum to clay soil causes flocculation, a grouping together of the clay particles so there is more oxygen between the particles. It makes the soil less clumpy.
Sometimes referred to as green manure, cover crops can be grown in clay soil, and then the crop turned into the soil. Crops such as barley, alfalfa or bell beans have tough roots that burrow into the clay soil. Before the cover crop comes to maturity, or goes to seed, you can turn the crop into the soil. This method naturally replenishes the soil with nutrients while loosening the tightly packed clay soil. The crops break down within the soil, encouraging friendly bacteria and fungus and may result in an increase in earthworm activity.
Turning the immature cover crop into the soil also works to aerate the soil. The soil then is not only amended with a naturally occurring compost material, but is also loosened when the crop is dug into the soil.
With the presence of friendly bacteria, fungus and earthworms in your clay soil, plants have a better chance of establishing healthy root systems and so may thrive in the soil. To keep the soil healthy, add a layer of much with each seasonal planting, and continue to add amendments as needed.