If your garden has clay soil, growing plants successfully may be difficult, and it will be a challenge to loosen the soil and prepare it for planting. Clay soil retains water for long periods, as it drains slowly. In sunny weather, clay soil dries to a hard, cracked surface that is difficult to penetrate. Some positive aspects to clay soil are that soil moisture levels will be high and it generally holds more nutrients than sandy soils.
The best time to loosen clay soil is prior to planting; you can check the soil pH levels at the same time. Use a home soil pH test kit or send samples of your soil to your local soil-testing laboratory. When you are aware of the pH levels for the soil, you can decide on the best soil amendments to work into the garden. Organic matter is the best addition for loosening clay soils. Working a 2- to 3-inch mixture of compost and aged manure into the top 6 to 8 inches of your soil will start the process of lightening the clay and improving the soil's drainage.
Once you have planted your garden beds, continue to add organic matter to the soil with mulches. Use grass clippings, leaves or compost to further loosen your clay soil. Regular cultivation and planting of clay soils will also assist in aerating the soil and loosening it.
Expanded shale is an additional means of loosening clay soils. Dr. Steve George of the Texas Cooperative Extension advises that following extensive research, expanded shale will open up and aerates clay soil quicker than any other material, since it provides "aeration from within the shale particles." Where clay soils are extremely heavy, gypsum may assist in improving soil structure; however, its effects are short-lived.
Clay Soils Under Lawns or Plantings
Where lawns are laid on top of clay soils or plants are already established, core cultivation may be the only way to loosen the soil. This entails drilling 1/2-inch holes into the soil to a depth of around 4 inches and no more than 2 inches apart. Good topsoil should then be worked into these holes. Core cultivation allows air and water to begin circulating around the root system of the plantings, allowing the roots to maintain growth.
It is a common misconception that adding sand to clay soils will amend the soil, giving it a sandier, loam-type texture. In fact, adding sand to clay soils will result in a concrete-like mix that will be more difficult to grow in than your original clay soil. This is because the clay particles will adhere to the sand and stick together when wet.