Clay soils are sticky, solid soils that can condense down tightly and smother plants that are trying to survive in it. Plants need oxygen as well as water to survive and a clay soil can compact down making it difficult for plants to grow. Adding amendments to the soil can help to rectify this problem if the amendments are added at the correct amounts.
Dig up and turn the soil that is going to be used for growing the garden to at least 12 inches deep--deeper if growing crops or plants with very deep roots. This turning process can be done by hand with a shovel or with a rototiller. Do not try to work the soil when it is wet because this will only cause more problems. If the ground is saturated, wait until it has dried out to begin.
Distribute a thick layer of coarse materials over the top of the soil. This material should be made up of about 50 percent sand and 50 percent compost and should be at least 6 inches thick. If you are only using sand, the mixture must be 50 percent sand to 50 percent clay soil. If the soil is not at least 50 percent sand, there is a great risk of making soil that can be worse than than either soil or sand alone. The bonding of these two materials creates a good soil mixture only when sand is mixed into the clay at high ratios. Using composted materials helps to prevent the need for so much sand and help to bring nutrients to the soil that will be needed by plants later.
Turn the sand and compost mix into the soil with a shovel or rototiller to mix well. Dig deeply into the soil, and turn the dirt over by the shovelful until the entire garden area has been mixed.
Using a garden rake, break up any clumps that form in the soil. It is essential that the soil, compost and sand are mixed well into the soil.
Test the soil by picking up a handful and forming it into a ball. If it is a hard clump that forms together well, it is probably still too sandy and more material should be added. If it clumps together but doesn't form a hard ball, the soil is at the correct density.