There is a simple way to tell if you have clay-based soil. Take a bunch of soil in your hand and squeeze. When your hand is opened, the soil should break apart and fall out of your hand easily. If it stays in a clump, the soil has too much clay and should be amended before you plant. If you leave the soil as it is, you could have a problem later. When the soil becomes wet, the water saturates the clay-based soil and sits on top and in the soil. A great many plants do not grow well with saturated roots, because of compaction and limited oxygen. The clay compaction makes it difficult for roots to grow, stunting a plant's growth. In some plants, it can cause root rot. In addition, it is sticky and clumpy, making it difficult to work with.
Work the clay soil with a shovel or tiller, depending on the size of the area to be amended. Dig down 12 to 18 inches, turning the soil as you work. Wait until the soil is fairly dry. When a clay soil is wet and soppy, it sticks to the shovel or machinery, making it more work than it need be.
Take out any rocks, roots, weeds or other litter.
Layer 3 inches of compost (decomposed organic material, found at nurseries or garden centers) on top of the worked area. Rework the soil with a shovel or tiller, until the compost is evenly mixed.
Conduct another hand squeeze test on the soil. If the soil is still too clumpy and doesn't break apart easily, pour an additional 2 to 3 inches of compost on top and work it in again. When the soil breaks apart easily, after squeezing it, the soil is amended.
Rake the garden area even with a rake.