Although clay soil is considered a problem by many gardeners, it can actually be beneficial if approached the right way. According to soil scientist Simon Leake at Sydney Environmental & Soil Testing Laboratory, mineral-rich clay can be an exceptional growing medium when well managed. Amending clay soil by adding organic material, such as compost, has proven to be effective. However, for best results, you should apply the compost to a large area. Organics added in small amounts will provide little benefit.
Determine whether your soil is mostly clay. This is best done by having a sample analyzed at a soil testing laboratory. However, you can perform a simple soil test by taking a handful of moist soil and squeezing it into a ball. If the soil holds the shape of the ball when you open your hand, it is either clay or loam. Loam will crumble when poked, whereas clay will remain a solid lump.
Turn the top 8 inches of the clay soil with a spade. If the ground is dry, water the area lightly to make the digging easier. Spread 2 inches of well-aged compost over the area and dig it into the soil. Repeat the process with a second 2-inch layer of compost. It is important to get the compost mixed in right to the bottom of the loosened soil.
Wait for at least a week before planting to allow the organic material to break down into the soil. Check the consistency of the soil by doing the squeeze test. If it retains shape but falls apart when prodded, the soil is ready for planting.