Clay soil is too compact to grow most vegetables. The top 8 to 10 inches of soil must have enough organic material in the mix to allow oxygen to the plant's roots. Adding organic matter to clay soils amends the heavy soil, according to North Carolina State University. Clay does have an advantage once organic material is amended into the soil; it has the ability to hold moisture. Appending materials into clay soil is not a one-time endeavor, however, it is an annual process.
Conduct a soil test of the clay soil. Take soil samples from several locations around the garden site. Mix the samples together, dry the soil thoroughly, then take the samples to your local agricultural extension service.
Wet the clay soil if it is dry. Hard clay will not respond well to any form of cultivation, mechanical or hand digging. Wait two days after adding moisture. Never work clay soil if excessively wet, as the clay will only compact further and become more difficult to work.
Layer organic material to a depth of 2 inches over the clay garden site. The amount of material is approximately 3 cubic yards per 1,000 square feet. Use a combination of materials that include peat moss, leaf mulch and compost. Wood by-products can also be utilized but must be broken down to 2 inches in diameter or less.
Work the organic material into the soil using the rototiller to a depth of 6 to 8 inches. You can dig the material into the soil using the shovel for smaller growing beds or sites with restricted access.
Add fertilizer and agricultural lime to the garden bed based on the soil test analysis. Using the soil test results is the most accurate method for adding proper nutrients to the soil.
Layer another 2 inches of organic material over the fertilizer and lime. Work this material into the clay soil.