According to Ohio State University, the first step to improving the quality of garden vegetables is to improve the soil. Soil that is heavy clay retains water and will not allow plants to grow well. Soils such as this appears rock hard when dry and slippery when wet. To improve the soil, the first step is to mix soil amendments into the garden with a tiller. But care must be taken when tilling clay soil.
Wait until clay is not too wet to till or too dry. Dry soil is rock-hard and difficult to till. Tilling wet soil will compact the soil and create clods that stick together.
Break up the soil with a rototiller to a depth of 8 inches by running the roto tiller in sections over your garden.
Spread compost over the soil of your garden to a depth of 2 inches.
Turn the compost into the soil by running the rototiller over the garden soil again.