The same qualities that make clay perfect for pottery--it sticks together well and it doesn't absorb water--make it the bane of gardeners. Clay compacts easily, making it difficult for plant roots to permeate and depriving those roots that do take hold of the oxygen they need to remain healthy. Clay soils often lack nutrients plants need. If you live in an area with clay soil, to grow a garden you must first address the problems inherent with clay. Over time, you can transform clay soils into a much more hospitable environment for plants.
Till the garden. You should till clay garden soil when it's dry, if possible. This will make it less likely to clump together. After tilling, go through the garden with a garden rake or hoe and break up any clumps and spread them out. Your goal is to separate the soil into as fine particles as possible.
Add organic matter to the soil. Mix manure, compost or rotted leaves into the clay to add nutrients, help the clay hold more water and improve the overall soil structure. Use 4 to 6 inches of organic matter over the whole garden.
Add nitrogen to compensate for a temporary nitrogen deficiency caused by the addition of slow-decomposing organic material, such as peat moss, straw or rice hulls. These materials can benefit the soil but also leach nitrogen needed for plant growth. Add 1/4 pound of ammonium sulfate or ammonium nitrate for each bushel of organic matter you incorporate into the soil.
Plant a winter cover crop, such as rye grass, in the fall. These crops add nitrogen to the soil. In the spring, till the cover crop into the soil to add more organic matter and nutrients to the clay.