Heavy clay soil has advantages and disadvantages for a gardener. While the clay holds nutrients well, it holds water too well. Plant roots can become waterlogged due to poor drainage, and clay gets hard and cracked when the weather is dry. Heavy clay soil also becomes easily compacted, which prevents roots from getting adequate oxygen. Improving clay soil for a good garden tilth and drainage takes time and patience, but it can be done without expensive amendments and the end result is worth the hard work.
Plant raised beds if you want to start growing in clay soil right away. Spread a 3-4 inch layer of mulch and compost over the soil and plant directly into the mulch layer. The plants' roots will start breaking into the clay layer, and the compost will attract earthworms that can begin the job of digging the organic material into the clay. Adding organic material is the first step to improving tilth and drainage.
Till in the fall when the soil is not too wet. Tilling wet soil is unnecessarily hard work for you or your tiller, and it can create rock-hard clumps of clay. Do your tilling when the soil is moist, and go down about 6 inches. This is a good time to add another layer of mulch or compost, as well as manure, lime and phosphorus to balance pH and increase sulfur, nitrogen and calcium levels. Spread the amendments over the top of the soil and till them in.
Consider using a cover crop to help break up clay and fix nitrogen in the soil. Cover cropping can be done on fallow ground, or you can plant a winter cover crop. Good cover crops include vetch, clover, and rye. Sweet clover and winter rye can be planted in fall and do well in cold weather. The root systems of cover crops help hold the topsoil in place while aerating the clay and improving drainage. Cover crops have the added advantage of helping to choke out weeds.
Work the cover crop into the soil in the spring several weeks before you want to start planting, but when the soil is not too wet. You shouldn't clear cover crops away before tilling, although cutting down tall grasses might make tilling easier. Cover crops are a type of green compost that will improve drainage and add nutrients to the soil.
Work a layer of mulch and compost into the soil right before you do your spring planting. The best way to do this is called double-digging. With a shovel, dig a trench that is about 1 foot across and 1 foot deep along one end of the bed. Hit the soil you've dug up to break it into smaller clumps, mix in some mulch, and set it aside. Use a digging fork to loosen the soil in the trench, and then dig another trench next to the first. Mix the soil from the second trench with mulch and put it into the first trench. Repeat this process until you've dug and mulched the entire bed, using the soil from the last trench to fill the first trench you dug. Double-digging is work-intensive, but the results are longer lasting, and gentler on your soil than using a tiller.