Clay soil is probably the most difficult type of soil to garden in. The clay minerals are flat, like plates, and when stacked up, they leave little space for water and air to penetrate. Clay is fertile and holds water well, but it is difficult to wet when dry and warms slowly in spring. Add organic matter every year to lighten and aerate your clay soil. Dig when dry for best results and be sure to avoid compacting it by walking on it. Use pathways instead.
Potatoes have strong roots that can push through clay and are often used as a first crop on these soils. They prefer acidic soil with a pH of 5.5 to 6.2 so don't add lime before you plant. Use middle or late-season varieties because the soil will be cold in early spring. Plant them in newly amended soil and let the organic matter rot in preparation for other crops next year.
The Cabbage Family
Most brassicas (cabbage family plants) grow well in clay soil, though it's best to amend it as much as possible. Cauliflowers are the exception. Again, because the soil warms slowly, use mid or late-season varieties. If your soil is acidic, add ground limestone since these plants prefer a pH of 6.5 to 7.0. To get a head start on the season, buy small plants at your local nursery or grow your own transplants from seed indoors.
Corn is a heavy feeder and will appreciate the fertility of clay soil. It dislikes wet soil, however, so mound your beds up to get the best drainage. Before you plant, add as much organic matter as you can and break up the clay when it's dry so water will drain between the clods. Sow the seeds when the soil is warm (at least 60 degrees F) and be careful to plant them no more than 1 inch deep. Cover with potting soil rather than soil from the bed so the seedlings will have loose soil to push through. Never let the ground dry out completely or you'll stress the corn plants, making them vulnerable to pests and diseases, but don't keep it soaking wet either.
Both winter and summer squash like warmth, requiring 70 degree F soil to germinate. They can be direct seeded or grown indoors to get a start on the season. Like corn, they are heavy feeders and, fortunately, the nutrients from compost or fertilizer that you add will "stick" to the clay rather than leaching through as they would in sandy soil. Squash like well-drained soil, however, so plant on top of small mounds and avoid over-watering. Give them a slightly acidic to neutral soil with a pH from 6.5 to 7.0.