Clay soil tends to have poor drainage because clay is the smallest particles that make up soil. In most gardening situations, improving clay soil drainage is a necessity. While loam--an equal mixture of sand, silt, clay and organic material--makes up the ideal soil, clay soil has more clay than any other material. The clay holds onto water and can drown your plants unless you do something about it. However, if you want to improve clay soil drainage you will have to do some work.
Test your soil to see how much clay is in it. Take a sample of soil, about half a cup, from your yard and put it in a jar. Fill the jar with water, close the lid and shake it. Let it sit for 2 to 3 days. Once everything has settled, look at the soil. Sand will be at the bottom, silt will be on top of the sand, clay will be on the silt and organic material will float on top of the water. If your layer of clay is more than half of the soil, you have heavy clay soil.
Measure the square footage of the area you want to improve using this formula: length x width = square footage. Write the number down.
Add an equal mixture of coarse sand and compost or aged manure to the soil at a depth of 3 to 4 inches each per square foot.
Work the compost or aged manure into the soil to a depth of 6 inches. Use a tiller to work the compost or aged manure into the clay soil. Organic material such as compost and aged manure improves the amount of air in the soil and aids drainage.
Work the coarse sand into the clay soil with a tiller on top of the compost or aged manure to a depth of 6 inches. Coarse sand also breaks up the clay and allows for better drainage.