Clay soil is soil that is comprised of mostly clay. When clay soil is 50 percent or more clay, it is defined as heavy clay.
Clay soil is effective because it retains moisture well and is rich in nutrients. It tends to drain slowly, however, and warms up more slowly in spring compared to regular soils. Although tree and shrub roots are hardy enough to break through clay soil, vegetable, perennial and annual roots have difficulty growing in it because it compacts easily. Bulbs left to winter in clay soil often rot from the excess water content. Improve clay soil by mixing in organic matter such as grass clippings, fertilizer and compost to the upper 6 to 12 inches.