Balanced, well-drained soil full of organic material is every gardener's dream, but unfortunately, soil is often less than perfect. Heavy, clay-based soil is a common problem in many parts of the country. Although clay soils are usually nutrient-rich, the heavy nature of the soil makes growing plants difficult, as the density limits air circulation and the soil's ability to drain well. Clay soil can be improved, but the process requires patience and persistence. Severely compacted soils may not be resolved the first year.
Test the soil to determine if the soil is dry enough to be worked in spring. Squeeze a handful of soil in the palm of your hand, then drop the soil on a hard surface such as a concrete sidewalk. If the ball of soil breaks into small pieces, the soil is ready to be tilled. Wait a few days if the clay doesn't shatter, as attempting to work the clay too soon will create hard dirt clods, making the situation worse.
Cultivate the soil thoroughly, as cultivation will improve the soil's air circulation and permeability to water. Use a spade or a rototiller to cultivate the soil to a depth of 7 to 8 inches. Cultivate the soil until the clods are about the size of marbles. Avoid overworking the soil, as finely-worked soil will crust over the top. Use a rake to break up any large clods that remain after cultivation.
Spread 2 to 4 inches of organic material and 1 to 2 inches of coarse sand on top of the cultivated soil, then dig the organic matter and sand into the top 6 to 8 inches of soil. Organic materials such as compost, rotten manure, leaf mould or other green plant waste are preferable. Other materials, such as sawdust, peat moss, straw and shredded bark, which are slower to decompose, are acceptable but less desirable.
Plant the improved area as desired. Spread 2 to 3 inches of mulch such as compost or dry grass clippings around established plants to avoid erosion and prevent the surface of the soil from crusting. Limit foot traffic on the clay area as much as possible, especially when the soil is wet, as too much walking will compress the soil.
Severely compacted clay soil should be cultivated every spring, with a fresh layer of organic material worked into the soil.