Many areas of the United States have soil that makes planting difficult because it is composed of small particles called clay. Clay soils are sticky when wet and hard clumps like concrete when dry. It is difficult for plants to establish roots and get nutrition from clay, so the soil must be correctly prepared before using for planting. Correcting a clay soil for planting takes time--often several growing seasons--but it can be done successfully. Once clay soils are adequately prepared, plants will have access to their rich nutrients and thrive.
Conduct a soil test two to three months before planting. This is essential because clay soils are not identical in their composition, and the correct preparation should be tailored to the amendments needed by the soil in your garden. Collect soil samples from at least three areas you want to plant in and three different soil depths. Mix together the sample into a bag and submit to a lab for testing. Most states offer soil testing through their agricultural university labs.
Analyze the soil test results, paying particular attention to pH, potassium, phosphorous, nutrient availability and cation--the soil's nutrient-holding capacity provided by calcium, magnesium, sodium and potassium. Plants prefer a slightly acid to neutral pH and high cation. Use this information to tailor your plan to prepare clay soil for planting.
Till the soil at least 6 to 12 inches deep. Wet clay soils should be dug with a spading fork rather than a shovel. Relatively dry clay soil can be effectively turned and prepared with an electric or gas-powered tiller.
Break up the larger clods or clumps of clay soil and allow them to air dry.
Amend the soil to correct pH using sulfur to raise the pH and lime to lower it. Add a fertilizer with the correct ratio of nitrogen, potassium and phosphorous, depending on your soil test results.
Add organic material, including brown manure from dried grass and leaves and green manure from plants. Oregon State University recommends adding to one-third of the soil you are amending to effectively change the soil composition.
Add expanded shale. Dr. Steve George of the Texas Cooperative Extension suggests adding 3 inches of expanded shale and tilling it in 6 to 8 inches deep.
Mix amendments and clay soil thoroughly. Let soil rest for two weeks before planting.