Vegetables grow best in loamy soil, which allows plenty of drainage and has high soil porosity so roots can access oxygen. Clay soil by nature has poor soil structure that compacts easily, restricting water and oxygen flow through the soil. Water might stand on the soil surface, unable to penetrate the soil, or the soil simply stays wet, which can cause root rot. Vegetables such as cabbage, broccoli and brussels sprouts do well in heavy soil, but frequent, heavy applications of organic matter are required before planting most vegetables. Start amending the soil one year before planting for best results.
Cover the ground with 4 inches of organic humus material in spring one year before beginning a garden in the following spring. If grass is growing in the garden area, leave the grass in place because this is additional plant material to add to the soil. Materials you might add include aged manure, grass clippings, straw, sawdust, shredded bark, and leaves or leaf mold. For best results, use an equal amount of green and brown materials to avoid nitrogen and carbon imbalances in the soil.
Add 3 to 4 inches of finished compost over the 4-inch layer of humus materials. Compost is important because it is already broken down, providing stabilized structure to the soil. The humus materials decay rapidly and must be added frequently before serious changes occur, but compost has an instant effect on soil structure.
Spread 2 inches of coarse sand over the compost layer to increase porosity and drainage in the soil. Fine sand particles mixed with clay makes a cement-like mixture, so you must add only coarse sand to the clay.
Broadcast a complete fertilizer, such as 12-12-12, which contains nitrogen, phosphorous and potassium, into the soil at a rate of 2 pounds per 100 square feet or roughly 3 ounces per 10 square feet of garden area. Fertilizer is especially important if the brown and green humus materials are imbalanced.
Till the layers of soil amendments into the top 8 to 10 inches of the native clay soil using a rototiller. Wait until the soil is dry before attempting to till the soil because wet clay soil forms large clods that are difficult to break up after they dry.
Apply additional layers of humus materials, compost and coarse sand about once monthly over the summer, and till the materials into the top 8 to 10 inches of soil. Repeated applications are not required, but the rate of improvement in clay soil is consistent with the rate of application of soil amendments.
Broadcast seed for a cover crop in mid to late fall at a rate of about 1/3 pound for every 100 square feet of garden space. Alfalfa, barley, rye and clover are common cover crops you might plant. Allow the cover crop to grow naturally, and leave it untouched over winter. Cover crops protect the ground over winter and provide additional humus to improve soil structure.
Add about 2 inches each of finished compost, aged manure and coarse sand over the cover crop. These materials are optional, but only further improve the clay soil.
Till the cover crop, compost, manure and sand into the top 10 inches of the soil in late winter to early spring after all danger of frost has passed and when the soil is dry.
Break up any soil clods with a spading fork. Allow about two weeks for the soil from the clods to dry. Till the soil a second time after the clods have dried.
Rake the ground smooth, using a bow rake and leaf rake.
Work the soil into straight rows, using a garden hoe to dig furrows as deep as the root ball of your vegetable transplants, usually no more than 6 inches. If you will direct sow seed into the garden, follow the specific planting instructions for each plant type. Seeds are generally planted to a depth of 1/2 to 1 inch; refer to your seed packet for planting depths.
Loosen the soil between planting rows with the rototiller, being careful not to disturb the plant roots. Use the garden hoe if you cannot navigate the tiller between rows without damaging your vegetable plants. This keeps the soil loose and prevents compaction. If possible, place a few stepping stones in the row to help limit foot traffic in the garden, which can also compact the soil.
Things You Will Need
- Humus materials
- Finished compost
- Coarse sand
- Broadcast spreader
- Cover crop seed
- Spading fork
- Bow rake
- Leaf rake
- Garden hoe
- Not everyone has an entire year to improve clay soil, but at the very least, you should begin your efforts in the fall before planting your vegetables in spring. Add the humus materials, compost and coarse sand, till it into the soil, then sow the cover crop seeds. As a general rule, the heavier the clay soil in your garden, the heavier and more frequent your applications of soil amendments should be to achieve better soil structure and tilth.
- North Carolina Cooperative Extension Service: Amending Clay Soils
- University of California Vegetable Research and Information Series: Managing Clay Soils in the Home Garden
- The Ohio State University Extension: Improving Soils for Vegetable Gardening
- Fine Gardening: Improving Clay Soils
- Oregon State University Extension Service: How to Improve Clay Soil
- Hemera Technologies/AbleStock.com/Getty Images