In many parts of the country, the native soil is a thick, sticky clay that becomes a slimy mess when it's wet and a rock-like impenetrable wall when it's dry. Clay might be good for making pots, but it's not so great for growing vegetables. But you don't need to trash your plans of a big backyard vegetable garden. With some soil amendments and a little hard work, you can turn your barren clay soil into something fertile and workable. To make a significant improvement, soil amendments should equal about one-third of the original soil you are working with.
Sand and Gravel
Sand and gravel improve drainage in clay soil by creating small air pockets and pathways for the water to seep through. However, too much sand can cause water to drain out too quickly, or the water may zip through the sand-amended soil and then collect and stagnate when it reaches the solid clay. For amending clay soils, sand and gravel are best used in conjunction with organic materials such as compost and manure.
Compost and Composted Manure
Compost and composted manure increase the fertility of clay soil and also help regulate its water retention abilities. Till compost or composted manure, mixed with sand or gravel if needed, into your clay soil to a depth of one foot. You may be able to plant vegetables directly into your soil. You could also build raised beds on top of the amended soil to give your vegetables a few extra inches of good soil. Fill your raised beds with a combination of compost or composted manure and high quality topsoil.
Adding a layer of mulch, such as dried leaves, shredded bark or straw, regulates soil temperature and moisture, and will help prevent clay-heavy soil from becoming muddy or drying out and cracking. Mulch also keeps down weeds and helps prevent fruit rot on vegetables such as squash, eggplant or cucumbers. Till the mulch into the soil at the end of the growing season; as it decomposes it will increase the fertility of the soil and add valuable nutrients.
If you've got the time, growing a cover crop---sometimes called green manure---is a great way to break up clay soils. Cover crops are planted in the fall and tilled under the following spring. While they are growing, the roots work to aerate the thick, heavy clay, and when they are tilled under they add valuable nitrogen to the soil. Cover crops should be sown very thickly to completely blanket the garden bed. You may plant your vegetables two weeks after you till under the cover crop. Good cover crops include crimson clover, fava beans and Austrian winter peas.