Heavy clay soil, made of fine particles, can become as hard as concrete in the heat and turn slimy and sticky when it rains. It blocks the penetration of water, air and nutrients to plant roots and can be a chore for gardeners. Treating clay soil to make it more congenial for gardening is called amending it. Additions to the soil are called amendments.
Problems with Clay
Clay is made of fine particles of minerals that are flat or flaky. Their large surface area means that they expand when they're wet. They don't drain well; poor drainage deprives plant roots of oxygen. Clay warms slowly in the spring, delaying planting times. Water in clay can freeze in the winter, causing the clay to buckle or hump, pushing plants out. In a drought, a crust may form on top, blocking seedlings and preventing roots from going deeper.
Organic vs Inorganic Amendments
Organic amendments are anything that was once living, such as treated sewage, compost, grass clippings, manure, sawdust, sphagnum, wood ash and wood chips. Inorganic amendments are either man-made or mined, including pea gravel, perlite, sand, tire chunks and vermiculite.
Organic amendments provide plant nutrients and energy for bacteria, earthworms and fungi that live in the soil. Inorganic matter helps your clay stay loose so you can more easily cultivate it; inorganic amendments improve aeration, promote drainage and improve the ability of clay to absorb water and nutrients.
Till or spade the clay about 8 inches deep. Spread a 2-inch layer of amendment on top and work it into the clay. Spread another 2-inch layer on top and work that in, too; this time you can add fertilizer or lime if you wish.
Fertilizer is not an amendment; it is a nutrient. Lime is not a fertilizer; it is added to acidic soils to make them better able to accept fertilizer. Contact your agricultural extension service to learn how to get your soil tested to see if it needs lime.
Pea-sized nuggets of pine bark sold as pine bark mulch is a good amendment. Manure that has been aged through composting is good.
Sawdust or wood chip will take nitrogen from your soil when they begin to rot. Do not till it into your clay until it is well decomposed.
Fresh manure is salty and will dry out plant roots. It's a good nitrogen source for compost, but not as amendment to clay.
Add gravel that is about the size of a pea. This will not break down and will help keep your clay soil loose and better able to drain. You can also add expanded shale available commercially.
Sand is a good amendment for some clay soils and causes others to turn rock hard. Check with your local agricultural extension service to find out which kind of clay you have.
Gypsum is good for alkaline clays, but not good for acidic clays.
Raised Planting Beds
When you add amendments to clay soil, you will add volume; that raises the level of the planting area. Keeping it raised is a good way to promote drainage; to keep it raised, build a retaining edge or rake the amended clay into a berm.