Clay soil can be both a blessing and a curse for most gardeners. Although it readily holds moisture and nutrients, it is often poorly drained and difficult to work with. During dry periods, clay can harden to the consistency of a cement sidewalk, and any following moisture turns it into a sticky, plant suffocating slurry. Lacking organic matter, most soil microorganisms necessary for a healthy garden are absent. Clay soil does retain nutrients and minerals far batter than other soil types. The negatives far outweigh the positives, but there are a number of simple remedies and modifications for this soil type.
Rehabilitating clay soil is a long term process. The first thing to do is improve drainage. Take a look at where water collects and persists after a rainstorm. The easiest fix is to landscape with plants that enjoy wet feet. If this is not possible, the installation of trenches and french drains is called for. A french drain is simply a three to four foot hole dug into the ground and filled with round gravel. These are extremely labor intensive, but it is amazing how much water they can hold, water that would normally persist in standing puddles. Best of all, this is not redirecting the water somewhere else, such as your neighbor’s yard.
Many gardening publications recommend tilling or breaking up the first 18-24″ of surface soil, then mixing in amendments. Good luck. This is exhausting, backbreaking work. It is far easier to let nature work for you, by applying a thick layer of rotten hay or straw as a mulch. A layer four to six inches deep will break down slowly over the course of a year, keeping the soil moist and allowing earthworms and soil microorganisms to do the hard work for you. A hay or straw mulch also prevents weeds. The best time to apply is in early spring, or in winter in areas with no snow cover. By the middle of summer, pulling back the mulch will reveal a plenitude of earthworms, and soil that is almost crumbly.
Some rudimentary planting can actually occur at this point. A few large trees and shrubs that tolerate clay soil can be installed, such as Magnolia grandiflora, paper birches, sugar maples, willows and shrub dogwoods. The roots of these plants will penetrate the subsoil further, breaking it up gradually so you don’t have to. In the fall, apply another layer of mulch. By spring, topsoil can be hauled in and applied over the mulch. This is not as counter intuitive as it sounds. The soil has been kept moist over two growing seasons, and worms and plant roots are breaking up the subsoil. The hay mulch adds much needed organic matter, and will continue to break down slowly. Some high nitrogen fertilizer such as composted steer or chicken manure can be applied around existing trees and shrubs. The underlying mulch, while breaking down, can tie up nitrogen levels in the soil temporarily and a little compost can counteract this. Fine landscape bark can be applied around this time, and will also add needed organic matter. Be sure to keep both mulch and topsoil away from the main trunks of trees. It can cause rot and kill them. Mulch should not be applied deeper than two or three inches within the tree dripline.
The next spring, a soil test is in order. This will provide an idea of what nutrients may be lacking. Mulch such as conifer chips can acidify the soil, and so some lime may be needed. Organic fertilizers such as cottonseed meal, steer or chicken manure, and other soil builders such as greensand may be indicated by the soil test.
For those who have more money than time, it may be best to have truckloads of topsoil hauled in, to form large berms or raised beds. This is quick and easy, but it is expensive. A soil test is still needed unless the soil in question comes from a superior source.
A quick vegetable garden can be planted and harvested the same season using the straw mulch method. Apply the mulch in spring as early as possible. A small load of compost may be required as well. Lay out the garden, mulch well, and when planting time comes push aside the mulch, add a few shovels of compost and start planting. Rows should be at least a foot wide, and the compost six to eight inches deep. The first season’s harvest will be surprisingly good. Clay retains many minerals that vegetables need. The next year, add more mulch. By this time the soil should be showing good structure, and a soil test and a few adjustments indicated by it are all that needs to be done. Watch out for pests that may like to use the mulch as habitat. Slugs and earwigs are notorious for liking dark, sheltered places. Organic controls such as beer traps in the case of slugs and rolled up newspapers for earwigs work well.
The things to remember about clay is that it needs organic matter to improve tilth, and good drainage.