Unearthing Clay Soil's Virtues

By Barbara Damrosch
Special to The Washington Post
Thursday, March 15, 2007

A hero with feet of clay is one with hidden flaws, but if your garden has feet of clay its flaws are obvious. In spring, clay soil remains cold. Puddles refuse to drain. If you squeeze a handful of wet clay soil it will remain in one piece, slippery, sticky and marked with your fingers' imprint. In dry weather it will seem more crumbly, but it can often form an impenetrable crust, cracked like the surface of an inhospitable planet. Seedlings struggle to break the surface. Plant roots are sometimes heaved out or ripped apart when deep fissures appear.

Gardeners who work with clay are loud in their laments, but for those who rise to the challenge, clay has outstanding virtues. It is composed of very tiny particles stacked together in horizontal plates. Because of their sheer numbers these particles give the soil a huge surface area that is negatively charged, enabling the particles to bond with positively charged elements such as potassium, calcium and magnesium -- essential nutrients for plants to grow -- so clay soils are often very fertile in terms of their mineral content. Their ability to hold on to moisture sustains plants during a drought and helps prevent the leaching out of soluble nutrients such as nitrogen and potassium during irrigation or rain.

Clay soil takes many forms. The red clay found in many parts of the Southeast is a familiar sight, but clay soils can also be gray, brown, greenish, bluish, yellow and near-black (darker ones are usually higher in organic matter and better for growing). The texture of your soil -- whether it is fine-particled clay, coarse-particled sand or the more garden-friendly silts and loams that lie in between -- is the result of massive forces such as retreating glaciers, upthrusting mountains, erosion, and the march of bulldozers and backhoes excavating the land on which your home now stands. Often what passes for soil in a typical yard is simply subsoil from which the better-textured topsoil has been removed during construction. Perhaps two inches or so have been replaced for the obligatory lawn, but what lies beneath is often devoid of organic matter and heavily compacted by machinery. Compaction is bad for any soil type but is especially bad for clay, whose plate structure is easily compressed, reducing the flow of water and air.

If your yard has little or no topsoil, you may need to have some brought in if you want to grow a garden. But if the problem is simply a clay topsoil that is hard to work, the solution is to add organic matter, and plenty of it. The most important thing organic matter does to clay is make it more permeable by getting its particles to clump together into crumbs. This happens because bacteria busily breaking down the organic matter and digesting it produce a gluelike substance called glomalin, a magic stickum that turns the dense matter of clay soil fudge into light, loamy brownies.

The best organic matter to add is a well-made compost, because it provides the most long-lasting structure. Other good sources include aged manure, leaf mold and peat moss. You could till some in this spring once the soil is workable, then fork in more in fall. Winter's freezing and thawing of the clay clods will incorporate these additions further. Turning under cover crops such as clover is a good idea. Even mulches that gradually decompose into the soil are helpful, though I'd avoid tilling under bark mulch or sawdust, which deplete soil nitrogen as they decompose. Amending clay soil is hard work that requires a strong arm and an optimistic spirit. Busting it up with a mattock is often the arduous first step. After five years or so of regular applications of organic matter, you will still have clay soil, but it will feel lighter, and both you and your plants will struggle less.

The sandy soil in my own garden is at the opposite end of the spectrum. It doesn't clod up. It's well-aerated and easy to dig. It emerges from mud season in good shape, warming up quickly when the sun shines. But it doesn't hold on to fertility and moisture well. Adding organic matter each year helps, but I will have to add it forever. A clay soil can be more permanently improved. When we dug our pond and found heavy blue clay during the excavation, we not only lined the pond's bottom with the clay, we also spread some on our gardens and tilled it in. I guess one gardener's curse is another's buried treasure.

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