After almost 30 years of gardening, I’ve learned that the limitations of climate, shade and space are nothing compared to working with heavy clay. Wise plant choices help mitigate the shortcomings of shade and a small lot. Careful cultural practices lessen the impact of drought and unwelcome frosts. But coping with sticky, dense, unrelenting, unforgiving, heavy clay has broken more than a few shovels—and has nearly broken my spirit as I’ve watched plants struggle in its grip. If only I had known then what I know now, I would have spent much more time improving my soil and less time buying plants at the nursery.
A short lesson in soil
Soil is a combination of minerals, organic matter, water, air and micro-organisms. The mineral component is made up of various-sized particles; clay is the smallest, sand is the largest, and silt falls somewhere in between. The proportion of each component determines the texture and structure of your soil.
The best soil is loam—a balanced mix of clay, sand and silt. I don’t believe it actually exists, and suspect it’s one of those goals dangled in front of tired gardeners, like slug-free hostas and weed-free rock gardens.
Soil composed mainly of clay is heavy and dense because its small particles fit closely together. That’s why it’s so difficult to dig (see Characteristics of clay, to find out if you have this type of soil). As well, water drains slowly because of clay’s compacted nature, and there is less oxygen available to plant roots. Although clay soil is often waterlogged in spring, during periods of drought it hardens, and water often rolls off the concrete-like surface before it has a chance to penetrate down to thirsty roots. As bleak as all of this sounds, there is one benefit: it usually holds more nutrients than sandy soil; the latter has bigger spaces between particles, which causes water and nutrients to leach away quickly.
While working with clay is tough on gardeners, the biggest issue for plants is the slow drainage. Improving its structure (see Clod busting,) improves drainage; planting in raised beds made with good soil also helps. With careful management and generous additions of organic material, clay soil will improve over time—lots of time. And remember that it’s an ongoing process. The soil will become more workable, but expect to spend a few years amending clay before you notice an appreciable difference.