Simple clay amending techniques for a clod-free garden
After almost 40 years of gardening, I’ve learned that the limitations of climate, shade and space are nothing compared to coping 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 unreachable goals presented to frustrated 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 to clay soil: 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.
Characteristics of clay
You know you have clay soil if:
- the soil is sticky and difficult to dig
- digging often results in huge clods that are hard to break up
- the soil is heavy and slow to warm up in spring
- during a stretch of dry weather, the surface hardens and cracks
- the soil is slow to drain
1. Clay soil is easily compacted. Avoid walking on cultivated soil, especially when it’s wet, or compressing it with heavy equipment. Digging wet clay also compromises its structure and will set back your efforts to improve it.
2. Amend clay soil by adding plenty of organic material, such as well-aged compost, sawdust (but not from pressure-treated wood), composted manure or leaf mould (partially decomposed shredded leaves). Coarse materials are better than fine ones. Do this repeatedly and as often as possible. For example, when making a new bed, spread several inches of organic material over the area and dig it in at least eight inches (20 cm) deep. When moving or adding new perennials to an existing bed, throw a shovelful of compost into the planting hole.
Organic materials prompt mineral particles such as clay to come together in clumps, called aggregates. The various sizes and shapes of these clumps help form larger pore spaces, which creates more room for oxygen and water to move around plant roots. Organic materials also attract earthworms and micro-organisms that will munch and digest their way around the soil, loosening up its structure even further.
3. Hand digging clay is preferable to using a tiller, which pulverizes soil into too fine a texture. Digging with a shovel leaves clumps of various sizes that allow a better exchange of oxygen in the plants’ root zone. Dig down at least eight inches (20 cm) when amending. Fall is the best time because soil is usually drier than in spring. Leave rough clods to allow snow to further break down the soil.
4. A two- to three-inch (5- to 8-cm) layer of organic mulch, such as shredded bark or leaf mould, helps keep the soil from forming a crust. It also ups the quotient of organic matter.
5. Adding sand can make matters worse; to have a positive effect, it must be coarse builder’s sand (various-sized particles; not horticultural or play type sand) and copious amounts are needed—at least one part coarse sand, one part organic matter and one part existing soil. Adding too little or too fine a sand creates the perfect conditions for making bricks!
Planting trees & shrubs in clay
- Dig a hole at least twice the diameter of the root ball, but no deeper than its height. Rough up the sides to make it easier for roots to penetrate. Don’t add gravel to the bottom in hopes of improving drainage. According to research done at the University of Minnesota, a layer of gravel creates a perched water table that causes the soil above the gravel to hold even more water.
- Position the root ball in the hole, ensuring that it sits at or slightly above grade; plants that settle below grade can suffer from root rot.
- Backfill with the soil you removed (if it’s relatively fertile) or amend it slightly by adding organic matter. If you amend the backfill material too radically, the resulting drastic change in soil fertility and texture will discourage roots from spreading out beyond the sides of the hole. It’s best to heavily amend the whole bed or border than individual planting areas wherever possible, but it’s better to amend a single planting hole than do nothing at all.