First Published 3/10/2005
Breaking the Crust

Up on the hill of my parents’ farm, the rich, clay loam soil has supported fantastic crops for many years, and continues to do so to this day. However, that clay loam had an alter ego: under the right conditions, it could transform from soft and supportive to unbearably crusty.

The Upper Crust

Anyone familiar with clay is familiar with soil crust. A soil crust is a thick layer of hard, impermeable clay that makes cultivation a nightmare. Crusts are particularly evident in the spring as the snow melts, or a day or two after a heavy rain. The soil develops a patchwork of irregularly-shaped plates that make the ground look a bit like a jigsaw puzzle, each “piece” separated by deep fissures.

Many plants, particularly seedlings, find it difficult to penetrate through crusts. Some seedlings escape via the fissures, but seeds, tubers or bulbs unlucky enough to be situated right underneath a plate labour so intensively to push their way through the crust that many simply burn out before ever seeing the light of day. I’ll never forget flipping over clods of crusted soil on our vegetable farm in the spring and discovering onion and carrot seedlings that had germinated perfectly, and yet were unable to emerge through the crust. All we could do was break up the soil and reseed.

Bulb Breakthrough

Some of the most popular spring flowering bulbs have varying degrees of success dealing with clay crusts. If you’re growing tulips, narcissus, or irises, take heart – these bulbs have adapted quite well to the problem of crusty soils. All three genuses have sheath leaves, which are tough, bayonet-like leaves that thrust upward and break through the crust, allowing the more tender leaves easier access to the surface. Other bulbs, such as anemones and aconitum, have no sheath leaves and have a much harder time breaking through soil crusts.

But even bulbs with sheath leaves have a hard time breaking through thick, heavy crusts. I’ve seen very distorted tulips with rough, twisted leaves growing in heavy clay soils, confirmation that the plants won the struggle to emerge through the crust, but did not escape that battle unscathed!

Correcting Clay

Clay is often demonized because of its tendency to produce crusts, but in moderation clay is a valuable soil component. High quality loam soils contain a fair bit of clay; part of the reason that such soils produce good crops is because the clay holds a lot of water in the soil, providing a reservoir for plants that doesn’t exist in sandy soils. It’s only when the clay content ventures into the 30-40% range and above that it starts to negatively affect soil quality.

The method for correcting a crust problem in the garden depends on the quantity of clay in the soil. A soil that is around 90% clay is unsuitable for gardening. It’s virtually impossible to amend soils so dominated by clay. Adding sand is often touted as a remedy for clay soils, but when clay content is that high, it just won’t work. Unless a tremendous amount of sand is added (about 50% sand by soil volume), all you end up with a bunch of sand particles surrounded by clay; the resulting soil has the consistency of concrete. The only option is to remove the clay and replace it with a truckload or two of high quality loam. This may be an expensive solution, but it’s far easier on the blood pressure than years of frustrated ambitions in the garden.

If, on the other hand, you have clay loam like that of my parents’ farm, with a clay content of around 30%, adding compost, well-rotted manure, or simply more loam to the soil will help. The humus in the organic matter binds itself to the clay particles, preventing those particles from binding together. The result is a nice, soft, permeable soil that lets water and nutrients flow into and through the soil, and allows seedlings up out of the soil.

Clay is not a villain in the garden, provided it isn’t overwhelmingly dominant in your soil. With clay, a little goes a long way.