PHOTO: Sam/Flickr
May 29, 2015

It’s no secret that garden soils come in many variations—silty, loamy, sand or clay—each with its own characteristics and challenges. Perhaps the most challenging to amend is clay soil, but all is not lost. You just need to work a different amendment angle.

First off, let me say that clay soil is not all bad. It may be difficult to work with and in need of some TLC, but it has some good things going for it. Many clay soils, for example, are rich in minerals. Plus the cation exchange capacity—aka, the soil’s ability to hold onto water and nutrients—is quite high, which is especially useful in drier parts of the country. Clay soil is made of tiny particles a mere 0.002 mm in size, causing it to drain slower and retain water better, meaning you can water and fertilize less often. The density also gives plants something firm to sink their roots into, enabling them to withstand temperature and moisture extremes. Also, plants overwintered in clay soil are less likely to heave out of the ground. So all in all, clay can be downright friendly.

Of course, there are some cons, and these are what we usually hear about. That denser quality that helps plants stay planted makes clay soil heavier, thus harder to till or dig, and if it’s too dense, it can even smother roots. Clay is also more likely to compact, as anyone who has encountered clay after rain can affirm as they scrape the gunk off their shoes and garden tools. In fact, digging or tilling in clay soil that is too wet actually compacts rather than aerates the soil. Not so good. And with that high CEC, clay can hold both good and bad minerals, salt being one of the primary offenders, which can take some time to properly amend. Finally, clay soil in boggy areas can keep plant roots from receiving the oxygen they need.

So what’s a gardener with clay soil to do?

Your main goal should be improving the aeration and drainage of your clay soil using a variety of soil amendments. “Adding organic soil amendments to the soil lightens soil texture, discourages compaction, adds nutrients, improves drainage and aeration, moderates soil temperature, and provides pore space, which is essential to plant growth,” according to the Cooperative Extension at North Carolina State-Durham. “Clay without organic matter is like a flattened deck of playing cards. Adding organic material jumbles the cards, permitting water and oxygen to enter the soil.”

You can amend the soil by working fibrous materials into it with a long-term goal of incorporating sufficient amounts of fiber into the soil to bring your humus level between 4 and 5 percent. Mulching can be part of this picture. While mulch doesn’t penetrate too far into the soil unless you till or work it under, it does keep water from running off easily, preventing moisture loss. Plus, it slowly adds compost as it breaks down, keeps your soil cooler, and also helps keep your shoes clean!

Here are a few great mulch options for clay soil to get you started on the way to a manageable garden bed.

1. Wood Chips

Wood chips are readily available at many commercial stores, and some areas even allow gardeners access to their piles of chipped branches resulting from around-town cleanup. Because wood chips have high carbon content, they need nitrogen during the decomposition process. Small wood particles, such as sawdust, will actually leach nitrogen from the soil to help with decomposition, which could ultimately hurt your nitrogen-loving plants. However, you don’t need to work about this as much with larger particles, like wood chips. When used as a mulch instead of being tilled into the soil, nitrogen tie-up is minimal. Wood chips are best left to the moisture and earthworms to slowly break down, enriching the soil over time, and being replenished as needed.

If you’re mulching around trees or shrubs, avoid contact with the trunk or stem, as this can provide habitat to slugs, sow bugs and mice that could damage the tree. Also keep in mind that wood chips weather with age: While they might start out a nice, rich color, they eventually end up graying. Cornell University says that just adding more mulch to “freshen the look” not only wastes mulch but can suffocate the roots of shallow-rooted species and cause cankers to develop around the bases of susceptible trees and shrubs. Instead, renew the mulch every two to three years, working the old chips into the soil before adding a new layer.

2. Tree Bark

Commercially available bark mulches are predominately by-products of fir, redwood, pine and spruce logs, and are sold shredded or as chunks or granules. Granules are more suited to working into the soil, while chunks and shredded bark work better for mulching.Not only is tree bark easy to obtain, it tends to hold its color better than wood chips and resists wind dispersion and compaction.As with wood chips, keep bark away from trunks and stems to discourage damage to the plant.Some barks, can be toxic to young plants. If you notice a sour smell, exposing the mulch to air can help leach the toxins out so the mulch is usable. Bagged bark has generally been weathered long enough that this is not a concern.

3. Hulls/Shells

A number of mulches on the market are made from the hulls or shells of various plants. Here’s a brief overview of some to consider:

  • Nut shells from pecans, hazelnuts, and others make long-lasting mulches that hold their rich color well and add texture to the landscape. These are available commercially, especially in regions where the nuts are grown.
  • Buckwheat hulls are long-lived with a neutral coloring that complements plantings. They are, however, finely textured, and can blow around in windy spots. Hot, humid weather may bring out a slight odor some people dislike.
  • Cocoa bean hulls are another great choice for mulching: They have a rich, chocolate color and a pleasing odor, and these roasted hulls look great around plantings. Cocoa bean mulch does tend to pack down over time, so stir it occasionally to ensure that roots get enough air.Also be aware that they are toxic to dogs, so this wouldn’t be a good choice if any neighborhood dogs have access to them.


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