Recently, I read about using crushed seashells to improve the soil in your garden. Gardening my entire life in the Midwest, I hadn’t heard of this method, so I did a little research. Should I be adding crushed seashells to my garden?

The quick answer is I probably shouldn’t, at least not on a large scale, because they aren’t readily available in my area. But if you live near a body of water where seashells are plentiful, you might consider giving this a try.

How Do Seashells Improve Soil?

Seashells improve soil in these ways:

To add organic matter

Some shells, particularly those of crustaceans like crab, lobster and shrimp, contain chitin. When broken down by various bacteria and fungi, chitin adds organic matter and slowly releases nutrients — including nitrogen, calcium and magnesium — into the soil, where plants can utilize it.

To add calcium to the soil

Many gardeners concerned about calcium deficiencies in their soil may have read seashells are a great source of calcium. That’s true. But the shells must be broken down over a long period before that calcium is available for plant growth. They’re not a quick fix.

To control root-knot nematodes

Some gardeners add seashells to their gardens to control root-knot nematodes which can stunt the growth of many vegetable plants. According to the Clemson University Cooperative Extension, “The addition of organic matter or fertilizers that contain chitin can increase the population of soil microorganisms in the soil that can aid in nematode control.”

Christy Wilhelmi, the author of Grow Your Own Mini Fruit Garden who gardens on the West Coast, says, “On occasion, I will suggest shrimp shell meal to treat root-feeding nematodes, but I’d rather we aerate the soil and apply beneficial nematodes to combat that problem instead.”

To raise soil pH

Seashells can also make acidic soils more alkaline. However, it can take a while for this to happen.

C.L. Fornari, an East Coast-based garden author and speaker, says, “Crushed shells and coral are usually used to alter the pH of water in hydroponic operations. Since water continually circulates in such systems, it can leach the carbonates fairly quickly and distribute them evenly.

“This is not the case in soil, however. Shells are pretty slow to break down in the ground, and it’s hard to make a homogeneous planting area or see results in raising pH that are very rapid.”

Test Before Adding Seashells, or Any Other Amendments, to Your Soil

Before adding seashells or anything else to your garden, test your soil to find out what’s really needed. Your local cooperative extension service can tell you where to send soil for testing, generally for a fee. Follow their instructions for gathering a good soil sample. The results should include suggestions for specific amendments you may need to add to your soil.

Lab soil testing is recommended, but DIY soil test kits are available. However, DIY test kits may not provide all of the information you need to determine what your soil lacks.

Other Uses for Seashells in the Garden

Fornari says few gardeners around her use crushed seashells in their gardens. “In this area crushed clam and oyster shells are more likely to be used for driveways, walkways and parking areas,” Fornari says.

Some gardeners also mulch crushed seashells around their plantings, adding a layer that is about two inches thick. And some put crushed seashells on top of the soil of their houseplants to help control fungus gnats.

Where Do You Buy Seashells?

Some craft stores and online merchants sell seashells. Seafood processing plants divert the shells from crab, lobster and shrimp to composting facilities that turn them into mulch or soil amendments rather than sending them to landfills.

As Fornari notes, “If people are thinking of using bulk crushed coral or shells in their garden, they might want to be sure that these are coming from farmed sources, not harvested from wild populations. Our sea life has it hard enough without being mined for garden supplies!”