Any food you can think of will break down if left outside long enough. This is even true of harder organic materials like bones and shellfish shells.
But just because it will eventually break down, does that mean you should compost it?
Shellfish shells can be composted but will break down very slowly unless they are crushed into smaller pieces or ground into a meal. At the same time, they are also a rich source of calcium and important plant micronutrients, and can therefore make a valuable and sustainable amendment to the home garden.
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Benefits of Composting Shellfish Shells
Shellfish shells can add considerable amounts of trace nutrients and even some nitrogen into your soil.
The hard shells of mussels, oysters, and clams are composed mainly of calcium carbonate and chitin. Calcium carbonate, which is also found in egg shells, will add much-needed calcium in the soil, and chitin is an organic polymer that forms the backbone of the shells of shellfish and insects. Chitin has nitrogen in its molecular structure, so as that breaks down, it will add nitrogen to the soil.
However, shellfish shells also have some protein (another nitrogen source) numerous trace minerals, such as magnesium, manganese, iron, and boron, which are all important for plant growth.
Minerals the soil eventually end up washed down streams and rivers into the ocean, which is very high in mineral content. As shellfish grow, they incorporate these minerals into their shells, which we can then use to compost and re-enrich our soil.
Preparing Shellfish Shells for Composting
Shells generally take a very long time to decompose in the soil. Household chicken egg shells can take years to break down completely, so thicker mussel or clam shells will take much longer. Nut shells will take a shorter time to break down, by comparison.
However, you can speed up the breakdown of your shellfish shells by crushing or grinding them. Research has shown that ground up egg shells are a very effective amendment to the soil. The finer you grind them, the more quickly they will break down in your soil.
Before you take out your mortar and pestle or blender, you should always rinse your shellfish shells before you compost them. Try to rinse off as much salt, sauces, or heavy oils off them.
Then you can either crush or grind them into a powder. Some people will bake them in an oven in order to make them as dry and brittle as possible beforehand. If you want to add whole shells to your compost pile you can do it but they will take years to fully break down.
How to Use Shellfish Shells in the Garden
There are a few ways to use shellfish shells in the garden, either in your compost pile or directly in your garden beds.
If you add shellfish shells to your compost pile, it’s better to put them somewhere in the middle of the pile, where it will be harder for animals to smell them and allow microorganisms to break them down faster.
Crushed shellfish shells can also be composted via trench composting, which involves digging a trench or hole several inches deep in your garden and spreading the shells inside before covering them up with soil. This is the best way to compost if you don’t have a compost pile, since they will break down right where the plants can take up the nutrients.
If the shells have been fully cleaned and ground up into a powder, you can even top dress your garden beds directly by sprinkling it and working it into the top few inches of soil. But if you do this, make sure to use the shells only and no meat, as the scent of meat and fish will attract pests. If there is some meat attached, it’s better to bury it deep in your bed or compost pile.
Related Questions About Composting Shellfish
Can You Also Compost Lobster, Crab, and Shrimp Shells?
Lobster and crab shells can be composted just like mussel, clam, or oyster shells. Because they are quite thick, they will take years to break down unless you crush or grind them.
Shrimp shells can be used in the same way, but because they are much thinner they will break down much faster.
If you are using shrimp shells, boil them for 20 minutes, drain them, and toast them in the oven until they are dry, which will make them easy to grind up into a meal which you can use in your garden. Although shrimp shells will break down much faster than lobster or crab, I still recommend burying them in your compost pile or several inches into your garden beds because of the strong smell when they start decomposing.
Can You Compost Uneaten Shellfish?
You can safely compost partially eaten or uneaten shellfish with meat still attached but there are a few caveats.
While cleaned, crushed shellfish can be used for top dressing or mixing in the top couple inches of soil, uneaten or leftover shellfish should be treated like composting any meat or fish and either buried at least several inches under the soil or inside a hot compost pile.
Any kind of meat will attract animals, such as rats, cats, or raccoons. Even if buried a couple inches in the soil of your garden beds, they can easily track the scent and start digging them up. As shellfish begin to decompose, they also have a very strong, unpleasant smell, which could attract even more pests into your garden.
You could also put it into a compost pile, but ensure that it’s turned frequently and keep the shellfish buried inside the pile, which will ensure the meat will break down faster. In the end, the shells should be cleaned of all their meat, and will themselves slowly break down.
So, for that reason, uneaten shellfish, along with fish and meat, should be either buried in a deep trench to mask the scent from animals or in a hot compost pile to decompose it faster.