Homesteading, Gardening, and Off-Grid Living

How to Compost Clam, Mussel, and Oyster Shells 

By: Zac Friedman

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You can compost seafood shells including clam shells, mussel shells, and oyster shells. You will have to clean them and crush them before adding them to your compost bin. If you don’t they will take too long to degrade. The shells will add valuable nutrients to your compost including nitrogen and calcium. They also contain a substance called chitin, which has anti-fungal properties.

In this guide, I will explain how to compost clam shells and other types of seafood shells, step-by-step. I’ll also outline a few benefits and drawbacks of composting clam shells to help you decide whether or not they belong in your compost bin. 

How to Compost Clam, Mussel, and Oyster Shells pin

Can You Compost Shellfish Shells?

Yes. Clam shells, mussel shells, oyster shells, and other seafood shells such as crab shells, lobster shells, and shrimp shells can all be composted. Shellfish shells are biodegradable. They are made from organic matter. They can be broken down by bacteria and other living organisms in the soil.

When these shells break down, they enrich the soil with chitin, nitrogen, proteins, calcium carbonate, and trace amounts of other minerals. These nutrients help your plants grow larger and healthier. Clam shells are considered green materials in composting due to their nitrogen content. 

Seafood shells are animal products. Generally, it is not advisable to compost animal products because they can create odors and draw pests. Shells are different. Once they’re cleaned, they have no odor and they do not attract pests.


Benefits of Composting Clam Shells

Clam shells offer a number of benefits for your compost. They add valuable nutrients to the soil, help to neutralize acidic soil, and offer anti-fungal as well as nematode control.

Clam shells and other seafood shells contain a substance called chitin. Chitin is a long-chain glucose molecule. It is one of the most common organic compounds on the planet.

There are several benefits of adding chitin to your garden. First, it is high in nitrogen. As the clam shells biodegrade, they slowly release nitrogen into the compost. 

This nitrogen acts as a catalyst in the composting process. It encourages the multiplication of beneficial bacteria and fungi by helping form protein and other essential cellular components. These microorganisms break down organic materials into compost. Nitrogen allows them to operate more efficiently. You can read more about how chitin and the decomposition of seafood shells in this scientific study.

Compost that is high in nitrogen improves also soil health and fertility. Plants grow stronger and faster when they have sufficient nitrogen. You won’t need to use as many chemical fertilizers if you compost clam shells. They work as a natural fertilizer. 

Chitin also offers antifungal properties. It helps to manage various fungal diseases that can develop in the compost heap or the garden. It basically creates an environment that is hostile toward harmful fungi. This helps your plants stay disease-free. 

In addition, chitin helps to combat nematodes. These are microscopic worms that can harm plant roots. Nematodes can weaken your plants and reduce yields. If left untreated, they can kill your plants. 

Chitin reduces nematode populations by making the soil toxic to nematodes. It encourages the growth of certain bacteria which produce substances that are harmful to nematodes, such as ammonia. This disrupts the lifecycle of the nematodes and reduces their population. 

Clam shells also add much-needed calcium to your compost. Seafood shells contain calcium carbonate. This is a natural liming agent. It is the key ingredient in garden lime. Calcium carbonate helps neutralize acidic soil by increasing the pH of your compost. It is an alkaline material.

If your garden soil is too acidic, plants can’t absorb the nutrients they need effectively. Adding calcium content from clam shells is a great way to help neutralize the acidity. 

In addition, calcium carbonate helps with new plant cell development by fortifying the cell walls of the plants. This helps with plant growth. 

Seafood shells also add some other essential plant micronutrients that may not be found in the soil. This reduces the need for fertilizers.

Composting shellfish shells also reduces the amount of food waste in the landfill. This is good for the environment. All of this makes clam shells a great addition to your compost.

compost bins

Drawbacks of Adding Clam Shells to Compost

There are several potential drawbacks to consider before you add shells to your compost.

First, composting clam shells takes some work. You have to break them down into smaller pieces before adding them to your compost material. If you add them whole, they will take years to degrade.

Breaking down clam shells is a hassle. You have to clean them, bake them to make them brittle, and then crush them with a rock or mallet. 

Another potential drawback is that clam shells could attract pests. This is because they can have an odor if they aren’t properly cleaned. The excess meat or sauces can start to stink.

To prevent this, be sure to clean the shells properly before adding them to your compost. It’s important to remove all uneaten shellfish meat and sauce so they don’t smell.

A person holding a handful of compost

How to Compost Clam Shells

Step 1:  Clean the Clam Shells.

First, rinse the shells to remove any meat, butter, seasonings, oils, marinades, or excess salt. Cleaning the shells helps to reduce odors that could attract pests. It also removes spices that could slow down the decomposition process. Decomposers don’t like spicy or citrusy foods. Boiling also kills any bad bacteria and removes any fat or oil that can slow decomposition. 

Step 2: Dry the Shells

Place the clam shells on a baking sheet and place them in the oven on low heat. Bake them until they are brittle. This step is optional but it will make the shells easier to break up.

Step 3: Crush the Clam Shells 

This is the hardest part of the process. Lay a dish towel over the shells and crush them with a hammer or a large rock. Crush them until they’re the consistency of gravel, if possible. You’ll want to do this outside on a hard surface so you don’t damage your home. Transfer the crushed shells into a bag or bucket. Crushing the shells will help them break down faster because they have more surface area for decomposers to work with. 

To speed up the decomposition process, you can put the crushed shells into a food processor or blender to turn them into powder. They will break down much faster when they’re in powder form.

It’s important to note that this will damage your food processor. It will dull the blades. I recommend you use an old food processor or buy a cheap one at a thrift store so you don’t damage your expensive one. 

Step 4: Prepare the Compost Bin or Pile

Add a layer of brown content to the compost about 6 inches deep. Brown materials are rich in carbon. Examples include dry leaves, newspaper, twigs, and wood chips. 

Next, add a layer of green content about 2 inches deep. Green materials are rich in nitrogen. Examples include fresh grass clippings, food scraps, coffee grounds, egg shells, manure, and yard waste. 

The ideal C/N ratio for your compost is 30:1 or 30 parts carbon for 1 part nitrogen by weight. Clam shells are considered nitrogen in composting.

Step 5: Add the Clam Shells to the Compost

Add a layer of crushed clam shells on top of the compost. Cover this layer with more green materials. 

If you have more clam shells, continue layering. Add a layer of brown, then green, then more shells, then more green. 

If you already have an established compost bin or pile, you can dig a hole into the compost and add the crushed shells then cover them with compost. Placing them near the center where the compost is the warmest and most active will speed up the decomposition process. 

Step 6: Maintain Moisture and Aeration

Regularly turn the compost with a pitchfork every couple of days. Also, make sure the compost remains moist but not too wet. This promotes a faster and more efficient decomposition of the shells. 

Another Option: Soil Incorporation

You can also simply bury or spread the crushed clam shells in your garden and let them decompose naturally in the soil. This is a great option if you live in a city that doesn’t allow compost bins or if you simply don’t want to deal with composting.

To incorporate clam shells in the soil, start by cleaning and crushing them, as outlined above. Next, find a place in your garden where you want to enrich the soil.

Dig a hole around 12 inches deep. Add the crushed clam shells. You want a layer around 3-4 inches thick. Finally, cover the shells with the soil that you removed. 

After 2-3 months, you can plant in the area. The shells will decompose and enrich the soil. 

If you do this, try to add small amounts of shells. If you add too many, they will take too long to decompose. 

How Long Does it Take for Clam Shells and Mussel Shells to Decompose?

If the clam shells are crushed into small pieces or ground into a powder, they can decompose in several weeks to several months. 

If you were to add the shells to the compost whole, they could take several years to fully decompose. Clam shells are hard and thick. It takes a long time for them to break down on their own.

The exact amount of time it takes for clam shells to decompose depends on a few factors including the state of the shells when you put them in your compost and the level of microbial activity in your compost.

You can speed up the composting process by making sure your compost is aerated, moist, and hot. A good hot compost pile that is well maintained will break them down much faster than cold compost, compost that is too dry, or compost that lacks aeration. 

Clam shells don’t break down quite as fast as other types of seafood shells such as shrimp tails or crab shells. This isn’t really an issue because clam shells don’t smell or attract pests if they’re clean. They should break down within a few months if they’re crushed decently and placed in an active compost bin. 

Other Ways to Use Clam, Mussel, and Other Seafood Shells

  • Seafood broth- Add some aromatics and water and simmer the clam shells for a few hours. They will make a nice stock that you can use for soups, sauces, stews, a nice risotto, and other dishes. After making the stock, you can compost the shells.
  • Mulch- You can apply the crushed shells to the top of the soil. This helps to deter snails and slugs. They don’t like the sharp shells. You could also set the whole shells in your flower pots. Over time, the shells will decompose and add nitrogen and calcium carbonate to the soil. You should make sure the shells are clean before using them as a mulch so they don’t create odors and attract pests.
  • Arts and crafts-  If you have kids, you can use your clam and mussel shells for various craft projects such as decorating picture frames or flower pots. You could use them to make jewlery, a soap dish, or a decorative wreath. The possibilities are endless. All you need is some hot glue and your imagination.
  • Use them around the house- If you have large shells, such as abalone shells, you can use them for holding various household items like your keys, jewelry, loose change, etc. The shell can function as a bowl. 

My Experience

When I eat clams and other shellfish, I compost the shells. I have a pretty active compost pile so crustacean shells usually degrade within a month or two if I pulverize them. 

I bought a cheap food processor at a thrift store that I use to grind the shells up. First, I bake them to make them brittle then smash them up with a hammer. This is the best way that I have found to process them. It’s a bit of a hassle but I think it’s worth it. 

I use the compost in my flower beds. Adding clam shells to compost is beneficial. They make excellent compost when they’re properly processed first. They enrich your compost with essential nutrients like nitrogen and calcium. This makes your soil more fertile. In addition, they contain chitin, which has its own benefits such as reducing nematode populations and harmful fungi. 

The only real drawback of composting the hard shells of mussels, clams, and oysters is that they take some effort to prepare. You can’t put them in your compost whole or they will take too long to degrade. 

Do you compost clam shells and mussel shells? Share your experience in the comments below!

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How to Compost Clam, Mussel, and Oyster Shells Step by Step
How to Compost Clam, Mussel, and Oyster Shells

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