Are crab shells compostable?

  • Date: May 14, 2022

If you live by the ocean, it is likely that shellfish is particularly fresh and abundant. Keen gardeners will be glad to know that crab shells do not need to go to waste – they will compost along with other types of shellfish shell. Added to the compost pile, crab shells will biodegrade. It may take a while but crushing them up will speed up the process significantly. In a few months they should have completely biodegraded – having added much needed nutrients to the compost.

What are crab shells made from?

Crab shells main components include calcium carbonate and chitin (a carbohydrate). As well as forming part of a durable exoskeleton, chitin also has the added advantage of being a natural insect repellent. Other shellfish shells (such as lobster and mussel shells) also contain chitin and calcium carbonate in varying degrees. These components break down into several valuable nutrients that will benefit your garden including calcium, phosphorus, magnesium and nitrogen.

Preparing crab shells for composting

The smaller the pieces of shell, the quicker they will break down in the compost pile. Before you begin crushing make sure you give them a good clean to ensure you get rid of any salt or foodstuffs you don’t want in the compost. Many people bake them in the oven too – this makes them brittle so they can be more easily broken down whilst killing off any remaining bacteria.

Use whatever you have at hand to break them down, a hammer does the trick nicely. The aim here is to get the pieces as small as possible to speed up the process. In theory, you can miss out this step. But you will be waiting a while longer!

Can crushed shells go in the garden beds?

If you do not have a compost pile, you can put them directly into the garden bed. Simply mix the crushed shells into the top couple of inches of the soil. Much like in the compost pile, these will break down and release their nutrients directly to the plants. It is even more important here that the shells were fully cleaned before crushing – this avoids the appearance of any pests attracted by the decaying shellfish meat.

Can you compost shellfish meat?

You can. But there are some conditions attached, if you don’t want a pack of animals turning up for feeding time. Being buried a few inches is not enough – the scent of the decomposing meat will be picked up by the local wildlife. Hot composting is highly advised to ensure the meat is broken down faster. If this is not possible, it must be buried very deeply to ensure the scent does not attract pests.

In our household, we just devour all the meat from the shellfish to avoid these issues!

Can you compost lobster shells and other types of shellfish?

Yes! As with crab shells, other shellfish can be added to the compost pile. Due to the different components making up the shells, the compost will be slightly different. For example, some shells contain more calcium carbonate than others. But all composted shells will provide a nutrient-rich fertilizer for your backyard.

Which plants benefit from composted shells?

All plants will benefit on some level from having the nutrients added to their environment. The pH level in the soil will be balanced – having a universal positive effect on the garden.

Compost which contains broken down crab shells, and shellfish in general, is particularly rich in calcium. This is due to shells comprising largely of calcium carbonate. Calcium is vital to plant function. Being a key component of the cell walls and membranes, it helps plant cells maintain their structure. It is also an important intracellular messenger.  

Some plants in particular benefit from this extra boost of calcium. This can help in the prevention of blossom end rot, a calcium deficiency characterized by dark patches. Due to the lack of calcium, the cell membranes are affected. Their permeability is altered, leading to the cells ultimately bursting.  Tomatoes and peppers are amongst the plants that particularly benefit from a injection of extra calcium.

The bottom line

Crab shells, as well as other shellfish shells, are a great addition to any compost pile. Comprised mainly of calcium carbonate and the carbohydrate chitin, they produce a nutrient-rich compost. The extra calcium is a much welcome boost too for some types of plants. It will perhaps take a bit more patience, and a bit extra work than some other compostable materials, but in a few months your garden will thank you for it!