Are eggshells biodegradable? Can they be composted?

  • Date: May 27, 2022

Despite the increasing popularity of veganism, every year the average American consumes nearly 300 eggs. And you can see why – quick and nutritious, they can be incorporated into every meal. However, from a waste point of view the egg consumption does not make for good reading. As well as the egg cartons, we each have to dispose of 300 eggshells every year! Rather than send them off to landfill, are eggshells biodegradable and can we pop them in the compost pile?

Firstly, what are eggshells made from?

Eggshells need to be hard to protect their contents. This hard coating is largely due to calcium carbonate that make up the vast majority of the shell – over 95%. The rest is comprised protein and other minerals.

Are eggshells biodegradable?

A material is biodegradable if it can be broken down by micro-organisms such as bacteria. Essentially, the components can be returned to nature. Eggshells are biodegradable but they do take a while – years rather than months. As mentioned, they are extremely durable, and this could the barrier that prevents a speedy decomposition.

Can eggshells be composted?

Composting is a great way to dispose of organic waste. It diverts waste from landfill as well as producing a nutrient-rich fertilizer for your garden. However, a common misconception is that anything that is biodegradable can be composted. This is not the case – the terms are not interchangeable. To be fit for the compost pile, an item must be able to contribute towards the production of a nutrient-rich compost. It must also not be laden with chemicals may contaminate the compost and cause harm to your garden down the line. Thankfully, eggshells can be composted – this is especially good news since eggshells cannot be recycled.

Composting materials are split into two main categories: ‘browns’ and ‘greens’. ‘Browns’ are carbon-rich and include items such as dried leaves and paper. ‘Greens’, on the other hand, are nitrogen-rich and can include many items – from food scraps to grass clippings. 

There seems to a bit debate as to which category eggshells fit into. Some sources state that eggshells do not fall into either category. Others say it is ‘green’ whilst others say it ‘brown’!  As you will likely be aware, the ratio of carbon-rich to nitrogen-rich materials in the pile is extremely important. A healthy pile needs to have an adequate mix of both ‘greens’ and ‘browns’.  We recommend a one part ‘browns’ to one part ‘green’ ratio – just to keep things simple. 

But how do eggshells fit into this ratio? If you are only adding a few eggshells here and there, whether they are ‘green’, ‘brown’ or neither will not have a big impact on your pile. As always, look out for signs that you compost pile is healthy and adjust where necessary!

Things to consider when composting eggshells….

There is an element of controversy when it comes to composting eggshells. Many people avoid it as they fear than any salmonella on the shell will be transferred into the pile and infect plants that you use the compost with. Experts suggest that the eggshells will likely be such an insignificant part of your pile that even if they did contain salmonella (which is unlikely to start with) that it would not overwhelm a compost pile. There is also plenty anecdotal advice from fellow composters that they have no issues with this and have been composting eggshells for many years. However, if you do not feel comfortable do not compost your eggshells – it is your call.

Eggshells can hang around a while in the compost pile. As with other items, you can speed up the process by breaking them down into smaller pieces. Many people bake them in the oven first to make them more brittle. Although this will do no harm – it is not necessary either. Smashing them with a heavy item, such as a rolling pin, will do the job nicely. Simply mix them in with the existing pile. Job done.   

The bottom line

Eggshells are biodegradable. Composting eggshells is a controversial subject in the composting world. Whilst some composters cite potential salmonella as a reason not to pop eggshells in the pile, many compost eggshells with no problems. It can several months for the eggshells to fully breakdown – patience is required! Since they can’t be recycled, using them in the garden is by far the best way to diver them from landfill.