Eggshells

In short

Eggshells compost, but far more slowly than people expect — a whole shell can still be recognisable a year later. Crushing them finely is what makes the difference. They add calcium to your soil and are a useful carbon-neutral addition to a heap, but they are not a fertiliser on their own and the popular claim that they stop slugs does not hold up well.

What it's made of

What something is made from is what decides where it can go — this is usually the reason an item can't be recycled.

• About 95% calcium carbonate — chemically the same material as limestone.
• A thin protein membrane on the inside, which is what actually breaks down quickly.
• Traces of magnesium and phosphorus.

Before you get rid of it

1. Rinse the shells so there is no egg white left, which removes both the smell and the salmonella risk.
2. Let them dry completely — a windowsill or a cooling oven works.
3. Crush them as finely as you can. A rolling pin is fine; a blender or mortar and pestle is better. This is the step that determines whether they break down in months or years.
4. Add the crushed shell to your compost, or work it straight into garden soil.

Common mistakes

• Tossing whole shells in the heap and expecting them to disappear. Calcium carbonate dissolves slowly, and surface area is everything — whole shells can last for years.
• Relying on crushed shells as a slug barrier. It is a very widely repeated tip, but trials have generally found slugs cross them without much trouble.
• Putting shells in the recycling bin. There is no stream for them.
• Adding unrinsed shells to an indoor compost caddy, which is what causes the smell people complain about.

Better than binning it

• Crushed finely and dug in, they slowly raise calcium levels in soil — useful for tomatoes, where calcium availability is linked to blossom end rot.
• Half shells make tidy little seed starters that go straight into the ground.
• Crushed shell works as a mild abrasive for scouring pans.
• Some people feed clean, baked, crushed shell back to laying hens as a calcium supplement.

What happens to it

In a compost heap the membrane goes quickly and the mineral shell does not — it dissolves gradually as the compost stays slightly acidic, releasing calcium into the finished product. This is why grinding matters so much: fine particles have vastly more surface area exposed, so the calcium actually becomes available to plants within a season rather than sitting there as recognisable shell.

Questions people ask