You’re probably heard that eggshells are a good source of calcium and are an excellent addition to compost or even to add directly to the soil in your garden. In fact, eggshells are even recommended as a way of preventing blossom end rot in tomatoes*, a disease related to the unavailability of calcium in the soil.
Now, this sounds very logical: eggshells are 95% calcium carbonate, after all. Doesn’t it stand to reason that they’d up the amount of calcium available to plants if you added them to compost or soil?
But that’s without considering the fact that eggshells don’t break down readily. In fact, archeologists regularly dig up middens (refuse mounds) hundreds of years old and the eggshells are still there, largely intact. As a result, very little calcium from eggshells is actually returned to the soil in any way useful to plants, at least during the lifetime of the gardener.
Actually, if you’re an astute composter, you’ll already have noticed this: when your compost is “ready” for garden use and all the other ingredients have decomposed, you’ll still see abundant bits of undecomposed eggshell.
Unusual Conditions Can Help Decomposition
Under special circumstances, eggshells will break down much faster.
Under very acid conditions (a pH of less than 4), for example, they will decompose quite readily, especially if reduced into powder, but you’ll find very few compost piles or garden soils in that range. (Actually, you wouldn’t want your garden soil to be that acidic: very few plants can tolerate extremely acid soils.)
A rather extreme way of actually getting some value out of the calcium carbonate eggshells are made of is to “melt” them in acid! Pour plain kitchen vinegar, highly acid (a pH of about 3), over ground-up eggshells and let sit overnight (or longer, depending on the quantity and the size of the shell fragments). The acid will cause the shell bits to dissolve, releasing carbon dioxide and leaving calcium and water you could then add to your garden’s soil.
They will also decompose faster in hot compost; that is compost where the temperature rises to 40 to 60 °C (100 to 140 °C). Even then, they’ll need to be ground up very finely to decompose within a reasonable amount of time. And of course, most home compost bins, as well as garden soils, don’t get anywhere near that level of heat.
If your municipality has a compost pickup program, definitely make sure your eggshells go there. They’ll be doing hot composting and will make sure your eggshells are properly composted.
Much Ado About Nothing?
It’s important to realize that, in fact, most garden soils do not lack calcium, so you almost never need to add more. It’s one of the most common soil elements, abundant nearly everywhere. So all this concern about the calcium in eggshells is largely theoretical. Your garden will probably get along fine if you never add calcium, whether it be from eggshells or another source.
That said, you can still add eggshells to soil or compost. They’re harmless, after all, and it’s a good way of recycling them at home, especially if the alternative is sending them off to a municipal dump where they’ll just take up space.
If you can, reduce eggshells to powder or at least to the smallest pieces possible. Try using a hammer, a pestle, a blender or, best of all, a coffee grinder. Small pieces will have the greatest chance of breaking down to something useful within your great-grandchildren’s lifetime.
As to the common garden myth that eggshells can be used to control slugs, read more about that here.
One truly useful thing you can do with eggshells is to feed them to chickens. They need a lot of calcium to produce eggs and their digestive system will break eggshells down. Yes, that does sound a bit cannibalistic, but chickens actively seek out anything containing calcium, even their own egg shells.
So, eggshells are not all that useful to the soil you garden in, but at least you can recycle them there and reduce your environmental footprint!