Recently, a member asked me if it was okay to compost eggshells. My short answer was yes. However, I also promised her a longer explanation and that's what we have below...
The majority of an eggshell is composed of calcium carbonate (CaCO3). This type of calcium is also referred to as high calcium lime, or lime, in the agricultural world. To be exact, the average dry eggshell weighs between 5.0-5.5 grams and approximately 2.2 grams of that is calcium1.
In terms of increasing calcium levels in your compost or garden soil, 2.2 grams of calcium is not a whole heck of a lot; however, over time, regular additions of eggshells to your compost pile will add up. But I'm talking years, not months.
If your soil test reveals a calcium deficiency in your garden, don't wait for the benefits of composting eggshells to kick in. Go get some calcium carbonate from your local farmer's co-op or garden center and apply as per your soil test recommendations. Calcium is far too important a mineral to be farting around with (technically-speaking) and prolonging the effects of a deficiency in your garden.
What other nutrients are in an eggshell?
The average shell of a chicken egg contains carbon, approximately 0.3% phosphorus, and 1.0% nitrogen. The shell also contains trace amounts of various other minerals such as zinc, iron, manganese, copper, sodium, and potassium.
Here is the technique I use to compost eggshells...
I always have several egg cartons on the go (1 full and 1-2 empty). After I crack an egg, I keep the top and bottom of the shell separated and I place them upright in an empty carton. After several days, the shells become dry and quite brittle - perfect for crushing.
I then place the shells in a small plastic bag and pound on them with the side of my fist. Or better yet, if you have children, give them the bag and tell them to step on it over and over again until they get tired. Make sure they wear shoes because the shells can be quite sharp sometimes.
You can stop at this stage and add the shells to your compost, or you can go one step further.
If you're keen, place the crushed shells into a coffee grinder and whiz them into a fine powder (see before and after images below). I prefer this method, especially if I plan to compost eggshells using my worm bins.
All in all, eggshells are yet another carbon-based ingredient that we can keep out of our landfill and instead, use to remineralize our gardens, albeit over many years and with lots of fist pounding and foot stomping.
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1 Butcher, G. D. & Miles, R. D. Concepts of Eggshell Quality, URL http://www.afn.org/~poultry/flkman4.htm
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