Eggshells are primarily made up of calcium carbonate, the same calcium carbonate the makes up the agricultural lime that many farmers add to their soil. Calcium carbonate helps plants develop healthy structures and is required for normal cell growth. It is often recommended for blossom-end rot or BER. BER develops when there is insufficient calcium in developing cells of the plants. If your soil needs calcium, then the eggshells a normal household produces will not be enough. In this situation, go ahead and buy lime or calcium carbonate to improve your soil composition. For those of us that are maintaining already balanced soil, the amount of calcium carbonate you will be adding is helpful over the long haul.
While calcium carbonate the primary gardening benefit of eggshells, it is not the only mineral they provide. Eggshells also contain phosphorus, nitrogen, as well as trace amounts of zinc, iron, copper, sodium, manganese, and potassium. All of which are needed for healthy soil composition. Adding eggshells on a consistent basis over time will help maintain the levels of these essential minerals in your garden soil.
Even before we understood the benefits, we were adding eggshells to our compost bin, but when we started composting with worms, I had to step up my game. Worms are not supposed to have animal protein, but the grittiness of the eggshells helps in their digestive process. So instead of just tossing the eggshells into my kitchen compost bin, I now rinse them and set them out to dry. When they are completely dry, I add them to a zip lock bag I keep just for eggshells. My six year old mashes them up with his fists and stomps on them. I have considered using a coffee grinder to turn them into a fine powder, but our old grinder is hiding somewhere out in the garage because of lack of use and I cannot find it. Maybe we donated it to one of those charities that collects clothes and household goods from your front porch. Anyway, a rolling pin works well too, as does a food processor. Whatever your method, the finer, the better for those tiny worm mouths. We mix the eggshell powder into the worm bedding when we are preparing a new tray, but you could just add the eggshells with your other food scraps or sprinkle it on and fluff the bedding to mix in the eggshells.