Plants need calcium, especially things like tomatoes which will suffer from blossom end-rot without sufficient amounts.
I save all my eggshells and my birds generate about 5 to 10 pounds worth per year. I dry them, crush them and every spring I spread them all over my gardens. Today was eggshell-spreading day! There are usually none left over, but when there are they get fed back to the birds who love them.
I really appreciate that I don’t have to purchase bone meal, which is a by-product of the beef industry. It’s nice using fertilizers that are generated right here on the property and it saves me money. I don’t bother crushing them to a powder, I just do the best I can and they decompose over time.
It’s a myth that eggshells will prevent slugs in your garden. Tests have shown that they actually attract them! If slugs are a concern for you, turn your eggshell into the soil so they’re covered. I don’t bother with this because my ducks eat most of the slugs around here and I like the way the shells brighten things up.