Before crushing up your eggshells, wash them thoroughly and, if possible, remove the membrane from the egg.
There are a variety of tips for removing the membrane from avid gardeners online.
Sharing in the HomeTalk forum, one gardener named Christine recommended leaving the eggshell in a container of warm soapy.
After allowing it to soak, rinse the shells under clean water, rub away the membrane and set them aside to dry.