Place one raw egg in the bottom of the pot before you add your plant. As the egg decomposes, it will serve as a natural fertilizer adding calcium to the soil.
Save those shells
Every time you use an egg, don't discard the egg shells, put them in a container in your fridge. When the container is full, crush up your shells into small bits and spread them all around the base of your plants. The sharp edges of the egg shells may stop some slugs, snails, and other bugs from nibbling on your plants and will add a touch of calcium to the soil.