It's a common gardening myth that egg shells can be used as fertilizer for plants. The reality is that while egg shells may provide a small amount of nutrients, they're not an effective way to fertilize your garden. Egg shells contain calcium carbonate, which is beneficial for plants, but they need other nutrients like nitrogen and phosphorus to thrive. If you want to use egg shells as fertilizer, it's best to grind them up into a powder and mix them with other compost ingredients.