Using Eggshells in the Garden
The calcium from eggshells is also welcome in garden soil, where it moderates soil acidity while providing nutrients for plants. Eggshells contain such an abundance of calcium that they can be used almost like lime, though you would need a lot of eggshells to make a measurable impact. By the time they are well ground, it takes 150 eggshells to make a cup of coarse eggshell powder.
http://www.growveg.com/growblogpost.aspx?id=328