Egg shells contain a lot of calcium, which is needed by plants. Tomatoes especially need calcium, and not getting enough will cause blossom end rot. Crush egg shells, and place in the bottom of plant holes before planting. Source.
The smaller you crush the shells, the more available the calcium will be to plants. You can use a mortar and pestle, coffee grinder, or anything else. You may want to let the shells dry out first, perhaps by heating them in the oven. This video provides details.
Note, many sources will also suggest using egg shells to deter slugs, however it is questionable whether or not this actually works and may actually attract slugs if there are remnants of the egg membrane.
This applies to Eggshells.
About this Tip
Added October 2, 2014.
Last updated October 2, 2014.
Applies to Everywhere.