Eggshells are perfect for planting seeds since they are naturally bio degradable and provide nutrition to the seed when while it's trying to grow. Once it serves its purpose it degrades away.
http://www.17apart.com/2012/01/how-to-plant-seeds-using-eggshells.html
http://www.17apart.com/2012/01/how-to-plant-seeds-using-eggshells.html