1 Answer

It has been long recommended that egg shells should be used as a compost for plants. I'm not sure if they would benefit trees but they sure help the spoil. Coffee grounds are beneficial to soil and plants (trees too). 

Why should you put egg shells in your garden?
Crushed egg shells add valuable nutrients to soil. Calcium is essential for cell growth in all plants. Calcium is especially important for fast growing plants because they quickly deplete the surrounding soil of calcium. Egg shells should be washed before you place them in a compose or into your garden.
Make sure you crush the egg shells before you add them to your garden. Egg shells that are not crushed will break down very slowly. http://www.mamashealth.com/garden/eggshells.asp