Dirt cheap

I know, it's been awhile, but did you really think you'd heard the last from me?  Come on, you know I love to dish the dirt!  Here's a riddle for you...

What do eggs shells, coffee grounds, cucumber skins, banana peels, apple cores, nut shells, and grass clippings have in common?

http://alacartegreen.blogspot.com/2013/09/dirt-cheap.html