My chickens are cooped up...no free range. What do I need to provide for their digestion they would normally get through digesting grit from the ground? I heard I need to purchase some stuff for them to eat, like gravel. What is it? Would sand work? I feed them back their shells. Is that enough? I have some gravel used to spread over the lawn called Pelletized Dolomite Lime that is a source of magnesium and calcium. It is not feed grade, but can they eat that or is that unsafe for animals?