Carbonatite

Available as a 40 lb. bag or a ton
Recommended method of application: Drop spreader, or some Broadcast spreaders
Material: a dusty sand
 
This is a natural, mined vocanic calcium mineral.  It is reactive (available) and contains many different minerals and trace elements in a non-leachable, but biologically available material.  It is roughly 33% calcium carbonate, 7.5% clay compounds like biotite, vermiculite, greensand, etc., 6% rock phosphate, and 4% trace elements.  Unless you have excessive Calcium, this is a great product to use.  Contains about 19.5% Calcium, 1.3% Magnesium, 0.8% Potassium, 3.8% Iron, and 3.2% Phosphate.  When you need calcium, this is a fantastic product to use. 
Apply 500-2000 lbs per acre (assuming you need the calcium).
Always check with your organic certifier before using any product.