This A-2 Medium-Grade Vermiculite is a medium particle size material. This organic material is OMRI-listed for use in organic gardening. This bag contains four cubic feet of vermiculite. For professional growers, Vermiculite is a great addition to any growing media. It is an excellent option for seed starting and other horticulture mixes. This versatile product is also useful for refractory, lightweight concrete, fire proofing, and packaging. Coarseness and size may vary depending on mine harvested from.