Hydroton Hydrokorrels are a unique, lightweight expanded clay aggregate. Made in Holland, this is the cleanest source (dust free) that we have found. Clay is mined, formed into pellets and fired at high temperatures to cause the clay to expand into lightweight balls that are porous but do not float like perlite. Clay naturally has balanced capillary action to accelerate plant growth with an ideal surface for root structures and beneficial bacterias. Hydroton Hydrokorrels drain freely, are pH adjusted, and free of harmful heavy metals and inert. For best use, rinse before planting.