Hydroton horticultural clay pebbles are made from 100% natural clay. They are clean, pH stable, and offer great aeration and drainage in hydroponics, especially in flood and drain, deep water culture, and drip feed systems. Due to their unique structure and ability to cover a large surface area, they offer the ideal environment to foster beneficial bacterial growth around the root zone, leading to naturally healthier plants. The original Hydroton brand expanded clay is a unique, lightweight expanded clay aggregate made in Germany. This natural clay is mined, formed into pellets, then kiln fired at high temperatures to cause the clay to expand into tough, lightweight balls. Clay naturally has balanced capillary action, with an ideal surface structure. Hydroton brand expanded clay is semi-porous and generally does not float like other products can. For best results, rinse all expanded clay before use. It is reusable as long as it is thoroughly cleaned between uses. 50 liter bags feature an easy pour spout. Size specification is 8-16 mm.