These marble sized balls of clay are unique type of soilless media. They are included with GH systems because they have a good balance between air and moisture, they are ecologically responsible, and easy to use. Clay, an organic and sustainable material, is shaped into balls, and then fired in an oven at about 1200 degrees. They then puff up like popcorn into air filled pebbles, each piece completely inert. Its the perfect media choice for ebb and flow and drain to waste. Water flows between the pebbles quickly, supplying the roots with nutrients, yet allowing the roots to stay oxygenated. Roots and plants develop healthy and vibrant.