Modern day soilfree mediums have come a long way since the use of river gravel and sand. Man made mediums crafted from natural and organic raw materials provide plants with nearly perfect environments for taking root and growing quickly. A perfect medium is able to hold a nearly equal concentration of air and water.
Your plants need both oxygen and nutrients to reach their roots. The determining factor in water/air holding capacity of a medium are the small spaces between each granule or fiber. The name for these "holes" in the medium is "interstitial spaces."

HYDRO MEDIA

Grow Rocks 1 Products Propagation-Cloning Plugs 6 Products Rockwool 26 Products Soilless Mix 6 Products STG Grow Mats/ Cubes/ Slabs 8 Products