In most hydroponic gardens, soilfree growing mediums are used primarily for starting seeds and when rooting cuttings. The less medium a system requires, the easier and less expensive it is to operate. This is a major consideration for those intending to make a profit from their hydroponic gardens. Modern day soilfree mediums have come a long way since the use of river gravel and sand in early systems. A perfect medium is able to hold a nearly equal concentration of air and water. As you have learned, your plants need both oxygen and nutrients to reach their roots. The water/air holding capacity of a growing medium is determined by the small spaces between each granule or fiber. These "holes" in the medium are known as "interstitial spaces."
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