growing mediums.
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.” Fine sand features very small interstitial spaces that cannot hold much air and water.
On the other hand, coarse gravel has large interstitial spaces that can hold a lot of both air and water. Yet, as science would have it, once the interstitial space gets to be too large for capillary action to hold the water in place, you guessed it, the water runs right through it. If your system constantly recirculates your nutrient solution, a fast draining medium would be acceptable.
But obviously, coarse gravel isn’t a good medium for a system that doesn’t constantly circulate nutrient to the plants.
The Perfect Medium
1. Holds a even ratio of air to water.
2. Helps to buffer pH changes over time.
3. Is easily flushed and re-wets easily after being completely dehydrated as would be the case during storage.
3. Is reusable or biodegradable to insure safe disposal.
4. Is inexpensive and easy to obtain.
5. Should be lightweight and easy to work with both indoors and out.
Source: How to hydroponics by Keith Roberto