There are many forms of growing media also know as growing medium. They all exhibit different qualities from the point of view of plant health and growth. Growing media’s used in hydroponic systems should always be sterile when first used. It is not recommended to re-use some medias due to build-up of salts and other unwanted residues and the fact that some breakdown effecting the air to water ratio. A good media should allow the right amount of air to water ratio. It should not ever become water logged or stoggy and needs to remain “open” whilst giving the plant the support it needs to grow and offering the plant just the right amount of water and nutrient.
Soiless growing medias hold and make available water, nutrients and oxygen to the plants root systems. They also support the plant.
The plant's root system's growth is affected by the CF, the pH and the actual texture of the substrate therefore the texture of the substrate is important. There is no doubt that certain crops prefer certain substrates and that can apply to seedling stage versus “growing on” stage.
Substrates such as scoria, perlite and expanded clays have greater surface areas and therefore hold more water. Medias with sharp edges should be avoided as they can damage the root systems and stem particularly in positions where the plant is moving such as wind.
Medias with large particles allow for greater aeration, however it is necessary in this situation to increase the number of watering cycles to compensate for the low water retention. Medias with finer particles pack together closely and whilst water retention is greater aeration is compromised. Therefore correct texture promotes good root health, penetration, oxygen availability, and nutrient uptake. Drainage is also essential as a lack of drainage can reduce the available oxygen.
Vermiculite, rockwool, coir, peat and peat moss are substrates that retain large amounts of moisture within their cells and are therefore ideal for NFT systems and others such as flood and drain where capillary action is required.
Sawdust used as a media deserves a special mention. It is one of the most popular and widely used medias. Sawdust has naturally occurring trichoderma, a bacteria that aids with root health, protecting against pythium however sawdust has draw backs - it uses nitrogen as it breaks down, it becomes water logged as it gets older and is very much a one crop media. It particularly suits one crop that grows for a short duration. As a media it is inexpensive and readily available.
Sand is not a good media. It has a very low porosity and becomes water logged. Sand can be helpful when used as an addition to other more course medias thus improving their water holding capacity.
Inert medias such vermiculite, perlite, rockwool do not change the integrity of the nutrients whilst medias such as limestone effect the pH. Coir although considered inert can effect the nutrients because it often has sodium issues due to where it was grown.
Some medias can be used again and again while others are best used once. Coir is probably the best media I have seen for reuse. I have seen 5 year old coir growing better crops than a fresh batch beside it.
When reusing medias its important to sterilize them before replanting the next crop. Products like Purity XL are organic natural products that kill 99.99% of harmful bacteria that have built up during the previous crop and eliminate the need for crop rotation.