Soil isn’t necessary to grow healthy, flourishing plants. In a hydroponic system, the plant is supported in an inert growth medium such as gravel, and a nutrient solution is periodically applied. The hydroponic growing medium supports the plant and its root structure. A good growing medium will have several key characteristics to allow for optimum plant growth.
- Density – the medium must be dense enough to anchor the plant roots, and at the same time not so dense that it will impede oxygen/nutrient flow to the roots.
- Sterile – a clean and sterile growing medium will minimize the spread of both diseases and pests. In addition, the medium should be clean so that it does not introduce additional nutrients to the roots.
- Porosity – the pieces of medium must stay damp from the nutrient flow long enough for the plant to absorb all of its required nutrients between cycles.
- Neutral pH – A neutral pH is crucial to the success of the growing system
- Easy to handle, clean, sterilize, and reusable.
You may be surprised at some of the potential growing mediums used for hydroponic gardening: straw, wood chips, sawdust, peat moss, sand, crushed brick, perlite, coconut fiber, vermiculite, rock wool, clay pellets, glass marbles, sponge, plastic foam scraps, broken dishes or pots, in addition to the wide variety of commercially developed mediums. The selection of the “best” medium depends largely on your environment and situation. The most traditional medium is likely a coarse gravel, probably dating back several thousand years. A fast-draining medium such as Hydroton or expanded shale works well in an ebb and flow system.
The growing medium should be selected so that it will complement the surrounding environment. If for example, in a greenhouse, the humidity is usually relatively high, then the ideal growing medium would be something easily aerated, which would suggest one of the more porous selections.
- Hydroton – is manufactured from clay formed into pellets and fired in a rotary kiln at high temperature. The result is a round, lightweight, porous, clay ball. It is reusable, washable, inert, and pH neutral. They drain freely, and will provide good oxygen levels around the roots. These are well suited for flood and drain systems.
- Rockwool – is produced by melting a combination of rock and sand, and blowing the mixture into a spinning chamber (very similar to the process of making cotton candy). Rockwool can hold 10-14 times more water than soil, and approximately 20% more air, and it could be used as a soil substitute for just about any hydroponic system. It is commonly used to propagate seedlings. It does have a slightly basic pH, some people prefer to soak the medium in water prior to use to minimize this effect. Close monitoring of pH will be necessary to ensure it is within an acceptable range.
- Straw – is self-sterilizing, if you get a bale of hay wet, the center will reach about 160 degrees; which will sterilize it of anything that might be harmful to your plants. You could plant something right in that straw and water with a nutrient solution. If you try this, be sure to use a thermometer capable of getting to the middle of the bale. You will need to cool down the hay bail once it reaches the high temperature, otherwise it will begin to break down. Once it does start to breakdown it is an invitation for a variety of fungus invasions. In hydroponic gardening we want to control as many variables as possible, therefore an inert growing medium is more attractive.