Clay Pellets – This man-made product is often called grow rocks. It is made by baking clay in a kiln. Inside the clay pellets are tiny air pockets (much like lava rock) which makes this a light weight medium (some of the pellets even float). The pellets are great for ebb & flow systems or other systems that have frequent watering cycles. Clay pellets do not retain much water so they need to be watered often so that the roots of your plants do not dry out. You can also mix pellets with other growing medium(s) which will increase aeration (increase water drainage decrease water retention). Expanded clay pellets are rather expensive but are reusable, which makes them a good choice for the long term. After you harvest your crop you can wash the clay pellets to remove all the old roots and then sterilize them with a 10% bleach and water mix (one part bleach to 9 parts water). Rinse the pellets with a lot of water before you use them again, so there is no bleach residue. The grow rocks can also be sterilized by using a 10% mixture of hydrogen peroxide and water. I have read that hydrogen peroxide is a more environmentally friendly way to sterilize things compared to bleach. Clay is a finite resource, but the fact that you can recycle the pellets indefinitely makes them green, if you do in fact reuse them indefinitely.
Gravel – Gravel was traditionally used as the only medium in hydroponics because it is easy to clean, never locks up nutrients and is cheap. It is also a good additive for heavy soils because it creates spaces for air. Gravel made of limestone should not be used; it will affect the pH of the medium. I have never used silica stone which is sold by HTGSupply.com but I imagine that it is a similar product in that it is suitable as a hydroponic medium and can be used as a soil amendment to increase aeration. Gravel and stone are a finite resource, but the fact that you can reuse them indefinitely makes them environmentally friendly. You can sterilize them the same as with clay pellets.
Rockwool – Rockwool is a very popular growing medium. I have used it for many years. Rockwool was originally used as insulation and was called "Mineral Insulation". It was developed for gardening in Denmark and is used extensively around the world for "Drip-Style" hydroponic systems.
Rockwool is a finite resource. It is made from stone which has been heated then extruded into thin stands which are like glass wool. The process is very similar to making cotton candy. You can buy Rockwool in blocks of different sizes. Small blocks with a preformed hole for seeds/ cuttings and medium and large blocks to transplant into as the plant grows. It can be used in any system but as I said is often used with drip emitters. I like to grow in dirt, but I used to start plants with a flat of Rockwool and then transplant each little one inch cube into a soil container. This made starting plants easy and minimized the transplant shock. Some other reasons I liked Rockwool is that it holds an incredible amount of water which gives you a "buffer" against power outages and pump (or timer) failure. Even better it also holds air, Rockwool holds at least 18 % air at all times (unless it is sitting directly in water). This means that it is practically impossible to over-water (Note I said practically impossible). As with any medium, there are disadvantages: Rockwool should be pre-soaked for 24 hours before use while most other growing medium only need to be well watered. Rockwool has a high (basic) pH which means if your plants don’t like a high pH you have to adjust your nutrient solution. As with any particulate matter, the fibers and dust from the Rockwool are bad for your lungs. Wear a dust mask when handling to prevent problems. Rockwool is hard to dispose of, if buried it will last indefinitely but so will sand, perlite and gravel. I have transplanted plants outdoors that were started in small one inch cubes and the cubes are still there years later. However, they were incorporated into the soil ecosystem and were showing no ill effect to the transplanted plant or surrounding plants. Rockwool can not be reused as the roots grow into and through it and since it is made from rock, it is not a sustainable resource. I have not used it in over a year.
Good Growing
Dr. E.R. Myers
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