Hydroponics 101: Rockwool
Rockwool has long been one of the most popular hydroponic growing mediums. The vast majority of Rockwool used in the world is used for insulation purposes (much like fiberglass). However, adjusting the mineral content can substantially change the properties of Rockwool. In the early 1960s it was found that following several modifications to the manufacturing process Rockwool would support and, under the right handling practices, promote plant growth. This produced horticultural rockwool, which is what is sold as a hydroponic substrate.
Grodan, the leading distributer of Rockwool, produces Rockwool by melting basaltic rock and limestone to temperatures as high as 3000°F (1600°C), at which point they become lava. They are then put into a spinning chamber and are spun together into fibers, much like cotton candy. Immediately following spinning, a binder is added to the fibers and they are compressed and cured into large slabs. By adjusting the amount of pressure, the density of the media is adjusted. The large slabs can be cut into smaller slabs and propagation blocks for easy handling. The spun fibers are also formed into a granulated product, which can be handled in a manner similar to bales of peat.
Rockwool is an inorganic substrate; therefore it maintains its structure over a long period of time. In general, Rockwool holds more water per unit volume than the other inorganic substrates and therefore has a greater buffering capacity. Rockwool is inert, so it has no built in nutrition and does not add or take anything away from plants. It can be used in either recirculating or drain to waste systems, and with either synthetic or organic nutrients.
Advantages of Rockwool:
- Retains Water – Since rockwool will easily give up water to the roots, even when it is almost dry, growers can allow more of the pore space in rockwool for air, while still maintaining a satisfactory supply of nutrient solution to the roots.
- Holds Air – Rockwool holds at least 18 % air at all times (unless it is sitting directly in water), which supplies the root zone with plenty of oxygen.
- Clean & Convenient – Rockwool holds together very well so it can’t spill. Rockwool also comes wrapped in plastic, which makes it easy to handle and keeps evaporation to a minimum.
- Comes In A Variety Of Sizes And Shapes – From 1″ cubes designed for use in propagation, to 3″x12″x36″ slabs capable of holding the root systems of huge plants, rockwool comes in dozens of shapes and sizes making it a versatile growing medium. Rockwool also comes “Loose” so you can fill pots or containers of any size.
Rockwool Conditioning Tips*
When rockwool is new it contains some residual lime from production. This has led to a mistaken belief that rockwool is alkaline and that one has to continuously adjust pH. In fact, once the lime is flushed out, rockwool is pH neutral.
Immediately before use, flush the rockwool with a pH 5.5 solution. This is done to flush out the dissolved lime. The lime will make the pH value rise to 6.0. From this point onwards rockwool does not change the pH in any way. Most rockwool cubes will have presoak instructions on the packaging.
How to pH condition:
- Saturate rockwool in no lower than pH 5.5 water for about half an hour to an hour, depending on the amount of rockwool you are conditioning. Higher quantities of rockwool will require longer flushing times to saturate evenly.
- Remove and let drain to waste.
- Flush through the rockwool with a normal nutrient solution at pH 5.5-6.0, just prior to planting.
There are also products such as Europonic Rockwool Conditioner that make the presoak process much easier by adjusting and stabilizing rockwool for maximum nutrient uptake. A unique blend of pH controls and minerals, use it for conditioning your rockwool before starting seed, clones or transplants. Use three ounces of Europonic Rockwool Conditioning Solution per gallon of water and mix thoroughly. Saturate dry rockwool with this mixture and let soak overnight.
It is important that you do not condition your rockwool with a solution at a pH lower than 5.5. If you do this, you can damage the actual fibers of the rockwool. If you use a pH 4.0 solution, you will find that your pH jumps all the way to 7.0. The lower the pH you use, the higher it jumps. If the fibers are damaged it can be difficult to re-establish a stable pH level, so never go below pH 5 with rockwool. To soak cubes, put them in a bucket filled with water. To soak slabs, cut a hole in the plastic bag they come in and fill it with water until totally saturated. After 24 hours, cut drainage slits in the bottom.
If handling rockwool in a dry state and working in confined spaces, always use a protective mask so that you aren’t breathing in the harmful dust. Although it is non-toxic, rockwool can cause skin irritation. Always wear a dust mask and gloves when handling dry rockwool. If skin irritation occurs, rinse the area with water.