Rockwool Facts and Usage

Courtesy of General Hydroponics

Q: What is rockwool?
A: Rockwool is spun mineral wool fiber. Various “rock” components, with a heavy dose of silicon oxides are melted at high temperature and fibers are formed as molten material is passed by a series of “spinners.” The finished product which is “bonded” via proprietary resins, is inert, meaning it has no cation exchange capacity. Approximately 97% v/v/ is air space. Bonded products (plugs, blocks and slabs) also contain a wetting agent, which may (absorbent) or may not (repellant) be present in granulated (also called flock) rockwool.

Q: What’s so good about rockwool?
A: Rockwool is considered by many commercial growers and their consultants to be the ideal substrate for hydroponic production. Since it is inert, fertilization can be controlled in a very exact manner. Because of its unique structure, rockwool can hold water and retain sufficient air space to promote optimum root growth. Since rockwool exhibits a slow, steady drainage profile, the crop can be manipulated more precisely between vegetative and generative growth without fear of drastic changes in EC or pH. End result: better yields and higher quality.

Q: Is one type of rockwool system (eg., ebb& flood) better than another?
A: As long as appropriate air/water ratios, fertilizer concentration, EC, pH and any other critical factors are maintained properly, the actual means of irrigating the plants does not matter. More appropriately care should be taken to choose the correct rockwool product (slab, block, BAB-12, Baby Leach) and size for the system. Commercial growers favor drip irrigation with growing blocks on top of slabs for tomatoes, peppers and cucumbers while lettuce, strawberries and many herbs are produced in NFT, water-table, ebb & flood and other systems set up for their specific growing conditions.

Q: Is rockwool harmful to the environment?
A: No. It’s tempting to be trite and leave it at that, but the Professor will expound. Rockwool, as the name implies, is produced from rock. Rock comes from the earth. Returning rock back to the earth after use (recycling) is a good idea. Many products are wrapped in plastic, however, and so the plastic wrap should be removed before tilling the rockwool into the garden or a farmer’s field. Disposing of rockwool in a landfill, while not an appetizing thought, also has its positive aspects. Landfills (we used to call them what they really are- DUMPS) have a problem with anaerobic bacteria. Rockwool mixed in the garbage helps to aerate the mix and enhance aerobic bacterial activity. So whether you plow it or dump it you are helping the environment! Category: Water Relations

Q: Why is rockwool considered a “wet” substrate?
A: Rockwool is approximately 97% air by volume when dry, but rapidly hydrates, filling the air spaces with water and nutrients. Because of the rapid hydration people sometimes tend to over-water, not realizing the structural difference between rockwool and inert media like pumice or fired-clay.

Q: How much should you irrigate rockwool?
A: The ideal moisture percentage in a block or slab is 65-70% during periods of light. Allowing for 30-35% air space in the rockwool promotes healthy root growth. Consult Prof von Hydro’s 1-2-3 Instructions for irrigation strategies.

Q: What happens if you water too much or too little with rockwool?
A: The answer is somewhat dependent on stage of crop growth. Continued saturation of the growing medium reduces root vigor. Too little solution stresses the plant as EC spikes. However, commercial growers use irrigation volume (along with pH balance and fertilizer ratio) to manipulate plants between vegetative and generative growth. For example, a young tomato plant with one-two fruit clusters requires about 1.6 liters of nutrient solution per day under long day length conditions. Reducing the volume of the feed slightly and raising the EC in the root zone stresses the plant and induces generative growth.

Q: Horizontal fiber slabs are wet and vertical fiber slabs are dry, right?
A: It’s interesting how the long, dark nights in northern latitudes often cause the line between fantasy and reality to blur! IF the horizontal and vertical slabs are made from the same material by the same manufacturer, then, and only then, is theabove statement somewhat correct (see next question for full explanation). Fiber length, fiber diameter, material density, product height and a host of other factors ultimately determine the water-holding capacity and drainage characteristics of a growing slab.

Q: Are “wet” and “dry” the correct terms to use when describing rockwool?
A: The answer is no, but often we use them to simplify explanations. Citing the question above about horizontal and vertical is the perfect example. The only difference between a given manufacturer’s horizontal and vertical growing slabs is the orientation of the cut when the raw material is made into horticultural product. The starting material is exactly the same. How could the exact, same material be “wet” in once instance and yet “dry” in another? Professor van Hydro would very much like to see this misleading terminology disappear from the scene, but alas, there are some who prefer to perpetuate the myth.

Q: How do commercial vegetable growers view this issue of “wet versus dry”?
A: They try not to fall prey to marketing gimics! Typically growers perform leach tests to determine how well a product hydrates, how quickly it drains, and then how rapidly it re-hydrates. They look at the rooting profile under both low and high light conditions to ensure that the rockwool product is compatible with their irrigation strategy. And last, but not least, they grow a crop on it!

Q: I want to use “continuous” drip in my rockwool system. Any suggestions?
A: Boy are you cheap! Get a timer! Still too cheap even after being embarrassed publicly? Try increasing the column height of your rockwool substrate. If you currently use a 3” high growing slab try going to 4”. If you use RB6 or RB10 growing blocks switch to a BAB-12. This will increase your air / water ratio under continuous drip (or excessive flooding) and result in improved root growth. Rockwool Fibers and Other Basics

Q: How is rockwool made? I heard it came from volcanoes!
A: Yes, a big volcano in Copenhagen (just kidding!). A variety of rock materials are are used (depending on the manufacturer) in the starting mix. Most rockwools are fairly high in silicon oxides. The materials are sized on large screens and then melted at about 2000 º C. before being blown across “spinners” that separate the molten material into fibers before it cools. Fibers are then treated with resins to “bond” the material before it passes through a curing oven. After that the largesheets of material are cut and packaged into the rockwool products you see at you local store.

Q: Can you explain to me what is meant by fiber orientation?
A: It’s as simple as it sounds! This terminology is used to describe the appearance and orientation of the fibers in the rockwool product relative to the ground (horizontal plane) and it is a simple matter of the axis on which the raw material is cut to make the finished product. Slabs usually are either vertical or horizontal. Growing blocks also have vertically-oriented fibers but prior to going through the curing oven these fibers are “scrunched” (or “re-oriented”) in order to provide maximum strength without having to increase the actual density of the rockwool.

Q: What is the real significance of horizontal versus vertical fiber orientation?
A: Typically roots tend to spread out more in a horizontal fiber which can be very helpful under stress conditions. Conversely roots tend to go “right to the bottom” of the bag with vertical fiber slabs. Manufacturers who make very low-density products will sometimes recommend that their customers use vertical fiber slabs to avoid breakdown (and breakage) of the slabs during normal use.

Q: Are all rockwools the same?
A: No. Manufacturers start with different rock components, use different bonding agents and wetting agents, melt at different temperatures, use different kinds of melters (coke-fired ovens versus electric melters), different numbers and arrangements of the “spinners,” have different target densities, fiber diameters, fiber lengths and even a few “trade secrets” that none of us will ever know. However, all of them are mineral wool products that are inert, have 96-97% air space and all of the constituents of the fiber are so tightly bound that laboratory analysis requires sulfuric acid / nitric acid digestions of the rockwool for 24 hours at high temperature to perform analytical procedures. A Little Chemistry

Q: What’s this about rockwool having a high pH that requires leaching before use?
A: Rockwool essentially has a neutral pH. All bonded horticultural rockwool (ie., slabs, blocks, plugs) are held together with resins that are employed in the curing process. These resins are > 8.0 pH BUT they do NOT affect the pH of the growing medium (they can be extracted by boiling in water). One manufacturerleaves residual lime in the material (a benign artifact of the production process) and so they require an extended soaking period prior to use. This is not required with General Hydroponics rockwool products.

Q: I just decided to switch from Promix to rockwool and hyroponics in my indoor garden. Do I fertilize differently?
A: You are changing from a system with relatively high organic matter in the root zone to one in which the growing medium is inert and has no cation exchange capacity. Rockwool systems require more vigilance in maintaining correct fertilizer ratios, EC, pH and irrigation cycles because there is no “buffer.” This sounds daunting but actually is a good thing, because it allows you to control the environment in which the roots are growing and increase both yield and quality.

Q: How do I leach my rockwool system when the EC is too high?
A: First of all be sure that EC in your nutrient tank is the culprit. Compare EC values in your slabs or blocks to that of the solution. If the EC in your rockwool is significantly higher than that of your tank, you probably are irrigating too infrequently or your solution pH has gotten way out of whack. If both the tank and rockwool values are elevated you should reduce you tank EC via dilution with distilled water, adjust pH if necessary and then flush the system several times to bring down EC in the rockwool and prevent further damage to the plants. Once you bring down the EC in the rockwool into the proper range, immediately change to a fresh nutrient solution in your tank and be certain that both the EC and pH are in the correct range.

Q: I heard that organic mediums and fertilizers result in better “taste” than in systems that use mineral fertilizers and mineral growing mediums (such as rockwool). Is this true?
A: If you look WAY back in the archives you will find that the original concept behind hydroponic growing (aside from greatly reduced environmental costs) was the fact that food quality actually could be improved in water-culture systems. However, it wasn’t until about 35 years ago that commercial systems were developed that actually produced vegetables that were of a quality equal to that of their field-grown cousins. The key ingredients to this change? High-quality mineral fertilizers and the development of rockwool as a growing substrate. This does not mean that “organics” won’t work in hydroponics, but remember that the definition of “organic” simply means a compound that contains elemental carbon. Different forms of nutrients are taken up by plants be it organic or inorganic. Not to become anthropomorphic here, but plants “don’t care” where it comes from, aslong as the required nutrients are in appropriate concentrations and are available (or can be converted to an available form). AND the plant has it’s own system for carbon assimilation (CO 2 fixation) so it actually has no need for the carbon in “organic” fertilizers. Whether the phosphate comes from bat guano or a large mine in Saskatchewan is immaterial to the taste of the fruit.