The irrigation of rock wool is slightly different from other solid substrates, because the way the material is manufactured has just the right water gradient, and it does provide a limited root zone for plants that eventually grow quite large. For this reason, most rock wool products are best irrigated with short, frequent applications of nutrient solution, with each irrigation amount just enough to bring the rock wool to "field capacity". Field capacity is a term indicating that the substrate has completely drained, but still maintains a good moisture level for plant roots to access until the next irrigation. At each irrigation, the rock wool material should have some drainage, but this should not be excessive. Feeding about 10-35% of the nutrient solution to the plants, with drainage of rock wool at each irrigation is considered better. A large amount of solution discharged will flush fresh nutrient solution directly through the rock wool slab, usually keeping the nutrition in the slab stable. Checking the salt concentration in the root zone is important for rock wool, just as for any other substrate. Although rock wool does not contain any natural minerals or salts that may affect the salt concentration level, as plants extract different proportions of water and nutrients from the root zone, the salt concentration of the nutrient solution within the growing substrate will change. Careful monitoring and control of the salt concentration and pH value of the nutrient solution in circulating rock wool systems is as important as for any other growing medium. Under warm growing conditions, plants can extract high levels of water from the nutrient solution, thereby rapidly increasing the salt concentration, and requiring more water replenishment to be added to the nutrient reservoir. Under cool and/or humid conditions, the salt concentration may decrease as plants extract nutrients, but not as much water is needed,
Rock wool has characteristics that allow growers to control the root zone to a greater extent, which can be used to help "guide" plants into a more vegetative or reproductive/generative growth state. By increasing the time between irrigations and allowing the salt concentration in the root zone to increase to dry the back of the rock wool slab, plants such as tomatoes can be pushed into a more reproductive state, while reducing leaf growth and more assimilates being directed to the fruit. The higher moisture content and lower salt concentration maintained in rock wool promote plants toward more lush vegetative growth. Skilled growers use these techniques in rock wool growing media to guide their crops and control the growth of leaves, flowers and fruits at different times.
Rock wool is a "sterile" product (only directly after production) and does not contain any naturally occurring beneficial microbial populations at first planting, but studies have shown that microbial life does develop in rock wool substrates in the same way as in other more "organic" media such as peat and coco. However, the accumulation of this beneficial microbial population is usually slower in rock wool, because the carbon source initially available for microorganisms to eat is limited. As roots develop and organic exudates are produced, microbial life inside rock wool gradually builds up, but rock wool can be inoculated with microbial products to assist this process and help form a healthy root zone. High levels of oxidation in well-managed rock wool systems also contribute to the establishment and reproduction of beneficial microbial populations.
Rock wool is essentially rock and will not decompose, break down or degrade over time, so growers can use it for many consecutive crops. However, it is recommended to steam or at least treat rock wool with boiling water before replanting to help prevent any carryover of root disease pathogens. Thorough leaching with clean water also helps remove excess salts from previous crops. Some smaller growers use chemical disinfectants to treat rock wool before use, but care needs to be taken to thoroughly rinse off these materials before replanting; steam or hot water is considered a safer option. Finally, used rock wool material must be disposed of - growers usually just dump it, but the material can be shredded and reused in other planting mixtures,
Rock wool has many advantages in hydroponic production: the plastic packaging for manufacturing rock wool fibers and growing slabs from molten rock ensures the product is sterile and free of weed seeds, pests and pathogens. High-quality rock wool brands, as finished products, are also consistent in quality and will not decompose or degrade over time like many other naturally growing substrates. Rock wool maintains its physical properties over time and with consecutive crops. Rock wool is lightweight, so it is easy to handle and move, and once fully irrigated, it becomes heavy and provides stability for crops.
Rock wool is available in a variety of sizes, from small propagation plugs of 2-3 cm connected into sheets for direct sowing crops such as lettuce and other seedlings, to large cubes of over 10 cm for more advanced transplants. Rock wool plugs are commonly used for cuttings to maintain ideal aeration and moisture levels to promote rapid root development. Rock wool can be inoculated with beneficial microorganisms, such as Trichoderma, in much the same way as with other substrates such as coco, but more frequent use of microbial products with rock wool substrate is recommended. Most rock wool products and reliable brands do not have any significant impact on the salt concentration, pH or composition of the nutrient solution used. Since rock wool does not provide naturally occurring nutrients, using a balanced nutrient product will provide optimal growth.
The manufacture of rock wool can provide near-ideal moisture and aeration levels in the root zone, which helps prevent overwatering and root suffocation due to lack of oxygen. Rock wool can be used for consecutive crops because its structure does not decompose rapidly with use or over time. Some commercial tomato growers reuse high-quality rock wool boards for up to 6 consecutive crops, using steam sterilization to control root pathogens between plantings. Rock wool products and growing slabs are ready to use, just need to thoroughly wet the substrate before planting. Rock wool can be monitored using a moisture content meter, which can accurately measure the moisture content, salt concentration and temperature in the plant root zone environment. These help fine-tune the application of nutrient solution to levels suitable for each growth stage.
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