Rockwool, a lightweight hydroponic substrate is made from spinning molten basaltic rock into fine fibers which are then formed into a range of cubes, blocks, growing slabs and granular products. Because rockwool and other stone or mineral wools used in horticulture originate from rock, they are considered by many to be a natural product.
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Rockwool is a widely utilized substrate in commercial horticulture for the production of crops as diverse as tomatoes, melons, cucumbers, peppers, strawberries, herbs and cut flowers; however it can also easily be used by smaller hydroponic growers wanting to take advantage of improved root zone technology.
Rockwool originally started as a thermal insulation material in the construction industry, its lightweight but highly aerated nature helps keep heat inside buildings, while being easy to handle, cut and install. Towards the end of the &#;s trials were carried out in Denmark to test the possibility of using stone wool as a substrate for hydroponic plants and since then rockwool as a growing media has seen continuing development and improvement.
Rockwool is used by both large scale commercial producers and small growers alike. Rockwool has also developed into a range of products each with different advantages and applications. apart from the selection of different sized rockwool cubes, blocks and plugs for propagation, growing slabs and granulated rockwool exist for the production of longer term crops and fruiting plants.
The way in which the molten rock fibers are stacked and the density inside the rockwool product determine the properties of the growing media such as the moisture holding capacity, the aeration or air filled porosity and the moisture gradient from the top to the base of the cube or growing slab. By altering these properties, rockwool products for different applications have been made available to growers.
One product for example maintains a slightly drier root zone and helps steer crops away from overly vegetative growth, while another is designed for ultra quick root growth and development. This allows growers to choose the rockwool product which best suits their system, crop, irrigation strategy and environment to maximize plant growth and development.
standard rockwool products drain freely after irrigation and will then typically contain 80% nutrient solution, 15% air pore space and 5% rockwool fibers, although these ratios differ slightly between rockwool brands and products. a typical rockwool slab, such as those used for tomatoes and other fruiting crops, contains around 2.4 gallons of nutrient solution immediately after irrigation, despite the drainage holes allowing free drainage of excess solution.
One of the most important characteristics of rockwool is plants are still able to extract water for growth at very low moisture tensions in the media. That means that plants can easily extract water when the rockwool is saturated from recent irrigation and when the rockwool slab has dried down considerably and lost as much as 70-80% of its moisture content, levels which in other growing media would cause severe wilting in the crop.
The moisture gradient between the top and base of a rockwool growing slab, cube or block is one of the important characteristics of the product. at the base of the rockwool there is plentiful moisture after irrigation, usually at media saturation levels, while the upper layers of the rockwool are held in a drier condition and hence have access to plenty of aeration and oxygen for root uptake and respiration. It is this moisture gradient from top to bottom of the rockwool material which make it such a good hydroponic substrate, but at the same time growers who are not aware of this property can make the mistake of thinking the rockwool is too dry on the surface and over irrigate the plants despite having plenty of nutrient solution help deep down in the root system.
Rockwool, when irrigated correctly should not sit in a pool of nutrient and be completely saturated from top to bottom like a sponge. It is essential that the rockwool is allowed to completely drain so that excess nutrient solution after being applied, leaves the slab or cube under the pull of gravity, in doing so, fresh air is drawn into the top layers of the material providing fresh oxygenation for the root zone. By allowing rockwool to drain freely, over watering becomes more difficult.
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Make sure you don&#;t over irrigate the plants. At the surface of the slab the rock wool may seem dry, but at the bottom there is plenty of nutrient solution.Irrigation of rockwool is a little different to other solid substrates because of the way the material is manufactured to have just the right degree of moisture gradient and because it does give a limited root zone for plants that eventually grow fairly large. For this reason, most rockwool products are best irrigated with short, frequent applications of nutrient solution, with just enough at each irrigation for the rockwool to reach `field capacity&#;. Field capacity is a term which means the substrate has drained fully but is still holding a good level of moisture for the plant roots to access until the next irrigation. at each irrigation, there should be some drainage from the rockwool material, however this shouldn&#;t be excessive. Having around 10-35% of the nutrient solution fed to the plants, drain from the rockwool at each irrigation is considered optimal. This amount of drainage of solution flushes fresh nutrient solution right through the rockwool slab and usually keeps the EC in the slab fairly stable.
Checking the EC in the root zone is important with rockwool just as it is with any other substrate. While rockwool doesn&#;t contain any naturally occurring minerals or salts which may influence EC levels, the EC of the nutrient solution inside the growing substrate changes as plants extract different ratios of water and nutrients from the root zone. Careful monitoring and control of both the EC and pH in the nutrient solution in recirculating rockwool systems is just as important as with any other growing media.
Rockwool has the characteristic of allowing a grower a greater degree of control over the root zone and this can be used to help `steer&#; plants into either a more vegetative or generative/reproductive growth. Drying back of the rockwool slab by increasing the time between irrigations and allowing the EC in the root zone to increase pushes plants such as tomatoes into a more generative state with less leaf growth and more assimilate being directed into the fruit. a higher level of moisture maintained in the rockwool and a lower EC pushes the plants towards more lush vegetative growth. Skillful growers use these techniques in rockwool growing media to direct their crop and control leaf, flower and fruit growth at different times.
Rockwool, being a &#;sterile&#; product (only directly after production) does not contain any naturally occurring beneficial microbial populations when first planted out, however research has shown that microbial life does develop in rockwool substrates in the same way as other more `organic&#; mediums such as peat and coco. This build up of beneficial microbial populations however is generally slower in rockwool as there are initially limited carbon sources for the microbes to feed on.
As root systems develop and produce organic exudate, microbial life inside rockwool gradually build, however rockwool can be inoculated with microbial products to assist this process and help develop a healthy root zone. The high level of oxygenation in a well managed rockwool system also help with the establishment and multiplication of beneficial microbe populations.
Rockwool, being essentially rock does not decompose, fracture or break down over time, hence growers can use it for many successive crops. However it is recommended that rockwool is steamed or at least treated with boiling water before replanting to help prevent any carry over of root disease pathogens. a thorough leaching with clean water also helps remove any excess salts from the previous crop. some smaller growers use chemical disinfectants to treat rockwool before use, however care needs to be taken to completely rinse these from the material before replanting and steam or hot water is seen as a much safer option.
Eventually, used rockwool material has to be disposed off &#; often growers simply dump this, however it is possible to shred the material and re use it in other growing mixes, or incorporate it into outdoor soils and gardens as a soil conditioner.
Rockwool has many advantages for hydroponic production: the manufacture of the rockwool fibers from molten rock and plastic wrapping of growing slabs ensures the product is sterile, and free from weed seeds, pests and pathogens. High quality rockwool brands, being a manufactured product are also consistent in quality and don&#;t decompose or break down over time in the way that many other natural growing substrates do. Rockwool maintains it physical properties over time and with successive crops. Rockwool is light weight and thus easy to handle and shift into place, once fully irrigated however it becomes heavy and provides stability to the crop.
Rockwool comes in a convenient range of sizes from small 2-3 cm propagation plugs joined in sheets for direct sowing crops such as lettuce and other seedlings, to large cubes of over 10cm for more advanced transplants. Rockwool plugs are often used for cuttings where they maintain the ideal levels of aeration and moisture for rapid root development. Rockwool can be inoculated with beneficial microbes such as Trichoderma in much the same way other substrates like coco are, however more frequent applications of microbial products are recommended with rockwool substrates. Most rockwool products and reliable brands don&#;t have any major influence on the EC, pH or composition of the nutrient solution applied. since rockwool provides no naturally occurring nutrients a well balanced nutrient product applied will give optimal growth.
Rockwool is manufactured to give a close to ideal level of moisture and aeration in the root zone, this helps prevent over watering and root suffocation from a lack of oxygenation.
Rockwool can be used for successive crops as its structure does not tend to break down rapidly with use or over time. some commercial tomato growers re use good quality rockwool for as many as 6 successive crops with use of steam sterilization to control root pathogens between plantings.
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Rockwool products and growing slabs come ready to use, the substrate only needs to be thoroughly wetted before planting. Rockwool can be monitored with a water content meter which gives accurate measurement of the water content, EC and temperature in the plant&#;s root zone environment. These assist with fine tuning the application of nutrient solution to just the right level for each stage of growth.
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Rock wool is sterile and inert and thus makes an excellent seed germination and growing medium.Rockwool, despite being light weight, is bulky to transport and store, unlike coco slabs which can be highly compressed and then expanded with water before use. Rockwool needs to be placed on a fully leveled surface to allow the moisture gradient inside the product to be even and prevent any saturated or overly dry patches from developing.
Despite being usable for more than one crop, and some recycling programs developed for used rockwool, disposal can still be a problem for many growers as rockwool does not decompose or break down over time. The fibers of rockwool can irritate the skin and a face mask is recommended if handling granulated rockwool or disposing of old rockwool products.
New or inexperienced growers need to determine the right frequency and amount of irrigation for rockwool systems as this can differ somewhat from other substrates such as perlite and coco.
Rockwool contains no naturally occurring nutrients (coco often contains levels of potassium and sometimes other minerals which are used to pre-condition the substrate), hence the plants are totally reliant on a well balanced and complete hydroponic nutrient solution at each stage of growth.
Rockwool being an inert substrate made from rock doesn&#;t contain naturally occurring growth stimulants such as humic acid, other organic compounds or naturally occurring beneficial microbes, although these can be added with the use of good quality hydroponic supplement products.
This post first appeared on the Rockwool R-Class blog. www.rclass.rockwool.com/blog
If you&#;ve been in the construction industry long enough, chances are you&#;ve had to remove an old mineral wool insulation product during a renovation. I know I have. It&#;s itchy, easily falls apart, and it often doesn&#;t completely fill a cavity bay. I&#;ve had many conversations with other builders who will not consider using a mineral wool product because of their past experiences with the older mineral wool insulations. I can tell you; the old stuff is nothing like modern stone wool.
I&#;m currently working on a room renovation in my own home where I removed all the existing wall and ceiling finishing materials and insulation. The existing &#;s era kraft faced mineral wool had a listed R-value of R-7 for the original 2&#; x 4&#; wall assembly. This same product was used for the sloped roof of my Cape Cod design. Ice dams and comfort issues were my main complaints. My goals for the project is to increase both the wall and ceiling R-values from the interior, (no exterior wall or roof insulation is included) and improve the air tightness of the space. (And get rid of the popcorn ceiling!)
During the demo process, what I found was someone had installed a second layer of drywall over the original drywall, which had some funky &#;s wallpaper. Water staining was present over a large portion of the original drywall and wallpaper. The water damage was caused by air leaks. Warm, moist air inside the home finding a cold, condensing surface during the heating season. Frost would accumulate on the interior side of the roof and wall sheathing until spring when temperatures would warm. The frost would then melt, water would run onto the interior surfaces of the exterior walls. Fortunately, there was no structural damage or rot, and only a very small amount of mold, mostly just water staining.
Removal of the existing insulation was the worst part of the entire job. The process required eye protection, a respirator, and long sleaved clothing with gloves, and I still ended up itchy and coughing. The 70-year-old insulation required removal from the home in small loads, if I tried taking too much outside at a time, the product would fall apart leaving a trail of shredded insulation. Once the old insulation was removed and everything cleaned up, the rest of the process was just like working on a new home.
The new cavity insulation for both the walls and ceiling is ROCKWOOL R-15 Comfortbatt®. The Comfortbatt® I was using is designed to fit inside a 16-inch-on-center wall cavity. The batts are also sized for a standard 8 foot tall, each batt is 47-inches long, two batts fit perfectly in the cavity. At 100 ½ inches, my wall height is a little taller than standard which required a small additional piece of insulation be added. I found it easier to install the small piece between the two full batts, rather than either at the floor or ceiling. Something else I did to simplify installation, I ran all my electrical cabling through the walls at 47 inches above the bottom plate, eliminating the need to split a batt to fit around the electrical wire.
Maximum performance of all fibrous insulations happens when the assemblies are airtight. Air sealing can happen on either the exterior or interior sides of the walls and ceiling. In the case of this project, I will be using both. As you can tell from some of the previous photos, my home has board sheeting, individual boards nailed to the studs on the exterior at a diagonal to add structural support (a common practice before plywood or OSB). Though board sheeting offers resiliency with its ability to take on and release moisture, it is poor at providing any type of air sealing. I chose to use a water resistive barrier (WRB) on the exterior that also functions as an air barrier. The product is a self-adhered WRB (basically a giant piece of tape) called Henry Blue Skin. This product is airtight but will allow some moisture in the form of vapor to move through when drying is needed.
The interior air (and vapor) control is provided by a product called a responsive membrane, also known as a smart vapor retarder. I chose to use Siga&#;s Majrex. When installed correctly, Majrex is very effective at stopping air, but similar to the Henry Blue Skin WRB, it also has the capability to allow water vapor to move out of a wall cavity when needed.
I&#;ve carefully selected the materials I&#;m using in my own home based on the attributes associated with each of the products. ROCKWOOL&#;s Comfortbatt® has the density to completely fill a cavity and can more easily provide a high-grade installation than other insulation products I&#;ve used in the past, thus maximizing the R-value. Both the WRB and interior air/vapor control products provide high levels of air tightness but still allow a good drying potential of the assembly if needed.
Renovation projects often have limitations to the amount of performance gains based on the work scope and budget. Choosing the right products for the project can help a home last, I&#;m hoping my home, originally constructed in the &#;s, will still be around in another 70 years.
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