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Rockin’ Rockwool: What You Need to Know About Using Rockwool to Get Bigger Yields

what’s the similarity between a popular hydroponics material and cotton candy? what’s the similarity between a popular hydroponics material and cotton candy?

Here’s a fun hydroponics riddle for you: what’s the similarity between a popular hydroponics material and cotton candy?

The answer is that both of them are spun from melted stuff. Cotton candy is spun sugar, and rockwool is spun basalt (sometimes referred to as “diabase”).

You can’t grow hydroponics plants in cotton candy (although you can use bud builder additives that make your flowers taste and smell like candy) but rockwool is among the most popular sterile soilless growing media that hydroponics gardeners use. For many years, rockwool was the only sterile soilless medium (or substrate) that was widely available for hydroponics growers.

Nowadays, growers can also choose coco coir, hydroton, perlite and a few other sterile substrates, but rockwool still holds predominant market share, in part because people have long used it and because it works.

Let’s take a closer look at rockwool…

The generic form of rockwool started out as an industrial material used for insulation. It was valued because it was dense but airy, lightweight, pliable, relatively easy to manufacture, and less expensive than other insulation.

When amateur and commercial hydroponics gardening became popular in Europe during the 1960’s, some brilliant innovators looked at rockwool and wondered if it could be a replacement for sand, gravel, sphagnum moss and other substrates used for plant growth.

Horticultural inventors experimented with rockwool by altering its mineral content and the process by which it’s spun, compressed and formed. Soon, they’d created an agricultural grade rockwool that had several desirable qualities.

By the way, this type of innovation is still at work today in the hydroponics industry, and it’s why you can get modernized digital ballasts, nutrients and other materials that were never available until the past couple of years.

One important rockwool quality for you is that rockwool has a porous, pliable internal structure that allows roots to easily penetrate it while also providing suitable anchoring material so your plants’ roots are firmly entrenched. Quality horticultural rockwool provides your plants approximately 13-20% pore space (the space in which oxygen can be held).

Why is pore space so important for your plants? Because your plants need to intake oxygen through their roots. One of the reasons that some substrates are unsuitable for gardening is that they lack pore space, especially when wet. The lack of pore space, combined with too much water, shuts oxygen out of your plants.

This is why overwatering is a big problem for your plants. It suffocates your roots so they can’t absorb enough oxygen. Plants need oxygen to sustain healthy metabolism. When rockwool is properly watered, it maintains adequate pore space for easy absorption of oxygen and provides your plants the water they need too.

Getting the Best Quality and Type of Rockwool

So you see that rockwool has beneficial pore space for your crops, and it also has other qualities that make it useful for your gardening. However, now is a good time to explain that not all rockwool is created equal.

As with most other equipment and supplies you’ll find in the hydroponics marketplace, the usefulness of rockwool depends on how it it’s manufactured, and the raw materials used in its manufacture.

For example, the best horticultural rockwool is made from pure basaltic diabase using a quality-controlled melting and spinning process that has precise supervision of fiber size, compression, and consistency.

Inferior versions of rockwool are created when these ideal conditions are compromised, either by incompetence or deliberately. Some manufacturers use inferior diabase, or a mix of quality diabase and other mineral materials, to create rockwool.

Other manufacturers don’t pay as close attention to the physical properties of their rockwool as they make it. In some cases, this results in rockwool with fibers that are too coarse for your roots, less air space, and unstable chemical factors that we’ll discuss later.

It’s interesting to note that the most excellent rockwool is made by manufacturers who understand that minerals contain mineral oils that repel water. That means manufacturers of the best rockwool remove the mineral oils during manufacture, and replace them with just the right amounts and types of wetting agents that help water penetrate the mineral fibers so the rockwool holds the right amount of water for your plants. You may already know that you can purchase organic or synthetic wetting agents that help your crops absorb nutrients via leaves and roots.

As with the manufacturing of hydroponics nutrients and other hydroponics products, experience and scientific knowledge make the difference between the best rockwool and inferior rockwool for you.

For example, some nutrient manufacturers use inferior source materials, or they don’t know about or use the right chelates and other substances that guarantee your crops absorb all the nutrition they need immediately. You could buy expensive nutrients that just don’t work well enough for your plants.

With rockwool, you have to be very sure you’re buying the best, because inferior rockwool can cause problems with watering, pH, oxygen availability and root growth. In extreme cases, the wrong rockwool (or the wrong version of any horticultural substrate) can slow or stop your plant growth.

You get the best rockwool by going to a trusted hydroponics store and asking the personnel what brand of rockwool they recommend and why they recommend it. Find out if there’s a demo garden where you can see and feel the rockwool in use.

Talk to other growers who are highly successful using rockwool, whose gardens are producing huge yields. Look for companies that are known for making products specifically for your garden. See if there’s a guarantee. I’ve found that if a company isn’t willing to stand behind its products with an ironclad guarantee, then those products might not be so good.

Those are some ways for you to get the best rockwool for your plants.

Rockwool and Water Revisited

We already talked about the need for rockwool manufacturers to remove water-repelling mineral oils from their rockwool and add the right wetting agents in the right ratios so you get optimum water-holding and water-releasing from your rockwool.

When rockwool is made properly, its ability to hold large amounts of water without drowning your plants, and to release water when your plants need it, make it very attractive in drip irrigation gardens using pump timers.

In fact you can give well-drained rockwool a good soaking so it’s sopping wet, and it will on its own properly drain, aerate and release water for several days.

Indeed, one of the interesting and beneficial features of rockwool is that even when it’s almost completely dried out, it still retains adequate water and pore space to feed your plants the water and oxygen they need. You can’t say the same for some other substrates.

The key is to make sure the rockwool is able to drain, and to ensure that you’re using high-quality water and properly-chelated nutrients.

Using Rockwool So You Get the Most From It

Rockwool comes in several configurations you can tailor to your gardening needs. You’ll find it in bricks, slabs, cubes and loose.

No matter what configuration you chose, you’ll need to pre-adjust your rockwool’s pH before you put plants into it. That’s because rockwool has a natural pH that’s not ideal for your plants’ roots.
Before I give you the info on how to adjust rockwool, a word of caution. Rockwool is similar to fiberglass and similar materials in that it can irritate your skin and lungs. Wear gloves when you’re handling it, and don’t inhale rockwool dust. It’s not good for you.

When I use rockwool slabs, I put them in a spare bathtub and fill the tub with reverse osmosis water. I let the slabs soak for a while. Then I measure the pH. Usually it’s around 7.5, or even higher.

I use a pH down solution, or a pH-lowering nutrient such as Grandma Enggy’s F1 fulvic acid, to bring the overall water down to about 5.7. I leave the slabs in the pH-adjusted water overnight, and then I measure the pH again.

If it’s still in target range, I drain the water, let the slabs drain, and then place them in my gardening system. If it’s not in the target range, I adjust, agitate the water, and let the rockwool sit a while longer before I put it into my gardening system.

Once the rockwool is properly adjusted and in my system, I drip irrigate the slabs with reverse osmosis water adjusted to 5.7 pH, measuring the pH and TDS of the runoff water to see if I’m staying in target range. If not, I adjust the water slightly. I let the slabs drain for a few hours, and then measure again. Pretty soon, I have created a suitable pH environment inside my rockwool.

As do all careful gardeners, you want to measure the pH and TDS of your runoff water from time to time to ensure that my substrate and nutrients solution are staying within ideal parameters.

You’ll see fluctuations in pH and TDS as your plants pick and choose the nutrients they need from your nutrients solution, but you want to ensure that your average pH and TDS run-off readings aren’t wildly out of range.

When your rockwool has its ideal and consistent pH, that’s the time to put plants in it and start running hydroponics nutrients solution at the proper pH and ppm. Please note that hydroponics gardening pH concerns are slated to be a thing of the past later this year when a new generation of hydroponics fertilizers debut. These nutrients have been scientifically engineered so they provide your plants ample supplies of full spectrum nutrition without you having to adjust pH.

You’ll figure out on your own the proper watering frequency, amount and scheduling for your rockwool but in general, after initial treatment you want your rockwool to be moist to the touch (but not gushing water) after each watering cycle. Monitor your rockwool and you’ll see that they change from darker brown when they have just been watered to a lighter shade of brown as they dry out. If your rockwool gets so dry that it feels like dry insulation material, you’re not watering frequently enough.

Rockwool is valued because it lasts a long time, holds water properly, provides aeration, and can be formed into shapes and textures that are ideal for your plants.

Unlike coco coir, it’s not as easy to clean rockwool using an enzyme product that eats organic debris and allows you to use your substrate over and over again. But coco coir has its own plusses and minuses.

For reliability, consistency and proven results, I favor rockwool or Sunshine Mix #4 in my garden. Different gardening climates and other factors influence the type of substrate you choose, so we’ll talk about other types of substrates in future articles, so be sure to stay tuned in to Rosebud so you get the exclusive information that helps you get the fastest growth and biggest yields.

And here’s a tip- the Grodan company (which makes a very high quality series of rockwool products), also makes a pH/TDS meter specifically for rockwool.  This is a very professional way of ensuring your rockwool substrate is in the sweet spot for the best absorption of nutrients.

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Last modified on Thursday, 07 April 2011 23:24

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