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GROWERS INFORMATION -
ARTICLE - ROCKWOOL FACTS |
Rockwool Facts
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. |
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