© bOnK: March 22, 1999

6. Growing Haze

In nature, most plants root in soil.
Rainwater dissolves elements contained in the medium, so the roots can provide the nutrients to the plants.
Already as early as in the 17th century scientists started laboratory experiments to find out about the factors that control plant growth.
The aristocracy who liked to grow exotic plant and trees from the 'New World' mostly did this in their 'orangeries', the hothouses of that time.
Because these plants were grown in pots containing soil, they soon started to develop deficiency diseases and people slowly started to find out about the necessary nutrients for plants.
During the ages that followed, the scientists found out that plants could be grown in different inert substrates or even water alone, provided the proper nutrients were available.
Around the 1930's studies on hydroponics were done in the USA.
Studies that during WW II proved fruitful when the army grew vegetables hydroponically with gravel as a substrate. This was done on remote islands in the Pacific where fertile soil was absent and transport was expensive and risky.

Any instance where plants are grown in a soilless medium is called hydroponics.
Although plants may flourish just by hanging with their roots in a water-nutrient solution, all kinds of inert substrates are used to support the roots. Among others sand, gravel, perlite, and peat were, and still are, in use. Experiments were done with peanut shells and lately promising results are being reported by using coconut fibers as a growing medium.
Maybe the weed grown on these materials would drive you nuts. Since I'd rather be stoned let's rock wool. (Well actually I'd rather be high, but there's no way one can grow weed on air alone ;-)

Rock wool ¹

This material was first found in Hawaii where it is natural volcanic product.
The locals thought it to be the hair of Pele, who was the goddess of the volcanoes (Haw lauoho-o Pele). She would pull out her hair in anger and throw it around, making a lot of noise and fire in the process.
Already in the midst of the 19th century rock wool was produced in the USA as an insulation material, mainly for steam engines. Nowadays it is still used a lot for the insulation of buildings, your own walls might be chock-full with the stuff.
In the late sixties, rock wool was first used commercially as a growing substrate in Denmark (of which Holland is not the capital, mind you!).
Today it's the most used material to grow plants and veggies in the Dutch greenhouses.
Once fabricated, it's a clean, inert material that contains no nutrients at all. This makes it an easy product to control. It has a more or less 'perfect ' ratio between air and water.

Preparing rock wool

Actually, the new rock wool slabs are a bit on the basic (notice the lowercase!) or alkaline side of the pH scale.
It's always a good idea to prepare the slabs one day ahead of the arrival of your plants.
Mix a mild solution of the nutrient that you are going to use. Half the normal strength should do the trick.
Now only for once, we are going to lower pH to a mortal value. Use nitric acid (HNO3) to lower the half-strength nutrient down to a pH just under pH 5.0.
Watch it here!
Dissolve only small amounts of acid at the time, stir the solution, measure it and if pH still shows high add another small amount of acid.
Special care should be taken once pH drops below 5.3 because the buffer of the bicarbonates in the water is broken around this figure.

The amount of acid needed to lower acidity from pH 5.3 down to pH 5.0 is very small compared to the amount needed to lower pH from 7.3 to 7.0.
If you manage to make the solution as sour as pH 4.5 you're out of luck. You have to throw it away which is very bad for sewer systems, gardens, and Mother Nature as a whole, so please try to stay on the safe side and be a little cautious here.

If pH is (more or less) in the right range wait for a couple of hours to give the starting nutrient time to really dissolve.
Check pH again, but do not make adjustments if figures are only a few tenths of 'ideal'.
When you are happy with the figures pour this solution in the new slabs, completely saturating them. Let the whole thing brew for at least a night.
This acidic solution will lower the pH in the rock wool to a 'healthy' pH 6.0. It's best to make enough of this 'preparing solution' so you can fill all of the slabs in one run since it is almost impossible to make a second with the same figures.
But remember that you just have to throw away all that's left over.

CAUTION! Never water your plants with a nutrient as acidic as this. For normal watering pH should be around 6.0.

I will talk about pH (a little ;-) more in-depth later on.
For the moment just keep in mind that on the pH scale 7.0 is neutral, higher figures mean more alkalinity, and lower figures mean a more sour solution.
Also bear in mind that, because we're talking logarithmic here, a solution with pH 6.0 is TEN times more sour than a pH 7.0, and pH 5.0 would mean ONE HUNDRED times more H+ ions than the neutral pH 7.0 solution.
There are nice formulas to compute how much acid of a given strength should be added to another liquid to get a certain pH.
Since those formulas make use of atomic weights and such, but mainly because I never found it necessary to learn them myself, I will not bore you with that.


Although they appear the same, the two varieties of rock wool are a completely different product. First of all, the insulation variant is water repellent while the horticultural grade absorbs an enormous amount of water. Furthermore, steel and copper slags and cinders from the steel mills are sometimes used together with the basalt and limestone that normally make up rock wool. These metals may react with the acidic nutrient solution making life very hard for your plants, to say the least. Use only horticultural rock wool of a good quality; never use the insulation grade.


Most of the time, commercial growers use nitric acid(HNO3) to lower pH in nutrient solutions.
Another acid used to lower pH is phosphoric acid (P2O5).
To prepare rock wool and to leach out the excess of nutrients during growth you could also use monohydrate citric acid (wich is not the same as lemon juice!).
Citric acid is never to be used in combination with nutrient though (i.e. to lower pH), as it will form sludge in the container. Besides this slime clogging your (eventual) drip-system, your plants can't cope with these (protein) molecules.
To rise pH you can use potassium oxide (K2O).
Though acids and bases to lower and rise pH work full proof, they aren't 'fool proof', so RTFM before you start experimenting. You should only use a base or an acid, in other words: don't use base to get pH up, once you've added to much acid.


Litmus paper can be used to determine the pH of your nutrient solution.
To get to know the values you have to compare colors, which of course should be done in natural (white) light. The modern meters do the same trick a lot easier, and are already sold for as little as $80. If there is no grow-shop in your area you can obtain one in a aquarium store. Remember that you also regularly need to calibrate your meter to a fluid with a known pH (mostly 7.0).

¹ Rock wool is a product. Amongst many others, Rockwool™ is a firm producing this material.


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