Quote:
Originally Posted by WaterWitchin
Camille, is this similar to wetting perlite before using it, instead of inhaling big puffs of dust when you use in the garden?
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Horticultural rockwool has a much more compact structure than the stuff used for insolation, so the amount of dust is far, far less. At work we use blocks or slabs which are partially or entirely wrapped in plastic, so there is only a small risk of skin irritation or inhalation. With the small cubes, you get a bit more dust but it's still nothing compared to insulation rockwool.
All rockwool products need to be soaked before use. One reason is to have a uniformly wet substrate for good initial root development. The second and most important reason is to lower the pH. During the manufacturing process residual lime is left in the rockwool, which increases the pH. The starting pH of rockwool is around 8, so unless it is sufficiently soaked beforehand, pH will stay high or even continue to increase during the early stages of cultivation. A good soak and flush of the cubes will remove a lot of lime, and if you water in sufficient amounts once the plants are potted, it will continue to flush it out.
Another option is to soak the rockwool in water at pH 5,5-6 to dissolve the lime and bring the cubes done to the proper pH immediately, taking care to avoid having the pH dip below 5, at which point the rockwool itself will start to degrade/dissolve (leading to a compacted mess similar to what DC describes). At work we soak out rockwool in the nutrient solution used during cultivation, with a pH of 5,8.
In french orchid forums it's often instructed to repeatedly soak the cubes over the course of several days until the pH no longer increases, but unless you are using a recirculating nutrient solution, this is not necessary. With standard watering practices (the excess water is thrown out) at each watering a little bit more lime will leach and drain out.
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