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Rockwool, sometimes referred to as stonewool, is a growing medium made of basalt melted back into lava and poured into a spinner in a process similar to making cotton candy.

The Do's of Rockwool

If you are watering and the amount of drainage exceeds 30%, you are probably overwatering. This may lead to algae growth. Most of the water volume in rockwool—as much as 85%—is available water for the plant, and the block can dry down to very dry conditions before the plant finds itself in water stress (as low as 10% in rockwool). The plant can literally tip over because the block is too light before you need to water again.

Don’t: Overwater Plants in Rockwool

The fact that the two materials look similar has given rise to urban legends regarding reuse, toxicity and disposal. At the end of the plant life cycle, horticultural rockwool can be reused to grow different plants, or you can shred it to be composted and then reused in potting mixes or in garden beds.

In all situations, if I find my rockwool presoak water is higher than 5.6–5.7 pH, I use pH-Down to lower it.

Soaking rockwool cubes and slabs is easier than soaking loose rockwool.

Rockwool Presoaking Drainage & Moisture Logistics

Grodan is based in the Netherlands and has in the past few years upped its game when it comes to rockwool. The company’s products are researched and tested in the Dutch nation’s thriving indoor hydroponics agriculture industry.

Because zero-parts-per-million water is tricky to pH adjust, I use beneficial microbe supplements, such as Voodoo Juice, Piranha and Tarantula, added to my presoak water for rockwool that will host new cuttings or marijuana seeds that have enough PPM for accurate pH metering. Along with these three supplements that guard roots and help them function better, I often also add House & Garden Roots Excelurator. This combination provides enough PPM so you can successfully set your presoak water pH to 5.5.

Understanding The Rockwool Presoak

In all cases, before you put seeds, clones or plants into presoaked rockwool, you want it to dry out from its pre-treatment phase so it’s moderately moist but not soaking wet.