Rockwool is not alkaline! It is a common belief that Rockwool is alkaline and that one has to continuously adjust the pH. This is not true. When Grodan rockwool is new it contains some residual lime from production.

Everybody should soak the rockwool in no lower than pH 5 water the day before use. This is done to dissolve the lime. The lime will make the pH value raise a full point. Immediately before use; you flush the rockwool with your nutrient solution. When you flush, you also flush out the dissolved lime.

From this point onwards rockwool does not change the pH in any way. Exception: If you condition your rockwool with a pH lower that 5, you may also damage the fiber itself. As an example: If you use pH 4 water for conditioning, the pH the day after will be 7. The lower pH you use the higher it jumps. Once the fibers are damaged it might take quite a while before the pH stabilizes. So never go below pH 5 with rockwool.

Why does pH change ? When the plant grows the pH goes up because of the root activity. If the pH is not going up, your plant is not growing! When the plant gets ready to set flowers or fruit, the pH will briefly drop. So when you notice this drop, it is time to change to your bloom-solution.

When your propagated plants have grown into the bigger DM blocks, you will often go into a period of rapid growth. This means that your pH will go up . Your EC (conductivity or TDS) will also tend to go up. Be sure to water the blocks enough. You need to check EC and pH in the blocks everyday. During this initial growing on, water the blocks every 6-7 hours, but only while the light is on. If the pH or the EC in the block is too high, the quickest way to change it is to top water with balanced nutrient solution.

As the plants get bigger 3-4 weeks after potting up the A0 cube, you will need to water approx. every 5 hours. (1000 W light) Set your first watering to start when your light switches on. Be sure to complete the last watering no less than 2 hours before you switch your light off so the blocks are not too wet during the night.