Never use raw rockwool! The pH is too high. Always soak rockwool before using it for the 1st time. Soak with water or a nutrient mixture with the pH adjusted to 5 and an EC of about 1.3 (ppm values). CANNA Start has been specially developed for this purpose. As well as correcting pH & EC, a pre-soak also ensures an even distribution of the nutrient solution throughout the slab.