Why is pH important?

The pH is a measure of the acidity or the alkalinity of the solution. Normally the pH ranges from 0 to 14. A low pH (lower than 7) means that a solution is acidic. Pure water has a pH of around 7 which is neutral. Higher than 7 is called alkaline. Acidic solutions can be neutralised with alkaline solutions and vice versa. Too acidic is fatal for any plant and so is too alkaline. One of the causes is that the nutrients change into a, for the plant, non absorbable form when the pH becomes too high or low.

The pH of the nutrient solution should be checked regularly with a pH meter.

pH too high: lower it with Nitric acid (during the growth period). Phosphoric acid (during the flowering period).

pH too low: increase it with an alkaline solution (during the whole period).

Take care with these acids because they are corrosive. The pH of a nutrient solution for growing your “favourite plant” should be between 5 and 5.8. Because stone wool (rockwool) is made from rock and rocks can be very alkaline, stone wool can also be very alkaline itself. This results in an immediate rise of the pH value of the nutrient solution as soon as it comes in contact with the stone wool. To minimize this effect, rockwool slabs should be soaked in nutrient solution in which the pH has been adjusted to 5.3, at least 24 hours before using them (one slab will need ca.10 litres). After 24 hours, measure the pH in the slab again and adjust if necessary. The EC of this solution should be 1.7 ms. The soaking is an art. You must learn how heavy and/or moist the slab should be. When the slab is too wet the young plant will experience difficulties. The rockwool slab has then absorbed so much water that there is hardly any room left for oxygen. This results in a plant that dies from root rot.

This shows how important the use of rockwool grow blocks are. These rockwool grow blocks are structured vertically (rockwool slabs have a horizontal structure) which greatly improves drainage and stimulates a rapid, healthy root development. Take all of these factors into account because the first stage determines the later quality level and growth strength of the plant. We can only give guidelines in this because the pH of the tap water differs from town to town, and might even be different from the morning to the evening. Also the factor with which the pH in the slab changes might differ from place to place. During the growth period we use a pH of between 5.2 and 5.8 in the slab.

N.B: All accurate measurements need to be carried out in the slab.

To provide for an average measurement of all the slabs, use the common drainwater. It is advisable to buy a hypodermic syringe to suck the liquid out of the slab. (ca.50ml is enough for a sample, but make sure you pour it back where it came from).

If for example the pH in the slab is 5.3 and you want it to be 5.8, then (5.3+0.5=5.8) you need to increase the pH in the reservoir with 0.5. So if for example the pH in the reservoir is 5.4 you would have to make it 5.9. Between 7 and 14 days after the flowering period is initiated (the moment you change from 18 to 12 hours of light) you gradually reduce the pH in the slab to 5.6 which enables the plant to absorb more phosphorus. At this time you also change from Reiziger Grow Food A&B to Reiziger Bloom Food A&B.