Rinsing your rockwool slabs With only the over drain you won't keep the EC and pH of the root system on a constant level. That's why you'll need to help the root environment a little bit. You do this by rinsing the slabs regularly. By rising the slabs, also the excess waste salts and waste acids of the slabs are rinsed out, so the roots can continue to do their work. The lower the pH in the slab, the more difficult it becomes for the roots. In the beginning, when the pH lies around 5.0, damage occurs to the roots. If the pH is around 4.5, the acidity begins to affect the rock wool which causes an irreversible reaction; the release of alkaline substances. These are substances which have a pH value of between 7 and 14. This reaction causes thehairs of the roots to burn, which are the ones that select the elements in the feeding solution that the marijuana plant needs. Conversely, the pH can also be too high. Most of the tap water has a pH value between 7 and 8. Aside from that, the EC may increase in the slab, which has disastrous consequences. A too high or too low pH means that the hair roots eventually die with an EC that's too high. How to act To begin, you'll need the right tools: measuring cup syringe of 50 ml pH meter EC-meter In order to perform the correct root environment measurements, measuring only the water that drains from the containers is not sufficient enough. In fact, it's highly inaccurate but can be a first indication. Take a water sample using a syringe at various places in the rock wool. The best spot to conduct a correct pH and EC measurement is directly at the roots. For instance, if you don't give them enough water, you'll notice that the respective values change quickly. You can measure the various samples from the slab individually before putting them all together in a measuring cup (and shake well), then you can measure the sample regarding pH and EC value. To measure all the slabs in the way described above is rather cumbersome, so it is best to choose a number of reference slabs. Oddly enough, you will see that the slab with the lowest pH value will maintain that value throughout the whole harvest. The importance of watering the rock wool well is emphasized here once again. You'll need to keep a close eye on this slab. Let it be a reference point to determine if, and when, you have to rinse them all. The moment of rinsing has come when the pH value derives 0.5 from the pH of your last feeding solution. Or when a similar deviation occurs with the EC value. When have you rinsed long enough? You can stop rinsing on the moment the value in the slab has the same value as the rinsing water, a pH of 5.5 and a feeding value of about 0.5 EC. If that means you have to make another batch then do so. Citric acid If you look at the photosynthesis from a micro biologic perspective, you will see that there's one specific type of acid that plays a key role here. I'll spare you all the details, but in a nutshell it comes down to this: we know that citric acid triggers the chain reaction called photosynthesis. After that, a whole range of reactions occur, like the creation of sugars and carbohydrates etc. Citric acid is created as a result of all that and, there you have it, the circle is complete and a new reaction can begin. So if you routinely rinse, you could use citric acid instead of phosphoric acid. At the grow shop you can buy a special type of citric acid which you can use as a booster during the flowering period. You can also choose from products like a Marijuana Booster, in which citric acid is mixed in order to speed up the photosynthetic process. That means that you stop adding nutrition to the water for more than a week in advance, but you have to make sure that you keep the pH values of the water between 5.5 and 5.8. This way, the marijuana plant will absorb the last nutrition from the slabs. More importantly is that the plant, that stores food reserves just like humans, is forced that way to use up these reserves before it's harvested.