@Ferenc, I order the light mix soil by biobizz which i think the pH its pre-buffered. But i don't know for sure. I also have some nutrients by biobiz so it migh helps.
@Royalgorillaxx, Don't you think it is too low and the plant can't absorb certain nutritients so stunt the growth? Too acidic?
Should be between 6.2-6.8 for soil i think but I might be wrong. I stopped checking. Just buy new soil time to time and that is working properly. Never use old soil again I have learnt from my previous mistake(s).