yellow leave in flowering CrystalCandy

Hi there Gino, it's not an NPK deficiency , you're right. This is a magnesium deficiency . That's because with super soil sometimes there is not enough soil bacteria to process the micronutrients. I know you are not feeding directly but you can fix this by adding calmag, or something to break up the micronutrients in the soil like enzymes , humic acid, ... In your case my best advice is to add beneficial bacteria like Bacilus subtilis or a product like hydro guard. This will increase the soil life which in turn should help solve the problem. Just remember in bio it takes a bit of time to see the results . Hope this helps ! 🚀