https://www.growweedeasy.com/magnesium-deficiency-cannabis
Read this page...growweedeasy.com should already be a bookmark for every grower, if you haven't read there nutrient deficiency guides you should do so. It helps you spot certain nutrient problems before they become severe, and allows you to recognize classic symptoms like, for example, interveinal yellowing on the lower leaves of your plant, as a magnesium deficiency. I'm sure you could have another problem at play here, maybe related to the Mg def, maybe not. If you're in promix, which I've heard is actually peat and not really soil, you're going to want to make sure your PH is in the proper range. Typically, Mg deficiency related to PH is from the PH being to low. Not always, but typically. Peat turns acidic as well. If the PH is not in the correct range, make ammendments and adjust it, and wait to see if symptoms continue spreading. If they do not, problem solved. If they do, you can either add a calmag supplement or you can add a well balanced base nutrient like, but not limited too, Floranova Bloom or Flora Trio. If you use Floranova, don't bother with the grow bottle all that you need is Floranova Bloom all the way through, it is Lucas Formula in a bottle. Some of your leaves are very dark green, so I would only add a calmag supplement as a last resort. If you do need to do so, use something like CaliMagic that has very little N (1-0-0), as compared to Botanicare's CalMag Plus which is great, but it has twice the N present in it (2-0-0).
Good luck.
If it doesn't, then we will go from there!
