LEMON KUSH HEADBAND First Grow

Day 78, Plant 1, adjusted LST to bring colas sites down lower, this plant is starting to take off. Going to put it into flower in the next couple days. Starting to show some signs of magnesium deficiency with small brown spots on older leaves and some yellowing in the centre of leaves. I've been lazy and haven't checked the ph of my water. When I tested it, it came up at 7.5. I think this is the issue and not a lack of nutrients in the soil. Used some ph down to get it to 6.5 for next watering to see if this corrects the problem. Plant 2, chugging along. Not absorbing water as quick as plant 1 and the Granddaddy Purple. However, still looks healthy and hoping it produces some quality bud.