When a plant yellows from the bottom up, 99% of the time it's an N deficiency. It's actually very common. Even though you're in flowering the plant still needs/uses nitrogen. What's happening is the plant is cannibalizing it's self. It's feeding off it's stored up nutrients.
Myself, if I had a plant in flowering showing N deficiency and I was using chem fertilizers, I'd give her a 1/2 strength dose of Veg nutes. Be careful though - if your veg fert is real high in N, use something lower. It's a fine line though. Usually with mj less is better, but too little and the plant continues to starve. Too much burns and slows growth to almost a dead stop.
Gnatty Reviewed by Gnatty on . yellowing of leaves mid flowering Hi, that's my second grow and I have some yellow leaves all around my plant and I wonder why. I've read that it's normal in the end of flowering but not in the end of 4th week ? I posted some pics to let you see how it goes.. My first grow was better than this one. Rating: 5