What kind of ph test did you use? The liquid test kits (color-coded results) are skewed by using tinted water, and those cheap soil probe meters are pieces of crap that are not trustworthy. Were you really getting 5.0 readings, all leaves would start to turn bright yellow...and it would happen fairly fast. Are you using any other additives? (CalMag, Superthrive, Sweet, molasses...) Spraying them with anything?

What size pot are they in, and when did you last transplant? (how long have they been in that same soil? Four weeks is a tad deep into flower, but if they're rootbound, transplant carefully...right now, using care not to harm the roots too much. Sometimes you gotta do, what you gotta do. :jointsmile:

If above questions are not helpful, fill-out the troubleshooting form as completely as possible. The more info you provide, the better the possibility of an accurate diagnosis. :thumbsup:
Rusty Trichome Reviewed by Rusty Trichome on . Low Soil PH.........How can I raise it ? Hi guys, I was hoping someone could help me this, I've got two plants about fours weeks into flowering. Both are are yellowing rapidly and have been for about two weeks now, apart from that they seem fairly healthy, buds developing nicely and the fan leaves sticking straight out. I know the leaves start yellowing during the flowering cycle, it just seems its happening a bit too soon. They are 18 inches tall, 1 gallon pots in FFOF, I know my pots are a bit small and that could be contributing to Rating: 5