Hey
@budsofgeorgia - How are your plants doing now?
I've had necrotic leaf tips a few times that looked just like what you're having. I suspect more than one thing can cause it. What probably confuses the issue sometimes is when the problem is due to two or more things together.
Seriously I could fix them overnight by simply transplanting into fresh soil
Perhaps, but doing so wouldn't help us learn what was wrong with the reused soil. It would only support the theory that there was something wrong with the soil, and we already suspect that's the problem. If we can figure out what the exact problem is, we can save some money by properly amending reused soil. That's my goal.
Anyway... I had trouble with reused soil this grow. Only one of three plants had necrotic leaf tips. I believe it's because I over adjusted the nutrients. It was also in a different tent for a while with a light I'm using for the first time. So, it's hard to say what exactly caused the problem. It's doing better now. Keeping the VPD lower and moving the humidifier also helped that plant. After I harvest, I'm going to rethink how that tent is configured.
Overall, though, I believe finding the right mix of nutrients to amend to reused soil is the key. I'm thinking that because the soil was last used for flowering, and the last thing I do is stop feeding the plant, then the most depleted nutrients are probably what the plant used most near the end of its life, and thus those nutrients are the most needed amendments. Phosphorous and potassium would be the most likely candidates. Phosphorous deficiency in particular is known to cause slow growth. So, as I was trying to solve the nutrient problem, I supplemented flowering nutrients during vegetation. That seemed to help. They're flowering well now and looking good, but I'm sure the yield will be low due to the slow growth during vegetation. I have some clones growing in a new soil mix that tests this line of thought.