I'm not using micronutirents. Im using fox farm nutrients...And I was going 1/2 of what it says on the bottle. I just brought the nutrients up, just because its organic nutes and most likely won't harm my plant!
I think this problem was there for a while and maybe i never noticed, it does take about 3 weeks before phosphorus starts to appear in the leaf.
This is what I'm thinking here, there is no other deficiency to explain my problem.
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And you are now telling me this "iron"
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I'd like to help but honestly you keep assuming you've figured it out as a single, simple deficiency of phosphorous... But it makes no sense to have a simple phosphorous deficiency while in vegetative growth that's severe enough to cause leaves to die while you are using complete nutrients that have plenty of phosphorous (even if at a light dose).
I personally would either assume (like Snow Crash nicely wrote out) there is a more basic problem going on that is resulting in some elements or microelements to be unavailable to the plant. If it's due to an imbalance of elements building up in the soil and locking out other elements (like phosphorous), simply giving them stronger nutrients is going to make the problem worse or not help. Also, if you are not supplementing microelements while using distilled water there's a good chance they could use a boost and a slight deficiency of one or more microelements is a good possibility (read Fox Farm's recommendations if using distilled or tap water w/ their products if you don't believe me personally).
Also, there's a possibility that the soil pH has changed over time. Checking just the pH of the nutrient mix when you feed doesn't cut it. You'll need to test the pH of the run-off to know anything about what is going on at the root zone.
Good luck in figuring it out.