Plant is starting to get this on the old growth what do I do?
I had this happen to me recently, you really have to think about what makes sense here… I started researching plants and nute problems in general, aside from cannabis. My brown spots showed up and I initially thought it was some type of a deposit in the leaves, so I flushed and assumed there were still nutes in the soil. WRONG, once my leaves started to yellow I knew it was a N deficiency, and the brown spots were a P and/or K deficiency. SO i prepared a weak solution and sprayed the leaves the next morning before turning my lights back on. I pruned the damage leaves to rid them of the spots and see if anymore showed up… within 3 days the leaves were green and no more spots appeared. I tend to under fertilize the first few weeks and get a feel for my new strains ability to absorb nutes. they all work so differently it amazes me. Even within the same strain each plant has its own regimen. Hope this helps. Just remember to give them a little shot of nutes every time you water (2-4 days apart). Mainly macro nutrients are the ones that need replenishing more often than you think, but don’t overdo IT! be gentle!
Frank…Welcome to ILGM this is by far the best forum on the net. Go and read some posts and you’ll feel right at home…lol
Anyway feel free and ask
B Safe
Will
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You said this was older growth. In the latter stages of flower Fan leaves start to deteriorate and turn yellow as the plant recognizes it is finishing, and the plant uses the stored nutrients from these leaves to live off.
One reason you may recognize this a s a N def. is because we use less N in flower, and the plant’s natural process will draw from the leaves what it is not getting 
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I believe it is a good idea to use CalMag throughout veg stage, and perhaps a week or 2 into flower. Remember that the sooner you find that you can eliminate certain minerals; The better the yield will taste.
Something that I believe in under certain circumstances and only when needed to save a plant is constantly mention on this forum. Flushing.
Flushing is OK to do once a month; Or, during the transition phase but, is not necessary in most potted cannabis grows.
If you provide fresh nutrients ph’d at 6.5, Once; And 1-2 watering periods, with pure ph’d water “6.5”; You should not have to worry about flushing.
Too many times growers get into trouble because they get too proactive. Sometimes patience proves to be a better formula for grow success. i.e. Flushing after an issue occurs can lead to more issues when the plant is over watered. This further confuses the issue.
Food for thought
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Thank you… so if I use cal Meg it should solve my problem?
How do I get the water and nuts down to 6.2 or 6.8 ? Keep adding plant food to water till it gets there?
Hi, it’s a little over a month in veg… the pH of my dirt is 7.0 water is 7.3 … it’s a G13 plant
You dont use plant food to adjust your pH, you use pH Up and Down.
There are also a couple of home recipies like vinegar people use but I know nothing about them
-good luck