There are purple petioles as well on some of the upper fan leaves..
This is tricky because your marijuana will exhibit vigorous growth so excessive height may be your first clue. Sometimes the tips of the large lower leaves will turn brown and die. Spots and areas of dead tissue (necrosis) may form on the blades, particularly on the edges. Leaves may be pale or yellow with red petioles and stems. Branches will break easily. Treat with a high K fertilizer. Recovery will be slow. New growth will not have the red color, and leaves will stop spotting after a couple of weeks.
What are your temps? In the winter, purple petioles are more often a result of cold than potassium deficiency. I would not arbitrarily decide to give a big does of K unless I knew that was what was wrong with it.
I do see signs of nute burn also. What is a "good dose" of bloom nutes? Are you on a regular feeding schedule? If you are growing organic and have been growing organic the entire way through (this means NO non organic nutes), pH shouldn't be a problem. However any non organic anything that you give plants could mess with the microbes in the soil and will cause problems.
A good dose was 1 tbl spoon per gallon of earth juice bloom, I am on a feeding schedule of sorts, just waited too long with these auto's going into flower so quickly. Temps seem to be good, 70' on the floor of the tent, mid 70's on top of the pots below the canopy. It is hard to tell temps ontop of the canopy, but my temp gun reads 78 on the plants. I planned on using meta-k (potassium nutes) a few feeds earlier, but I never did. The plants would of got some K from the 2-1-1 grow and the 0-3-1 bloom so I wasn't overly concerned, I thought I could add it later if I had to sort of a thing. Apparently that was a bad choice on my part
edit: 1tbl spoon / gal = 4ml/L so they got half a tbl spoon worth.
Both mixes I also added some EWC, and aerated them for about 36-48hrs. Before those 2 feeds, they were on a light veg feedings. I started out with .5ml of the grow and slowly increased it to 2ml/L, I added xatalyst in approx the same portions, as well as .5ml microblast every other feed. They have also had a few waterings with hygrozyme, mostly when they were younger.
I am somewhat inclined to think that those couple leaves that are deep in the lower part of the plant are older leaves that will naturally die off as the canopy gets bigger and shades those leaves. That is something that will happen to the healthiest plants. however, the little bit of yellow tips along the margins of the leaves could be a sign of a problem. If it is on only 1-3 leaves at the very bottom of the plant, I would say do nothing different and wait. Watch the leaves to see if this continues to occur on newer leaves and/or fforms a pattern such as all of the lower half of the plant's fan leaves begin to show this symptom. That will tell us if there is an actual problem or not.
So, LongTimelisnr1stTimecallr it has been awhile since you last posted. Welcome back.....what have ya been up to. By the way BigSur is our local Cal/mag specialist. Maybe he will chime in with his vast knowledge on the subject.