is it a potassium deficiency? or Cal Mag deficiency ?

Hey Growmie, This is a potassium deficiency, my friend. With a potassium deficiency, you’ll generally see symptoms on older leaves, but not always. Sometimes you’ll see the symptoms at the top of the plant. Leaves with a potassium deficiency get yellow, brown, or burnt edges and tips. The burnt edges may look a little like nutrient burn, except the affected leaves also start turning yellow in the margins. The reason most growers see potassium deficiencies is because potassium is best absorbed at lower pH ranges. When the pH gets too high, your plant may exhibit signs of a potassium deficiency even if it’s physically there near the roots. In soil, potassium is best absorbed by the roots in the 6.0 – 7.0 pH range but I would try to get it around 6 - 6.5. Hopefully, I answered your question and remember Happy Growing and Free your mind one puff at a time.