Is this potassium definitely ? Week 8

This is fairly typical for cannabis at this stage of flowering and is nothing really to worry about and is often referred to as "the fade". Seeing as you are close to harvest, I would not be changing anything you are doing and she does not need any boosted/extra feeding to get her through to maturity. Just don't defoliate. What is occurring is that your plant is supplementing its supply of carbs, sugars, amino acids etc by taking these elements out of her older leaves to help and grow her flowers. It is more efficient to do this rather than grow flowers and make these elements from new during this energy dense period of growing flowers. Had the leaves been cut off, there would have been none of these "reserve" elements to be had and your flowers would have suffered due to this fact, a fact that defoliation advocates fail to understand. Try not to flush too early either, this just removes even more goodness from the soil. All that is really required for a flush is to give just plain water for the last 3 irrigations before harvest. In future you can give "grow" nutrients a bit longer before switching to bloom nutrients and an extra week or two of grow nutrients will keep your plants green for longer and will not affect your plants adversely. Personally I like to keep giving grow nutrients for the first two weeks of flowering, before changing to half grow and half bloom for the next two weeks, before then again changing to just bloom nutrients through to maturity. This seems to prevent any excess yellowing from occurring too early and boosts bud growth a bit too, with the slight increase of nitrogen that is available during those crucial first few weeks as the plant starts the flowering process. I have been doing this for over 20 years now and am convinced of its effectiveness. Others may disagree, but it works for me and I am sticking with it. Hope this helps,........ Organoman. (and hater of defoliation!)