Ok. From here, you can see the yellowing lower leaves. However, notive their overall health. No tip burn or curl anywhere on the plant that would indicate K def. The leaves are all healthy and standing at attention. With K def, it affects the stomata, which is why you get that curl like wind burn or overwatering, if you've seen that. K tox will ghost your tips. They'll start out as an ashy gray and move to a brown burn. The gray is subtle, but it's there if you look.
We know it's not N because the color variations. N tox would cause the entire leaf to be dark green and the tips would take a bow. It's a bow because at least you didn't go too far and roast them. They recover in that state. It could be N def, but no other symptoms suggest anything of the nature.
If it were P def, there would be splotches on the mid and lower leaves from the breakdown. The leaves would become weak and waffly, if that's a word. Since it's P tox, there is an over abundance of stored energy for the plant which is responsible for the dark green edges. Progression of this tox will lock out K next and lead to K, Ca, Mg, and Fe deficiencies.
