Potassium supports thick stem growth, photosynthesis, water restoration and disease resistance and can be found in the entire plant, necessary for all actions dealing with water transportation. This is necessary in all stages, but mostly in bud development. Retarded growth with scorched tips and edges on leaves is a sign, as well as stretching plants that become thin and brittle. This is confused with iron, but with potassium the tips of leaves curl while edges burn and die.
Older leaves may show red color and curl upwards. Dead patched (Necrosis) may appear on larger fan leaves, killing them and turning them brown. Older leaves also show different patches of color (mottle), turning yellow between veins, followed by whole leaves turning dark yellow and dying. Overall growth is slow in vegetative and during flowering. Your first sign is dark edges around the leaves.
Really low humidy is a major cause since your potassium is sweat out through the plant to keep it cool. Too much calcium or ammonium nitrogen as well as cold weather make it harder to be absorbed as well as too much sodium causing it to be displaced. Effects will mainly be on older leaves and margins. Too much potassium leads to salt damage and acid fixation of roots as well as calcium deficiency.
Leaves will show a light to dark yellow color between veins. Molecular imbalance can cauce reduced uptake and lead to deficiencies magnesium, manganese, zinc, calcium and iron. Potassium absorbs best around pH levels of 6-9.5 in soil and in soilless mediums between 4.7-5.3 and 6.7-8.5, try not to go over 6.5
Peters All Purpose plant food 20-20-20, Miracle Grow tomato plant food, Miracle Grow All purpose plant food, wood ashes, which are fast absorption, kelp meal, which is medium absorption, Greensand, which is slow absorption, and granite dust, which is slow absorption will do. sulfate of potash, sulfate of potash magnesia, and muriate of potash are medium absorption. FOXFARM GROW BIG HYDROPONIC CONCENTRATE, is fast absorption and can bring your pH down .Earth Juice Meta-K, which is fast acting (can bring down your pH as well). Leaves will never recover, but the plant will show recovery after about 4 to 5 days when using a fast acting nutrient.