Phosphorus role and signs
Jobs
Root and stem growth, seedling vigor, and the reproductive stage
Mobile
Older leaves show trouble first
Look
Slow, stunted plants with small, dark, weak leaves that pick up dull blue or purple tones
Edges
Leaf margins tan or brown and curl down as the shortage moves inward
Stems
Petioles may go purple or red, but some cultivars do that naturally
Phosphorus in practice
Flower demand
Low phosphorus in bloom cuts yield
Cold lock
Below about 50 F (10 C), phosphorus uptake gets difficult even when the bottle is correct
Soluble bloom
Water-soluble high-P formulas help in cool weather
Do not flood
Excess phosphorus can burn plants or make harsh, overfed flowers
Slow sources
Rock phosphate and greensand release P gradually and suit prevention more than rescue
Potassium role and signs
Jobs
Water transport, photosynthesis, thick stems, disease resistance and bud development
Mild look
Plants can look tall and vigorous while lower leaf tips and edges tan, die or spot
Worse look
Chlorotic mottling, red and yellow patches between green veins, red stems, then smaller leaves
Mobile
Damage starts on older fans. Dead tissue does not recover
Excess
Too much K yellows or whites the tissue between veins
Why potassium runs short
Organic gap: Guano, fish emulsion, alfalfa, cottonseed, blood meal and many manures are light on K versus N and P.
Cold: Low temperature slows potassium uptake.
Competition: Extra calcium or ammonium, and high sodium, push potassium aside.
Hydro note: True K shortage is less common in hydro than in soil or potting mix.
Potassium fixes
Bloom bottles
Flower formulas are usually high in K and also use alkaline K to balance acidic P
Fast sources
Liquefied kelp, wood ash, potassium bicarbonate, potassium sulfate and potassium dihydrogen phosphate
Silicon pair
Potassium silicate supplies silicon plus a little K and helps cell walls
Slow sources
Granite dust and greensand prevent shortage more than they rescue one
Timeline
Fast products show healthier new growth in four to five days