Potassium is a macronutrient taken up as the K+ ion. It is not built into structures; it stays dissolved in the cell sap, where it regulates osmotic pressure, opens and closes stomata, activates dozens of enzymes and moves sugars through the phloem. It is highly mobile.
In practice
Deficiency starts on older and middle leaves as yellowing and then brown, crispy margins and tips, sometimes with rust-like spotting. Hydro feeds commonly carry about 150-300 mg/L (ppm) K. Saloner and colleagues found cannabis needs less K than many bottle labels suggest, and that very high K mostly crowds out calcium and magnesium uptake rather than adding yield.
Labels state potassium as K2O. Elemental K = K2O × 0.830. Potassium nitrate (13-0-46) is about 38% K; monopotassium phosphate is about 28.7% K.
Don't confuse with
- Nutrient burn: tip burn from overall high EC, not specifically low K.
- Magnesium deficiency: interveinal yellowing on older leaves; K deficiency hits the leaf edges first.
- Potash: a trade word for potassium salts, not a separate nutrient.
A macronutrient is an essential element plants need in large amounts, measured in tens to hundreds of mg/L: nitrogen, phosphorus and potassium, plus calcium, magnesium and sulfur.Microscopic pores, mostly on the underside of leaves, each opened and closed by two guard cells to control gas exchange and water loss.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.Potassium nitrate (KNO3) is a soluble fertiliser salt supplying about 13% nitrogen as nitrate and 38% potassium, used in both veg and flower feeds.Monopotassium phosphate (MKP, KH2PO4) supplies about 22.7% phosphorus and 28.7% potassium with no nitrogen, and is the base of most PK boosters.Nutrient burn is browning and crisping of leaf tips, then margins, caused by a root-zone salt concentration (EC) higher than the plant can handle.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.Magnesium (Mg) is a mobile secondary nutrient at the centre of every chlorophyll molecule; deficiency causes interveinal yellowing on older leaves.Potassium sulfate (K2SO4) supplies roughly 41-45% potassium and 17-18% sulfur with no nitrogen, useful for adding K while tapering N in late flower.Nutrient antagonism is when an excess of one element reduces the uptake or use of another, such as high potassium suppressing magnesium and calcium.The guaranteed analysis is the legally required label statement of minimum nutrient content, giving P as P2O5 and K as K2O; multiply by 0.436 and 0.830 for elements.
