Phosphorus is the macronutrient at the heart of energy transfer: it forms ATP, nucleic acids and the phospholipids in every cell membrane. Plants take it up as phosphate ions (mainly H2PO4- in the usual root-zone pH range). It is mobile, so deficiency shows on older leaves first.
In practice
Deficient plants grow slowly, with dark, dull or purplish older leaves and sometimes red petioles. Cannabis trials put the useful range roughly at 25-60 mg/L (ppm) P in solution; above that, yield and cannabinoids plateau and the extra simply leaves in the runoff. Excess P can also tie up zinc, iron and copper.
Labels state phosphorus as P2O5. Elemental P = P2O5 × 0.436. A "0-52-34" salt contains about 22.7% P.
Don't confuse with
- Phosphate: the ionic form the plant absorbs, not the element.
- Purple stems from genetics or cold: anthocyanin colour is not proof of P deficiency.
Myth check
"More phosphorus in flower means bigger buds" is not supported. Independent cannabis trials found a plateau past moderate P (Evidence: Established for the plateau). See bloom boosters and Mythbusters.
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.Phosphate is the ionic form in which plants absorb phosphorus, mainly H2PO4- at the normal root-zone pH of 5.5-6.5.The pH scale runs from 0 (strongly acidic) to 14 (strongly alkaline) with 7 neutral; it is logarithmic, so each whole unit is a tenfold change in acidity.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.The stalk that attaches a leaf to the stem at a node; in cannabis it carries all the leaflets of a palmate leaf.A cannabinoid is any compound that binds the body's cannabinoid receptors or shares the typical cannabis terpenophenolic structure; the plant-made ones are phytocannabinoids such as THCA and CBDA.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.Monopotassium phosphate (MKP, KH2PO4) supplies about 22.7% phosphorus and 28.7% potassium with no nitrogen, and is the base of most PK boosters.A bloom booster is a high-phosphorus, high-potassium additive sold for flowering; cannabis trials show a P plateau, so past moderate levels it adds cost and runoff, not yield.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.
