Phosphorus (P)

Marijuana Horticulture

by Jorge Cervantes

Cannabis uses the highest levels of phosphorus during germination, seedling, cloning, and flowering. Super Bloom fertilizers, designed for flowering, have high levels of phosphorus.

Phosphorus is necessary for photosynthesis and provides a mechanism for the energy to transfer within the plant. Phosphorus – one of the components of DNA, many being enzymes and proteins – is associated with overall vigor, resin, and seed production. The highest concentrations of phosphorus are found in root growing tips, growing shoots, and vascular tissue.

A lack of phosphorus causes stunted growth and smaller leaves; leaves turn bluish-green and blotches often appear. Stems, leaf stems (petioles), and main veins turn reddish-purple starting on the the leaf’s underside. The reddening of the stems and the veins is not always well pronounced. The leaf tips of older leaves turn dark and curl downward. Severely affected leaves develop large purplish black necrotic (dead) blotches. These leaves later become bronzish-purple, dry, shrivel up, contort, and drop off. Flowering is often delayed, buds are uniformly smaller, seed yield is poor, and plants become very vulnerable to fungal and insect attack. Phosphorus deficiencies are aggravated by clay, acidic, and soggy soils. Zinc is also necessary for proper utilization of phosphorus.

Deficiencies are somewhat common and are often misdiagnosed. Deficiencies are most common when the growing medium pH is above 7 and phosphorus is unable to be absorbed properly; the soil is acidic (below 5.8) and / or there is an excess of iron and zinc; the soil has become fixated (chemically bound) with phosphates.

Treat deficiency by lowering the pH to 5.5-6.2 in hydroponic units; 6 to 7 for clay soils; and 5.5-6.5 for potting soils so phosphorus will become available. If the soil is too acidic, and an excess of iron and zinc exists, phosphorus becomes unavailable. If you are growing in soil, mix a complete fertilizer that contains phosphorus into the growing medium before planting. Fertigate with an inorganic, complete hydroponic fertilizer that contains phosphorus. Mix in the organic nutrients – to add phosphorus to soil. Always sue finely ground organic components that are readily available to the plants.

Toxic signs of phosphorus may take several weeks to surface, especially if excesses are buffered by a stable pH. Marijuana uses a lot of phosphorus throughout its life cycle, and many varieties tolerate high levels. Excessive phosphorus interferes with calcium, copper, iron, magnesium, and zinc stability and uptake. Toxic symptoms of phosphorus manifest as a deficiency of zinc, iron, magnesium, calcium, and copper; zinc is the most common.

Treat toxicity by flushing the growing medium of affected plants with a very mild and complete fertilizer. Severe problems require more water to be flushed through the growing medium. Flush a minimum of three times the volume of water for the volume of the growing medium.