Phosphorus (The “P” in N-P-K) is considered a macro-nutrient because it is essential to almost every aspect of healthy plant growth including photosynthesis, respiration, and formation of complex chemical compounds.
During photosynthesis, plants manufacture their own food. A planet-wide system called the Phosphorus Cycle transforms minerals with the help of specialized bacteria into available soluble forms which plant roots can access under proper conditions of pH, moisture levels, and total dissolved solids.
Like nitrogen, phosphorus is considered “mobile” in plants and can be transferred to newer growth high in the canopy in times of scarcity. At first glance, it can be difficult to diagnose phosphorus deficiency in Cannabis since, as with nitrogen, there may be a yellowing and dying of the lower leaves which progresses upwards. The telling factors include green veins (even though the leaf material has yellowed) along with some blue or bronze tinting, mostly on the undersides of the leaves. There can also be some purpling of the stems, but this can be misleading since some strains develop this naturally, especially with abundant sunshine.
Cannabis requires varying levels of phosphorus for optimum vigor at distinct life stages. Too much can cause nutrient lock-up of iron and zinc, resulting in deficiencies of these important micro-nutrients. Too little can result in a general malaise with yellowing leaves and pitiful bud formation. Phosphorus availability is critically important in the flowering phase. Let’s review those “Cannabis industry standard” N-P-K ratios for the most important life milestones:
- Clones / Seedlings 1-2-2
- Vegetative Growth 3-1-2
- Flowering 1-4-5
- Ripening 0-2-0
Bat guano and bone meal are two examples of organic substances rich in available phosphorus. Specialized bloom formulations of granules, concentrates or nutrient pre-mixes contain high concentrations of phosphorus which may be especially valuable in cooler weather when natural absorption slows. Remember that 100 pound bag of granules with an N-P-K label certifying a guaranteed analysis of 8-8-8 from our chat about nitrogen? Whereas nitrogen as a salt or organic compound is “available ” to the plant, phosphorus and potassium are usually packaged in mined rock mineral form which must be broken down, usually by symbiotic mycorrhizal fungi, to become available to the plant. So for instance in the case of phosphorus the bag usually contains phosphate rock expressed as an equivalent quantity of phosphorus pentoxide (P2O5) which when broken down results in a little more than 4.5 pounds of material from your 100 pound bag of 8-8-8.
Adding mycorrhizal inoculant products such as great white, mykos, or rootella to soil grown Cannabis might seem expensive, but it pays you back with water efficiency, tilth, and phosphorus availability which results in bigger buds for less fertilizer application. These good fungi and their hyphae essentially extend the root system of obligate mycotrophs, like Cannabis, by a factor of ten to as much as one hundred!
Disclaimer: Any advice and opinions offered about the cultivation of cannabis by Bruce N. Goren are his own and do not represent the University of California or the Master Gardener Program.