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Phosphorus and Potassium in Cannabis Production – Nutrient Science 

 

It is important that growers don’t overuse P and K in cannabis production.

 

Similar cations compete for uptake with other cations. As a result, an excess of one nutrient can result in nutrient antagonism, also called “lock out”, of one or more other nutrients.

 

By way of example, the cations potassium (K+), calcium (Ca++) and magnesium (Mg++) can antagonise each other when one of them is present in solution in excess. It is, therefore, important to understand that the overuse of potassium will lock out calcium and magnesium resulting in suboptimal crop nutrition and yield losses.

 

Additionally, excessive phosphorus (P) in solution can reduce iron and zinc uptake.

 

Other than this, excess P can potentially interfere with Ca uptake, although the interaction between calcium and phosphate is complex because these ions both support and counteract each other. The supporting effect is due to a simultaneous uptake and translocation of calcium and phosphate. The counteracting effect is caused by potential precipitation of less soluble calcium phosphates at the vicinity of nutrient-absorbing roots.

 

Fertilisers are not plant food – more does not equal more!!

 

It is important that growers understand fertilisers are not plant food. They are no more food for plants than iron, calcium or zinc supplements are food for people. Green (autotrophic) plants produce their own food through photosynthesis, a process that utilises the energy of the sun or artificial light to create sugars (photosynthates) from carbon dioxide and water. If you need to think of plants eating something, their food is light because light is the energy of photosynthesis. Either way, the food (sugars) produced by plants during photosynthesis is combined with plant nutrients to produce proteins, enzymes, vitamins, and other elements essential to growth. Despite this, the terms “plant food” and “feeding” are all too commonly used when discussing hydroponic nutrients and additives. This has led far too many growers to believe that the more fertilisers (or the more of certain fertilisers) they provide to a plant, the fatter and heavier that plant will become. This is a myth, generally circulated by those with interests in selling overpriced hydroponic nutrients, or by those who have a very limited understanding of basic plant science.

 

P and K additives are widely overused by many indoor hydroponic growers

 

There have been several recent studies that have demonstrated cannabis requires far lower, particularly K, but also P, than has been widely promoted as optimum. That is, often when lab analysing hydroponic feed solutions, based on hydroponic industry manufacturer feed charts, there is often more than 120 ppm of P and more than 300 ppm of K in the feed solution.

 

K Supply – The Science 

 

Optimal K in cannabis production appears to be cultivar specific.  [1]

 

However, where K supply during flower (the reproductive phase) is concerned, a 2022 study where two cultivars of medical cannabis were grown hydroponically (perlite substrate) in controlled environmental conditions, under five levels of K supply: 15, 60, 100, 175, and 240 mg L−1 K, found:

 

  • Plant function and biomass yield production were impaired under a low K supply of 15 mg L−1 (15 ppm)
  • K over-supply damage was caused at 240 mg L−1K in one cultivar, suggesting 60–175 mg L−1 K (60 -175 ppm) as the optimal range for the cultivation of medical cannabis.
  • No beneficial effects were observed for the elevation of K supply from 60 to 175 mg L−1K C for cannabinoid and terpenoid content
  • A significant decline in Ca and Mg concentrations with the elevation in K supply demonstrated a competitive interaction between K and these cations for plant uptake
  • Plant response to K supply at the flowering phase was in accord with the response formerly observed for the vegetative growth phase in a previous study by the same researchers [2]
  • The deficiency response under low K supply was more moderate at the flowering stage than at the vegetative stage. [3]

 

This study also found that low/deficient (and not high) K during flower increased cannabinoid production due to nutrient stress.  This said, there is an inverse relationship between cannabis yield and potency, with cannabinoid concentrations decreasing on a %w/w basis as plant inflorescence yield increases, consistent with what is termed the “yield dilution effect”. Therefore, growers should, first and foremost, strive to provide nutrients at levels that promote optimal flower yields/weight relative to cultivar. In short, inducing any form of nutrient stress, whether deficiency or excess, will likely result in reduced flower/weight yields (while potentially increasing cannabinoid production on a %w/w basis). [4]

 

In another study (2021) that looked at the response of a high delta-9-tetrahydrocannabinol (THC) Cannabis sativa cultivar “Gelato” grown in deep-water culture with different nutrient solution treatments varying in their concentrations (mg L–1) of N (70, 120, 180, 250, 290), P (20, 40, 60, 80, 100), and K (60, 120, 200, 280, 340) the authors found:

 

  • Cannabis inflorescence yield responded to increasing N and P supply but did not respond to K within the tested range.
  • Thus, K within the tested range of 60–340 mg L–1 had no effect on inflorescence yield.
  • This lack of response (at least with Gelato strain) suggests that 60 mg L–1K is not low enough to cause nutrient deficiency, and 340 mg L–1K is not high enough to cause toxicity. however, given no increase in inflorescence yield occurred above 60 ppm K, 60 ppm of K was sufficient to promote optimal flower yield in Gelato.
  • There were no nutrient treatment effects on the inflorescence cannabinoid content. I. e. all the nutrient treatments resulted in about the same cannabinoid content in flowers. Thus, 60 ppm of K resulted in the same levels of cannabinoids as higher K treatments. [5] The bottom line is slamming cannabis plants with high K will neither increase yields nor potency. In fact, quite the opposite is likely true.
  •  

These studies demonstrate 1) Optimal K is cultivar specific and 2) there is a wide tolerance range for K in cannabis; however, as K in solution increases, Ca and Mg in the tissue decreases (provided Ca and Mg in solution remains the same while K is increased).

 

P Supply – The Science  

 

Optimal P supply for cannabis appears to be cultivar specific.[6]

 

A recent study conducted by Bevan et al., found P at approximately 59 mg L-1 (59 ppm), produced maximal inflorescence yield in Deep Water Culture (DWC) recycling where the feed solution was replaced once weekly. The authors of this study concluding on P supply; “Many commercial cannabis cultivation operations currently use fertilizer formulations that contain very high levels of P (more than 200 mg L-1 P in some cases). This practice is based on anecdotal evidence that P enhances inflorescence production. These concentrations are much higher than the optimal rate of 60 mg L-1 P found in our study, and at the higher range could cause reduction of both plant growth and inflorescence yield.” (Bevan L, Jones M, Zheng Y. 2021)

 

Another P and cannabis study tested two commercial medicinal cannabis cultivars, “Royal Medic” (RM) and “Desert Queen” (DQ) (Teva Adir Ltd., Israel), representing two chemotypes—(i) high THC and low CBD (DQ) and (ii) balanced THC and CBD (RM). The plants in each group, which were grown hydroponically, received one of five P concentrations (5, 15, 30, 60, and 90 mg L–1 P), and short photoperiod was applied (12/12-h light/dark) using high-pressure sodium bulbs (980 μmol m–2s–1) for 63 and 68 days for DQ and RM, respectively. The study found that to achieve maximum inflorescence yield, a minimum supply of 30 mg L–1 P, and an optimum P supply range of 30–90 mg L–1 P (30 – 90 ppm or mg/L) was recommended for cannabis production. [7]

 

The authors of this study also found the amount of cannabinoids produced per plant increased with P (5, 15, 30, 60, and 90 mg L–1 P) in RM for all cannabinoids tested. However, in DQ, such an increase was apparent only up to 30 mg L–1 P supply.

 

Another study which tested P concentrations of 3.75, 7.50, 11.25, 15.0, 22.50, and 30.0 mg·L−1 (ppm) in Hemp (low THC, high CBD genotype) concluded that cannabinoids didn’t increase above 11.25 ppm of P. However, plants grown with a higher rate of P fertility (30.0 mg·L−1) had greater plant width and may result in more buds per plant. The authors concluding (perhaps open to interpretation given other data and “the yield dilution effect”); “These results indicate that, if a crop is being produced under greenhouse conditions, specifically for cannabinoid production, an excessive P supply did not result in higher cannabinoid production” [8]

 

P and K Myth V. Science 

 

Myth #1: High P leads to higher cannabinoid production 

 

The Science.

This one still needs further investigation; however, it appears as if P supply and cannabinoid production is cultivar specific. For now, one study found in low THC, high CBD hemp, increasing P above 11.25 ppm had no effect on cannabinoids. However, another study found that where two high THC varieties were tested, cannabinoids produced per plant increased as P increased between 5, 15, 30, 60, and 90 mg L–1 P in one cultivar (Royal Medic) while no increase in cannabinoids was recorded above 30 ppm P in the other cultivar (Desert Queen). The authors of the latter study concluded optimal P for cannabis production was 30–90 mg L–1 P, which is lower than has been previously promoted by some.

 

Myth #2. High P leads to higher yields

 

The Science. Higher levels of P can lead to higher bud production up until a certain point. However, studies tell us that the levels of P (> 120ppm) that have long been promoted by the hydroponics industry are excessive and beyond what is necessary to achieve optimum yields. In fact, these excessive levels of P are likely reducing and not increasing yields.  In general, based on current scientific findings, cannabis seems to have similar P requirement to other flowering/fruiting dicots within the ranges of 60 – 90 ppm (during flower) for high THC varieties, dependent on growing methodology and cultivar (Shiponi and Bernstein, 2021 and Bevan et al., 2021).

 

Myth #3. High K leads to improved yields

 

The Science. Optimal K appears to be cultivar specific.  However, many feed charts tend to promote the overuse of K. Studies have shown that high K decreases concentrations of Ca and Mg in cannabis leaf tissue. Excessive K could, thus, lead to Ca and Mg deficiency resulting in lower and not higher yields.

 

Myth #4. High K leads to higher cannabinoid production

 

The Science.  Increasing evidence supports the involvement of macro and micronutrients in cannabinoid production (Shiponi and Bernstein 2021). However, no studies, to date, have found excessively high K increases cannabinoid production. In fact, one expert in the field of cannabis production has found that low/deficient (and not high) K during flower increased cannabinoid production due to nutrient stress (Bernstein).  This said, there appears to be an inverse relationship between cannabis yield and potency, with cannabinoid concentrations decreasing as plant inflorescence yield increases, consistent with what is termed the “yield dilution effect”. Therefore, growers should, first and foremost, strive to provide nutrients at levels that promote optimal flower yields relative to cultivar. In short, inducing any form of nutrient stress, whether deficiency or excess, will likely result in reduced flower yields.

 

Refs

 

[1] Saloner A, Sacks MM, Bernstein N. Response of Medical Cannabis (Cannabis sativa L.) Genotypes to K Supply Under Long Photoperiod. Front Plant Sci. 2019 Nov 18;10:1369. doi: 10.3389/fpls.2019.01369. PMID: 31803198; PMCID: PMC6876614.

[2] Saloner, A.; Sacks, M.M.; Bernstein, N. Response of medical cannabis (Cannabis sativa L.) genotypes to K supply under long photoperiod. Front. Plant Sci. 201910, 1369.

[3] Saloner A, Bernstein N. Effect of Potassium (K) Supply on Cannabinoids, Terpenoids and Plant Function in Medical Cannabis. Agronomy. 2022; 12(5):1242. https://doi.org/10.3390/agronomy12051242

[4] Saloner A, Bernstein N. Effect of Potassium (K) Supply on Cannabinoids, Terpenoids and Plant Function in Medical Cannabis. Agronomy. 2022; 12(5):1242. https://doi.org/10.3390/agronomy12051242

[5] Bevan, L., Jones, M., & Zheng, Y. (2021). Optimisation of Nitrogen, Phosphorus, and Potassium for Soilless Production of Cannabis sativa in the Flowering Stage Using Response Surface Analysis. Frontiers in plant science12, 764103. https://doi.org/10.3389/fpls.2021.764103

[6] Shiponi, S., & Bernstein, N. (2021). The Highs and Lows of P Supply in Medical Cannabis: Effects on Cannabinoids, the Ionome, and Morpho-Physiology. Frontiers in plant science12, 657323. https://doi.org/10.3389/fpls.2021.657323

[7] Shiponi, S., & Bernstein, N. (2021). The Highs and Lows of P Supply in Medical Cannabis: Effects on Cannabinoids, the Ionome, and Morpho-Physiology. Frontiers in plant science12, 657323. https://doi.org/10.3389/fpls.2021.657323

[8] Cockson P, Schroeder-Moreno M, Veazie P, Barajas G, Logan D, Davis M, Whipker BE. Impact of Phosphorus on Cannabis sativa Reproduction, Cannabinoids, and Terpenes. Applied Sciences. 2020; 10(21):7875.

 

 

 



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