The Best NPK Ratio For Cannabis in The Flowering Stage

I’m probably going to receive some heavy criticism for writing this article; if there is one thing I know about cannabis growers, we are all stubborn. At the time of writing, I am yet to find another blog providing new information on this subject; we’re stuck within an echo chamber where everyone regurgitates the same retired knowledge.

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Hi, my name is Jason. And I know what you’re thinking, “we already know the correct NPK proportions for cannabis during bloom; stop adding nitrogen and amp up the levels of phosphorus and potassium”. Well, I am here to be that guy to tell you to reconsider.

Now I understand that typically when we are attempting to grow fruits and flowers, adding excessive nitrogen is counter-productive. However, I feel like cannabis growers have taken this to an unnecessary extreme; we recognize nitrogen as a potential problem and have responded by removing almost all of it, which is too extreme, in my opinion.

Researching Cannabis Has Been Difficult

The legal restrictions surrounding cannabis cultivation mean it’s only recently that science has been able to tell us what NPK proportions our plants want.

Thankfully, researchers at the University of Guelph in Canada took steps to do exactly that. The paper is titled “Optimization of N, P, K for soilless production of Cannabis sativa in the flowering stage using response surface analysis” and provides data that all cannabis growers should consider.

The report acknowledges that the current community consensus on how cannabis plants should be fertilized (particularly in bloom) is flawed, based on little scientific backing.

The team was trying to figure out the ideal concentrations of each element, not just the NPK ratio. However, I’m sceptical of how I should process an “ideal concentration.”

I read my plant’s nutritional needs on a “plant by plant” basis and rarely try to adopt strict amounts; I am more interested in the NPK ratio.

Cannabis Potency Response in Regards to Nitrogen

The report cited previous studies investigating nitrogen application rates on cannabis grown in organic media. The highest application rate (284 mg/L) showed a 20% decrease in potency, but application rates between 226 and 57 mg/L did not affect the cannabinoid concentration.

A follow-up study confirmed that sentiment by reporting that buds from plants fed 160 mg/L instead of 30 mg/L of N contained approximately 30 % and 20 % less THCA and CBDA.

In both those studies, even though potency increased, the yield halves when comparing the highest and lowest nitrogen application rates.

The study of discussion (published in 2021) couldn’t confirm that higher nitrogen applications decreased potency even at the highest application rate. Given that the 2021 study is unique in that the researchers used a more modern and accurate analytical technique to gather the data, I think it is reasonable to hold the results from the more aged studies with a higher level of scepticism.

It is important to note that we discuss the ideal NPK ratio for maximum yield. Even though the recent study couldn’t confirm the losses of potency, it is still worth considering the possibility that the nutrient proportions may also affect the quality of the flower; perhaps the highest yielding NPK ratio won’t produce the highest quality crop? I don’t subscribe to that sentiment, but im not going to leave it out.

Cannabis Yield Response to Increasing Levels of Nitrogen Potassium and Phosphorus

So the first point to take away from the report was that cannabis yields did respond to increasing levels of Nitrogen and Phosphorus. But what’s important is the proportion in which these elements are used.

The team found the optimal range to lie within the concentrations of 160–230 and 40–80 mg/L of Nitrogen and Phosphorus, respectively.

Many commercial bloom fertilizers use a lot of Phosphorus, usually a lot higher than the amounts used in this study. It’s based on anecdotal evidence that increased phosphorus levels enhance bud production.

However, this data suggests that pumping our plants full of Phosphorus reduces plant growth and yield and contributes to environmental pollution.

The main takeaway from those results that I feel we should respect is that nitrogen is the element most in demand, even during bloom.

Furthermore, the team noted that for the specific strain they were growing, they narrowed the optimum amount of nitrogen and phosphorus to 194 and 59 mg/L, respectively. The more you deviated from these amounts, the more the yield suffered.

In regards to potassium, the results were a lot less clear. The team witnessed no effects on the yield within the concentration ranges they tested, 60–360 mg/L. Suggesting that amounts of 60 mg/L were not low enough for deficiency symptoms, and quantities of 360 mg/L were not high enough to cause toxicity.

Reports from other studies suggest that the yield response to potassium may be strain-specific; the study cited a report that noted one strain supplied with 240 mg/L of K had reduced shoot and root biomass compared to when it was fed only 175 mg/L. Conversely, another strain responded by increasing its biomass in the same conditions.

You Still Have to be Flexible With the Specific Amounts of Fertilizer you Feed your Cannabis.

It is difficult to recommend the optimum amount and proportion of nutrients to deliver to a crop. For cannabis, studies have focused on maintaining a fixed NPK ratio in different concentrations or feeding excesses of one element while keeping the others constant.

Neither of these methods can account for the interactions between the nutrient minerals, which also affect nutrient uptake. For example, high potassium levels can reduce calcium and magnesium uptake in cannabis, thought to be due to an antagonistic relationship between these ions.

Similarly, new growers often overapply fertilizers and or boosters. Instead of ending up with toxicities, they end up with deficiencies; if you overapply a particular element, it may lead to nutrient lockout; there needs to be a balance.

I wouldn’t say the concentrations reported here are perfectly optimized for all grow setups. I feel like these levels will vary somewhat depending on the strain and the substrate but can serve as a good approximation.

The Study used plants that were grown in DWC. Due to the unique interactions of various mineral solutions with differing growing media, the optimum nutrient levels will likely vary from substrate to substrate.

Similarly, different strains may have different nutritional needs and tolerances, which affects the optimal feed concentration. So we should remain flexible.

So What NPK Ratio Should you Aim for?

Now, if you want to get anal about it, you can use the concentrations advised in this Study to figure out the ideal NPK ratio to maximize your yield.

It should be between 10:3:18 and 10:3:3; with those proportions, you should be able to approach the levels mentioned in the Study to some degree.

It will be likely that the macronutrient profile of whatever fertilizer won’t match the optimal concentrations exactly, but you don’t need to be that specific.

You can use other bloom boosters to enhance the quality and quantity of your yields that work better than PK bloom boosters.

Beneficial microbe supplements are one that I recommend.




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Jason Maverick

Jason Maverick

Autistic and Opinionated

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