Nitrogen Source Matters: High NH4/NO3 Ratio Reduces Cannabinoids, Terpenoids, and Yield in Medical Cannabis

Great study:

I have found over years of testing it’s better to use less than more NH4. Plant’s can’t regulate their NH4 uptake, unlike NO3. So, NH4 kind of acts like plant mainlining:

giphy (16)

The study:

10% NH4 to total-N (NH4 + NO3) was the best of the worst regarding effects from NH4. The 10% NH4 of TN equates to a NO3:NH4 ratio of 9:1. Results at 10% NH4:

  • Increase in most tested cannabinoids and monoterpenes. The study shows no increase in THC, but that’s only THC; when we look at total-THC (i.e., eq. for THCa–>THC + THC), there’s a notable increase in side/bottom flowers.

  • Marginal positive and negative effects on tested sesquiterpene4s

  • Reduction in biomass

  • Variable effect (increases and decreases) in plant tissue nutrient concentration based on tissue type (flower, sugar leaves, sun leaf, stem, and roots)

I use 5.25% to ~6.25% NH4 to total nitrogen, dictated by my target NO3:NH4 ratio of 15:1 to 18:1. I have tested many NO3:NH4 ratios and have fried my fair share of plants. Anything close to NO3:NH4 of 6:1 (14% NH4) is dangerous territory. I never drop below NO3:NH4 of 12:1 (7.7% NH4).

Adding a bit of NH4 is beneficial for many reasons, such as stabilizing substrate pH, increasing CO2 fixation, and long-term CO2 use efficiency. Now, at low % NH4 we can add increasing some cannabinoids, monoterpenes, and modulating nutrient contenion in plant tissue.

It would be interstesing to correlate nutrient macronutrient ratios to plant tissue macronutrient ratios.

After reading this study, I will conduct a rigorous scientific study looking at the same metrics as this study (flowers, sugar lealves, and sun leaves) under 1%, 2.5%, 5%, and 7.5% NH4 ratios in a plant growth research chamber.


I forgot to mention the growth conditions. The extremely high VPD may have skewed the outcome:

Flowering photoperoid:

  • Lumainare = HPS
  • PPFD = 980
  • Temp = 25’C
  • RH = 38%
  • VPD-room = 1.97 kPa
  • VPD-leaf = 2.36 kPa (assuming a generous +2’C LST above air temp; a conservative estimate would be closer to +3’C, or even +4’C with low air velocity)
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Very interesting. I’ve always tried to exclude ammoniacal nitrogen completely but there’s always a bit present in technical grade nitrates. I wonder if it’s time to try a very mild increase to 5-8%

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The fertigation data should be taken in to consideration. It’s fubar.

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Damn, fubar is too kind a word, lol. Can I get some table salt with that flower and water fast, please? :roll_eyes:

I didn’t thoroughly read the paper yet; I only skimmed it. I found it an hour before I posted. I should have read it more thoroughly before posting. Thanks for pointing this out!

When combining that wacky nutrient profile with the insanely high VPD-leaf, it’s hard to draw much value from the study. I’ll dig into this weekend and see if I can draw any worthwhile inferences or data points.

In my past R&D on NO3:NH4 ratios, I had ideal VPD and a good nutrient profile in coco coir and rockwool. So I’m confident in recommending 15:1 (6.25% NH4) to 20:1 (4.75% NH4), but I wouldn’t go below 12:1 (7.7% NH4), that’s for sure.

Shoddy methods are the bane of research. So many cannabis studies use crap methods, throwing the whole paper into question. Researchers do themselves no favors when they don’t at least email expert growers asking for their opinion on the study design. It’s so easy to identify companies, consultants, or growers they could trust. It blows my mind they don’t conduct such due diligence; how long does it take to send a few emails FFS. To make matters worse, the authors of that paper co-authored a handful of recent studies on cannabis nutrition that have been discussed in this forum, like:


You have my vote. If you use PureCal (0% NH4) and ammonium sulfate, you hit any % NH4 or NO3:NH4 ratio you want.

There are many reasons to provide NH4 for indoor cannabis production, but don’t overdo it. I wouldn’t hit 8% NH4 to total-N at first (NO3:NH4 11.5:1). I’d suggest going with 5% (NO3:NH4 of 19:1) or, at most, 7.5% (NO3:NH4 of 12.3:1) because some strains are more sensitive than others. Plus, depending upon your substrate temperature, you could drive the substrate pH too low. Make sure to track your substrate pH (Pour Thru) or at least run off pH if you’re going to use 8% or higher NH4 to total-N.

Let us know how it works out for you! :pray:

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Yeah, that particular set of authors like to go all over the place. For some reason they like NaCl and they run obscenely low Ca whenever they do a N study, but sane Ca when they do a K study.

Several greenhouses out here in the desert run at 2.0+ vpd all summer. Takes a lot higher Ca:K ratio to get decent results in that environment.

Most of my client’s facilities run right at 6% Nh4. That typically gives a .1 pH rise through the medium.


I agree with increasing the Ca:K ratio under high VPD. I’d also increase B and other mass flow nutrients. I read a number studies a few years ago about mid-day depression of photosynthesis (caused by high PPFD, high temp, and low RH during the mid-day). A simple way to reduce or even eliminate the mid-day depression of photosynthesis is spraying the plants with water every so often (or pulling shade cloth) - I love simple and cheap solutions.

I agree with hitting 6% NH4, or about 15.6:1 NO3:NH4; that’s an optimal value IME.


I learned the value of high plant metabolism using Ca from @emdub27 , Grant Mahy and a consult with Dr. Daniel Fernandez as an overall theme.

10:1 NO3-:NH4+ keeps the pH more stable (Thanks, J. Benton Jones!), but I like that ratio @Ralf . I used to use a carbonate/citrate pH buffer, but realized that I can do the same thing with inputs like MKP while trying to hit target numbers.


At 10:1 I typically see falling pH in the medium.

In the commercial stuff with relatively inexperienced operators I much prefer to see a slow rise. That allows us to run a lower pH in the pipes to keep the orthophospate speciation H2PO4 dominant and therefore avoid reactions with other anions. Not a concern if running polyphosphate.


Great comments!

Thanks, @AgTonik, and nice to interact with you. I’ve read a lot of your posts :+1:

10:1 NO3:NH4 (9.09% NH4 to total-N) can work well, but IME it takes an experienced grower, monitoring substrate pH (Pour Thru), and testing each strain. IME 10:1 doesn’t provide benefits vs. 12:1, but 10:1 can reduce substrate pH over time (rather than stabilizing pH), benefiting alkaline substrates (some coco brands) and calcareous soil. Especially if the substrate is on the cool and wet side (reducing nitrifying bacteria conversion of NH4 to NO3) and the plants aren’t vigorously growing.

Looking at this study where they used 10% NH4, the graphs suggest that 5% or 7.5% NH4 would provide better results. The study was pretty flawed, but AFAIK, it’s the only cannabis-specific study on NO3:NH4 ratios’ effect on yield, flower size, potency, aromatics, and tissue nutrient concentrations.

Before settling on 12:1 as the minimum ratio I use, I ran run 8:1 (11% to NH4), but I noticed many strains didn’t respond well. I then went to 10:1 and still found a few finicky strains. Once I hit 12:1, I didn’t find any strains that reacted poorly, and Pour Thru analyses showed stable pH (minor pH increase over two months).

Correct me if I’m wrong, but AFAIK, many of the data we have, and claims by experts like Jones, are based on tomatoes and other vegetable crops. I have found cannabis doesn’t follow those trends. For example, many authors and papers suggest 20% NH4 (4:1) or 25% NH4 (3:1) to stabilize pH, but that is too much NH4 for cannabis.

I found the same as @emdub27, that 10:1 tends to reduce pH over time. It did work well for me with sufficient run-off per day and keeping substrate on the warmer side. The main concern is plant sensitivity to NH4; some strains are very sensitive, while others can handle more NH4 without issue.

It would be valuable if people started reporting their NO3:NH4 ratios and weekly Pour Thru pH results. We could crowdsource a solid data set to derive robust recommondetions by strain, growth conditions, substrate, irrigation method/frequency, substate moisture content (VWC) or tension (kPa), and nutrient solution profile.

I think the biggest take-home for people reading who want to play with NO3:NH4 is to go slow, starting at 12:1 or 10:1 to be aggressive, adjusting from there.

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I might be wrong, but the Dr. Jones recommendation in his book was for determinant tomatoes? I think both of your thoughts here are great observations on pH falling at 10:1 nitrogen sources. My source water has carbonates and I do plenty of runoff, which may prevent huge pH fluctuations. I really should upgrade and use the water quality function in the newer version of Hydrobuddy to be more proactive.

I wouldn’t listen to me. I’m just a wannabe armchair agronomist!

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