Nutrient Interactions in Cannabis: Where Feeding Schedules Are Really Decided

Lead: In the last post we went through the N/P/K numbers that actually hold up. But no feeding schedule works in isolation. The research of the past two years is clear: if you want to optimize fertilization, you have to think about light, water regime, climate and the root microbiome at the same time. And the order of those levers is not arbitrary.

What this is about

The first fertilization post focused on the direct N/P/K studies: Saloner and Bernstein, Bevan, Caplan. That was the groundwork. But growers in a CSC know from practice that the same ppm values produce very different results in two different setups. And that difference is not a measurement error, it is an interaction with other variables.

We have widened our research library considerably since then: 50 papers in total, with a clear focus on studies that explicitly combine two or more variables. We analyzed six of them in full text. Four interaction findings came out of that, plus two negative studies that can save a club money.

Water and nitrogen cannot be separated

Tang and colleagues (2018) measured industrial hemp in whole-canopy gas exchange chambers, a method that captures the entire stand in real time rather than a single leaf. The result: drought stress and nitrogen deficiency reinforce each other disproportionately. Drought stress on its own lowers photosynthesis mainly through stomatal closure. Nitrogen deficiency on its own lowers it through reduced Rubisco and chlorophyll levels. When both arrive together, the plant has no reserves left. In practice that means: if you run deliberate drought stress in late flower (as Caplan and colleagues showed in 2019 for raising cannabinoid content), you need to be sure beforehand that nitrogen supply is not collapsing in parallel. Otherwise you are combining two stresses that can each be useful in the right phase, but that ruin the yield when combined.

Sheldon and colleagues (2021) add the other side: drought stress tolerance varies clearly by genotype. What counts as intentional ripening stress in one cultivar is a yield loss in the next. Same rule here: observe first, standardize second.

Phosphorus is the underrated lever

Two recent studies suggest that common P practice is probably set too high. Shiponi and Bernstein (2021) tested five P levels (5 to 90 mg/L) on two cultivars in flower and found an opposing effect: more P raises flower yield, but lowers cannabinoid biosynthesis at the same time. That matters, because the optimum for yield and the optimum for active compounds do not coincide. If you push for maximum dry mass, you give up cannabinoid concentration.

Hershkowitz, Westmoreland and Bugbee (2025) go a step further and test whether doubling the total nutrient solution concentration (from EC 2 to EC 4 mS/cm) buys anything. The answer is sober: no significant effect. Neither yield nor cannabinoids went up. More than that, the study found no further response above roughly 15 mg/L P when phosphorus was raised in isolation either. That is well below what many fertilizer manufacturers recommend on their data sheets. One qualifier: this applies to recirculating hydro setups. In drain-to-waste systems with heavy leaching, higher nominal doses still make sense, because a large share simply runs off unused. But if you recirculate or fertigate conservatively, there is money and environmental load to save here without losing quality.

Light spectrum affects cannabinoids, but not all of them equally

Brousseau and colleagues (2021) review the light spectrum and cannabinoid axis and deliver a differentiated picture. Cannabinoids are produced in the trichomes as photo-protective secondary metabolites. That is the mechanistic reason why manipulating the spectrum can do anything at all. UV-B can stimulate biosynthesis, visible LED spectra increase THC and terpene accumulation, CBD responds much more weakly. Morello and colleagues (2022) contribute a practical concept with “cannabinoid production efficiency”: what counts is not absolute cannabinoid content, but cannabinoid per mol of photons, in other words cannabinoid per unit of electricity invested.

But: Rodriguez-Morrison and colleagues (2021) show the other side. In their controlled study, short-wave UV-B increased neither yield nor cannabinoid concentration. That is an important negative study, because plenty of commercial UV lamps are marketed on exactly that effect. The truth is probably somewhere in between: UV effects depend on cultivar, dose and growth phase, and blanket advertising claims do not survive a careful test.

What does that mean for a club? If you are thinking about switching LED fixtures, cannabinoid efficiency per kWh is a more useful yardstick than the yield figure in the brochure.

Microbiome: big promise, thin evidence

Two reviews, Lyu and colleagues (2019) on PGPR (plant growth-promoting rhizobacteria) and Ahmed and Hijri (2021) on the microbiota in general, arrive at the same verdict: plausibly effective, but barely established causally for drug-type cannabis. The transferable evidence from rice, wheat and maize is impressive. The direct cannabis data are thin. Lyu et al. single out Pseudomonas and Bacillus strains, because the mechanisms (nitrogen fixation, phosphorus mobilization, phytohormones) are well understood, and because cannabis is a highly regulated crop for which few or no chemical plant protection products are approved in most jurisdictions, which makes biological disease control particularly attractive.

One caveat: Lyu et al. is a perspective, not an original experiment, and one of the co-authors is CEO of a cannabis bioproduct company. The conflict of interest is properly documented. Even so, it is not enough as a basis for recommendations to clubs. Filho, Thomason and colleagues (2020) as well as Filho, Chim and colleagues (2024) provide original data on integrated nutrient and microbe systems that are far more concrete. Bottom line for practice: treat biostimulants as an experiment, document them, collect your own data. Read marketing claims critically.

Fertilization × climate × pathogens: the unpopular coupling

Punja and colleagues (2019) cataloged cannabis pathogens and molds across several sites in Canada over three years. That is the standard reference, but it is often overlooked in fertilization debates. The connection is banal and important: over-fertilization, nitrogen over-fertilization above all, produces soft, lush tissue with thin cell wall architecture. That is an open invitation to pathogens. Botrytis cinerea (bud rot) in high humidity and poorly ventilated canopies, Fusarium in waterlogged root zones, powdery mildew on soft plants. The follow-up studies (Mahmoud et al. 2023, Punja et al. 2023, Buirs & Punja 2024) back that up.

From a club perspective this is one more argument against nitrogen over-fertilization. The “more is not better” point from the first post gains another dimension. Over-fertilization is expensive, it lowers quality, and it raises risk. With a Botrytis outbreak in late flower, an entire batch can be lost fast.

If you want to reduce inputs

Massuela and colleagues (University of Hohenheim, 2023) asked a question that comes up in clubs all the time: can we cut inputs without losing active compounds? They ran five fertilization levels × two fertilizer types (organic and mineral) in flower and found a clear trade-off: reduced fertilization lowers flower mass, but CBD concentration rises. That balances out CBD yield up to a certain stress threshold. Only under very strong reduction does the yield of active compounds break down. Important side findings: no significant interaction between fertilizer type and concentration, but organic fertilization was consistently less efficient. That confirms the practical knowledge that organic schedules need higher nominal doses to keep up.

Method discussion: what interaction studies do (and do not) deliver

Three points to take away. First: most interaction studies test exactly two variables, nitrogen × water or spectrum × phase. Full factorial designs with three or more variables are rare, methodologically expensive, and only just getting started in cannabis research. What we call “interactions” is usually two separate studies linked together, not the direct measurement of a three-way interaction. Second: negative studies (such as Hershkowitz 2025 on P and Rodriguez-Morrison 2021 on UV-B) are worth their weight in gold, but they get cited less than positive findings. If you are assessing feeding schedules or lighting systems, go looking for the negative studies on purpose. Third: the lab-to-practice gap is wider for interactions than for single variables, because in a real grow many variables fluctuate at once.

Tell us which interaction is the number one practical question in your setup: spectrum × fertilization, climate × nitrogen, substrate × pH? We will take it into the next research cycle.

Take-aways

Nitrogen and water cannot be separated: combined stress lowers photosynthesis disproportionately. Genotypes respond differently.
P is usually overdosed. In recirculating systems, studies found no further gain in yield or cannabinoids beyond roughly 15 mg/L P, and higher P levels can even lower cannabinoid content. In drain-to-waste, higher nominal doses still make sense.
Light spectrum affects cannabinoids, but depending on cultivar and phase: no blanket recommendation. Cannabinoid per kWh is the better yardstick than yield.
Microbial biostimulants are plausible, but barely established causally for drug-type cannabis. Treat them as an experiment and collect your own data.
Over-fertilization raises pathogen risk (bud rot, powdery mildew). Less is not only cheaper, it is often safer.
Reduced fertilization in flower lowers mass but raises the concentration of active compounds, up to a stress threshold.

Sources

Shiponi, S. & Bernstein, N. (2021). The Highs and Lows of P Supply in Medical Cannabis. Frontiers in Plant Science. doi:10.3389/fpls.2021.657323
Hershkowitz, J., Westmoreland, F. & Bugbee, B. (2025). Elevated root-zone P and nutrient concentration do not increase yield or cannabinoids in medical cannabis. Frontiers in Plant Science. doi:10.3389/fpls.2025.1433985
Massuela, D., Münz, S., Hartung, J. et al. (2023). Cannabis Hunger Games: nutrient stress induction in flowering stage. Frontiers in Plant Science. doi:10.3389/fpls.2023.1233232
Tang, K., Fracasso, A., Struik, P. C. et al. (2018). Water- and Nitrogen-Use Efficiencies of Hemp Based on Whole-Canopy Measurements and Modeling. Frontiers in Plant Science. doi:10.3389/fpls.2018.00951
Lyu, D., Backer, R., Robinson, W. G. & Smith, D. L. (2019). Plant Growth-Promoting Rhizobacteria for Cannabis Production. Frontiers in Microbiology. doi:10.3389/fmicb.2019.01761
Brousseau, V. D., Wu, B.-S., MacPherson, S. et al. (2021). Cannabinoids and Terpenes: Photo-Protectants Manipulation. Frontiers in Plant Science. doi:10.3389/fpls.2021.620021
Morello, V., Brousseau, V. D., Wu, B.-S. et al. (2022). Light Quality Impacts Vertical Growth Rate, Phytochemical Yield and Cannabinoid Production Efficiency. Plants. doi:10.3390/plants11212982
Punja, Z. K., Collyer, D., Scott, C. et al. (2019). Pathogens and Molds Affecting Production and Quality of Cannabis sativa. Frontiers in Plant Science. doi:10.3389/fpls.2019.01120
Caplan, D., Dixon, M. & Zheng, Y. (2019). Increasing Inflorescence Dry Weight and Cannabinoid Content via Controlled Drought Stress. HortScience. doi:10.21273/hortsci13510-18
Saloner, A., Sacks, M. M. & Bernstein, N. (2019). Response of Medical Cannabis Genotypes to K Supply. Frontiers in Plant Science. doi:10.3389/fpls.2019.01369
Rodriguez-Morrison, V., Llewellyn, D. & Zheng, Y. (2021). Cannabis Inflorescence Yield and Cannabinoid Concentration Are Not Increased With UV-B. Frontiers in Plant Science. doi:10.3389/fpls.2021.725078
Cockson, P., Landis, H., Smith, T. et al. (2019). Characterization of Nutrient Disorders of Cannabis sativa. Applied Sciences. doi:10.3390/app9204432

Topic hub with all 50 papers from this research cycle: cannabis duengung wechselwirkungen