“Recent studies have already demonstrated the significant effects of nitrogen and phosphorus on medicinal cannabis function, yield, and cannabinoid and terpenoid profiles. However, there is an urgent need to study the effects of other plant macronutrients, such as potassium (K), on plant function and secondary metabolism. The available information that there already is, is mainly for cannabis genotypes grown in the field for fiber or seed production, rather than medical types.” Therefore, Prof. Nirit Bernstein of the Volcani Center in Israel, who is the research leader and supervisor of PhD student Avia Saloner, decided to do something about this knowledge gap. They studied two cultivars of medical cannabis grown in controlled environmental conditions under five levels of K supply: 15, 60, 100, 175, and 240 mg L−1 K.
The medical cannabis cultivars ‘Royal medic’ (RM) and ‘Desert queen’ (DQ) were used as a model system in this study. They represent two distinct chemotypes: a balanced THC: CBD concentration (about 5%; RM) and a high THC level (DQ). After planting, for 17 days, the plants received a uniform K fertilization treatment as was found optimal for the vegetative stage of growth in the researchers’ previous study. After 17 days of vegetative growth, the plants were randomly divided into five treatments of increasing K supply: 15, 60, 100, 175, and 240 mg L−1 K, with five plants per treatment.
The optimal K concentration
There was significant damage to both cultivars under the low K supply of 15 mg L−1 K and with the over-supply of K at 240 mg L−1 K in the cultivar DQ. The impaired physiological function reduced biomass accumulation and yield production, and was accompanied by visual appearance of moderate leaf chlorosis under 15 and 240 mg L−1 K. This decrease in physiological function and plant biomass indicates that the supply of 15 mg L−1 K is insufficient for medical cannabis cultivation. Higher supply inputs of 60–175 mg L−1 K improved plant function in both cultivars but demonstrated genotypic variability in the response of specific physiological functions. It cannot be ignored that cannabinoid and terpenoid content decreased with the elevation of K supply, and no beneficial effects were observed for the elevation of K supply from 60 to 175 mg L−1 K. Therefore, the integration of the results suggests that 60 mg L−1 K is the optimal supply level to maintain high function, yield, and secondary metabolite profiles in medical cannabis.