Julie A Hershkowitz F. Mitchell Westmoreland Bruce Bugbee ^*

• Utah State University, Logan, United States

Elevating nutrients above levels adequate for other crops is thought to increase yield and quality of medical cannabis. Legalization has heightened awareness of the environmental impact of overfertilization. Phosphorous (P) has been particularly elevated, but high levels of other elements have also been applied to create osmotic stress. Because high levels of one element can inhibit the uptake of other elements, elevated levels of P can interact with elevated levels of other elements. Here we report the effects of increasing P from 15 to 90 mg per L) and elevated levels of other elements (solution concentration electrical conductivity of 2 and 4 mS per cm) on yield, cannabinoids, and nutrient partitioning of medical cannabis in closed-system hydroponics. There was no difference in yield of quality among either EC or P treatments. Reducing P in the refill solution from 90 to 15 mg per L reduced P in solution at harvest from 300 to less than 0.1 mg per L. Despite the low steady-state concentration of P in solution in the 15 mg per L treatment, uptake of P was adequate to support rapid growth and yield. In tissue, leaf P doubled and flower P increased 70% when the P input increased from 15 to 90 mg per L. Doubling nutrient input from 2 to 4 mS per cm increased nutrient accumulation in solution, but did not significantly increase leaf and flower nutrient concentrations. These data indicate cannabis tolerates high nutrient concentration, but excessive fertilization does not improve yield or quality.