Optimal Rate of Organic Fertilizer during the Flowering Stage for Cannabis Grown in Two Coir-based Substrates

In the expanding North American medical cannabis industry, growers lack reliable and systematically investigated information on the horticultural management of their crops, especially with regard to nutrient management and growing substrates. To evaluate organic substrates and their optimal nutrient management, five rates that supplied 57, 113, 170, 226, and 283 mg N/L of a liquid organic fertilizer (2.00N–0.87P–3.32K) were applied to container-grown plants Cannabis sativa L. ‘WP:Med (Wappa)’ in two coir-based organic substrates. The trial was conducted in a walk-in growth chamber and the two substrates used were ABcann UNIMIX 2-HP (U2-HP) with lower container capacity (CC) and ABcann UNIMIX 2 (U2) with higher CC. U2-HP produced 11% higher floral dry weight (yield), 13% higher growth index (GI), 20% higher ∆ 9 -tetrahydrocannabinol (THC) concentration, 57% higher THC yield (per plant), 22% higher Δ 9 -tetrahydrocannabidiolic acid (THCA) yield, and 20% higher cannabigerolic acid (CBGA) yield than U2. Increasing fertilizer rate led to increased growth and yield but also to a dilution of THC, THCA, and CBGA. In U2-HP, to maximize both yield and cannabinoid yield, the optimal organic fertilizer rates were those which supplied 212–261 mg N/L. For U2, the highest applied rate, that supplied 283 mg N/L, maximized yield; although lower rates delivered higher cannabinoid concentrations in dry floral material. The results on these substrates and recommended fertilizer rates can serve as a guide when using other organic fertilizers and substrates; although results may differ with cannabis variety.