HORTSCIENCE 52(9):1307–1312. 2017. doi: 10.21273/HORTSCI11903-17 but the medicinal effects of CBD have re- cently been identified (Russo, 2011) leading some breeders to select for high CBD. Most Optimal Rate of Organic Fertilizer indoor production of cannabis occurs in two growth stages, vegetative and flowering, during the Vegetative-stage for which are controlled by photoperiod (Farag and Kayser, 2015). Modern day cultivation of Cannabis Grown in Two Coir-based cannabis takes place almost exclusively in- doors under artificial lighting using either solution culture systems or soilless growing Substrates substrates (Leggett, 2006; Potter, 2014). In Deron Caplan, Mike Dixon, and Youbin Zheng1 addition, many cannabis growers favor or- School of Environmental Sciences, University of Guelph, Guelph, ON N1G ganic production practices because consumers 2W1, Canada and regulating bodies often demand pesticide- free cannabis (Canada Gazette, 2016). Additional index words. Cannabis sativa, cannabis growth, floral dry weight, marijuana, THC, Online horticultural resources are avail- CBN able for cannabis production; however, lim- ited information is available in peer-reviewed Abstract. Cannabis producers, especially those with organic operations, lack reliable scientific literature. Furthermore, there is information on the fertilization requirements for their crops. To determine the optimal scant published scientific research on any organic fertilizer rate for vegetative-stage cannabis (Cannabis sativa L.), five rates that aspect of organic cannabis production. Be- supplied 117, 234, 351, 468, and 585 mg N/L of a liquid organic fertilizer (4.0N–1.3P– cause of a lack of systematic horticultural 1.7K) were applied to container-grown plants with one of two coir-based organic research, current cannabis producers rely substrates. The trial was conducted in a walk-in growth chamber and the two substrates on cultivation methods derived largely used were ABcann UNIMIX 1-HP with lower water-holding capacity (WHC) and from anecdotal information. Information on ABcann UNIMIX 1 with higher WHC. No differences in growth or floral dry weight fiber-type cannabis cultivation techniques (yield) were found between the two substrates. Pooled data from both substrates showed allows for some parallels to be drawn; how- that the highest yield was achieved at a rate that supplied 389 mg N/L (interpolated from ever, fiber-type cannabis is field-grown yield-fertilizer responses) which was 1.8 times higher than that of the lowest fertilizer and has been selectively bred for fiber pro- rate. The concentration of Δ9-tetrahydrocannabinol (THC) in dry floral material was duction rather than for essential oil content maximized at a rate that supplied 418 mg N/L, and no fertilizer rate effects were observed (Amaducci et al., 2015). A chemotaxonomic on D9-tetrahydrocannabidiolic acid (THCA) or cannabinol (CBN). The highest yield, study found low gene flow between drug- and cannabinoid content, and plant growth were achieved around an organic fertilizer rate fiber-type cannabis (Hillig and Mahlberg, that supplied 389 mg N/L during the vegetative growth stage when using the two coir- 2004) and was supported by a recent genomic based organic substrates. study comparing fiber and drug-type canna- bis (van Bakel et al., 2011). This makes it difficult to relate cultivation techniques be- Cannabis (Cannabis sativa L.) legislation use of cannabis became widespread, with tween the two crops (Amaducci et al., 2015). in North America continues to move rapidly evidence of cultivation and use in ancient Fertilization is one of the most important toward liberalization and in some instances Egypt around 2800 BCE and in China around factors for indoor organic cannabis produc- legalization, shifting cultivation from a largely 2000 BCE (Russo, 2007). The medicinal tion. For fiber-type cannabis, the suggested illicit practice to one that is not only legal, but value of cannabis is attributed primarily to fertilization rate is around 50–200 kg N/ha in high demand. In the United States, with a group of secondary metabolites called (Aubin et al., 2015; Ehrensing, 1998; Vera only a handful of states having legalized cannabinoids which are concentrated mostly et al., 2004), which is similar to other high- recreational cannabis as of 2017, the market in the essential oils of unfertilized female yielding field crops such as wheat (Triticum for legal cannabis was estimated at $2.7 billion flowers (Potter, 2014). spp.; Baxter and Scheifele, 2008). It is USD in 2014, and it is expected to reach $11 More than 100 unique cannabinoids have difficult, however, to estimate fertilizer re- billion by 2019 (ArcView Market Research been identified (Ahmed et al., 2008, 2015; quirements of drug-type cannabis based on and New Frontier, 2014). The current Cana- ElSohly and Slade, 2005), although Δ9-THC fiber-type cannabis or other crops because of dian government has pledged to follow suit and cannabidiol (CBD) are considered the the differences in species and growing con- and pass legislation to legalize cannabis for primary psychoactive and medicinal compo- ditions (Wright and Niemiera, 1987). Fur- recreational purposes beginning in spring of nents (Elzinga et al., 2015; Mechoulam et al., thermore, it is well-known that different 2017. Until then, current legislation allows 1970). In live plants, cannabinoids exist pri- growth stages of the same species have a limited number of private, licensed facilities marily as carboxylic acids such as D9-THCA varying nutrient demand; when the demand to produce and distribute cannabis for medic- and cannabidiolic acid (CBDA) (Muntendam is met, plant performance is improved (Raviv inal purposes as well as conduct scientific et al., 2012). These acids undergo decarbox- and Lieth, 2007; Wang, 2000). Most studies research (Canada Gazette, 2016). ylation during storage (Ross and ElSohly, on fertilizer application in other crops have Cannabis is an annual dioecious species, 1997; Taschwer and Schmid, 2015) and upon been conducted using conventional fertil- producing separate male and female plants. heating (Kimura and Okamoto, 1970) to izers, and there are few on the use of organic Archeological evidence of cultivation dates become neutral cannabinoids such as THC fertilizers for container crops. Fertigation to 10,000 BCE in China where cannabis was and CBD. Varieties of cannabis with low rates of 190–400 mg N/L have been reported used primarily for fiber. Later, the medicinal THC and high CBD are termed hemp or fiber- for container production of organic greenhouse- type cannabis, whereas those with high THC grown tomatoes (Solanum lycopersicum L.; and low CBD are termed marijuana or drug- Surrage et al., 2010; Zhai et al., 2009). To type cannabis, hereafter referred to as canna- our knowledge, neither organic nor conven- We thank ABcann Medicinals Inc. for providing bis (van Bakel et al., 2011; Vollner et al., tional fertilizer application rates have been funding as well as materials, expertise, and ground-level support. We would also like to thank 1986). Selective breeding has produced hun- published for indoor cannabis production in Millenniumsoils Coir and EZ-GRO Inc. for pro- dreds of varieties of cannabis with varying scientific literature. viding materials and technical support. chemical compositions and growth character- Appropriate choice of a growing substrate 1Corresponding author. E-mail: yzheng@uoguelph. istics (Vollner et al., 1986). Selection has is essential for soilless crop production be- ca. mostly been for high floral THC concentration, cause it directly affects root zone water, air, HORTSCIENCE VOL. 52(9) SEPTEMBER 2017 1307 and nutrient availability and balance (Zheng, element concentrations of Nutri Plus Grow and leaves were trimmed thereafter. Floral 2016). While there are no experimental data were (in mg·L–1): 0.0 Ca, 0.0 Mg, 14.5 Zn, fresh weight was measured before the floral on growing substrates for cannabis, the in- 0.0 Mn, 12.0 B, 2.6 Mo, 2.1 Cu, and 8.5 Fe. material was placed in paper bags for drying formation we collected from the industry At the end of the vegetative growth period at 21 °C and 40% RH for 5 d until moisture indicates that many North American canna- (21 d after transplanting), six plants with content reached 11 ± 1%. Dry material was bis producers are using either coir- or peat- representative height and canopy size from then cured at 18 °C and 60% RH for 14 d based substrates, or inert substrates such as each treatment were selected and transferred before determining the floral dry weight rockwool. Different substrates have different into a growth chamber for the flowering (yield). physical and chemical properties; therefore, stage. Plants were potted into 6 L blow- Substrate electrical conductivity and pH it is essential to fertigate plants accordingly to molded black pots (22 cm diameter · 22 cm measurement. Substrate pH and electrical ensure an adequate root zone environment height) containing a custom blended organic conductivity (EC) were determined weekly (Zheng, 2016). growing substrate (60% sphagnum peat and using the pour-through method (Wright, The objective of this study was to de- 40% bulk coconut coir; Premier Tech, 1986) during the vegetative stage and at 4 termine the optimal organic fertilizer rates for Riviere-du-Loup, QC, Canada). Agricultural and 5 weeks of the flowering stage. Pour- growing vegetative-stage cannabis plants in dolomitic lime (Premier Tech) was incorpo- through solutions were measured for pH and two coir-based organic growing substrates in rated at a rate of 3.0 kg·m–3 of substrate. EC using a HI991300 portable pH/EC/TDS/ a controlled environment growth chamber. Plants were spaced on tables to a density of Temperature Meter (Hanna Instruments, 6.5 plants/m2. The PAR was maintained at Woonsocket, RI). Materials and Methods 500 ± 50 mmol·m–2·s–1 with a 12-h photo- Floral cannabinoid analysis. Dried, cured period. Irrigation was administered with one floral material was stored in dark and cool Plant culture and treatments.