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

Abstract

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 Δ 9-tetrahydrocannabinol (THC) concentration, 57% higher THC yield (per plant), 22% higher D 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.

Full text

HORTSCIENCE 52(12):1796–1803. 2017. doi: 10.21273/HORTSCI12401-17
Optimal Rate of Organic Fertilizer
during the Flowering Stage for
Cannabis Grown in Two Coir-based
Substrates
Deron Caplan, Mike Dixon, and Youbin Zheng
1
School of Environmental Sciences, University of Guelph, Guelph, ON N1G
2W1, Canada
Additional index words. Cannabis sativa, cannabis growth, floral dry weight, marijuana, THC,
THCA, CBGA
Abstract. 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 Δ
9
-
tetrahydrocannabinol (THC) concentration, 57% higher THC yield (per plant), 22%
higher D
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.
The North American market for
government-regulated cannabis (Cannabis
sativa L.) is expanding at an increasing pace.
In 2016, spending on medical cannabis was
reported at 4.9 billion USD and is projected
to exceed 7 billion USD by 2020 in North
America (ArcView Market Research, 2017).
Cannabis is an annual herbaceous species
which has been widely cultivated and used as
a medicinal plant since around 2800 BCE
(Russo, 2007). The medicinal value of this
species is attributed to a group of secondary
metabolites called cannabinoids which are
concentrated in the essential oils of unfertil-
ized female flowers (Potter, 2014). Over 100
unique cannabinoids have been identified
(Ahmed et al., 2008, 2015; ElSohly and
Slade, 2005; Radwan et al., 2015); although
Δ
9
-THC and cannabidiol (CBD) have been
most widely studied for their psychoactive
and medicinal properties (Elzinga et al.,
2015; Mechoulam et al., 1970; Vemuri and
Makriyannis, 2015). In live plants, cannabi-
noids exist predominantly as carboxylic acids
such as D
9
-THCA and cannabidiolic acid
(CBDA) (Muntendam et al., 2012). These
acids decarboxylate during storage (Ross and
ElSohly, 1997; Taschwer and Schmid, 2015)
and upon heating (Kimura and Okamoto,
1970) to become neutral cannabinoids such
as THC and CBD. Some varieties of cannabis
have been selectively bred and cultivated
mainly for fiber or seed production; these
are characterized by low THC and high CBD
concentrations and are generally termed
hemp or fiber-type cannabis. Varieties with
high THC and low CBD are termed marijuana
or drug-type cannabis (van Bakel et al., 2011;
Vollner et al., 1986), hereafter referred to as
cannabis.
Our communications with Canadian med-
ical cannabis producers, relevant horticulture
literature relating to cannabis (Knight et al.,
2010; Potter and Duncombe, 2012; Vanhove
et al., 2011, 2012) and reviews on global
cannabis production (Farag and Kayser,
2015; Leggett, 2006; Potter, 2014) suggest
that modern day production occurs primarily
in controlled environments using artificial
lighting and either soilless growing substrates
(Caplan et al., 2017) or solution culture.
Furthermore, most indoor production of can-
nabis occurs in two growth stages, vegetative
and flowering, which are controlled by pho-
toperiod (Potter, 2014). Cannabis production
has been and continues to be illegal in much
of the world which has limited scientific
research on this species, particularly with
regard to its production. Growers have access
to horticultural guides and online resources
but few are based on scientific research.
Published information on hemp production
allows for some parallels to be drawn; how-
ever, hemp is a field-grown crop and has been
selectively bred for fiber or seed production
rather than for flower and essential oil pro-
duction (Amaducci et al., 2015). In addition,
recent studies have found low gene flow
between cannabis and hemp (Hillig and
Mahlberg, 2004; van Bakel et al., 2011)
making it difficult to relate cultivation tech-
niques between the two crops (Amaducci
et al., 2015). The lack of horticultural in-
formation on cannabis limits producers and
patients seeking to grow or consume consis-
tent, high quality medicine.
Fertilization is one of the most important
factors for indoor cannabis production. Over-
fertilization can lead to salt accumulation in
the root zone, whereas under-fertilization can
cause nutrient deficiency and lower yields
(Bar-Yosef, 1999). The suggested fertiliza-
tion rate for hemp ranges from 50 to 200 kg
N/ha (Aubin et al., 2015; Ehrensing, 1998;
Vera et al., 2004) which is similar to other
high-yielding field crops such as wheat (Tri-
ticum spp.; Baxter and Scheifele, 2008). It is
difficult, however, to base fertilizer rates for
cannabis on suggestions for hemp or other
crops because of the differences in species
and growing conditions (Wright and Niemiera,
1987). Furthermore, it is common for nutri-
ent requirements to vary based on growth
stage in flowering plants. The vegetative
growth stage in flowering plants is charac-
terized by an exponential growth rate (bio-
mass increase) and often a higher nutrient
demand (Raviv and Lieth, 2007). The flower-
ing stage is characterized by a linear growth
rate. In this stage, carbohydrates are trans-
located to reproductive organs such as
flowers and seeds and nutrient demand de-
creases (Raviv and Lieth, 2007). Varying
fertilizer requirements by growth stage have
been reported in greenhouse-grown crops
such as sweet peppers (Capsicum annum
L.), for which the highest total plant and fruit
yield was achieved by supplying 30% N from
NH
4
+
-N in the vegetative stage and using
only NO
3
–
-N in the flowering stage (Xu et al.,
2001). A recent evaluation on the effects of
organic fertilization during the vegetative
stage for cannabis suggests that over-
fertilization during the vegetative stage may
decrease both THC concentration in floral
material and floral dry weight (yield; Caplan
et al., 2017) on harvest. An optimal fertilizer
rate of 389 mg N/L was proposed using
a liquid organic fertilizer (4.0N–1.3P–1.7K)
in two coir-based organic substrates. To our
knowledge, there is no research on flowering-
stage fertilizer rates for cannabis.
Received for publication 15 Aug. 2017. Accepted
for publication 24 Sept. 2017.
We thank ABcann Medicinals Inc. for providing
funding as well as materials, expertise, and ground-
level support. We also thank Millenniumsoils Coir
and EZ-GRO Inc. for providing materials and
technical support.
1
Corresponding author. E-mail: yzheng@
uoguelph.ca.
1796 HORTSCIENCE VOL. 52(12) DECEMBER 2017

Appropriate choice of a growing substrate
is also important for indoor cannabis pro-
duction. Substrates vary in physical and
chemical properties; therefore, to ensure
a suitable root zone environment it is impor-
tant for fertigation to be tailored to the
growing substrate (Zheng, 2016). Substrates
with low CC (drier substrates) require more
frequent irrigation to keep moisture levels
constant, whereas substrates with higher CC
(wetter substrates) require less frequent irri-
gation and may conserve irrigation water
(Raviv and Lieth, 2007). Although there is
scant research on growing substrates for
cannabis, the information we collected from
the industry indicates that many North
American cannabis producers are using ei-
ther coir- or peat-based substrates, or inert
substrates such as stone wool. Caplan et al.
(2017) evaluated two coir-based substrates
for vegetative-stage cannabis production. At
the end of the vegetative stage, plants were
transferred into a growth chamber for the
flowering stage under similar conditions to
determine if treatment effects carried forward
to harvest. The substrates differed in CC by
11% but no differences in growth, yield, or
cannabinoid content were reported between
the two. Vegetative-stage cannabis may grow
well in substrates with CCs within a certain
range; however, there are no similar evalua-
tions for flowering stage cannabis production.
The objectives of this study were to
1) evaluate two coir-based organic growing
substrates for flowering-stage cannabis pro-
duction in a controlled environment growth
chamber, and 2) determine the optimal or-
ganic fertilizer rates for these substrates.
Materials and Methods
Plant culture. Fifteen-day-old rooted cut-
tings (10 cm high with 6leaves)ofC.
sativa L. ‘WP:Med (Wappa)’ were trans-
planted into round peat-based pots (9.5 cm
diameter ·10.2 cm high; Jiffy Products
N.B. Ltd., Shippagan, Canada) filled with
ABcann UNIMIX 1-HP growing substrate
(ABcann Medicinals Inc., Napanee, Can-
ada) with one plant per pot. Pots were placed
in a walk-in growth chamber (15 m
2
)ata
density of 97 plants/m
2
.
The chamber air temperature was main-
tained at 24/23 C(
SD ± 0.04/1.0 C) and
relative humidity (RH) was 76/76% (SD ± 3.8/
3.9%) during the light/dark period through-
out the vegetative stage. Carbon dioxide
(CO
2
) concentration was maintained at 545
ppm (SD ± 45 ppm) from 1 to 4 d after
transplant (DAT), 570 ppm (SD ± 18 ppm)
from 5 to 10 DAT and 613 ppm (SD ± 18 ppm)
from days 11 to 19 DAT during the light
period and 529 ppm (SD ± 18 ppm) during the
entire dark period. Using dimmable fluorescent
lighting (Philips Lighting, Markham, Canada)
with an 18-h photoperiod, the photosyntheti-
cally active radiation (PAR)atthetopofthe
canopy was maintained at 100 mmol·m
–2
·s
–1
(SD ±1mmol·m
–2
·s
–1
)from1to4DAT,199
mmol·m
–2
·s
–1
(SD ±6mmol·m
–2
·s
–1
)from5to7
DAT, 300 mmol·m
–2
·s
–1
(SD ±2mmol·m
–2
·s
–1
)
from 8 to 10 DAT, and 337 mmol·m
–2
·s
–1
(SD ±
49 mmol·m
–2
·s
–1
)from11to19DAT.
Beginning 3 DAT, plants were hand-
fertigated, as per Caplan et al. (2017), using
Nutri Plus Organic Grow liquid organic
fertilizer (4.0N–1.3P–1.7K; EZ-GRO Inc.,
Kingston, ON, Canada) at a rate that supplied
389 mg N/L amended with 1 mL·L
–1
of
calcium–magnesium supplement (0.0N–
0.0P–0.0K–3.0Ca–1.6Mg; EZ-GRO Inc.),
and with a 20% leaching fraction. Other
nutrient element concentrations of Nutri Plus
Organic Grow were (in mg·L
–1
): 14.5 Zn,
12.0 B, 2.6 Mo, 2.1 Cu, and 8.5 Fe. Irrigation
was administered when mean substrate mois-
ture was 30%, measured using a WET-2
soil moisture sensor (Delta-T Devices Ltd.,
Cambridge, UK).
Treatments. At 19 DAT, 60 plants with
similar height and canopy size were selected
and transferred into a larger walk-in growth
chamber (130 m
2
) for the flowering stage.
This was considered the first day of the
flowering stage (DFS). Four additional plants
were harvested at this stage to measure initial
growth attributes. Plants were up-potted into
6 L blow-molded black pots (220 mm di-
ameter ·220 mm height) filled with one of
two growing substrates, ABcann UNIMIX
2-HP (U2-HP) or ABcann UNIMIX 2 (U2)
(physical and chemical properties presented
in Tables 1 and 2, respectively; ABcann
Medicinals Inc.). The substrates were
coir-based organic substrates with two dis-
tinct CCs: U2-HP with lower CC and better
drainage than U2. Coir weed control disks
were used on top of the growing substrate to
prevent algae growth.
The experiment was a completely ran-
domized design with two factors: five fertil-
izer rates and two substrate types, with six
replicates for each factor combination. Each
potted plant was an experimental unit. Plants
were fertilized at one of five rates that
supplied 57, 113, 170, 226, and 283 mg N/L
using Nutri Plus Organic Bloom (2.00N–
0.87P–3.32K; EZ-GRO Inc.), diluted with
RO water. Other nutrient element concentra-
tions in Nutri Plus Organic Bloom were (in
mg·L
–1
) 100 Mg, 10.0 Zn, 12.8 B, 0.1 Mo,
2.3 Cu, and 6.8 Fe. All fertigation solutions
were amended with 1 mL·L
–1
of calcium–
magnesium supplement (3.0Ca–1.6Mg;
EZ-GRO Inc.) and with Organa ADD micro-
nutrient supplement, at a rate that supplied
22.9 mg N/L (2.0N–0.0P–0.0K; EZ-GRO
Inc.). Other nutrient element concentrations
in Organa ADD were (in mg·L
–1
) 100.0 Ca,
29,851 Zn, 4892 Mn, 1239 B, 12.7 Mo, 2419
Cu, and 2917 Fe. Fertilizer rates were se-
lected based on recommendations for
vegetative-stage organic fertilizer rates from
Caplan et al. (2017) and previous studies on
organic fertigation of greenhouse-grown to-
matoes (Solanum lycopersicum L.; Surrage
et al., 2010; Zhai et al., 2009).
Plants were spaced on growing tables at
a density of 5.3 plants/m
2
.PAR was maintained
at 581 mmol·m
–2
·s
–1
(SD ±93mmol·m
–2
·s
–1
)
throughout the flowering stage under MAS-
TER GreenPower Plus 1000W high pressure
sodium lamps (Philips Lighting) with a 12-h
photoperiod. Growth chamber air tempera-
ture was maintained at 22/21 C(
SD ± 0.8/0.5
C) from 1 to 6 DFS, 20/20 C(
SD ± 0.5/0.8
C) from 7 to 9 DFS and 18/17 C(
SD ± 0.7/
1.0 C) from 10 to 53 DFS during light/dark
periods. RH was maintained at 70/76% (SD ±
2.9/2.3%) from 1 to 6 DFS, 64/66% (SD ± 2.5/
2.7%) from 7 to 43 DFS, and 56/62% (SD ±
2.6/1.5%) from 44 to 53 DFS. Chamber CO
2
was maintained at 594/659 ppm (SD ± 56/42
ppm) from 7 to 9 DFS, 673/705 ppm (SD ± 83/
44 ppm) from 7 to 9 DFS, and 781/838 ppm
(SD ± 83/78 ppm) from 44 to 53 DFS during
light/dark periods.
During the first 11 DFS, plants were hand-
fertigated at a rate that supplied 389 mg N/L
of Nutri Plus Organic Grow and from then
on, with the corresponding nutrient solution
of Nutri Plus Organic Bloom (including
calcium–magnesium and micronutrient sup-
plements) whenever the mean substrate
moisture content reached 30%. Between
Table 1. Physical properties of growing substrates ABcann UNIMIX 2-HP (U2-HP) and ABcann UNIMIX
2 (U2).
Growing substrate
Total porosity
z
CC
z
Air space
z
Bulk density
z
(g·cm
–3
)(%)
U2-HP 83 ± 0.5 49 ± 0.4 34 ± 0.4 0.10 ± 0.001
U2 91 ± 0.9 55 ± 2.2 35 ± 1.3 0.09 ± 0.001
z
Data are means ± SEM (n= 3). CC = container capacity.
Table 2. EC, pH, and nutrient content measured using the saturated media extract procedure (Warncke, 1986) for growing substrates ABcann UNIMIX 2-HP
(U2-HP) and ABcann UNIMIX 2 (U2).
Growing substrate EC
z
(mS·cm
–1
)pH
z
Nitrate N P K Ca Mg SO
4
2–
Na Cl
-
Zn Mn Cu Fe B Mo
(mg·L
–1
)
U2-HP 2.2 ± 0.02 6.2 ± 0.03 5 11.8 423 <1 4.0 34.0 118.9 534 <0.01 0.02 <0.01 0.2 0.09 0.01
U2 1.9 ± 0.04 6.4 ± 0.02 5 9.8 353 <1 3.2 37.0 109.4 488 <0.01 0.02 <0.01 0.7 0.12 <0.01
z
Data are means ± SEM (n= 3). EC = electrical conductivity; N = nitrogen; P = phosphorus; K = potassium; Ca = calcium; Mg = magnesium; SO
4
2–
= sulfate; Na =
sodium; Cl
-
= chloride; Zn = zinc; Mn = manganese; Cu = copper; Fe = iron; B = boron; Mo = molybdenum.
HORTSCIENCE VOL. 52(12) DECEMBER 2017 1797
|
SOIL MANAGEMENT,FERTILIZATION,AND IRRIGATION

days 45 and 53 in the flowering stage, no
fertilizer was applied and the substrates were
flushed, as per current industry practice, with
RO water when mean substrate moisture
content reached 30%. Fertigation solution
pH was adjusted to maintain substrate pH
between 5.5 and 6.3, measured using the
pour-through method (Wright, 1986) during
both vegetative and flowering stages.
Substrate electrical conductivity (EC) and
pH measurement. Substrate pH and EC dur-
ing the flowering stage were determined weekly
using pour-through method. Pour-through solu-
tions were measured using a HI991300 portable
pH/EC/TDS/Temperature Meter (Hanna Instru-
ments, Woonsocket, RI).
Growth and yield measurements. During
the flowering stage, branch number, canopy
area, and plant height were measured every
7 d on five randomly selected plants from
each treatment. Repeated measurements
were made on the same plants throughout
the vegetative stage. During the flowering
stage, branch number, canopy area, and plant
height were measured on all plants at 15, 27,
and 53 DFS. Canopy area for each plant was
calculated using two perpendicular length
measurements at the widest part of the
canopy using plant tags as reference points
for repeated measurements. Growth index for
each plant was calculated as height (cm) ·
length (cm) ·width (cm) ·300
–1
(Ruter,
1992). At 34 DFS, leaf greenness was mea-
sured as chlorophyll content index (CCI),
using a CCM-200 chlorophyll content meter
(Opti-Sciences Inc., Hudson, NH) from the
center of the most recent fully expanded leaf.
Plants were harvested at 53 DFS when floral
resin on most plants had 50% amber color-
ation. Stems were cut at substrate level;
above-ground fresh weight was measured;
large leaves were removed from stems and
plants were hung to dry at 18 C(
SD ± 0.1 C)
and 49% RH (SD ± 4.4%) for 6 d then cured at
18 C(
SD ± 0.5 C) and 58% RH (SD ± 3.5%)
for 11 d. Floral material was then cut from
stems and leaves were trimmed using
a Twister T4 mechanical trimming machine
(Keirton Inc., Surrey, BC, Canada) before
floral dry weight (yield) measurement.
Floral cannabinoid analysis. The dried
and cured floral material was stored in dark
and cool conditions according to United
Nations Office on Drugs and Crime (2009)
before being analyzed by an independent
laboratory (RPC Science and Engineering,
Fredericton, NB, Canada). Analysis of the
neutral cannabinoids Δ9-THC, CBD, canna-
binol (CBN), cannabichromene (CBC), and
cannabigerol (CBG) as well as acid forms,
Δ9-THCA, CBDA, and cannabigerolic acid
(CBGA) were conducted by high-
performance liquid chromatography as de-
scribed in section 5.4.8 of United Nations
Office on Drugs and Crime (2009).
Statistical analysis. Data were analyzed
using JMP Statistical Discovery Version 13.0
(SAS Institute Inc., Cary, NC) at a Type 1
error rate of #0.05. Full-factorial analysis of
variance with repeated measures was used to
determine the effects of substrate, fertilizer
and their interaction on substrate EC, pH,
plant height, GI, leaf number, and branch
number over time. Differences among means
were tested using Tukey’s multiple means
comparison test. Pearson correlation coeffi-
cients were calculated to compare cannabi-
noid concentrations with yield and CCI with
GI index and yield. Orthogonal partition and
regression analysis (Bowley, 1999) was used
to relate substrate EC, pH, plant growth,
yield, and cannabinoid yield/concentrations
with fertilizer rate. If the partitioning vari-
ance analysis indicated a significant treat-
ment effect, then the treatment effects were
partitioned into one or more regression ef-
fects followed by an estimation of regression
parameters for the best-fit regression. If there
was no significant treatment effect, then data
were presented as the average of all the
treatments (pooled). If cannabinoid concen-
trations were below the detection limit
(<0.05%), the values were excluded from
the analysis. The residuals of the above
analyses were tested for normality and equal-
ity of variance using The Shapiro–Wilk test
and Bartlett’s test, respectively.
Results
Growth
During both the 19-d vegetative stage and
the 53-d flowering stage,plants grew normally
and without any symptoms of nutrient disor-
der. Pistillate flowers were visible around 12
DFS in all treatments. During the flowering
stage (from 0 to 53 DFS), the average above-
ground fresh weight increased from 7.1to 296
g/plant, branch number increased from 4.5 to
11 branches/plant, and GI increased from 29
to 462 combined across treatments.
Four plants were removed during the trial
because of root rot, three in U2 (two at the
rate that supplied 57 mg N/L and one at the
rate that supplied 113 mg N/L), and one in
U2-HP (at the rate that supplied 57 mg N/L).
In these cases, both younger and older leaves
began to show interveinal purpling, leading
to chlorosis and necrosis. There was also
visible leaf purpling in treatments of U2, at
the rates that supplied 223 and 286 mg N/L;
most of these plants had entirely purple
leaves from 32 DFS until harvest.
Generally, plants with higher fertilizer
rates had greener leaves. At 34 DFS, there
was a positive linear relationship between
fertilizer rate and CCI measurements taken
from the center of the newest fully expanded
leaf (Fig. 1; pooled from both substrates), and
there was no difference in CCI between sub-
strate treatments. There was also a positive
correlation between CCI and floral dry weight
(r=0.64,P< 0.0001) and GI at 53 DFS (r=
0.39, P= 0.0184). Taking CCI measurements
became difficult after 34 DFS when trichomes
were abundant on the proximal region of
leaves which interfered with the readings.
There were signs of foliar senescence begin-
ning around week 6 of flowering when older
leaves became chlorotic and ultimately ne-
crotic before harvest. These signs of senes-
cence are typical for cannabis.
Growth index did not show any treatment
effect at 15 DFS, but increased linearly with
increasing fertilizer rate at 27 DFS and
exhibited a quadratic response at 53 DFS
(Fig. 2) for both substrates. At the final
Fig. 1. Response of leaf ‘greenness’ [chlorophyll
content index (CCI) readings] measured on the
newest fully expanded leaf to organic fertilizer
rate [indicated by nitrogen (N) concentration]
at day 34 of the flowering stage. Data were
pooled from both substrates. Values are means ±
SEM (n= 8) and line is the best-fit regression
relationships at P<0.05.
Fig. 2. Response of cannabis growth index to
organic fertilizer rate [indicated by nitrogen
(N) concentration] in two growing substrates
(U2-HP and U2) measured on different days
during the flowering stage (DFS). Values are
means ± SEM (n= 5 for U2-HP at the rate that
supplied 170 mg N/L on day 53; for U2 at the
rate that supplied 57 mg N/L on day 15 and at
the rate that supplied 113 mg N/L on day 53;
n= 4 for U2 at the rate that supplied 57 mg N/L
on day 27 and 53 and n= 6 for all other means)
and lines are the best fit regression relationships
at P< 0.05.
1798 HORTSCIENCE VOL. 52(12) DECEMBER 2017

measurement (53 DFS), the interpolated
maximum GI (558) for U2-HP was achieved
at a rate supplying 211 mg N/L, and for
U2 the interpolated maximum (510) was
achieved at a rate supplying 239 mg N/L.
Averaged across all fertilizer rates, plants
grown in U2-HP had a 13% higher final GI
than in U2 (F= 9.6, P< 0.01). No interactive
effect was detected between substrate and
fertilizer rate on GI.
Branch number did not differ among
fertilizer rates or between substrate treat-
ments, and there was no interactive effect of
these treatments on branch number. Pooled
across all treatments, branch number was 11
± 0.1 (± SEM) at 53 DFS.
Yield
In both substrates, yield increased with
increasing fertilizer rate; however, in U2-HP
yield reached a maximum, and in U2 yield
increased linearly (Fig. 3). The interpolated
maximum yield in U2-HP was 50 g/plant at
a rate supplying 261 mg N/L. In U2, the
highest yield was 47 ± 3.0 g/plant (mean ±
SEM) at the rate that supplied 283 mg N/L. At
the highest fertilizer rate, yield was 2.1
times greater than at the lowest administered
rate in U2-HP, and 2.2 times higher in U2.
Pooled across fertilizer rates, yield was 11%
higher in U2-HP than in U2 (P=0.0013;n=
29 for U2-HP and n= 27 for U2) with means
(± SEM) of 40 ± 2.2 g and 36 ± 2.0 g,
respectively. No interactive effect was de-
tected between substrate and fertilizer rate
on yield.
Cannabinoids
Fertilizer rate. Of the analyzed cannabi-
noids, only THC, THCA, CBG, and CBGA
were above the detection limit (0.05%). Also,
CBG concentration data were not normally
distributed and could not be fit into the
model; therefore, only treatment means are
presented for this data.
As fertilizer rate increased, there were
varied responses in floral concentrations and
cannabinoid yield per plant (Fig. 4). In U2,
THC concentration had a quadratic response
to increasing fertilizer rate with a minimum
of 0.36% at a rate supplying 193 mg N/L.
Averaged across fertilizer rates, THC con-
centration in U2-HP was 0.44% ± 0.018%
(mean ± SEM) and in this substrate, there was
no effect of fertilizer rate on THC concen-
tration. In both substrates, THCA concentra-
tion decreased linearly with increasing
fertilizer rate. THCA concentration from
lowest to highest fertilizer rate was 21.6% ±
0.64% to 16.7% ± 0.51% for U2-HP and
21.0% ± 0.63% to 18.1% ± 0.85% for U2
(mean ± SEM). In U2, CBGA concentration
decreased linearly as fertilizer rate increased,
ranging from 0.64% ± 0.02% to 0.54% ±
0.03% from lowest to highest fertilizer rate.
In U2-HP, floral CBGA concentration aver-
aged across fertilizer rates was 0.57% ±
0.01% (mean ± SEM) and in this substrate,
there was no effect of fertilizer rate on CBGA
concentration. Mean CBG concentration
pooled across all treatments was 0.06% ±
0.002% (± SEM).
Fertilizer effects were more evident when
analyzing cannabinoid yield (g/plant) as
a function of fertilizer rate (Fig. 4, right). In
U2-HP yield of floral THC responded qua-
dratically to increasing fertilizer rate, reach-
ing a maximum of 0.27 g/plant at a rate
supplying 223 mg N/L and in U2, reaching
minimum of 0.11 g/plant at a rate supplying
103 mg N/L. Yield of floral THCA increased
linearly in U2 and responded quadratically in
U2-HP with a maximum of 9.4 g/plant at
a rate supplying 212 mg N/L. For both sub-
strates, yield of floral CBG increased linearly
with increasing fertilizer rate. Finally, yield
of floral CBGA responded quadratically to
fertilizer rate in U2-HP with a maximum of
0.29 g/plant at a rate supplying 228 mg N/L
and increased linearly in U2.
To determine if increasing yield influ-
enced cannabinoid concentrations, Pearson
correlation coefficients were calculated to
relate cannabinoid concentrations with floral
dry weight. Pooled across treatments, floral
THCA concentration decreased with increas-
ing dry floral weight (r= –0.44, P= 0.0047),
but there was no correlation between THC,
CBG, or CBGA concentration and dry floral
weight.
Substrate type. In presenting the effects of
substrate on cannabinoids, means were
pooled across fertilizer rates; however, sta-
tistical analysis was based on the complete
model. There were no differences in the floral
concentration of THCA or CBGA between
the two substrates; although, U2-HP had
a THC concentration 20% higher than U2
[0.53% ± 0.016% and 0.44% ± 0.025%
(means ± SEM), respectively; F= 16.9, P=
0.0008; n= 20]. There was also an interactive
effect of substrate and fertilizer rate on THC
concentration (P= 0.01852; n= 4). At the
lowest and highest fertilizer rates, THC
concentrations did not differ between sub-
strates; but, at all other rates, THC concen-
tration was higher in U2-HP.
As with fertilizer rate, differences be-
tween substrates were more evident when
comparing cannabinoid yield per plant. THC
yield, THCA yield, and CBGA yield were all
higher in U2-HP than in U2: THC yield by
57% (F= 9.3, P#0.0001; n= 20), THCA
yield by 22% (F= 10.7, P#0.0001; n= 20),
and CBGA yield by 20% (F= 8.3, P#
0.0001; n= 20). THC yield in U2-HP was
0.22 ± 0.017 g/plant (means ± SEM) and in U2
was 0.14 ± 0.010 g/plant; THCA yield in U2-
HP was 7.9 ± 0.47 g/plant and in U2 was 6.5 ±
0.44 g/plant; and CBGA yield in U2-HP was
0.24 ± 0.016 g/plant and in U2 was 0.20 ±
0.013 g/plant. Substrate had no effect on
CBG yield [0.019 ± 0.002 g/plant (mean ±
SEM) across all treatments].
Substrate EC, pH, and irrigation. In both
substrates, EC increased linearly or
responded quadratically over time at each
fertilizer rate except at the lowest rate that
supplied 57 mg N/L, in which EC decreased
linearly (Fig. 5). At the final measurement
before harvest (at week 6 of the flowering
stage), EC from the lowest to the highest
fertilizer rate was 1.6 ± 0.01 to 6.0 ± 0.31
mS·cm
–1
for U2-HP and 1.9 ± 0.08 to 6.3 ±
0.35 mS·cm
–1
for U2 (means ± SEM). Aver-
aged across fertilizer rates and time, EC was
6.3% higher in U2-HP than in U2 [3.1 ± 97.2
mS·cm
–1
and 3.3 ± 119.0 mS·cm
–1
(means ±
SEM), respectively; F= 4.8, P= 0.03].
Substrate pH decreased over time in all
fertilizer rates and in both substrates (Ta-
ble 3). Starting at 4 weeks in the flowering
stage (WFS) until the end of the trial, pH was
lower in U2-HP than in U2, differing by 0.4–
0.9 during this time. There were no interac-
tive effects of substrate and fertilizer rate on
substrate EC or pH.
To maintain a minimum substrate moisture
content of 30%, plants grown in U2-HP were
fertigated 17 times during the flowering stage
compared with 13 times for U2. Fertigation
volume was 1 L/plant each time; therefore,
during the flowering stage, plants grown in U2-
HP were given 31% more water (4Lmore)
and fertilizer than those grown in U2. For
example, at the rate supplying 170 mg N/L,
plants grown in U2-HP received 2.89 g N/L of
Nutri Plus Organic Bloom during the flowering
stagecomparedwith2.21gN/LforU2.
Discussion
Both growing substrates, U2-HP and U2,
performed well for cannabis production dur-
ing the flowering stage; however, plants
grown in U2-HP had higher GI, yield, THC
concentration, THC yield, THCA yield, and
CBGA yield than those grown in U2.
Yield increased with increasing fertilizer
rate, reaching a plateau in U2-HP and until
the highest applied rate for U2; however, as
fertilizer rate increased, the concentration of
most measured cannabinoids (THC, THCA,
and CBGA) decreased. This suggests that for
cannabis, high fertilizer rate during the flow-
ering stage may have a dilution effect on
THC, THCA, and CBGA. Although the di-
lution effect was apparent with increasing
fertilizer rate, it did not have a substantial
Fig. 3. Response of cannabis yield to organicfertilizer
rate [indicated by nitrogen (N) concentration]
applied during the flowering stage in two growing
substrates (U2-HP and U2). Values are means ±
SEM and the curves are best-fit regression relation-
ship with P< 0.05. For U2-HP, n=5attherate
that supplied 170 mg N/L and n= 6 at each other
fertilizer rate; for U2, n= 4 at the rate that
supplied 57 mg N/L, n= 5 at the rate that supplied
113 mg N/L and n= 6 at each other rate.
HORTSCIENCE VOL. 52(12) DECEMBER 2017 1799

impact on the total per-plant yield of most
cannabinoids. This was evidenced by a lack
of correlation between yield and THC, CBG,
or CBGA concentrations. There was, how-
ever, a negative correlation between floral
THCA concentration and yield, suggesting
a slight dilution of THCA as yield increased.
This illustrates that growing higher yielding
cannabis plants is appropriate to maximize
the yield of THC and CBGA with only minor
losses of THCA. To maximize floral yield
without sacrificing THC, THCA, and CBGA
concentration due to the dilution effect, it is
recommended that excessive organic fertil-
izer application during the flowering stage be
avoided.
In U2-HP, peak yield, THC yield, THCA
yield, and CBGA yield were at rates supply-
ing 261, 223, 212, and 228 mg N/L, re-
spectively. Therefore, for U2-HP, the
optimal rate of this organic fertilizer is
between 212 and 261 mg N/L, depending
on grower preference for high yield or indi-
vidual cannabinoid yield. For U2, yield did
not reach a plateau over the applied fertilizer
rates; therefore, growers may choose to
maximize yield using the highest fertilizer
rate that supplied 283 mg N/L or chose
a lower rate to maximize cannabinoid con-
centration. Growing substrate U2-HP is pref-
erential to achieve maximum yield and
cannabinoid content, whereas U2 has the
potential to reduce water and fertilizer use
if the nutrient solution is not reused.
Some of the differences between the two
substrates may have been accounted for by
differences in irrigation frequency. Because
irrigation frequency was based on substrate
moisture content, the drier substrate (U2-HP)
was irrigated, and consequently fertilized,
more frequently than U2. The differences in
performance between the two substrates were
greater than that could be accounted for by
the additional fertilizer applied in U2-HP
over the duration of flowering stage. For
example, yield was higher in U2-HP at the
rate that supplied 170 mg N/L than yield in
U2 at the rate that supplied 226 mg N/L,
whereas the total fertilizer applied was 2.89
and 2.94 g N/L, respectively. High irrigation
frequency is known to increase plant growth
in some species (de Kreij and Straver, 1988;
Katsoulas et al., 2006; Morvant et al., 1998;
Silber et al., 2003, 2005). In greenhouse-
grown roses (Rosa hybrid L., ‘First Red’),
doubling irrigation frequency while main-
taining constant total irrigation volume in-
creased the dry weight of cut flower shoots by
30% over a lower frequency irrigation
(Katsoulas et al., 2006). Similar effects have
been reported in greenhouse-grown
Codiaeum variegatum L.; using a flood and
drain irrigation system, de Kreij and Straver
(1988) found that high irrigation frequency
combined with high-porosity substrates in-
creased plant growth and reduced substrate
nutrient leaching. Increased irrigation fre-
quency can also increase aboveground bio-
mass in lettuce (Lactuca sativa L., ‘Iceberg’;
Silber et al., 2003) and peppers (C. annum L.,
‘Selika’; Silber et al., 2005). The increased
Fig. 4. Relationshipbetween cannabinoid concentration in dry floral material (left) and cannabinoidyield per
plant (right) and organic fertilizer rate applied during the flowering stage in two substrates (U2-HP and
U2). Fertilizer rate is indicated by nitrogen (N) concentration. Values are means ± SEM. The curve is the
best fit regression relationship with P< 0.05. In U2-HP, for CBG concentration and yield at the rate that
supplied 113 mg N/L, n= 3 and at the rate that supplied 283 mg N/L, n= 2. In U2, for CBG concentration
and yield at the rates that supplied 57, 170, and 283 mg N/L, n= 3 and at the rate that supplied 113 mg
N/L, n= 2. For all other values, n=4.THC=Δ9-tetrahydrocannabinol; THCA = D9-tetrahydrocanna-
bidiolic acid; CBG = cannabigerol; CBGA = cannabigerolic acid.
Fig. 5. Response of growingsubstrate electrical conductivity(EC) to organic fertilizer rateapplied during the
flowering stage for cannabis in two substrates (U2-HP and U2). Fertilizer rateis indicated by nitrogen(N)
concentration. Dataare means ± SEM (For U2-HP, n= 4 at weeks 2, 4, and 6 a nd n= 8 at weeks 3 and 5; for
U2, n= 4 at each week) and lines are the best-fit regression relationships with P<0.05.
1800 HORTSCIENCE VOL. 52(12) DECEMBER 2017

growth from frequent irrigation has been
attributed to enhanced nutrient uptake, spe-
cifically improved P mobilization and uptake
(Silber et al., 2003, 2005).
Another factor that may have contrib-
uted to the differences in growth and canna-
binoid concentrations/yield between the
two substrates is root zone oxygen availabil-
ity. With a higher CC (Table 1), U2 holds
more water, consequentially displacing more
air and reducing the oxygen diffusing rate
in the root zone compared to U2-HP. A well-
oxygenated root zone is vital for good plant
health, improving nutrient uptake, root
growth and preventing root-borne disease
(Jackson and Colmer, 2005; Zheng et al.,
2007). Furthermore, there was some evi-
dence of root disease in the present study
that may have been caused by low root zone
oxygen. While there was no statistical dif-
ference, three plants were removed during
the trial due to root rot in U2 compared with
one in U2-HP. To our knowledge, there is
no research into the effects of irrigation or
root zone oxygen on cannabinoid concentra-
tions, but it is speculated that these factors
may have contributed to the higher cannabi-
noid concentrations seen in U2-HP. Results
from the present study suggests that cannabis
may benefit from high irrigation frequency
and/or high root zone oxygen; however,
further study is required to control for these
variables and to optimize irrigation fre-
quency and root zone oxygen concentration
for cannabis.
Plant growth responded as expected to
varying fertilizer application rates; plants
increased in size as fertilizer rate increased
until a maximum at a rate supplying 211 mg
N/L for U2-HP and at a rate supplying
239 mg N/L for U2 at final harvest. The
fertilizer rates which delivered optimal
growth and yield in this trial were generally
higher than typical recommended synthetic
fertilizer rates used in container crop pro-
duction, which rarely exceed 200 mg N/L
(Raviv and Lieth, 2007); this was likely
due to the slow releasing and less soluble
nature of N and P in organic fertilizer when
compared with most synthetic fertilizers
(Prasad et al., 2004). Improved performance
from high organic fertilizer rates (at rates
supplying around 389 mg N/L) was also
reported for vegetative stage cannabis
(Caplan et al., 2017), suggesting that canna-
bis has high organic fertilizer requirements
in both growth stages.
Caplan et al. (2017) also demonstrated
that with varying fertilizer rate and substrate
type applied to vegetative-stage cannabis,
that cannabinoid yield and concentration
did not differ substantially in the variety
‘OG Kush ·Grizzly’ when the plants were
grown under similar flowering-stage condi-
tions. During the vegetative stage, substrate
had no effect on floral cannabinoid concen-
tration, and of all the detected cannabinoids
only THC concentration responded to fertil-
izer rate, increasing to a maximum at a rate
supplying 418 mg N/L. Fertilizer effects were
more substantial in the present study, likely
because cannabinoid concentration increases
mainly during the flowering stage when
glandular trichome development is at its peak
(Aizpurua-Olaizola et al., 2016; Muntendam
et al., 2012). In addition, Caplan et al. (2017)
found that an increase in yield due to optimal
vegetative-stage fertigation was associated
with an increase in the concentration of
THC and cannabinol (CBN) in dry floral
material. In the present study, there was
negligible CBN detected in the floral material
of the variety ‘WP:Med (Wappa)’, and re-
sults differ from Caplan et al. (2017) with
regards to THC concentration. The can-
nabinoid dilution effect, attributed to in-
creased fertilizer rate, may therefore only
apply to the flowering stage. This difference
illustrates a varying effect between vegeta-
tive stage and flowering stage fertilizer rate
for cannabis.
The relationship between fertilizer rate
and yield differed between the two substrates.
In U2-HP, yield decreased at rates that
supplied above 261 mg N/L, yet yield in-
creased linearly with increasing fertilizer rate
in U2. The concentration of fertilizer in the
irrigation water remained constant; therefore,
more fertilizer was used for U2-HP than U2
over the 53-d period. It was presumed that
since more fertilizer was administered for
U2-HP, that substrate EC would accumulate
to a higher level in U2-HP compared with U2.
In fact, the opposite was observed; substrate
EC was slightly higher in U2 than in U2-HP
(Fig. 5). Plants grown in U2-HP also grew
larger, potentially because of increased irri-
gation frequency and/or substrate aeration as
previously described. Increased plant growth
may have facilitated greater nutrient uptake,
accounting for lower EC accumulation in U2-
HP. Silber et al. (2003) found that in lettuce,
increased irrigation frequency improved the
availability of immobile elements such as P
and K allowing for increased uptake. In U2-
HP, at rates that supplied above 261 mg N/L,
plants may have accumulated nutrients to
above-optimum levels, which could account
for the yield reduction at the higher rates. By
contrast, the lesser amount of total adminis-
tered fertilizer in U2 might have not reached
an optimum level.
Low substrate pH may have also con-
tributed to the yield reductions seen at the
highest fertilizer rate in U2-HP. The opti-
mal pH range for cannabis that is suggested
by grey resources (Cervantes, 2006) and
foundtobeacceptableinourprevioustrial
during the vegetative stage (Caplan et al.,
2017) is 5.8–7.2. The pH in both substrates
remained mostly within this range; how-
ever, at the highest fertilizer rate at 6 WFS,
pH in U2-HP was 5.1. Within the measured
range of substrate pH (means of 5.1–7.4 in
the flowering stage) there were no visual
signs of pH-induced disorders. This obser-
vation overlaps the acceptable range cited
in Caplan et al. (2017) of 6.7–7.2 for the
flowering stage. If the lowest substrate pH
measured (5.1) is excluded, a range of sub-
strate pH of 5.5–7.4 appears suitable in the
flowering stage for container production of
organic cannabis. More research is needed
to confirm the optimal range for multiple
varieties.
Leaf ‘greenness’ increased linearly with
increasing fertilizer rate and was positively
correlated to the proceeding GI measurement
as well as yield. The CCI is often a reliable
indicator of leaf N-status; however, the re-
lationship must be determined for a given
species using leaf tissue analysis for accurate
results (Xiong et al., 2015), and to our
knowledge has not been characterized for
cannabis. Current results suggest that CCI
may be an indicator of plant N-status for
Table 3. Response of substrate pH to organic fertilizer rate applied during the flowering stage for cannabis.
WFS
y
Growing substrate
Substrate
x
U2-HP U2
Fertilizer rate [indicated by nitrogen (mg·L
–1
N) concn]
57 113 170 226 283 All
z
57 113 170 226 283 All
z
Growing substrate pH
2 7.4 a
w
7.2 a 7.3 a 7.2 a 7.2 a 7.3 ± 0.04 7.4 a 7.3 a 7.1 a 7.6 a 6.9 a 7.2 ± 0.09 NS
3 7.0 a 6.7 ab 6.4 b 6.7 ab 6.5 ab 6.7 ± 0.06 7.0 a 6.4 a 6.6 a 6.4 a 6.3 a 6.6 ± 0.09 NS
4 6.0 a 6.0 a 5.7 a 6.0 a 5.9 a 5.9 ± 0.10 6.6 a 6.2 a 6.3 a 6.3 a 6.0 a 6.3 ± 0.09 **
5 5.9 a 5.6 a 5.6 a 5.9 a 5.8 a 5.8 ± 0.08 6.9 a 6.4 a 6.1 a 6.4 a 6.4 a 6.5 ± 0.09 ***
6 6.0 a 5.5 ab 5.5 ab 5.8 ab 5.1 b 5.6 ± 0.10 6.9 a 6.4 a 6.5 a 6.5 a 6.4 a 6.5 ± 0.07 ***
All
v
6.5 ± 0.13 6.2 ± 0.14 6.1 ± 0.15 6.3 ± 0.13 6.1 ± 0.14 6.2 ± 0.06 7.0 ± 0.08 6.5 ± 0.12 6.5 ± 0.10 6.6 ± 0.12 6.4 ± 0.09 6.6 ± 0.05
z
Data are means of substrates pooled across fertilizer rates.
y
WFS = weeks in the flowering stage.
x
NS,
*, **, ***
Nonsignificant, or significant at P< 0.05, 0.01, and 0.0001, respectively, within week intervals among substrate treatments.
w
Data followed by the same letter within the same column do not differ at P< 0.05.
v
Data are means of 6 weeks; for U2-HP, n= 4 at 2, 4 and 6 WFS and n= 8 at 3 and 5 WFS; for U2, n= 4 at each WFS.
HORTSCIENCE VOL. 52(12) DECEMBER 2017 1801

cannabis since yield and growth generally
increase with optimal N fertilization.
Finally, further research is required to
evaluate the effects of fertilizer rate and
substrate on other cannabinoids of interest,
such as CBD, which was not detected in the
current variety ‘WP:Med (Wappa)’. This
would require the use of additional cannabis
varieties.
Conclusions
Our results suggest that to produce high-
yielding, cannabinoid-rich, organic cannabis
plants, lower CC coir-based substrates, such
as U2-HP, are preferable to those with higher
CC, such as U2. The drier substrate produced
higher floral yield, GI, THC concentration,
THC yield, THCA yield, and CBGA yield
than U2, possibly because of higher fertiga-
tion frequency or adequate root zone oxygen
leading to a more favorable root zone envi-
ronment. Both substrates generally main-
tained suitable substrate pH between 5.5
and 7.4 and were effective for cannabis pro-
duction during the flowering stage. Increas-
ing fertilizer rate was found to have a dilution
effect on THC, THCA, and CBGA; therefore,
excessive organic fertilizer application dur-
ing the flowering stage should be avoided
despite increased biomass yield. To maxi-
mize both yield and cannabinoid yield, the
optimal organic fertilizer rate for U2-HP was
determined to be within a range supplying
212–261 mg N/L using the Nutri Plus Or-
ganic Bloom liquid organic fertilizer during
the flowering growth stage. Higher rates
within this range favor increased floral yield,
whereas lower rates favor higher yield of
some cannabinoids. For U2, the optimal rate
was one supplying 283 mg N/L to maximize
yield; although, lower rates may be desirable
to increase cannabinoid concentrations.
These recommendations could be general-
ized for similar organic fertilizer and sub-
strates; however, results may vary with
cannabis variety.
Literature Cited
Ahmed, S.A., S.A. Ross, D. Slade, M.M. Radwan,
I.A. Khan, and M.A. ElSohly. 2015. Minor
oxygenated cannabinoids from high potency
Cannabis sativa L. Phytochemistry 117:194–
199.
Ahmed, S.A., S.A. Ross, D. Slade, M.M. Radwan,
F. Zulfiqar, and M.A. ElSohly. 2008. Can-
nabinoid ester constituents from high po-
tency Cannabis sativa L. Planta Med. 74:536
542.
Aizpurua-Olaizola, O., U. Soydaner, E.
€
Ozt€
urk, D.
Schibano, Y. Simsir, P. Navarro, N. Etxebarria,
and A. Usobiaga. 2016. Evolution of the
cannabinoid and terpene content during the
growth of Cannabis sativa plants from different
chemotypes. J. Nat. Prod. 79:324–331.
Amaducci, S., D. Scordia, F.H. Liu, Q. Zhang, H.
Guo, G. Testa, and S.L. Cosentino. 2015. Key
cultivation techniques for hemp in Europe and
China. Ind. Crops Prod. 68:2–16.
ArcView Market Research. 2017. The state of legal
marijuana markets. 5th ed. ArcView Mkt. Res.,
San Francisco, CA.
Aubin, M-P., P. Seguin, A. Vanasse, G.F. Tremblay,
A.F. Mustafa, and J-B. Charron. 2015. Industrial
hemp response to nitrogen, phosphorus, and
potassium fertilization. Crop. Forage Turf-
grass Mgt. 1:1–10.
Bar-Yosef, B. 1999. Advances in fertigation. Adv.
Agron. 65:1–77.
Baxter, W.J. and G. Scheifele. 2008. Growing
industrial hemp in Ontario. Ontario Ministry
of Agriculture, Food and Rural Affairs.
Queen’s Printer for Ontario, Toronto, Canada.
Bowley, S. 1999. A hitchhiker’s guide to statistics
in plant biology. Any Old Subject Books,
Guelph, Canada.
Caplan, D., M. Dixon, and Y. Zheng. 2017.
Optimal rate of organic fertilizer during the
vegetative-stage for cannabis grown in two
coir-based substrates. HortScience 52:1307–
1312.
Cervantes, J. 2006. Marijuana horticulture: The
indoor/outdoor medical grower’s bible. Van
Patten Publishing, Vancouver, WA.
de Kreij, C. and N. Straver. 1988. Flooded-bench
irrigation: Effect of irrigation frequency and
type of potting soil on growth of Codiaeum and
on nutrient accumulation in the soil. Acta Hort.
221:245–252.
Ehrensing, D.T. 1998. Feasibility of industrial
hemp production in the United States Pacific
Northwest. Oregan State Univ. Agr. Expt. Sta.
Bul. 681.
ElSohly, M.A. and D. Slade. 2005. Chemical
constituents of marijuana: The complex mix-
ture of natural cannabinoids. Life Sci. 78:539–
548.
Elzinga, S., J. Fischedick, R. Podkolinski, and J.C.
Raber. 2015. Cannabinoids and terpenes as
chemotaxonomic markers in cannabis. Nat.
Prod. Chem. Res. 3:1–9.
Farag, S. and O. Kayser. 2015. Cultivation and
breeding of Cannabis sativa L. for prepa-
ration of standardized extracts for medici-
nal purposes. Medicinal and aromatic plants
of the world. Springer, Dordrecht, The
Netherlands.
Hillig, K.W. and P.G. Mahlberg. 2004. A chemo-
taxonomic analysis of cannabinoid variation
in cannabis (Cannabaceae). Amer. J. Bot.
91:966–975.
Jackson, M.B. and T.D. Colmer. 2005. Response
and adaptation by plants to flooding stress.
Ann. Bot. 96:501–505.
Katsoulas, N., C. Kittas, G. Dimokas, and C.
Lykas. 2006. Effect of irrigation frequency on
rose flower production and quality. Biosyst.
Eng. 93:237–244.
Kimura, M. and K. Okamoto. 1970. Distribution of
tetrahydrocannabinolic acid in fresh wild can-
nabis. Experientia 26:819–820.
Knight, G., S. Hansen, M. Connor, H. Poulsen, C.
McGovern, and J. Stacey. 2010. The results of
an experimental indoor hydroponic cannabis
growing study, using the ‘‘Screen of Green’’
(ScrOG) method-yield, tetrahydrocannabinol
(THC) and DNA analysis. Forensic Sci. Intl.
202:36–44.
Leggett, T. 2006. Review of the world cannabis
situation. Bul. Narc. 58:1–155.
Mechoulam, R., A. Shani, H. Edery, and Y.
Grunfeld. 1970. Chemical basis of hashish
activity. Science 169:611–612.
Morvant, J.K., J.M. Dole, and J.C. Cole. 1998.
Irrigation frequency and system affect poinset-
tia growth, water use, and runoff. HortScience
33:42–46.
Muntendam, R., N. Happyana, T. Erkelens, F.
Bruining, and O. Kayser. 2012. Time de-
pendent metabolomics and transcriptional
analysis of cannabinoid biosynthesis in
Cannabis sativa var. Bedrobinol and Bediol
grown under standardized condition and with
genetic homogeneity. Online Intl. J. Med.
Plants Res. 1:31–40.
Potter, D.J. 2014. A review of the cultivation and
processing of cannabis (Cannabis sativa L.) for
production of prescription medicines in the
UK. Drug Test. Anal. 6:31–38.
Potter, D.J. and P. Duncombe. 2012. The effect of
electrical lighting power and irradiance on
indoor-grown cannabis potency and yield. J.
Forensic Sci. 57:618–622.
Prasad, M., P. Simmons, and M.J. Maher. 2004.
Release characteristics of organic fertilizers.
Acta Hort. 644:163–170.
Radwan, M.M., M.A. ElSohly, A.T. El-Alfy, S.A.
Ahmed, D. Slade, A.S. Husni, S.P. Manly, L.
Wilson, S. Seale, S.J. Cutler, and S.A. Ross.
2015. Isolation and pharmacological evalua-
tion of minor cannabinoids from high-
potency Cannabis sativa.J.Nat.Prod.78:
1271–1276.
Raviv, M. and J.H. Lieth. 2007. Soilless culture:
Theory and practice. 1st ed. Elsevier B.V.,
Burlington, MA.
Ross, S.A. and M.A. ElSohly. 1997. CBN and Δ9-
THC concentration ratio as an indicator of the
age of stored marijuana samples. Bul. Narc.
49:139.
Russo, E.B. 2007. History of cannabis and its
preparations in saga, science, and sobriquet.
Chem. Biodivers. 4:1614–1648.
Ruter, J.M. 1992. Influence of source, rate, and
method of applicating controlled release fer-
tilizer on nutrient release and growth of
‘‘Savannah’’ holly. Fert. Res. 32:101–106.
Silber, A., M. Bruner, E. Kenig, G. Reshef, H.
Zohar, I. Posalski, H. Yehezkel, D. Shmuel, S.
Cohen, M. Dinar, E. Matan, I. Dinkin, Y.
Cohen, L. Karni, B. Aloni, and S. Assouline.
2005. High fertigation frequency and phospho-
rus level: Effects on summer-grown bell pepper
growth and blossom-end rot incidence. Plant
Soil 270:135–146.
Silber, A., G. Xu, and R. Wallach. 2003. High
irrigation frequency: The effect on plant
growth and on uptake of water and nutrients.
Acta Hort. 627:89–96.
Surrage, V.A., C. Lafreni
ere, M. Dixon, and Y.
Zheng. 2010. Benefits of vermicompost as
a constituent of growing substrates used in the
production of organic greenhouse tomatoes.
HortScience 45:1510–1515.
Taschwer, M. and M.G. Schmid. 2015. Determina-
tion of the relative percentage distribution of
THCA and D(9)-THC in herbal cannabis seized
in Austria—Impact of different storage tempera-
tures on stability. Forensic Sci. Intl. 254:167–171.
United Nations Office on Drugs and Crime. 2009.
Recommended methods for the identification
and analysis of cannabis and cannabis products,
manual for use by national drug analysis
laboratories. Laboratory and Scientific Section,
United Nations Office on Drugs and Crime,
Vienna, Austria.
van Bakel, H., J.M. Stout, A.G. Cote, C.M. Tallon,
A.G. Sharpe, T.R. Hughes, and J.E. Page. 2011.
The draft genome and transcriptome of Can-
nabis sativa. Genome Biol. 12:1–17.
Vanhove, W., T. Surmont, P. Van Damme, and B.
De Ruyver. 2012. Yield and turnover of illicit
indoor cannabis (Cannabis spp.) plantations in
Belgium. Forensic Sci. Intl. 220:265–270.
Vanhove, W., P. Van Damme, and N. Meert. 2011.
Factors determining yield and quality of illicit
indoor cannabis (Cannabis spp.) production.
Forensic Sci. Intl. 212:158–163.
1802 HORTSCIENCE VOL. 52(12) DECEMBER 2017

Vemuri, V.K. and A. Makriyannis. 2015. Medicinal
chemistry of cannabinoids. Clin. Pharmacol.
Ther. 97:553–558.
Vera, C.L., S.S. Malhi, J.P. Raney, and Z.H.
Wang. 2004. The effect of N and P fertiliza-
tion on growth, seed yield and quality of
industrial hemp in the Parkland region of
Saskatchewan. Can. J. Plant Sci. 84:939–947.
Vollner, L., D. Bieniek, and F. Korte. 1986. Review
of analytical methods for identification and
quantification of cannabis products. Regulat.
Toxicol. Pharmacol. 6:348–358.
Warncke, D.D. 1986. Analyzing greenhouse
growth media by the saturation extraction
method. HortScience 21:223–225.
Wright, R.D. 1986. The pour-through nutrient ex-
traction procedure. HortScience 21:227–229.
Wright, R.D. and A.X. Niemiera. 1987. Nutrition
of container-grown woody nursery crops. Hort.
Rev. 9:75–101.
Xiong, D., J. Chen, T. Yu, W. Gao, X. Ling, Y. Li,
S. Peng, and J. Huang. 2015. SPAD-based leaf
nitrogen estimation is impacted by environ-
mental factors and crop leaf characteristics.
Sci. Rpt. 5:13389.
Xu, G., S. Wolf, and U. Kafkafi. 2001. Effect
of varying nitrogen form and concentration
during growing season on sweet pepper
flowering and fruit yield. J. Plant Nutr.
24:1099–1116.
Zhai, Z., D.L. Ehret, T. Forge, T. Helmer, W.
Lin, and A.P. Papadopoulos. 2009. Organic
fertilizers for greenhouse tomatoes: Produc-
tivity and substrate microbiology. HortScience
44:800–809.
Zheng, Y. 2016. Root zone environment
management in container crop pro-
duction. Proc. for the Veg., Potato, Green-
house, Small Fruit & Gen. Session
Mid-Atlantic Fruit & Veg. Convention,
Hershey, PA.
Zheng, Y., L. Wang, and M. Dixon. 2007. An
upper limit for elevated root zone dissolved
oxygen concentration for tomato. Sci. Hort.
113:162–165.
HORTSCIENCE VOL. 52(12) DECEMBER 2017 1803