August 2020 - 22

CONTROLLED ENVIRONMENT AGRICULTURE
day throughout the experiment. Flats were placed on top of
expanded metal rolling benched in a glass-glazed greenhouse
with 75° F days and 70° F nights and a daily light integral
(DLI) between 12 and 14 mol∙m−2
∙d−1
.
Seedling data were collected 15 (pac choi), 16 (arugula
and Swiss chard) or 17 days (kale) after seeding. Heights
of 10 seedlings were taken from the top of the germination
substrate to the top of the growing tip. Then, 40 seedlings
were harvested by cutting at the top of the substrate, triple
rinsing with deionized water, and placed into a brown paper
bag to dry for dry weight. After dry weights were recorded,
shoot tissue was submitted for nutrient concentrations
analyses at a commercial laboratory.
In the second experiment, seedlings of arugula, 'Starbor'
kale, 'Win-Win' pac choi, and 'Fordhook Giant' Swiss chard
were transplanted into deep-flow technique (DFT) hydroponic
systems. Nutrient solutions were maintained at an EC of 0.5,
1.0, 2.0, 3.0, 4.0 or 5.0 mS·cm-1
using the same complete,
balanced, water-soluble fertilizer (Jack's Hydro FeED
16N-4P-17K; J.R. Peters) as in the seedling experiment. The
pH was maintained at 5.8 using potassium hydroxide.
Throughout the entire experiment, the EC and pH were
constantly maintained at target values using auto-dosers
that measured and corrected EC and pH as needed. The DFT
systems were grown in a similar glass-glazed greenhouse as
the seedlings were, with 73° F days and 66° F nights and a
daily light integral (DLI) between 12 and 14 mol∙m−2
∙d−1
.
Finished plant data were collected two (arugula), three (pac
choi and swiss chard), or four weeks (kale) after transplanting
seedlings into DFT systems and initiating treatments. Height,
width and leaf greenness were measured, after which plants
were harvested and fresh and dry weights were collected.
INCREASING FERTILIZER PROMOTES
SEEDLING GROWTH
In the first experiment, fertilizer concentration had varying
effects on plant growth. For all four species, height was
unaffected by fertilizer concentration. Alternatively, the dry
weight increased with fertilizer concentration for arugula,
kale and Swiss chard up to 200-ppm nitrogen, but not for
pac choi. Although we did not collect data on leaf area, as
fertilizer increased, so did leaf size.
Mineral nutrient concentrations were affected differently
by fertilizer concentration. For example, nitrogen (N),
phosphorus (P), potassium (K), and magnesium (Mg)
increased with increasing fertilizer concentration, whereas
calcium (Ca) and sulfur (S) decreased. For micronutrients,
while copper (Cu) and boron (B) were largely unaffected by
fertilizer concentration, iron (Fe), manganese (Mn) and zinc
(Zn) increased with increasing fertilizer concentrations.
Based on our research, there are a few pointers to
fertilizing specialty herb seedlings. First, seedling growth is
promoted by increasing fertilizer concentration, up to 200ppm
nitrogen. The effect of seedling size is not totally clear
- does a larger seedling finish more quickly? This makes
sense, though we do not have data to support that fact.
One challenge associated with growing large seedlings is
separating them for transplanting. The large leaves on bigger
seedlings can get tangled together when trying to separate
sheets of seedlings to transplant individual plugs.
22 AUGUST 2020 GPNMAG.COM
Figure 1. Arugula (top row), kale (middle row), and Swiss chard (bottom
row) seedlings grown in a 162-cell sheet of phenolic foam propagation
cubes and fertilized with 0- to 300-pm nitrogen from a complete, balanced
hydroponic fertilizer daily.
In addition promoting growth, increasing fertilizer concentration
increased tissue nutrient concentrations for most elements (N, P, K, Mg,
Fe, Mn and Zn). This makes sense, as increasing EC increases mineral
nutrient concentrations available for plants to take up. On the other hand,
increasing EC resulted in decreasing Ca and S tissue concentrations. The
likely cause of reductions in tissue Ca and S was the " dilution effect, "
whereby the increase in plant growth caused the percentage of Ca and
S in tissue to decline. The total Ca and S in specialty green seedlings did
not decrease with increasing EC, supporting our hypothesis. Regardless of
tissue nutrient values, all seedlings had a healthy appearance and had no
deficiency or toxicity symptoms.
Healthy specialty green seedlings can be grown with 25- to 200ppm
nitrogen. As the fertilizer concentration increases, so does shoot
growth. Therefore, fertilizer concentrations can be used to push growth of
overhead-irrigated seedlings. If sub-irrigation is used to fertilize seedlings,
using 150- or 200-ppm nitrogen solutions may not be advisable, since
young plants can be sensitive to high salt levels; in our study, the overhead
irrigation leached excess salts, whereas sub-irrigated seedlings would
accumulate them.
CROPS VARY IN THEIR NUTRIENT REQUIREMENTS
DURING FINISHING
In the second experiment, the height and width of plants were largely
unaffected by nutrient solution EC. Alternatively, plant weight responded
differently to increasing fertilization among specialty green species. Fresh
and dry weight of arugula and pac choi decreased as EC increased.
Alternatively, as EC increased from 0.5 to 2.0 mS·cm-1
swiss chard
weight increased, but decreased as EC further increased from 2.0 to
5.0 mS·cm-1
between 1.0 and 5.0 mS·cm-1
. While there was no difference between kale fresh weight
, plants grown in 0.5 mS·cm-1
weighed less
than the other EC treatments.
The tissue nutrient concentrations of finished plants were similar in the
finishing experiment as they were in the seedling experiment, in that tissue
nutrient concentrations increased for most macro- and micronutrients.
Similarly, Ca decreased with increasing EC. However, Mg decreased with
increasing EC, while S increased. Only Mn in arugula and pac choi and B
and Fe in swiss chard, decreased with increasing EC.
Nutrient solutions for specialty greens can be managed differently
for the different species. For arugula and pac choi, the EC should be
maintained between 0.5 and 2.0 mS·cm-1
, as greater concentrations
50
N concentration (ppm)
100
150
200
300
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August 2020

Table of Contents for the Digital Edition of August 2020
