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horticulturae
Article
Shoot Production and Mineral Nutrients of Five Microgreens
as Affected by Hydroponic Substrate Type and
Post-Emergent Fertilization
Tongyin Li *, Geoffrey T. Lalk, Jacob D. Arthur, Madeline H. Johnson and Guihong Bi
Citation: Li, T.; Lalk, G.T.; Arthur,
J.D.; Johnson, M.H.; Bi, G. Shoot
Production and Mineral Nutrients of
Five Microgreens as Affected by
Hydroponic Substrate Type and
Post-Emergent Fertilization.
Horticulturae 2021,7, 129. https://
doi.org/10.3390/horticulturae7060129
Academic Editor: Nazim Gruda
Received: 30 April 2021
Accepted: 31 May 2021
Published: 2 June 2021
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This article is an open access article
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Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Department of Plant and Soil Sciences, Mississippi State University, Starkweli, MS 39762, USA;
gtl31@msstate.edu (G.T.L.); jda360@msstate.edu (J.D.A.); mj1215@msstate.edu (M.H.J.);
gbi@pss.msstate.edu (G.B.)
*Correspondence: tl665@msstate.edu
Abstract:
As a new specialty crop with high market value, microgreens are vegetable or herb seedlings
consumed at a young age, 7–21 days after germination. They are known as functional food with high
concentrations of mineral nutrients and health beneficial phytochemicals. Microgreen industry lacks
standardized recommendations on cultural practices including species/variety selection, substrate
choice, and fertilization management. This study evaluated shoot growth and mineral nutrient
concentrations in five microgreens including four Brassica and one Raphanus microgreens as affected
by four hydroponic pad types and post-emergent fertilization in two experiments in January and
February 2020. The five microgreens varied in their shoot height, fresh, dry shoot weights, and
mineral nutrient concentrations with radish producing the highest fresh and dry shoot weights.
Radish had the highest nitrogen (N) concentration and mustard had the highest phosphorus (P)
concentrations when grown with three hydroponic pads except for hemp mat. Hydroponic pad type
altered fresh, dry shoot weights, and mineral nutrients in tested microgreens. Microgreens in hemp
mat showed the highest shoot height, fresh, dry shoot weights, and potassium (K) concentration, but
the lowest N concentration in one or two experiments. One time post-emergent fertilization generally
increased shoot height, fresh, dry shoot weights, and macronutrient concentrations in microgreens.
Keywords: shoot growth; yield; mineral nutrients; hydroponic pad; fertilization
1. Introduction
Microgreen is an emerging new specialty crop with high market value of USD
30–50 per pound
(454 g) [
1
]. It is a collective term for vegetable, herb, grain, or flower
seedlings consumed at a young stage [
2
–
5
]. Microgreens are harvested 7–21 days after
germination with expanding cotyledons or first pair of true leaves [
6
]. A number of
plant species in the families including Amaranthacea, Apiaceae, Asteraceae, Brassicaceae,
Fabaceae, and Lamiaceae, have been grown as microgreens, among which Brassica crops
are of the most grown species [
7
–
11
]. Microgreens are used by chef and consumers to
enhance flavor, color, and texture in various foods and have become increasingly popular
in recent years as consumer awareness of microgreen dietary value increases [
1
,
3
,
8
]. The
high market value, increasing customer demand, and short production cycle had drawn
interest among vegetable growers to produce microgreens [7].
Microgreens are reported to be four to six times nutrient denser than their mature
counterparts [
12
–
15
]. They are considered functional food with high mineral nutrient con-
centrations and health-beneficial phytochemicals [
15
]. Xiao et al. [
16
] reported microgreen
greens in the Brassicaceae family are most abundant in macronutrients of K and calcium
(Ca) and micronutrients of iron (Fe) and zinc (Zn) after evaluating 30 varieties. They also
contain high levels of antioxidant phytochemicals including ascorbic acid, carotenoids,
glucosinolates, and polyphenols with substantial variations within and between species [
6
].
Horticulturae 2021,7, 129. https://doi.org/10.3390/horticulturae7060129 https://www.mdpi.com/journal/horticulturae
Horticulturae 2021,7, 129 2 of 17
Microgreen lettuce (Lactuca sativa) have higher mineral concentrations including Ca, Mag-
nesium (Mg), Fe, manganese (Mn), Zn, selenium (Se), and molybdenum (Mo) and lower
nitrate concentration than mature lettuce [
10
]. Basil (Ocimum basilicum) microgreens can be
biofortified with Se to satisfy the human dietary need for this micronutrient [
17
]. There
lacks standardized recommendations for cultural practices including species/variety and
substrate choice, fertilization, and control of microenvironment in relation to shoot yield
and nutrient compositions of microgreens [4,9,18,19].
Fertilization was considered optional in microgreen production since seeds have stored
nutrient for initial seedling growth. However, fertilizer application, premixed in growing
substrate, or applied post-emergent was reported to result in fast growth and high yield of
microgreen shoots [
20
]. The most economic fertilization treatments for arugula (Eruca vesi-
caria subsp. sativa) microgreens were daily post-emergent fertigation with
150 mg·L−1N
or
daily application of 75 mg
·
L
−1
N plus preincorporation of
1000 mg·L−1N
[
18
]. Preharvest
daily spray of 10 mM calcium chloride solution on broccoli (Brassica oleracea var. italica) mi-
crogreens increased shoot biomass, Ca concentration, and improved visual quality during
storage. Plant species/variety, slow versus fast growing, may differ in their requirements
for fertilization [
1
]. With an increasing number of species and varieties grown as micro-
greens, crop-specific fertilizer requirements with respect to microshoot yield and mineral
nutrients largely remain unclear.
Microgreens can be produced with peat-based potting mix or hydroponic pads that
are made from synthetic or recycled fibrous materials [
7
]. A variety of hydroponic pads
including biostrate (made from felt fiber), jute mat, and hemp mat are commercially
available, some of which are compostable and can serve as a sustainable alternative to a
peat-based substrate [
21
,
22
]. Physical and chemical properties including container capacity,
air filled porosity, bulk density, pH, and electrical conductivity (EC) vary among substrate
types and affect growth of microgreen crops [
21
]. Efficacy of using hydroponic mats to
grow microgreens is species or variety dependent. Hydroponic pads made from textile
fibers and jute-kenaf fibers were reported to produce similar fresh shoot yield of rapini
(Brassica rapa L.) microgreens [
21
]. Hydroponic substrate type also affects nutritional facts
of microgreens and food safety in microgreen production due to the humid conditions
and warm temperatures. The latter has become one of the most important concerns in
microgreen production [
7
], where hydroponic mat type was found to affect microbial
populations on microgreens [21].
The objective of this study was to investigate shoot production and mineral nutrients
of five microgreens grown with four types of hydroponic pads as affected by post-emergent
fertilization.
2. Materials and Methods
2.1. Plant Materials and Cultivation
Shoot growth and mineral nutrient concentrations of five microgreens were evaluated
(Table 1). Microgreen seeds of selected species were purchased from the True Leaf Market
(Salt Lake City, UT, USA). Seed sowing rate for each microgreen was determined by supplier
recommendation and summarized in Table 1. Hundred-seed weight of each microgreen
was measured with three replications. This study was conducted in a greenhouse at
Mississippi State University, USA. (33.4552
◦
N, 88.7944
◦
W) and included two experiments
with the first conducted on 29 January 2020 and then repeated on 25 February 2020. The
temperature in the greenhouse was set at 25 ◦C with natural light.
Each microgreen was grown with four types of hydroponic pad including biostrate,
hemp mat, micro-mats, and jute mat (Table 2). Each hydroponic pad was precut or manually
cut into the size of approximately 25 cm by 25 cm to fit the bottom of the growing tray. Black
plastic trays without drainage holes (width 25.72 cm, length 25.72 cm, depth 6.03 cm; T.O.
Plastics, Clearwater, MN, USA) were used to grow microgreens in this study. Hydroponic
pads were then hydrated by soaking them into tap water, drained with excessive water, and
placed into each growing tray. Seeds of appropriate weight were measured and manually
Horticulturae 2021,7, 129 3 of 17
sown onto the hydrated hydroponic pads, and covered with another black tray to provide
a dark environment and maintain moisture during germination. Microgreen seeds were
misted with a spray bottle every 12 h to moisturize the seeds and substrate, when the cover
tray was removed shortly and put back on after misting. Four days after sowing, the cover
tray was turned upside down with the bottom being placed on top of microgreen shoots for
another 24–48 h. This practice was to provide some resistance and encourage microshoot
elongation as recommended by the seed supplier. The cover tray was then removed at
approximately 7 days after planting (DAP).
Table 1. Common name, scientific name, seeding rate, 100 seed weight, and harvest date of five microgreens.
Common
Name Scientific Name Seeding Rate
(g·m−2)
100 Seed Wt.
(g)
Harvest Date
(DAP) Harvest Stage 1
Broccoli Brassica oleracea var. italica cv.
‘Waltham’ 98.3 0.44 ±0.015 12–14 Stage 1
Cabbage Brassica oleracea var. capitata cv. ‘Red
Acre’ 83.1 0.44 ±0.016 14–15 Stage 1
Kale Brassica napus var. pabularia cv. ‘Red
Russian’ 75.6 0.23 ±0.008 14–16 Stage 1
Mustard Brassica juncea cv. ‘Red Garnet’ 60.5 0.18 ±0.003 15–16
Mixed with
70–80% stage 1
shoots
Radish Raphanus sativus cv. ‘Rambo’ 189.0 1.10 ±0.03 12–14 Stage 1
1
Microgreens were harvested with the expanding cotyledons (microgreen stage 1) or with the first pair of true leaves (microgreen stage 2).
Table 2.
Product name, constructed material, supplier, and manufacturer information of the four types of hydroponic pads
used in this study.
Hydroponic Pad Product Materials Supplier Manufacturer
Biostrate®Felt fiber True Leaf Market Quick Plug, South Portland, ME, USA
Hemp mat Hemp fiber Amazon Terrafibre, Drayton Valley, Canada
Jute mat Jute fiber True Leaf Market Handy Pantry, Salt Lake City, UT, USA
Micro-Mats Wood fiber True Leaf Market Handy Pantry, Salt Lake City, UT, USA
After covers were removed, half of the trays from each microgreen were fertigated
once with 120 mL of water-soluble fertilizer 20N-8.7P-16.6K (Peters
®
Professional 20-20-
20 General Purpose, also containing (wt/wt) 0.05% Mg, 0.05% Fe, 0.025% Mn, 0.013%
boron (B), 0.013% copper (Cu), 0.005% Mo, and 0.025% Zn; ICL Specialty Fertilizers, Tel-
Aviv, Israel) at a rate of 100 mg
·
L
−1
N. The fertilizer solution had a pH of 6.56 and EC
of
0.41 mS·cm−1
. As a control to the fertilization treatment, the other half of trays were
irrigated with the same volume of water (pH 7.54; EC 0.15 mS·cm−1).
2.2. Data Collection and Shoot Harvest
Plant height was measured in each tray before shoot harvest from the substrate surface
to the highest point of shoot growth. Microgreen shoot in each tray were carefully harvested
above the substrate surface, with the expanding cotyledons (microgreen stage 1) or with
the first pair of true leaves (microgreen stage 2) as described by Waterland et al. [
5
]. Fresh
shoot weight of microgreens harvested from each tray was measured. Freshly harvested
microgreen shoots were then oven dried at 60
◦
C until constant weight and measured for
dry shoot weight (DW). Dry weight percentage (%) was also determined for each tray.
2.3. Mineral Nutrient Analyses
Dry microgreen samples were ground to pass a 1-mm sieve with a grinder (Wiley mini
mill, Thomas Scientific, Swedesboro, NJ, USA) for mineral nutrient analyses. Combustion
analysis was used for the determination of total N concentration with 0.25 g of dry tissue
Horticulturae 2021,7, 129 4 of 17
using an elemental analyzer (vario MAX cube; Elementar Americas Inc., Long Island,
NY, USA). A dry tissue sample of 0.5 g was digested with 1 mL of 6 M hydrochloric acid
(HCl) and 50 mL of 0.05 M HCl for the concentrations of P, K, Ca, Mg, Cu, Fe, Mn, Zn,
and B using inductively coupled plasma optical emission spectrometry (SPECTROBLUE;
SPECTRO Analytical Instruments, Kleve, Germany). Microgreen samples were tested at
the Mississippi State University Extension Service Soil Testing Laboratory. Concentrations
of macronutrients (mg
·
g
−1
) and micronutrients (
µ
g
·
g
−1
) in microgreens were presented
on a dry weight basis.
2.4. Experimental Design and Statistical Analyses
This experiment was conducted in a randomized complete block design with a fac-
torial arrangement of treatments. Microgreens (5 species), hydroponic pad (4 types), and
fertilization (fertilizer or not) were the three main factors contributing to 40 treatment com-
binations. Each treatment combination had five replications with an individual growing
tray as the experimental unit. Significance of any main effect or the interaction among
main factors were determined by analysis of variance (ANOVA) using PROC GLMMIX
procedure of SAS (version 9.4; SAS Institute, Cary, NC, USA). Where indicated by ANOVA,
means were separated by Tukey’s honest significant difference (HSD) at
α≤
0.05. Data
from the two experiments were compared as repeated measures. All statistical analyses
were performed using SAS.
3. Results
The experiment date affected all measured dependent variables in this study, showing
varying trends between the January and February experiments (data not shown). Therefore,
data from the two experiments were presented separately. There was no three-way interac-
tion among microgreen species, hydroponic substrate type, and fertilization treatment for
any measured variable.
3.1. Shoot Height
Shoot height was affected by the interaction between microgreen species and hydro-
ponic pad type in both experiments (Tables 3and 4). Hemp resulted in the largest shoot
height within each microgreen, with biostrate and jute mat resulting in similar shoot height
in four microgreens next to hemp in January and February 2020. Micro-mats resulted in
the lowest height in broccoli, kale, and radish in both experiments. Shoot height of radish
and broccoli were higher than cabbage, kale, or mustard grown with any substrate type in
both experiments. One time post-emergent fertilization increased shoot height by 3.2% and
2.5% in January and February 2020, respectively compared to the no fertilizer treatment
(Table 5).
3.2. Fresh and Dry Shoot Weights
Microgreen shoots were harvested with expanding cotyledons or the first pair of true
leaves with varying days ranging from 12 to 16 DAP (Table 1, Figure S1). Fresh, dry shoot
weights, and dry weight percentage were affected by the interaction between microgreen
species and hydroponic pad type in January 2020 (Table 3). Hemp mat resulted in higher
fresh and dry shoot weights in each microgreen than other substrates in January (
Table 3
).
Micro-Mats resulted in the lowest fresh and dry shoot weights in broccoli, kale, and radish.
Among the five microgreens, radish produced the highest fresh and dry shoot weights
when grown with each substrate type, with broccoli being the second highest, higher
than kale, cabbage, or mustard. Dry weight percentage ranged from 5.48% in kale grown
with hemp to 9.21% in mustard grown with Micro-Mats. The comparable highest dry
weight percentages were found in cabbage, kale, and mustard grown with Micro-Mats and
cabbage grown with biostrate. Hemp resulted in the lowest dry weight percentages in
cabbage (5.81%), kale (5.48%), and mustard (6.78%) among the four substrate types.
Horticulturae 2021,7, 129 5 of 17
Fresh shoot weight and dry weight percentage were also affected by the interaction
between species and fertilization treatment in January 2020, when one time post-emergent
fertilization increased fresh shoot weight and decreased dry weight percentage in kale and
radish, but resulted in similar values of both variables in broccoli, cabbage, and radish
(Table 6). One time fertilization also increased overall dry shoot weight by 2.7% in the
February experiment (Table 5).
Table 3.
Shoot height, fresh shoot weight, dry shoot weight, and dry weight percentage of five microgreens affected by the
interaction between species and hydroponic pad type in January 2020.
January 2020
Microgreens Hydroponic Pad Shoot Height 1,2 Fresh Shoot Weight Dry Shoot Weight Dry Weight Percentage
(cm) (g·m−2) (g·m−2) (%)
Broccoli
Biostrate 6.69 cd 1322 ef 89.1 ef 6.76 de
Hemp 8.32 b 1843 c 109.8 d 6.04 efg
Jute 6.87 c 1512 de 100.1 de 6.63 def
Micro-Mats 5.78 ef 1136 fg 76.7 gh 6.77 de
Cabbage
Biostrate 4.85 ghi 533 j 46.0 l 8.78 ab
Hemp 6.71 cd 1462 de 84.9 fg 5.81 fg
Jute 5.3 fgh 817 hi 64.7 ij 7.92 bc
Micro-Mats 4.61 hij 539 j 48.5 kl 9.10 a
Kale
Biostrate 5.24 fgh 1003 gh 70.7 hi 7.23 cd
Hemp 6.9 c 1487 de 81.5 fgh 5.48 g
Jute 5.35 efg 1000 gh 63.6 ij 6.55 def
Micro-Mats 3.92 jk 656 ij 58.2 jk 9.06 a
Mustard
Biostrate 4.28 ijk 533 j 42.4 l 7.99 bc
Hemp 6.05 de 941 gh 63.8 ij 6.78 de
Jute 4.34 ij 612 ij 48.1 kl 7.87 bc
Micro-Mats 3.58 k 468 j 43.0 l 9.21 a
Radish
Biostrate 7.25 b 2278 b 152.2 b 6.77 de
Hemp 9.71 a 3158 a 186.4 a 5.92 efg
Jute 8 b 2414 b 160.8 b 6.69 def
Micro-Mats 6.01 de 1676 cd 122.9 c 7.46 cd
p-value Species <0.0001 <0.0001 <0.0001 <0.0001
Substrate <0.0001 <0.0001 <0.0001 <0.0001
Species *
Substrate 3<0.0001 <0.0001 <0.0001 <0.0001
1
Different lower case letters suggest significant difference among means within a column indicated by the Tukey’s HSD test at p
≤
0.05.
2Each
mean was obtained by averaging data from both the fertilized and no-fertilizer treatments.
3
Species * Substrate suggests interaction
between species and substrate type.
Table 4.
Shoot height, fresh shoot weight, dry shoot weight, and dry weight percentage of five microgreens affected by the
interaction between species and hydroponic pad type or the main effect of specie in February 2020.
February 2020
Microgreens Hydroponic Pad Shoot Height 1,2 Fresh Shoot Weight Dry Shoot Weight Dry Weight Percentage
(cm) (g·m−2) (g·m−2) (%)
Broccoli
Biostrate 7.08 c
1289 b 84.9 b 6.68 a
Hemp 8.31 a
Jute 7.45 b
Micro-Mats 6.55 cd
Cabbage
Biostrate 4.59 f–i
968 c 67.8 c 7.20 a
Hemp 5.79 e
Jute 5.01 f
Micro-Mats 4.18 hi
Horticulturae 2021,7, 129 6 of 17
Table 4. Cont.
February 2020
Microgreens Hydroponic Pad Shoot Height 1,2 Fresh Shoot Weight Dry Shoot Weight Dry Weight Percentage
(cm) (g·m−2) (g·m−2) (%)
Kale
Biostrate 4.75 fgh
929 c 71.2 c 7.90 a
Hemp 6.53 cd
Jute 4.93 f
Micro-Mats 3.96 i
Mustard
Biostrate 4.26 fhi
626 d 46.9 d 7.71 a
Hemp 6.25 de
Jute 4.83 f
Micro-Mats 4.25 ghi
Radish
Biostrate 7.24 b
2325 a 156.3 a 7.97 a
Hemp 8.92 a
Jute 7.24 b
Micro-Mats 6.3 de
p-value Species <0.0001 <0.0001 <0.0001 0.58
Substrate <0.0001 <0.0001 <0.0001 0.24
Species *
Substrate 30.0002 0.81 0.18 0.39
1
Different lower case letters suggest significant difference among means within a column indicated by the Tukey’s HSD test at p
≤
0.05.
2Means
for shoot height were obtained by averaging data from both the fertilized and no-fertilizer treatments; means of each microgreen
species for fresh, dry shoot weights, and dry weight percentage were obtained by averaging data from both fertilization treatments and all
four substrate types. 3Species * Substrate suggests interaction between species and substrate type.
Table 5.
Microgreen shoot growth and mineral nutrient concentrations affected by the main effect of post-emergent
fertilization.
January 2020 February 2020
Fertilization Shoot Height 1Dry Shoot Weight Mg 2S Shoot Height N P
(cm) (g·m−2) (mg·g−1) (mg·g−1) (cm) (mg·g−1) (mg·g−1)
Fertilized 6.08 86.8 5.24 7.60 5.99 49.6 9.48
Not fertilized 5.90 84.5 5.38 7.87 5.85 48.6 8.74
p-value 30.004 0.035 0.034 0.015 0.017 0.0003 <0.0001
1
Means of the fertilized or not fertilized treatment were obtained by averaging data over five tested microgreens grown with four types of
hydroponic substrates.
2
Mg = magnesium; S = sulfur; N = nitrogen; P = phosphorus.
3
p
≤
0.05 suggests significant difference between
means within a column indicated by Tukey’s HSD test.
In February 2020, fresh and dry shoot weights were affected by the main effects of
species and hydroponic pad type without interaction (Table 4). Ranking for both fresh
and dry weights among microgreens are radish > broccoli > kale or cabbage > mustard.
Hemp resulted in the highest fresh and dry weights of 1551 g
·
m
−2
and 101.9 g
·
m
−2
in
microgreens, with jute and biostrate producing similar fresh and dry shoot weights. Micro-
mats resulted in the lowest fresh shoot weight among all substrate types, but similar dry
weight to biostrate or jute (Data not shown). Dry weight percentage was not affected by
any experimental factor in February.
3.3. Macronutrient Concentrations
Macronutrient concentrations including N, P, K, Ca, Mg, and S in microgreens were
affected by the interaction between species and hydroponic pad in January and February
2020 (Tables 7and 8). Concentrations of N, P, and K in January and K concentration in
February were also affected by the interaction between species and fertilization (Table 6).
Horticulturae 2021,7, 129 7 of 17
Table 6.
Fresh shoot weight, dry weight percentage, and macronutrient concentrations in five microgreens affected by the
interaction between species and post-emergent fertilization treatment in January or February 2020.
January 2020 February
2020
Microgreens Fertilization Fresh Shoot Weight 1,2 Dry Weight Percentage N3P K K
(g·m−2) (%) (mg·g−1) (mg·g−1) (mg·g−1) (mg·g−1)
Broccoli Fertilized 1498 c 6.4 d 55.4 b 10.1 cd 18.8 b 14.8 c
Not fertilized 1408 c 6.7 cd 54.8 b 9.9 d 16.9 cd 13.2 d
Cabbage Fertilized 851 ef 7.8 a 51.1 c 9.5 de 18.9 b 13.3 d
Not fertilized 824 f 8.1 a 49.2 de 8.5 f 17.9 bcd 11.0 e
Kale Fertilized 1112 d 6.6 cd 50.7 cd 9.6 de 16.2 de 11.6 e
Not fertilized 961 e 7.6 ab 47.9 e 8.8 ef 14.7 ef 9.5 f
Mustard Fertilized 651 g 7.9 a 42.8 f 12.0 a 21.6 a 20.0 a
Not fertilized 626 g 8.0 a 41.3 g 10.8 bc 18.3 bc 17.0 b
Radish Fertilized 2235 a 6.3 d 61.0 a 11.0 b 14.5 ef 10.8 ef
Not fertilized 2227 b 7.1 bc 60.8 a 11.2 ab 14.0 f 10.3 ef
p-value Species <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
Fertilization <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
Species *
fertilization 3,4 0.0002 0.0012 0.0059 0.0016 0.014 0.013
1
Different lower case letters suggest significant difference among means within a column indicated by the Tukey’s HSD test at
p≤0.05
.
2Each
mean was obtained by averaging data over four hydroponic pad types.
3
N = nitrogen; P = phosphorus; K = potassium.
4
Species *
fertilization suggests interaction between species and fertilization treatment.
Table 7.
Macronutrient concentrations in five microgreens affected by the interaction between species and hydroponic pad
type in January 2020.
January 2020
Microgreens Hydroponic Pad N P K Ca Mg S
(mg·g−1)1,2,3 (mg·g−1) (mg·g−1) (mg·g−1) (mg·g−1) (mg·g−1)
Broccoli
Biostrate 58.1 b 10.1 d–g 13.9 de 7.2 hi 5.00 g–j 7.07 ef
Hemp 50.2 fg 9.7 efg 30.1 b 12.7 cd 5.75 c–f 9.00 ab
Jute 55.9 bc 10.2 d–g 14.0 d 11.0 de 5.53 d–h 8.97 abc
Micro-Mats 56.2 bc 9.9 d–g 13.3 d–g 5.4 ij 4.30 jk 8.78 a–d
Cabbage
Biostrate 57.8 b 9.9 d–g 14.4 d 9.1 fg 5.23 f–i 8.77 a–d
Hemp 41.5 j 7.8 h 35.2 a 19.6 a 6.38 bc 8.11 b–e
Jute 50.2 fg 8.8 gh 12.2 d–h 14.4 c 5.68 c–g 8.68 a–d
Micro-Mats 51.1 efg 9.4 g 11.7 d–h 5.1 j 3.67 k 9.93 a
Kale
Biostrate 52.5 def 9.2 g 10.5 gh 4.9 k 4.99 g–j 6.02 fg
Hemp 42.6 ij 9.3 g 31.7 b 11.5 de 6.05 bcd 8.72 a–d
Jute 53.2 de 9.4 fg 9.9 h 8.9 gh 5.52 d–h 7.70 cde
Micro-Mats 48.8 g 9.0 gh 9.8 h 3.1 k 3.78 k 8.05 b–e
Mustard
Biostrate 45.2 h 12.2 ab 14.4 d 11.8 de 6.55 b 7.56 de
Hemp 35.3 k 9.3 g 37.3 a 14.4 c 5.95 b–e 5.89 fg
Jute 44.7 hi 12.5 a 13.7 def 17.2 b 7.59 a 8.57 bcd
Micro-Mats 42.8 hij 11.6 abc 14.3 d 6.4 ij 4.72 ij 8.96 abc
Radish
Biostrate 61.7 a 11.9 abc 13.3 d–g 7.2 hi 5.03 ghi 4.31 h
Hemp 54.7 cd 10.8 cde 21.8 c 10.8 ef 5.31 e–i 8.66 a–d
Jute 63.1 a 10.7 c–f 10.9 fgh 9.1 fg 4.84 hij 5.64 g
Micro-Mats 64.1 a 11.1 bcd 11.1 e–h 5.4 ij 4.30 jk 5.34 gh
p-value Species <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
Substrate <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
Species *
Substrate 4<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
1
Different lower case letters suggest significant difference among means within a column indicated by the Tukey’s HSD test at
p≤0.05
.
2
Each mean was obtained by averaging data over both the fertilized and no-fertilizer treatments.
3
N = nitrogen; P = phosphorus;
K = potassium; Ca = calcium; Mg = magnesium; S = sulfur. 4Species * Substrate suggests interaction between species and substrate type.
Horticulturae 2021,7, 129 8 of 17
Table 8.
Macronutrient concentrations in five microgreens affected by the interaction between species and hydroponic pad
in February 2020.
February 2020
Microgreens Hydroponic Pad N P K Ca Mg S
(mg·g−1)1,2,3 (mg·g−1) (mg·g−1) (mg·g−1) (mg·g−1) (mg·g−1)
Broccoli
Biostrate 59.9 ab 8.7 efg 13.5 cd 8.23 efg 4.75 e–h 6.14 bcd
Hemp 49.6 fg 7.6 hi 19.4 b 14.3 ab 5.20 b–e 7.58 ab
Jute 57.8 b 9.0 d–g 11.6 def 7.24 f–i 4.93 def 7.0 ab
Micro-Mats 53.7 de 8.8 d–g 11.6 def 3.58 jk 3.98 ij 6.78 abc
Cabbage
Biostrate 50.8 ef 7.9 gh 11.8 de 10.2 de 4.59 fgh 2.93 ef
Hemp 39.3 k 6.1 j 19.8 b 13.4 bc 5.07 c–f 3.08 ef
Jute 45.9 hi 6.7 ij 8.8 g 7.57 e–h 4.22 hij 4.54 def
Micro-Mats 49.1 fg 7.5 hi 8.2 g 2.35 k 2.93 k 2.72 f
Kale
Biostrate 47.4 gh 8.8 def 9.1 fg 3.87 jk 4.68 e–h 4.38 def
Hemp 43.0 ij 8.5 e–h 13.9 cd 4.75 ijk 5.38 bcd 3.75 ef
Jute 48.2 fgh 9.9 cd 10.1 efg 9.49 def 5.75 b 7.93 ab
Micro-Mats 48.6 fgh 9.3 c–f 9.1 fg 3.32 jk 3.8 ij 6.94 ab
Mustard
Biostrate 43.9 ij 11.9 a 14.0 cd 11.5 cd 6.40 a 6.92 ab
Hemp 30.3 l 8.3 fgh 34.6 a 13.3 bc 5.52 bc 6.13 bcd
Jute 39.7 k 11.4 ab 12.4 cde 16.4 a 6.75 a 7.09 ab
Micro-Mats 42.1 jk 12.2 a 13.1 cd 5.75 g–j 4.33 ghi 8.55 a
Radish
Biostrate 61.3 a 9.9 cd 10.2 efg 5.52 hij 4.58 fgh 4.48 def
Hemp 54.7 cd 9.9 cd 14.6 c 10.1 de 4.90 d–g 6.87 ab
Jute 58.4 ab 10.3 bc 8.5 g 8.53 ef 4.76 e–h 4.86 cde
Micro-Mats 57.4 bc 9.5 cde 9.0 g 4.49 jk 3.74 j 4.53 def
p-value Species <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
Substrate <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
Species *
Substrate 4<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
1
Different lower case letters suggest significant difference among means within a column indicated by the Tukey’s HSD test at
p≤0.05
.
2
Each mean was obtained by averaging data over both the fertilized and no-fertilizer treatments.
3
N = nitrogen; P = phosphorus;
K = potassium; Ca = calcium; Mg = magnesium; S = sulfur. 4Species * Substrate suggests interaction between species and substrate type.
3.3.1. Nitrogen
Nitrogen concentration ranged from 35.3 mg
·
g
−1
in mustard grown with hemp
to
64.1 mg·g−1
in radish grown with micro-mats in January 2020 (Table 7) and from
30.3 mg·g−1
in mustard grown with hemp to 61.3 mg
·
g
−1
in radish grown with biostrate
in February 2020 (Table 8).
Radish microgreens had higher N concentration than other microgreen species when
grown with any substrate type in January 2020 ranging from 54.7 mg
·
g
−1
in hemp to
64.1 mg·g−1
in micro-mats. Mustard had the lowest N concentration among all microgreens
grown with each substrate in both experiments ranging from 35.3 mg
·
g
−1
in hemp to
45.2 mg·g−1
in biostrate in January, and from 30.3 mg
·
g
−1
in hemp mat to 43.9 mg
·
g
−1
in biostrate in February. In February, higher N concentrations were found in radish and
broccoli than kale, cabbage, or mustard when grown with any substrate type. Hemp
resulted in lower N concentrations in all five microgreens than the other three substrates in
both experiments. One time post-emergent fertilization increased overall N concentration
by 2.0% in February 2020 (Table 5).
3.3.2. Phosphorus
Phosphorus concentration ranged from 7.8 mg
·
g
−1
in cabbage grown with hemp
to 12.5 mg
·
g
−1
in mustard grown with a jute mat in January 2020 (Table 7) and from
Horticulturae 2021,7, 129 9 of 17
6.1 mg·g−1
in cabbage grown with hemp to 12.2 mg
·
g
−1
in mustard grown with micro-
mats in February 2020 (Table 8).
In January, four substrate types generally resulted in similar P concentrations in each
microgreen, except that P concentration in mustard was lower when grown with hemp
than other substrate types. Ranking of P concentration among microgreens varied in
different substrate types, where mustard had the highest P concentrations of
12.2 mg·g−1
,
12.5 mg·g−1
, and 11.6 mg
·
g
−1
among the five species in three substrates including biostrate,
jute mat, and micro-mats, respectively (Table 7). Phosphorus concentration in January was
also affected by the interaction between microgreen species and fertilization treatment,
where fertilization increased P concentration in cabbage and mustard, but resulted in
similar P concentrations in broccoli, kale, and radish (Table 6).
In February, mustard grown with biostrate, jute, and micro-mats had comparable
highest P concentrations ranging from 11.4 to 12.2 mg
·
g
−1
(Table 8). Hemp resulted in
the lowest P concentrations of 7.6 mg
·
g
−1
and 8.3 mg
·
g
−1
compared to other substrate in
broccoli and mustard, respectively. Fertilization increased overall P concentration by 8.4%
compared to the no-fertilization treatment (Table 5).
3.3.3. Potassium
Potassium concentration ranged from 9.8 mg
·
g
−1
in kale grown with micro-mats
to 37.3 mg
·
g
−1
in mustard grown with a hemp mat in January 2020 (Table 7) and from
8.2 mg·g−1
in cabbage grown with micro-mats to 34.6 mg
·
g
−1
in mustard grown with a
hemp mat in February 2020 (Table 8).
Hemp resulted in higher K concentrations than other substrate types in each micro-
green in both experiments. Among all treatment combinations, the highest K concentrations
were found in mustard grown with hemp in both experiments, 37.3 mg
·
g
−1
in January
and 34.6 mg
·
g
−1
in February respectively. Cabbage had comparable K concentration with
mustard in each substrate type in January. In February, broccoli and cabbage grown with
hemp had the second highest K concentrations next to mustard grown with hemp.
Potassium concentration was also affected by the interaction between species and
fertilization in January, where fertilization increased K concentrations in broccoli and
mustard by 11.2% and 18.0%, but not in the other three microgreens (Table 6).
3.3.4. Calcium
Calcium concentration ranged from 3.12 mg
·
g
−1
in kale grown with micro-mats
to
19.6 mg·g−1
in cabbage grown with a hemp mat in January 2020 (Table 7) and from
2.35 mg·g−1
in cabbage grown with micro-mats to 16.4 mg
·
g
−1
in mustard grown with a
jute mat in February 2020 (Table 8).
The five microgreens varied in their Ca concentrations, and substrate type altered con-
centrations for each species. In January, cabbage and mustard had higher Ca concentrations
than broccoli, kale, or radish when grown with biostrate, hemp, or jute mat. When grown
with micro-mats, broccoli had higher Ca concentrations than the other four microgreens.
The hemp mat resulted in the highest Ca concentrations of 19.6 mg
·
g
−1
in cabbage and
11.5 mg
·
g
−1
in kale, whereas the jute mat resulted in the highest Ca concentration of
17.2 mg·g−1in mustard.
There was not as much separation of means in February, where broccoli, cabbage,
kale, and radish had similar Ca concentrations when grown with a jute mat or micro-
mats. The hemp mat resulted in higher Ca concentrations in broccoli (14.3 mg
·
g
−1
) and
cabbage (
13.4 mg·g−1
) than other substrates, whereas the jute mat resulted in higher Ca
concentrations in kale (9.49 mg
·
g
−1
) and mustard (16.4 mg
·
g
−1
) than other substrate types.
3.3.5. Magnesium
Magnesium concentration ranged from 3.67 mg
·
g
−1
in cabbage grown with micro-
mats to 7.59 mg
·
g
−1
in mustard grown with a jute mat in January 2020 (Table 7) and from
Horticulturae 2021,7, 129 10 of 17
2.93 mg
·
g
−1
in cabbage grown in micro-mats to 6.75 mg
·
g
−1
in mustard grown with a jute
mate in February 2020 (Table 8).
Broccoli, cabbage, and kale had similar Mg concentrations when grown with any of
the four substrate types in January, and grown with biostrate or a hemp mat in February.
Mustard had higher Mg concentrations than other microgreens when grown with biostrate
or a jute mat in both experiments. Micro-mats resulted in lower Mg concentrations than
the other three substrate types in each microgreen in January, and in broccoli, kale, cabbage,
and mustard in February. One time fertilization decreased Mg concentration by 2.6%
compared with the no-fertilization treatment (Table 5).
3.3.6. Sulfur
Sulfur concentration ranged from 4.31 mg
·
g
−1
in radish grown with biostrate to
9.93 mg·g−1
in cabbage grown with micro-mats in January 2020 (Table 7) and from
2.72 mg·g−1
in cabbage grown with micro-mats to 8.55 mg
·
g
−1
in mustard grown with
micro-mats in February 2020 (Table 8), affected by the interaction between species and
substrate interaction in both experiments.
In general, separation among microgreens within a substrate type, or among substrates
within one microgreen were not as much in January or February. In January, radish had
lower S concentrations of 4.31 mg
·
g
−1
to 5.64 mg
·
g
−1
than other microgreens when grown
with biostrate, a jute mat, or micro-mats. The hemp mat, jute mat, and micro-mats resulted
in similar S concentrations in broccoli (8.78–9.00 mg
·
g
−1
) and kale (7.70–8.72 mg
·
g
−1
). One
time fertilization decreased S concentration by 3.6% compared with the no-fertilization
treatment (Table 5). In February, four substrate types resulted in similar S concentrations in
broccoli (6.14–7.58 mg
·
g
−1
) and cabbage (2.72–4.54 mg
·
g
−1
). Kale and cabbage had lower
S concentrations than other microgreens when grown in a hemp mat. Broccoli, kale, and
mustard had similar S concentrations when grown with a jute mat or micro-mats (Table 8).
3.4. Micronutrient Concentrations
Micronutrient concentrations including Cu, Fe, Mn, Zn, and B were all affected by the
interaction between species and hydroponic pad type, but not affected by the fertilization
treatment.
3.4.1. Copper
Copper concentration ranged from 0.51 to 1.92
µ
g
·
g
−1
in mustard grown with a hemp
mat in January 2020 (Table 9) and from 0.39 to 5.09
µ
g
·
g
−1
in cabbage grown in a hemp
mat in February 2020 (Table 10).
Table 9.
Micronutrient concentrations in five microgreens affected by the interaction between species and hydroponic pad
type in January 2020.
January 2020
Microgreens Hydroponic Pad Cu Fe Mn Zn B
(µg·g−1)1,2,3 (µg·g−1) (µg·g−1) (µg·g−1) (µg·g−1)
Broccoli
Biostrate 0.51 d 94.8 d–i 35.1 i–l 78.8 de 16.5 h–k
Hemp 0.51 d 86.2 f–i 45.7 ghi 74.4 def 34.6 bc
Jute 0.71 cd 119.5 bc 62.7 de 110.5 b 23.9 e–h
Micro-Mats 0.51 d 116.9 bcd 33.4 jkl 87.3 cd 17.1 g–j
Cabbage
Biostrate 0.51 d 113.3 b–e 58.4 def 76.1 de 19.9 f–i
Hemp 0.51 d 134.7 ab 118.6 a 84.8 d 45.0 a
Jute 0.51 d 156.3 a 116.5 a 133.3 a 28.0 cde
Micro-Mats 0.51 d 134 ab 51.5 efg 78.5 de 23.9 d–g
Kale
Biostrate 1.29 bc 84.8 c–h 45.3 ghi 53.3 hij 9.65 k
Hemp 0.67 d 97.4 c–h 65.9 cd 68.6 efg 37.3 b
Jute 1.29 bc 90.3 e–i 74.3 c 75.3 def 14.5 ijk
Micro-Mats 1.29 bc 90.4 e–i 39.0 h–k 50.8 ij 11.9 jk
Horticulturae 2021,7, 129 11 of 17
Table 9. Cont.
January 2020
Microgreens Hydroponic Pad Cu Fe Mn Zn B
(µg·g−1)1,2,3 (µg·g−1) (µg·g−1) (µg·g−1) (µg·g−1)
Mustard
Biostrate 1.29 bc 107.3 c–f 49.4 fgh 67.6 e–h 26.3 def
Hemp 1.92 a 101 c–g 42.7 g–k 61.1 f–i 52.0 a
Jute 1.29 bc 112.8 b–e 88.4 b 100.5 bc 30.4 bcd
Micro-Mats 1.29 bc 107 c–f 33.4 jkl 59.6 g–j 24.4 def
Radish
Biostrate 1.72 ab 83.4 ghi 31.5 kl 53.6 hij 11.5 jk
Hemp 1.29 bc 79.8 ghi 44.6 g–j 57.4 g–j 22.8 f–h
Jute 1.29 bc 76.9 hi 44.3 g–j 65.3 e–i 10.6 jk
Micro-Mats 1.29 bc 74.0 i 25.3 l 47.3 j 11.5 jk
p-value Species <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
Substrate 0.5972 <0.0001 <0.0001 <0.0001 <0.0001
Species * Substrate 4<0.0001 <0.0001 <0.0001 <0.0001 <0.0001
1
Different lower case letters suggest significant difference among means within a column indicated by the Tukey’s HSD test at
p≤0.05
.
2Each
mean was obtained by averaging data over both the fertilized and no-fertilizer treatments.
3
Cu = copper; Fe = iron; Mn = manganese;
Zn = zinc; B = boron. 4Species * Substrate suggests interaction between species and substrate type.
Table 10.
Micronutrient concentrations in five microgreens affected by the interaction between species and hydroponic pad
type in February 2020.
February 2020
Microgreens Hydroponic Pad Cu Fe Mn Zn B
(µg·g−1)1,2,3 (µg·g−1) (µg·g−1) (µg·g−1) (µg·g−1)
Broccoli
Biostrate 2.82 abc 80.4 b–e 29.2 f–i 63.7 bc 12.9 f–k
Hemp 2.82 abc 71.1 def 40.8 c–f 60.0 bcd 26.4 bc
Jute 1.20 bcd 81.2 b–e 47.5 bc 68.0 ab 19.5 c–g
Micro-Mats 0.82 cd 75.9 c–f 25.8 hi 55.6 bcd 16.5 e–j
Cabbage
Biostrate 3.50 ab 62.7 fg 34.9 e–h 24.8 hg 12.0 g–k
Hemp 5.09 a 70.6 def 56.6 b 30.9 fgh 21.3 b–f
Jute 4.92 a 99.9 a 71.8 a 61.3 bcd 18.3 c–h
Micro-Mats 0.88 cd 41.4 h 27.5 ghi 17.0 h 7.7 k
Kale
Biostrate 0.50 cd 42.6 h 37.5 c–h 26.3 hg 9.7 ijk
Hemp 4.26 a 42.8 h 48.2 bc 21.0 hg 15.2 e–j
Jute 0.39 d 83.3 bcd 76.3 a 70.2 ab 17.8 d–i
Micro-Mats 0.39 d 65.7 efg 37.8 c–g 45.4 def 11.0 h–k
Mustard
Biostrate 0.39 d 94.8 ab 43.5 cde 66.4 ab 25.7 bcd
Hemp 0.39 d 69.0 d–g 32.9 e–h 48.7 cde 37.9 a
Jute 0.71 cd 90.1 abc 81.9 a 83.3 a 28.2 b
Micro-Mats 0.39 d 94.5 ab 29.1 f–i 55.4 bcd 23.4 b–e
Radish
Biostrate 0.39 d 54.3 gh 20.2 i 48.6 cde 15.4 e–k
Hemp 0.39 d 44.5 h 45.5 bcd 47.1 c–f 15.0 e–k
Jute 0.39 d 53.7 gh 42.4 cde 61.9 bcd 10.2 h–k
Micro-Mats 0.39 d 40.6 h 19.4 i 37.1 efg 8.7 jk
p-value Species <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
Substrate <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
Species * Substrate 4<0.0001 <0.0001 <0.0001 <0.0001 <0.0001
1
Different lower case letters suggest significant difference among means within a column indicated by the Tukey’s HSD test at
p≤0.05
.
2Each
mean was obtained by averaging data over both the fertilized and no-fertilizer treatments.
3
Cu = copper; Fe = iron; Mn = manganese;
Zn = zinc; B = boron. 4Species * Substrate suggests interaction between species and substrate type.
Four hydroponic pad types resulted in similar Cu concentrations in radish and broccoli
in both experiments, in cabbage in January, in mustard in February. Biostrate, jute mate,
Horticulturae 2021,7, 129 12 of 17
and micro-mats resulted in similar Cu concentrations in Kale in both experiments and in
mustard in January.
Radish, kale, and mustard had similar Cu concentrations when grown with biostrate
or micro-mats, higher than broccoli or cabbage in January but lower than broccoli or
cabbage in February when grown with biostrate. When grown with a hemp mat, broccoli,
kale, and cabbage had similar Cu concentrations in both experiments. Micro-mats resulted
in similar Cu concentrations among all five microgreens in February.
3.4.2. Iron
Iron concentration ranged from 74.0
µ
g
·
g
−1
in radish grown with micro-mats to
156.3 µg·g−1
in cabbage grown with a jute mat in January 2020 (Table 9) and from
40.6 µg·g−1
in radish grown with micro-mats to 99.9
µ
g
·
g
−1
in cabbage grown with a jute mat in Febru-
ary 2020 (Table 10).
Four substrates resulted in similar Fe concentrations in radish, kale, and mustard
in January and in radish and broccoli in February. Cabbage grown with a jute mat had
the highest Fe concentrations of 156.3
µ
g
·
g
−1
in January and 99.9
µ
g
·
g
−1
in February
among all treatment combinations, respectively. In addition, cabbage grown with a hemp
mat and micro-mats also had higher Fe concentrations than other microgreens in January
2020. Mustard had higher Fe concentrations of 94.8
µ
g
·
g
−1
and 94.5
µ
g
·
g
−1
than other
microgreens when grown with biostrate and micro-mats in February.
3.4.3. Manganese
Manganese concentration ranged from 25.3
µ
g
·
g
−1
in radish grown with micro-mats
to 118.6
µ
g
·
g
−1
in cabbage grown with a hemp mat in January 2020 (Table 9) and from
19.4 µg·g−1
in radish grown with micro-mats to 81.9
µ
g
·
g
−1
in mustard grown with a jute
mat in February 2020 (Table 10).
In January, cabbage had higher Mn concentrations of 118.6
µ
g
·
g
−1
, 116.5
µ
g
·
g
−1
,
and 51.5
µ
g
·
g
−1
than other microgreens when grown with hemp, jute, or micro-mats,
respectively. Hemp and jute mats resulted in similar Mn concentrations in kale, cabbage,
and radish, higher than those in biostrate or micro-mats.
In February, hemp and jute mats resulted in higher Mn concentrations than biostrate
or micro-mats in kale, cabbage, and radish, while a jute mat resulted in the highest Mn
concentrations in broccoli and mustard.
3.4.4. Zinc
Zinc concentration ranged from 47.3
µ
g
·
g
−1
in radish grown with micro-mats to
133.3 µg·g−1
in cabbage grown with a jute mat in January 2020 (Table 9) and from
17.0 µg·g−1
in cabbage grown with micro-mats to 83.3
µ
g
·
g
−1
in mustard grown with a jute mat in
February 2020 (Table 10).
In January, broccoli and cabbage had higher Zn concentrations than kale, mustard,
or radish when grown with a jute mat or micro-mats. The jute mat resulted in the highest
Zn concentration of 110.5
µ
g
·
g
−1
, 133.3
µ
g
·
g
−1
, and 100.5
µ
g
·
g
−1
than the other three
substrate types in broccoli, cabbage, and mustard, respectively.
In February, jute mat also resulted in higher Zn concentrations of 61.3
µ
g
·
g
−1
,
70.2 µg·g−1
,
and 83.3
µ
g
·
g
−1
than other substrate types in kale, cabbage, and mustard, respectively.
Separation among microgreens was not as much when grown with a certain substrate.
3.4.5. Boron
Boron concentration ranged from 9.65
µ
g
·
g
−1
in kale grown with biostrate to
45.0 µg·g−1
in cabbage grown with a hemp mat in January 2020 (Table 9) and from 7.7
µ
g
·
g
−1
in cab-
bage grown with micro-mats to 37.9
µ
g
·
g
−1
in mustard grown with a hemp mat in February
2020 (Table 10).
In January, cabbage had higher B concentrations of 45.0
µ
g
·
g
−1
and 28.0
µ
g
·
g
−1
than
other microgreens when grown with a hemp or jute mat, respectively. Kale, mustard, and
Horticulturae 2021,7, 129 13 of 17
radish grown with biostrate, jute, and micro-mats had similar B concentrations, lower
than most other treatment combinations. Hemp mat resulted in higher B concentrations of
34.6 µg·g−1
, 45.0
µ
g
·
g
−1
, 37.3
µ
g
·
g
−1
, 52.0
µ
g
·
g
−1
, and 22.8
µ
g
·
g
−1
than other substrates
in broccoli, cabbage, kale, mustard, and radish, respectively.
In February, mustard had higher B concentrations of 23.4
µ
g
·
g
−1
in micro-mats to
37.9 µg·g−1
in jute mat than other microgreens when grown with each hydroponic pad type.
Hemp resulted in higher B concentrations than other substrates in broccoli and mustard.
4. Discussion
Fresh shoot yield is of the most limiting factor in microgreen production [
4
,
20
,
23
–
25
].
Microgreens varied in their fresh shoot yield, which was altered by hydroponic substrate
type and fertilization treatment. Fresh, dry shoot weights, and dry weight percentage of
tested microgreens in this current study are in similar ranges as our previous study when
similar microgreen species/varieties were grown with a peat-based soilless substrate [
26
],
suggesting selection of high-yielding microgreen species or varieties can potentially be
applied to different growing systems including using peat-based substrate, or hydroponic
pads of various types. Microgreen shoot yield (fresh or dry) results are also in agreement
with previously reported ranges [9,20,21].
Microgreen yield varied between the two experiments, with higher fresh shoot weight,
macro-, and micronutrient concentrations except for Cu in the January experiment than
February (data not shown). This is likely due to the fluctuating microenvironment condi-
tions in the greenhouse including temperature, relative humidity, and light conditions [
26
].
Air temperature was set to be 25
◦
C in the greenhouse, and was observed to fluctuate
within 8
◦
C of the setting. Relative humidity ranged from 20% to 80%, and daily light
integral ranged from 3 to 15 mol
·
m
−2·
d
−1
within the experiment duration. Producers
could experience similar changes in microgreens between production cycles. Supplemental
lighting may help offset such fluctuations.
Fresh microgreen yield was shown to increase with increasing seed sowing rate.
However, as seeding rate increases, individual shoot weight decreases and production cost
increases since seeds contribute to a significant portion of microgreen production cost [
4
,
18
].
The sowing rates used in this study was recommended by the seed supplier ranging from
60.5 g
·
m
−2
in mustard to 189 g m
−2
in radish (Table 1), equivalent to 17,182 seeds per m
2
in
radish to 33,611 seeds per m
2
in mustard. This sowing rate was generally in agreement with
reported ranges of 10,000–40,000 seeds per m
2
for microgreens, yet crop-specific optimum
sowing rate needs to be determined based on germination percentage, seed weight, and
desired shoot density [
27
]. Besides fresh shoot yield, choice of microgreen species is also
affected by appearance, texture, flavor, nutritional values, and phytochemical compositions,
with genetic variability between and within taxa for traits of interest [9,28].
Compared to a number of studies investigating the effect of light on minerals and
phytochemical concentrations in microgreens [
19
,
29
,
30
], there lacks standardization in
cultural practices including fertilization application, or guidelines for microgreen qual-
ity [
4
,
20
,
24
,
25
]. One time post-emergent fertigation increased overall shoot height in
both experiments, increased overall dry shoot weight and fresh shoot weight in kale
and radish in January 2020, and increased various macronutrient concentrations in dif-
ferent microgreens. This is in agreement with our previous results evaluating ten micro-
greens using a peat-based soilless substrate, when one time post-emergent fertigation
with
100 mg·L−1N
also increased overall fresh shoot yield, macro-, and micronutrient
concentrations in selected microgreen species. Murphy et al. [
4
] recommended daily fer-
tigation with
150 mg·L−1N
for optimal yield in arugula microgreens [
18
]. In addition to
increased yield, large shoot height is a desirable characteristic in assisting with easy harvest
of microgreens manually or mechanically [
11
]. Hydroponic pads generally have lower
water holding capacity than peat-based substrate [
21
], microgreens grown in a hydroponic
system may benefit from more frequent fertigation. Slow growing species may require
more frequent fertilization than fast growing ones [1].
Horticulturae 2021,7, 129 14 of 17
Mineral nutrient profiles varied among microgreens. For example, radish and broccoli
had the highest N concentrations and mustard had the highest P concentration when grown
in three substrate types except for hemp. Mustard also had the lowest N concentration
among tested microgreens when grown with each substrate type. Ranking of most other
mineral nutrients between microgreens varied among hydroponic pad types, suggesting
substrate type can affect nutrient profile of microgreens, which should be taken into consid-
eration in microgreen production. Analyses of 30 varieties of Brassica microgreens revealed
that they are a rich source of mineral nutrients, especially K, Mg, Fe, and Zn [
16
]. Concentra-
tions of macro- and micronutrients in the current study were generally in consistent ranges
with previous reports. The five tested microgreens had lower concentrations of K, Cu,
Fe, and B in this study grown with hydroponic pads than when grown with a peat-based
potting mix in our previous study [
26
]. This agrees with Di Gioia et al. [
21
] in that peat
resulted in higher nitrate, potassium, and sodium concentrations in microgreen rapini than
three hydroponic pads including Sure to Grow (made from polyethylene-terephthalate),
textile fiber, and jute-kenaf fiber. This may result from the facts that water holding capacity
of peat was tested to be higher than hydroponic pads, and that peat itself contains higher
mineral nutrient concentrations than the hydroponic pads [
21
]. Mineral nutrients may also
change among harvest stages from microgreen stage 1, microgreen stage 2, baby leaf, to
adult stage, with microgreen stages having higher mineral nutrient concentrations than the
adult stage [5].
An ideal hydroponic growing substrate should be readily available, relatively inexpen-
sive, and derived from renewable materials [
21
]. They should also have an adequate ratio
between micropores to macropores to have high container capacity and sufficient air filled
porosity. The four tested hydroponic pads are all constructed with biodegradable fibers
with different physical and chemical characteristics. Hemp mat was superior in producing
the highest shoot height, fresh and dry shoot weights in each microgreen species, or overall,
in one or two experiments compared with the other three pad types. Hemp also resulted in
the lowest N concentrations among substrate types in each microgreen in both experiments,
which could result from the dilution effect of nutrient due to the highest fresh and dry
shoot weights in microgreens. Fast growth and high yield of microgreens may benefit
from fertilization with a high N formula, with caution on the balance between nitrate and
ammonium forms of N since some microgreen crops like arugula tends to accumulate more
nitrate [
20
]. However, microgreens generally have lower nitrate concentrations than their
mature counterparts [
10
,
12
]. Compared to the hemp mat, jute, and micro-mats have denser
texture, likely contributing to a higher water holding capacity but less air filled porosity,
while biostrate is thin and lose moisture easily as observed.
Hydroponic pads constructed with different fiber materials can be sources of certain
mineral nutrients [
21
]. A hemp mat could be a source of K and B, resulting in the highest
K (in both experiments) and B concentrations (in January) in each microgreen. Highest
micronutrient concentrations were mostly found in the jute or hemp mat if there was
significant difference among substrate types, where the higher nutrient concentrations
may also result from better shoot growth in these two substrate types. Further analyses of
mineral compositions of different hydroponic pads will be required to confirm their effects
on microgreen mineral nutrient concentrations.
Food safety concerns including microbial contamination by human pathogens in
microgreen production has emerged with the expanding industry [
7
]. Microbial contamina-
tion can be introduced through seeds, growing substrate, equipment, or a lack of hygienic
practices by workers [
31
]. Microgreens are more vulnerable to internalization of bacteria
than mature vegetable plants [
32
]. However, foodborne pathogen outbreaks have not
been associated with microgreen consumption as much as in sprouts, where sprouts are
generally grown in a dark and moist condition that are more conductive to microbial
proliferation [
9
,
33
–
35
]. We did observe mold-like occurrence on hydroponic pads, with
higher mold counts on micro-mats and hemp mats, less in our previous study using a
peat-based substrate [
26
]. It was recently reported that microgreens grown in a hydroponic
Horticulturae 2021,7, 129 15 of 17
system and soil-substitute are both subject to pathogen proliferation with contaminated
seeds, with higher microbial population in the hydroponic system [
33
,
36
,
37
]. Effective
treatments for seed surface sterilization and antimicrobial action need to be developed for
sustainable microgreen production [
9
,
38
]. Disinfecting practices should also be identified
for hydroponic pads since they could be potential source of microbial contamination [
21
].
Growers should also select reputable supplier for certified microgreen or sprout seeds with
high germination quality and potentially lower health risks.
5. Conclusions
Microgreens varied in their shoot growth and mineral nutrient concentrations. Hydro-
ponic pads with varying physical and chemical properties affected shoot yield and mineral
nutrients of microgreens. Microgreens in the hemp mat showed the highest fresh and dry
shoot weight, likely due to the appropriate balance between water holding capacity and air
filled porosity. One time post-emergent fertilization increased overall shoot height of tested
microgreens in both experiments. Recommendation for fertilization should be adjusted to
species/varieties and production conditions.
Supplementary Materials:
The following are available online at https://www.mdpi.com/article/10
.3390/horticulturae7060129/s1, Figure S1: Growth stage of five microgreens at harvest including
‘Waltham’ broccoli (A), ‘Red Acre’ cabbage (B), ‘Red Russian’ kale (C), ‘Red Garnet’ mustard (D), and
Rambo radish (E).
Author Contributions:
Conceptualization, T.L.; investigation, G.T.L., J.D.A., M.H.J. and T.L.; writing
—original draft preparation, T.L.; writing—review and editing, G.B. and J.D.A.; funding acquisition,
T.L. and G.B. All authors have read and agreed to the published version of the manuscript.
Funding:
This work was supported by the United States Department of Agriculture (USDA) Missis-
sippi Department of Agriculture and Commerce Specialty Crop Block Grant Program (G00003962)
and the United States Department of Agriculture (USDA) National Institute of Food and Agriculture
Hatch Project MIS-112040. Mention of a trademark, proprietary product, or vendor, does not consti-
tute a guarantee or warranty of the product by Mississippi State University or the USDA and does
not imply its approval to the exclusion of other products or vendors that also may be suitable.
Institutional Review Board Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Treadwell, D.D.; Hochmuth, R.; Landrum, L.; Laughlin, W. Microgreens: A New Specialty Crop. Univ. Florida IFAS Ext. 2020,
HS1164. Available online: https://edis.ifas.ufl.edu/pdffiles/HS/HS116400.pdf (accessed on 7 December 2020).
2.
Gerovac, J.R.; Craver, J.K.; Boldt, J.K.; Lopez, R.G. Light intensity and quality from sole-source light emitting diodes impact
growth, morphology, and nutrient content of Brassica microgreens. HortScience 2016,51, 497–503. [CrossRef]
3.
Bachman, G.R. Growing Microgreens for the Mississippi Gardener; Mississippi State University Extension Service: Starkville, MS,
USA, 2014; p. 2857.
4.
Murphy, C.J.; Llort, K.F.; Pill, W.G. Factors affecting the growth of microgreen table beet. Int. J. Veg. Sci.
2010
,16, 253–266.
[CrossRef]
5.
Waterland, N.L.; Moon, Y.; Tou, J.C.; Kim, M.J.; Pena-Yewtukhiw, E.M.; Park, S. Mineral content differs among microgreen, baby
leaf, and adult stages in three cultivars of kale. HortScience 2017,52, 566–571. [CrossRef]
6.
Xiao, Z.; Rausch, S.R.; Luo, Y.; Sun, J.; Yu, L.; Wang, Q.; Chen, P.; Yu, L.; Stommel, J.R. Microgreens of Brassicaceae: Genetic
diversity of phytochemical concentrations and antioxidant capacity. LWT Food Sci. Technol. 2019,101, 731–737. [CrossRef]
7.
Turner, E.R.; Luo, Y.; Buchanan, R.L. Microgreen nutrition, food safety, and shelf life: A review. J. Food Sci.
2020
,85, 870–882.
[CrossRef]
8.
Bachman, G.R. Microgreens Varieties for the Mississippi Gardener; Mississippi State University Extension Service: Starkville, MS,
USA, 2015; p. 2884.
9. Kyriacou, M.C.; Rouphael, Y.; Di Gioia, F.; Kyratzis, A.; Serio, F.; Renna, M.; De Pascale, S.; Santamaria, P. Micro-scale vegetable
production and the rise of microgreens. Trends Food Sci. Technol. 2016,57, 103–115. [CrossRef]
10.
Pinto, E.; Almeida, A.A.; Aguiar, A.A.; Ferreira, I.M.P.L.V.O. Comparison between the mineral profile and nitrate content of
microgreens and mature lettuces. J. Food Compos. Anal. 2015,37, 38–43. [CrossRef]
Horticulturae 2021,7, 129 16 of 17
11.
Ying, Q.; Kong, Y.; Zheng, Y. Growth and appearance quality of four microgreen species under light-emitting diode lights with
different spectral combinations. HortScience 2020,55, 1399–1405. [CrossRef]
12.
Xiao, Z.; Lester, G.F.; Luo, Y.; Wang, Q. Assessment of vitamin and carotenoid concentrations of emerging food products: Edible
microgreens. J. Agric. Food Chem. 2012,60, 7644–7651. [CrossRef] [PubMed]
13.
Kyriacou, M.C.; El-Nakhel, C.; Graziani, G.; Pannico, A.; Soteriou, G.A.; Giordano, M.; Ritieni, A.; De Pascale, S.; Rouphael,
Y. Functional quality in novel food sources: Genotypic variation in the nutritive and phytochemical compostion of thirteen
microgreens species. Food Chem. 2019,277, 107–118. [CrossRef]
14.
Nadihira, S.; Ramar, A.; Jegadeeswari, V.; Srinivasan, S. Microgreen production in herbal spices. J. Pharmacogn. Phytochem.
2021
,
10, 168–170.
15.
Johnson, S.A.; Prenni, J.E.; Heuberger, A.L.; Isweiri, H.; Chaparro, J.M.; Newman, S.E.; Uchanski, M.E.; Omerigic, H.M.; Michell,
K.A.; Bunning, M.; et al. Comprehensive evaluation of metabolites and minerals in 6 microgreen species and the influence of
maturity. Curr. Dev. Nutr. 2021,5, nzaa180. [CrossRef] [PubMed]
16.
Xiao, Z.; Codling, E.E.; Luo, Y.; Nou, X.; Lester, G.E.; Wang, Q. Microgreens of Brassicaceae: Mineral composition and content of
30 varieties. J. Food Compos. Anal. 2016,49, 87–93. [CrossRef]
17.
Puccinelli, M.; Malorgio, F.; Rosellini, I.; Pezzarossa, B. Production of selenium-biofortified microgreens from selenium-enriched
seeds of basil. J. Sci. Food Agric. 2019,99, 5601–5605. [CrossRef] [PubMed]
18.
Murphy, C.J.; Pill, W.G. Cultural practices to speed the growth of microgreen arugula (roquette; Eruca vesicaria subsp. sativa). J.
Hort. Sci. Biotechnol. 2010,85, 171–176. [CrossRef]
19.
Brazaityt
˙
e, A.; Vaštakait
˙
e, V.; Viršil
˙
e, A.; Jankauskien
˙
e, J.; Samuolien
˙
e, G.; Sakalauskien
˙
e, S.; Noviˇckovas, A.; Miliauskien
˙
e, J.;
Duchovskis, P. Changes in mineral element content of microgreens cultivated under different lighting conditions in a greenhouse.
Acta Hort. 2018,1227, 507–516. [CrossRef]
20.
Bulgari, R.; Baldi, A.; Ferrante, A.; Lenzi, A. Yield and quality of basil, Swiss chard, and rocket microgreens grown in a hydroponic
system. N. Z. J. Crop Hort. Sci. 2017,45, 119–129. [CrossRef]
21.
Di Gioia, F.; De Bellis, P.; Minini, C.; Santamaria, P.; Serio, F. Physicochemical, agronomical and microbiological evaluation of
alternative growing media for the production of rapini (Brassica rapa L.) microgreens. J. Sci. Food Agric.
2017
,97, 1212–1219.
[CrossRef]
22.
Rajan, P.; Lada, R.R.; MacDonald, M.T. Advancement in indoor vertical farming for microgreen production. Am. J. Plant Sci.
2019
,
10, 1397–1408. [CrossRef]
23.
Kou, L.P.; Luo, Y.G.; Yang, T.B.; Xiao, Z.L.; Turner, E.R.; Lester, G.E.; Wang, Q.; Camp, M.J. Postharvest biology, quality and shelf
life of buckwheat microgreens. LWT Food Sci. Technol. 2013,51, 73–78. [CrossRef]
24.
Kou, L.; Yang, T.; Luo, Y.; Liu, X.; Huang, L.; Codling, E. Pre-harvest calcium application increases biomass and delays senescence
of broccoli microgreens. Postharvest Biol. Technol. 2014,87, 70–78. [CrossRef]
25.
Sun, J.; Kou, L.; Geng, P.; Huang, H.; Yang, T.; Luo, Y.; Chen, P. Metabolomic assessment reveals an elevated level of glucosinolate
content in CaCl2treated broccoli microgreens. J. Agric. Food Chem. 2015,63, 1863–1868. [CrossRef]
26.
Li, T.; Lalk, G.T.; Bi, G. Fertilization and Pre-sowing seed soaking affect yield and mineral nutrients of ten microgreen species.
Horticulturae 2021,7, 14. [CrossRef]
27.
Di Gioia, F.; Santamaria, P. Microgreens: Novel, Fresh and Functional Food to Explore All the Value of Biodiversity; ECO-Logica: Bari,
Italy, 2015.
28.
Xiao, Z.; Lester, G.E.; Park, E.; Saftner, R.A.; Luo, Y.; Wang, Q. Evaluation and correlation of sensory attributes and chemical
compositions of emerging fresh produce: Microgreens. Postharvest Biol. Technol. 2015,110, 140–148. [CrossRef]
29.
Alrifai, O.; Hao, X.; Marcone, M.F.; Tsao, R. Current review of the modulatory effects of LED lights on photosynthesis of secondary
metabolites and future perspectives of microgreen vegetables. J. Agric. Food Chem. 2019,67, 6075–6090. [CrossRef]
30.
Kopsell, D.A.; Sams, C.E. Increases in shoot tissue pigments, glucosinates, and mineral elements in sprouting broccoli after
exposure to short-duration blue light from light emitting diodes. J. Am. Soc. Hort. Sci. 2013,138, 31–37. [CrossRef]
31.
Yang, Y.; Meier, F.; Lo, J.A.; Yuan, W.; Sze, V.L.P.; Chung, H.L.; Yuk, H.G. Overview of recent events in the microbiological safety
of sprouts and new intervention technologies. Compr. Rev. Food Sci. Food Saf. 2013,12, 265–280. [CrossRef]
32.
Warriner, K.; Ibrahim, F.; Dickinson, M.; Wright, C.; Waites, W.M. Internalization of human pathogens within growing salad
vegetables. Biotechnol. Genet. Eng. Rev. 2003,20, 117–136. [CrossRef] [PubMed]
33.
Xiao, Z.; Bauchan, G.; Nichols-Russell, L.; Luo, Y.; Wang, Q.; Nou, X. Proliferation of Escherichia coli O157:H7 in soil-substitute
and hydroponic microgreen production systems. J. Food Prot. 2015,78, 1785–1790. [CrossRef] [PubMed]
34.
Xiao, Z.; Nou, X.; Luo, Y.; Wang, Q. Comparison of the growth of Escherichia coli O157: H7 and O104: H4 during sprouting and
microgreen production from contaminated radish seeds. Food Microbiol. 2014,44, 60–63. [CrossRef]
35.
Ebert, A. Spouts, microgreens, and edible flowers: The potential for high value specialty produce in Asia. In High Value Vegetables
in Southeast Asia: Production, Supply, and Demand, Proceedings of the SEAVEG 2012, Chiang Mai, Thailand, 24–26 January 2012; Asian
Vegetable Research and Development Center: Tainan, Taiwan, 2013; pp. 216–227.
36.
Reed, E.; Ferreira, C.M.; Bell, R.; Brown, E.W.; Zheng, J. Plant-microbe and abiotic factors influencing salmonella survival and
growth on alfalfa sprouts and Swiss chard microgreens. Appl. Environ. Microbiol. 2018,84, e02814-17. [CrossRef] [PubMed]
Horticulturae 2021,7, 129 17 of 17
37.
Wright, K.M.; Holden, N.J. Quantification and colonisation dynamics of Escherichia coli O157:H7 inoculation of microgreens
species and plant growth substrates. Int. J. Food Microbiol. 2018,273, 1–10. [CrossRef] [PubMed]
38.
Ding, H.; Fu, T.J.; Smith, M.A. Microbial contamination in sprouts: How effective is seed disinfection treatment? J. Food Sci.
2013
,
78, 495–501. [CrossRef] [PubMed]
