Evaluation of the Resource Use Efficiencies
of Small-Scale Vertical Hydroponic Structures against Growing Plants in Soil
Zikhona Buyeye1*, Gareth Lagerwall1, Aidan Senzanje1,
Alistair Clulow2 and Sipho Sibanda3
1Bioresources
Engineering Programme, School of Engineering,
University of Kwa Zulu-Natal, Pietermaritzburg 3209, South Africa
2Department of Agrometereology, University of Kwa Zulu-Natal,
Pietermaritzburg 3209, South Africa
3Institute of
Agricultural Engineering, Agricultural Research Council, Pretoria 0184, South
Africa
*For correspondence: z.buyeye@yahoo.com
Received 27 March 2021; Accepted 15 November 2021; Published 30 January
2022
Abstract
The aim of
this research was to investigate the use of small-scale vertical farming
structures as an alternative to improving resource use efficiency in
agriculture. To achieve this, Fordhook Giant Swiss
chard was grown over two cropping seasons between February and November 2019 at
the Ukulinga Research Farm in Mkondeni,
Pietermaritzburg. The main factor was the growing method. The sub-factors were
light provision and nutrient solution concentration. The hydroponic structures
had a significantly higher land use efficiency than the plant pot setup (3
041.05 g.m-2 vs 405.89 g.m-2 in cropping season one
(CS1), and 3 106.41 g.m-2 vs 464.53 g.m-2 in CS2), p <
0.0005. The crop water productivity of the hydroponic structures was
significantly higher than that of the plant pot setup (8.45 g.L-1 vs
5.72 g. L -1 in CS L and 8.44 g.L-1 vs 6.59 g.L-1
in CS2), p < 0.0005 in CS1 and p = 0.014 in CS2. The energy use efficiency
of plants grown hydroponically under sunlight (104.25 g. KWh-1 in
CS1 and 103.43 g. KWh-1 in CS2) was significantly higher than that
of plants grown hydroponically under grow lights (12.30 g. KWh-1 in
CS1 and 12.80 g. KWh-1 in CS2). It was also significantly higher
than plants grown in soil under grow lights (8.16 g. KWh-1 in CS1
and 9.29 g. KWh-1 in CS2), p < 0.0005. The vertical hydroponic
structures had higher resource use efficiencies than soil planting. The
research showed that small-scale hydroponic structures can be used to improve
agricultural productivity. © 2022 Friends Science Publishers
Keywords: Vertical
hydroponics; Resource use efficiency; Land use efficiency; Crop water productivity;
Energy use efficiency
Introduction
Vertical
farming is a method of farming where fungi, plants, animals and other life
forms are cultivated by artificially stacking them vertically (Banerjee and Adenaeuer 2014). Eigenbrod and Gruda (2015) presented vertical farming on a large scale as
a technique that can be used to improve agricultural productivity. Vertical
farming is conducted under controlled conditions. The elimination of natural,
thus unpredictable, conditions eradicate weather-related plant damage and
facilitates season-independent crop production. However, several challenges
will be encountered with the establishment of large-scale commercial vertical
farms. The highly controlled environments will require costly precision control
and monitoring. Consequently, vertical farms will have high energy- and skilled
labor requirements. Jenkins (2018) questioned the sustainability of vertical
farms, proposing that the high energy requirements may be counterproductive to
emission reduction. These limitations may deter current and potential producers
from moving away from conventional farming systems (Çİçeklİ
2013; Banerjee and Adenaeuer 2014; Sarkar and
Majumder 2015).
Barbosa et al. (2015) suggested that, in place
of expensive large-scale hydroponic systems, simplified hydroponics could be
used for food production as they are able to produce up to three or four times
more crops than conventional agriculture on an area basis. The performance of
hydroponic systems can be improved by making use of small-scale vertical
farming structures. These structures would maximize the efficiency which with
space is used by making use of multiple growing levels. Small-scale vertical
farming systems can contribute to increasing land use efficiency (LUE) by
extending food production into the vertical plane, thus increasing yield per
unit area.
Currently,
there is limited quantitative research on the applicability of small-scale
vertical farming systems in replacing conventional farming systems for future
food production. Cho (2015) attributed this information deficiency to the short
time span that vertical farms have been in operation. The owners had no
incentive to collect and record
operational data. Furthermore, due to competitive reasons, owners are reluctant
to reveal too much information.
Therefore, the
objective of this research was to assess the resource use efficiency of
small-scale vertical hydroponic structures compared to ground-based planting,
in terms of land-, water- and energy-use efficiency, under sunlight and Light
Emitting Diode (LED) grow lights. This was done to evaluate the potential use
of small-scale vertical farming systems in agricultural intensification. The
motivation for investigating small-scale vertical farming structures was that
they would be able to produce more yield per unit area than soil growth with
lower energy requirements than large-scale vertical farming systems.
It was postulated that the resource use efficiencies of
vertical hydroponic structures under sunlight and LED grow lights would not
differ from the resource use efficiencies of soil planting under sunlight and
LED grow lights. The results obtained in the study disproved the null
hypothesis. The vertical hydroponic structures had higher resource use
efficiencies than soil planting. Small-scale vertical hydroponic structures
have higher agricultural productivity than conventional farming, in terms of
land, water and energy use.
Materials and Methods
Experimental details and treatments
A hydroponic
vertical design was selected for this study. Swiss chard (Spinacea oleracea) was selected because it is a highly nutritious leafy
vegetable and it can be grown in a wide range of hydroponic systems (Parkell et al. 2016).
The trials were conducted inside the Engineering Practicals
Laboratory at the Ukulinga Research Farm in
Pietermaritzburg over two cropping seasons, one starting in February and ending
in March, and the other starting in October and ending in November 2020.
Experimental design
The study made
use of a randomized complete block design and consisted of three factors, each
comprising two levels. The main factor was the growing method: planting in soil
vs vertical hydroponics. The first sub-factor was light provision: natural
sunlight vs artificial light. Red – blue LED strip grow lights were used in a
ratio of 4:1, thus providing light in the photosynthetically active radiation
range. The light was provided at an intensity of 260 µmol.m-2.s-1
for an 18 h photoperiod as proposed by (Kang et al. 2013). The second sub-factor was the concentration of the
nutrient solution. Concentration level one (C1) was 1.4 g.L-1 and
concentration level two (C2) was 1.9 g.L-1 (Kumari et al. 2018). These concentration levels
were used to observe whether there would be significant variation between the
treatments across low and high nutrient concentrations. The treatments were
replicated four times.
Potting soil
and plant pots were used for the soil-grown plants. A total 160 plant pots were
used for plants for each lighting condition, each plant pot carried one plant.
Within each lighting condition, 80 plant pots were irrigated with C1 and the
remaining 80 were irrigated with C2. The plant pots were marked to indicate
which would be irrigated with which concentration level. The marked pots were
then placed randomly within each light treatment. The total area occupied by
the plant pots was 5 m2 for each lighting setup. The pots were kept
in position after sampling to not change total area occupied by the plant pots.
For the
vertical setup, 1200 mm long polyvinyl chlorine (PVC) pipes with a 120 mm
diameter were attached to a frame made of 38x38x3 mm steel sections. In each
lighting treatment, the vertical hydroponic structures carried a total of 160
plants. Each PVC pipe column carried twenty plants. For each lighting
treatment, each structure had a 45L reservoir, where a 2 m (maximum head), 1
200 L.h-1 (maximum flow rate) submersible fountain pump was used to
recirculate the nutrient solution. In the first reservoir, the nutrient
solution concentration was C1 and in the second, it was C2. Micro-sprayers were
used to deliver the nutrient solution to the plant roots. The nutrient solution
flowed down to the bottom of the pipes by gravity, where it was collected in
gutters and returned to the reservoir. The plants were secured in 50 mL net
pots, with 55 mm top- and 35 mm bottom diameters and a 52 mm height. Each net
pot carried one plant. Expanded clay pellets were used as the growing medium.
Fertigation and irrigation
The nutrient
solution selected for the experiment was Nutrifeed by
Stark Aryes (Xego et al. 2016). This nutrient solution
comprised the following macro- and micro-nutrients: Nitrogen (6.5%), Phosphorus
(2.7%), Potassium (13%), Calcium (7.0%), Magnesium (2.2%), Sulphur (7.5%) and
Iron, Manganese, Boron, Zinc, Copper and Molybdenum. Trichoderma was used in
conjunction with the nutrient solution at the beginning of transplanting as
biological control, as it can protect against diseases such as leaf spot and
wilt in leafy vegetables (Bhale et al. 2012). Diatomaceous earth was coated bi-weekly onto the
plants to control pests such as aphids and thrips (Buss and Brown 2006). The
Irrigation Design Manual (Burger et al.
2003) was used to calculate irrigation requirements of the soil grown plants.
It was determined that the plants would need to be irrigated with 5 mm of water
every 3 days.
The nutrient
solution was applied with every second irrigation. The TEROS 21 soil water
potential meter (METER Group, Inc. USA) with ± 10% accuracy (Eliades et al. 2018)
in conjunction with a Decagon Pro Check readout device (Bart et al. 2015) were used to monitor soil
moisture to ensure that the plants were not under- or over irrigated. For the
hydroponic structures, the nutrient solution was replenished every week. When the
nutrient solution was replenished, the solution from the previous week was
discarded to prevent the pumps from being clogged by the nutrient solution
precipitate. An Eco Testr EC meter (Stanley et al. 2014) with a ± 1% accuracy (Eutech Intsruments, Singapore)
was used to monitor the electrical conductivity to ensure that it was within
recommended values of 1.5 and 2.5 dS.m-1 (Kumari et al. 2018). When the solution was
above this range, it would be diluted with water. When it fell below these
values, more nutrient solution would
be added. The pH was also checked to ensure that it ranged between 5.5 and 6.5
as recommended for hydroponic systems (Sardare and Admane 2013). In cases where these values were exceeded, a
pH up/down solution was used to return the solution to permissible values.
Data collection
The area
occupied by each treatment was determined using a measuring tape. Dry matter
output per unit area was used as a measure of the land use efficiency. The land
use efficiency (LUE) was determined using Equation 1.
Where is the land
use efficiency (g.m-2), is the total number of plants, is the area occupied by the growing system (m2)
and is the plant dry weight (g). The water use per
plant was determined using Equation 2. The dry matter produced per liter of total water used
(g.L-1) was used as a measure of crop water productivity (CWP).
(2)
Where is the average
crop water productivity (g.L-1) and is the total water used for irrigation during
growing period (l). For the hydroponic systems under sunlight, the pump energy
consumption was calculated to determine the energy use efficiency (EUE). For
the hydroponic systems under sunlight, the pump energy consumption was
calculated to determine the energy use efficiency (EUE). For the hydroponic
system under LED grow lights, the pump and grow lights energy consumption were
calculated to determine the EUE. For the soil setup under grow lights, the
lights’ electricity consumption was used to determine the EUE.
Equation 3 was
used to determine the EUE of the different treatments.
(3)
Where is the energy use efficiency (g. KWh-1),
is the power rating of equipment (KW) and is the total
run time of equipment (hours).
Data analysis
An analysis of
variation (ANOVA) at a 95% confidence interval using IBM SPSS Statistics 26 (Dytham 2011) was used to conduct statistical analysis of
the results of two cropping seasons.
Results
This section presents the comparison of the resource use
efficiencies (RUEs) of the different treatments. Land-, water-, and energy use
efficiencies were used as measures of resource use efficiencies. The three-way
interaction between the growing method, light provision and nutrient solution
concentration level was not statistically significant for all the RUEs that
were evaluated in the study. This was because the two-way interactions between
the growing method and light provision did not differ significantly across the
two levels of nutrient solution concentration for all the RUEs. Furthermore,
the main effects of growing method and light provision did not differ
significantly across the two levels of nutrient solution concentration. This
result means that the level of nutrient solution concentration did not have a
significant effect on the RUEs. Therefore, the interpretations of the results
obtained in the study are admissible across recommended nutrient solution
concentration levels.
Land use efficiency
There was a statistically significant difference between
the LUE of plants grown hydroponically and those grown in soil, p < 0.0005
in both cropping seasons. The difference between the LUE of plants grown
hydroponically under grow lights and plants grown in soil under grow lights was
statistically significant, p < 0.0005. In both cropping seasons, the LUE of
plants grown hydroponically under grow lights was significantly higher than
that of plants grown hydroponically under sunlight, p < 0.0005. Even though
both systems occupied the same space, thus producing the same number of plants
per unit area, the dry mass per unit area of the plants grown hydroponically
under grow lights was significantly higher than that of plants grown
hydroponically under sunlight. Fig. 1 and 2 display the land use efficiencies
for the different treatments in CS1 and CS2, respectively. The LUE of plants
grown hydroponically under sunlight was significantly higher than that of
plants grown in soil under LED grow lights, p < 0.0005.
Crop water productivity
Plants grown hydroponically and those grown in soil used
a comparable amount of water in terms of total water consumption over the
growing period. This was because in the study, the nutrient solution used by
the hydroponic systems was discarded weekly. This was done to prevent the pumps
from being clogged by the nutrient solution that would collect at the bottom of
the reservoir due to precipitation of salts in the nutrient solution. However,
the difference between the mean CWP of plants grown hydroponically and those
grown in soil was statistically significant in both cropping seasons, p <
0.0005 in CS1 and in CS2, p = 0.014. Fig. 3 and 4 display the CWP for the
treatments in CS1 and CS2, respectively.
Fig. 1: The land use efficiency in g.m-2 of the
different treatments in cropping season one. The same letters indicate that the
difference was not statistically significant between the treatments at P
≤ 0.05
Fig. 2: The land use efficiency in g.m-2
of the different treatments in cropping season two. The same letters indicate
that the difference was not statistically significant between the treatments at
P ≤ 0.05
Although the water consumption for both treatments was
the same, there was a significant difference in both seasons (p < 0.0005)
between the mean CWP of plants grown hydroponically under LED grow lights and
plants grown hydroponically under sunlight. There was no significant difference
between the CWP of plants grown hydroponically under sunlight and those grown
in soil under grow lights, p = 0.969 in CS1 and p = 0.099 in CS2.
Energy use efficiency
The difference between the mean EUE between plants grown
hydroponically under sunlight and those grown hydroponically under grow lights
was statistically significant, p < 0.0005 in both cropping seasons. Fig. 5 displays
the EUE for the plants grown hydroponically under the different light settings
in CS1 and CS2.
The mean EUE of plants grown hydroponically under grow
lights was significantly different from the mean EUE of soil grown plants under
grow lights, p = 0.002 in CS1 and p = 0.013 in CS2. Fig. 6 displays the EUE of
the plants grown hydroponically and those grown in soil under LED grow lights. There was a
significant difference (p < 0.0005 in both cropping seasons) between the EUE
of plants grown hydroponically under sunlight and plants grown in soil under
grow lights. Fig. 7 illustrates the EUE of the hydroponic system under sunlight
and the soil system under grow lights.
Fig. 3: The water use
efficiency in g.L-1 of the different treatments in cropping season
one. The same letters indicate that the difference was not statistically
significant between the treatments at P ≤ 0.05
Fig. 4: The water use
efficiency in g.L-1 of the different treatments in cropping season
two. The same letters indicate that the difference was not statistically
significant between the treatments at P ≤ 0.05
Discussion
Use of the vertical plane enabled the hydroponic
structures to occupy a smaller horizontal area whilst producing more plants per
unit area. The vertical structures were able to produce 5 times more plants per
unit area than the soil set up for both light treatments. This result is
important because, with increasing food requirements and dwindling arable land,
small-scale hydroponic vertical structures use occupied space more efficiently
than conventional planting in soil. In contrast, Rufí-Salís
et al. (2020) reported a much higher LUE for large-scale vertical farms,
stating that such systems could have LUE values that are 12.5 – 25 times higher
than the conventional greenhouse production of lettuce. However, the LUE of
small-scale vertical farming structures could be increased by increasing column
height. But factors, such as column stability, shading effects, ergonomics for
planting/harvest time and ergonomics for maintenance would have to be
considered before scaling up column
Fig. 5: The energy use
efficiency in g.KWh-1 of the plants grown hydroponically under
different light treatments in cropping season one (CS1) and two (CS2). The same
letters indicate that the difference was not statistically significant between
the treatments at P ≤ 0.05
Fig. 6: The energy use efficiency in g. KWh-1 of the
different growing methods under LED grow lights in cropping season one (CS1)
and two (CS2). The same letters indicate that the difference was not
statistically significant between the treatments at P ≤ 0.05
sizes.
The addition of grow lights resulted in higher plant
mass production. This is because grow lights produce controlled and more
consistent radiation than sunlight. Unlike sunlight, the radiation from grow
lights is not influenced by factors such as weather conditions. Additionally,
artificial grow lights have the added benefit of providing a photoperiod that
can be altered to suit plant needs. In the study, plants grown under grow
lights had an 18 h photoperiod, whereas the photoperiod for plants grown under
sunlight depended on when the sun rose and set.
The results indicate that when comparing hydroponic
systems under sunlight with plants grown in soil under grow lights, the use of
the vertical plane outperforms the benefits associated with grow lights. This
is because the vertical structures occupied a smaller horizontal area than the
plant pots. Therefore, they produced more plants per unit area than the plants
grown in soil under grow lights. This is an important result for controlled
environmental agriculture (CEA) where grow lights are used to supplement
sunlight during dark hours. Grow lights have been identified as one of the
highest energy consumers in
Fig. 7: The energy use
efficiency in g. KWh-1 of the different treatments in cropping
season one (CS1) and two (CS2). The same letters indicate that the difference
was not statistically significant between the treatments at P ≤ 0.05
CEA. Therefore, this result suggests that vertical
hydroponic structures could be an alternative to soil-based plant growth in
cases where grow lights supplement sunlight, as they use space more
efficiently.
In this study, it was observed that the leaf areas of
plants grown hydroponically under sunlight decreased in size along the length
of the columns from top to bottom. Due to sample number restrictions, it could
not be determined whether this variation was significant. This appears to be a
limitation that is inherent to column-type vertical farming structures, as Touliatos et al. (2016) noted similar trends for
lettuce grown in vertical columns under metal halide lamps. In the study by Touliatos et al. (2016), the grow lights were placed
above plants. However, in this undertaken study, the leaf area sizes did not
vary along the column lengths of plants grown hydroponically under grow lights
because the lights were placed in front of the structures. This limitation,
therefore, only poses a restriction for increasing column height for plants
grown under sunlight or in cases where grow lights are placed above the
columns.
Barbosa et al. (2015) reported that
hydroponically- and soil-grown lettuce consumed a comparable volume of water,
but the hydroponically grown lettuce consumed less water per plant or,
inversely, produced more yield per liter of water than the soil system. A
similar result was found in this study. This can be attributed to the fact
that, in hydroponic systems, a higher fraction of the supplied water is
allocated to plant production than in soil systems. In soil systems, some of
the water supplied is lost to the soil environment surrounding the plant. In
hydroponic systems, because the nutrient solution is collected at the roots and
recirculated, there is minimal water loss to the surroundings. This is an
important finding for agriculture because fresh-water availability is an important
concern. These results show that vertical hydroponic structures produce
significantly higher yields than soil planting from the same volume of water.
The mean CWP of the vertical hydroponic structures could
further be improved if the ‘old’ nutrient solution was not discarded every
week, as was done in the study, but rather recovered and reused. Rufí-Salís et al. (2020) investigated techniques for
nutrient solution recovery in hydroponics and found that direct leachate
recirculation was the best option in terms of nutrient solution re-use, and it
had a lower carbon footprint than the other options investigated. In this
system, the recovered nutrient solution could be filtered and sterilized. The
remaining nutrients would then be analyzed to assess which nutrients needed to
be re-added to meet plant requirements. Such a system would have a two-fold
impact as it would not only decrease the amount of water added to the system,
but it would also decrease the overall amount of nutrients supplied as well.
The presence of LED grow lights improved the hydroponic
system’s yield production per liter of water consumed. In their study of
greenhouse tomatoes, Li et al. (2019) also found that plants grown under
supplementary LED lighting had a higher CWP than plants grown under sunlight,
even though water consumption was similar. The higher CWP by the hydroponic
system under grow lights was because the LED grow lights enhanced plant
photosynthesis, thereby increasing CWP without changing water consumption (Li et
al. 2019).
Despite the resulting increase in LUE and CWP, indoor
lighting has been identified as one of the largest energy consumers in CEA
(Sparks 2016). Barbosa et al. (2015) documented that the large-scale
hydroponic production of lettuce can require 82 times or more energy per
kilogram than conventional lettuce production. The high energy consumption in
this study was because the grow lights operated for 18 h a day to meet the
recommended photoperiod for plants. Although this has been proven to be beneficial
in terms of LUE and CWP, the large difference in EUE is a concern, especially
when the aim of vertical farming is to minimize negative environmental impacts.
Even though the hydroponic system under grow lights had
additional energy consumption from the pump, its EUE was still significantly
higher than that of the soil grown plants under grow lights. This meant that,
despite the vertical hydroponic system under grow lights having higher energy
consumption than soil-based growth under grow lights, the hydroponic system was
more productive in terms of EUE. Use of the vertical plane in conjunction with
water recirculation resulted in a more efficient use of energy. This result is
important for soil-based CEA where artificial grow lights are used to completely
replace sunlight. Since these types of CEA applications already need artificial
grow light, replacing soil plant growth with small-scale vertical hydroponic
systems would result in more efficient use of energy.
Whilst the energy consumption of the grow lights
resulted in low EUE for both the hydroponic and the plant pot systems, the
study demonstrated that the presence of grow lights can improve resource use
efficiency. Pennisi et al. (2019) also reported that use of grow lights can increase
the overall resource use efficiency of plant production. In the LUE graphs and
CWP graphs, the difference between the light treatments is more distinct in the
hydroponics treatment than in the soil treatment. This observation suggests
that hydroponic systems use LED grow lights more efficiently than growing
plants in soil.
There are several ways in which the energy consumption
of systems that use grow lights could be decreased. In regions with sufficient
sunlight radiation, these systems could be designed such that grow lights are
used seasonally or during times of low radiation. This presents an opportunity
for the development of affordable devices that can detect radiation and produce
instantaneous results about whether the use of grow lights is necessary at a
certain period.
Furthermore, grow lights can be used to mitigate the
variance of plant size along the length of columns in vertical hydroponic
systems. In such systems, grow lights can be applied to the lower sections to
supplement sunlight. Another alternative would be to use grow lights for a
shorter photoperiod. That is, to have the grow lights operate for a few hours
during dark hours to extend time for photosynthesis whilst reducing electricity
consumption. For example, Frąszczak (2013) found
that exposing dill plants to red LED light or white light at the end of the
night for 30 min stimulated plant growth. Therefore, there are several options
that can be explored where LED grow lights can be used in such a manner that optimizes
plant growth, whilst decreasing energy consumption in vertical hydroponics.
Conclusion
Research
presents vertical farming as a possible solution to the challenges associated
with conventional farming. Use of the vertical plane and artificial inputs has
been stated to improve yield quantities and qualities. However, thus far, there
has been a deficit of information on whether yield improvements are achieved at
a greater input cost than the conventional growth of plants in soil. This study
has contributed to increasing knowledge on the resource use efficiencies of
small-scale vertical hydroponic systems. The research conducted has proven that
small-scale vertical hydroponic structures use resources more efficiently than
growing plants in soil under LED grow lights as well as under sunlight for
recommended nutrient solution concentrations.
Acknowledgements
The first author acknowledges the financial sponsorship
from Agricultural Research Council and the Department of Science and
Technology, South Africa.
Author
Contributions
ZB conceptualized the work, collected and interpreted
the data, drafted and conducted critical revision of the article. GL and AS
contributed to the conceptualization of the work and critically revised the
article. AC critically revised the article, and SS contributed to the
conceptualizing of the work.
Conflicts of
Interest
The authors hereby declare that they do not have competing
financial interests or any relationships of a personal nature that could have
had influence on the work reported in this paper.
Ethics Approval
The experiments conducted in the study were not conducted on
animal or human subjects. Therefore, ethics approval was not required for the
study.
Funding Source
The research was made possible by funding from The Department
of Science and Technology through The Agricukltural Research Council.
References
Banerjee C, L Adenaeuer (2014). Up, up and away! The economics of
vertical farming. J Agric Stud 2:40‒60
Barbosa GL, FDA
Gadelha, N Kublik, A Proctor, L Reichelm,
E Weissinger, GM Wohlleb, RU
Halden (2015). Comparison of Land, Water, and Energy Requirements of Lettuce
Grown Using Hydroponic vs. Conventional Agricultural Methods. Intl J Environ Res Pub Health 12:6879‒6891
Bart D, T Davenport, Q Carpenter (2015). Stress and land‐use
legacies alter the relationship between invasive‐and native‐plant
richness. J Veg Sci 26:80‒88
Bhale U, M Ambuse, V Chatage, J Rajkonda (2012). Bioefficacy of Trichoderma isolates against pathogenic
fungi inciting spinach (Spinacea oleracea L.). J Biopest 5:222
Burger J, P Heyns, E Hoffman, E Kleynhans, F Koegelenberg,
M Lategan, D Mulder, H Smal,
C Stimie, W Uys, F van der Merve,
I van der Stoep, P Viljoen (2003). Irrigation
Design Manual. Agricultural Research Council – Institute for Agricultural
Engineering, Pretoria, South Africa
Buss E, S Brown (2006). Natural Products for Insect Management. Available at: https://www.researchgate.net/profile/Eileen_Buss/publication/237391484
Natural_Products_for_Insect_Pest_Management1/links/00b4952a5dbef7e2b4000000.pdf
(Accessed: 02 May 2020)
Cho R (2015).
How sustainable is vertical farming? Available at: https://phys.org/print369046891.html
(Accessed: 10 February 2018)
Çİçeklİ M (2013). Vertical farming. Ege Üniversitesi Ziraat
Fakültesi Dergisi Special
Issue: 383‒388
Dytham C (2011). Choosing
and Using Statistics: A Biologist's Guide. Wiley-Black, West Sussex, United
Kingdom
Eigenbrod C, N Gruda (2015). Urban
vegetable for food security in cities. A review. Agron Sust Dev 35:483‒498
Eliades M, A Bruggeman, H Djuma, MW Lubczynski (2018). Tree water dynamics in a semi-arid, Pinus
brutia forest. Water 10:1039‒1060
Frąszczak B (2013). Effect of short-term exposure to red and blue
light on dill plants growth. Hortic Sci 40:177‒185
Jenkins D
(2018). Rise to the Challenge Vertical Farming within the Urban Environment. MSc Dissertation, Architecture, Unitec
Institute of Technology, Auckland, New Zealand
Kang JH, S KrishnaKumar, SLS Atulba, BR Jeong, SJ Hwang (2013). Light intensity and photoperiod
influence the growth and development of hydroponically grown leaf lettuce in a
closed-type plant factory system. Hortic Environ Biotech
54:501‒509
Kumari S, P
Pradhan, R Yadav, S Kumar (2018). Hydroponic techniques: A soilless cultivation
in agriculture. J Pharmacog
Phytochem 7:1886‒1891
Li Q, Y Liu, S
Tian, Z Liang, S Li, Y Li, M Wei, D Zhang (2019). Effect of supplemental
lighting on water transport, photosynthetic carbon gain and water use
efficiency in greenhouse tomato. Sci Hortic 256:108630
Parkell NB, RC Hochmuth, WL Laughlin (2016). Leafy Greens in Hydroponics and
Protected Culture in Florida. Institute of Food and Agricultural Sciences.
Available at: https://growables.com/information/documents/HydroLeafyGreenUF.pdf
(Accessed: 19 June 2017)
Pennisi G, F Orsini, S Blasioli, A
Cellini, A Crepaldi, I Braschi,
F Spinelli, S Nicola, JA Fernandez, C Stanghellini
(2019). Resource use efficiency of indoor lettuce (Lactuca sativa L.) cultivation as affected by red: blue ratio provided by
LED lighting. Sci Rep 9:1‒11
Rufí-Salís M, MJ Calvo, A Petit-Boix, G Villalba, X Gabarrell (2020).
Exploring nutrient recovery from hydroponics in urban agriculture: An
environmental assessment. Resourc, Conserv Recycl 155:104683
Sardare MMD, MSV Admane (2013). A
review on plant without soil-hydroponics. Intl
J Res Eng Technol 2:299‒305
Sarkar A, M Majumder
(2015). Opportunities and Challenges in Sustainability of Vertical Eco-Farming:
A Review. J Adv Agric Tech 2:98‒105
Sparks RE
(2016). Mapping and analyzing energy use and efficiency in a modified
hydroponic shipping container. Unpublished
Dissertation thesis, Engineering, Purdue University, West Lafayette, USA
Stanley JN, DW Lamb, G Falzon, DA Schneider
(2014). Apparent electrical conductivity (ECa) as a
surrogate for neutron probe counts to measure soil moisture content in heavy
clay soils (Vertosols). Soil Res 52:373‒378
Touliatos D, IC Dodd, M McAinsh (2016).
Vertical farming increases lettuce yield per unit area compared to conventional
horizontal hydroponics. Food Ener Sec
5:184‒191
Xego S, L Kambizi, F Nchu
(2016). Threatened medicinal plants of South Africa: case of the family Hyacinthaceae. Afr J
Trad Compl Alt Med 13:169‒180