Hydroponic pH Modifiers affect Plant Growth and Nutrient Content in Leafy Greens

Abstract

Use of hydroponics is increasing because of its ability to be used for year round vegetable production using an environmentally sustainable system. Management of solution pH is an important challenge in hydroponics systems. Our objective was to quantify the effects of various pH modifiers on growth and nutrient uptake of leafy greens and stability of nutrient solution’s pH. Lettuce, basil, and Swiss chard were transplanted into an Ebb and flow system, and nutrient solution pH was maintained using three different pH modifiers (pH Down, lime juice, or vinegar). The nutrient solution’s pH was maintained between 5.5 and 6.5. pH Down resulted in the most stable solution pH and required the least amount of product used when compared to lime juice and vinegar. The cost of using phosphoric acid or lime juice was greater than that of using vinegar. Vinegar reduced the yield of all crops in comparison to pH Down. When compared to pH Down, lime juice reduced the yield of basil and Swiss chard but not that of lettuce. Therefore, growers can use lime juice as an alternative to pH Down in lettuce production but not for basil and Swiss chard, while vinegar would not be recommended for any of the crops studied.

Full text

Journal of Horticultural Research 2019, vol. 27(1): 31-36
DOI: 10.2478/johr-2019-0004
_______________________________________________________________________________________________________
*Corresponding author:
e-mail: hardeep.singh@okstate.edu
HYDROPONIC pH MODIFIERS AFFECT PLANT GROWTH
AND NUTRIENT CONTENT IN LEAFY GREENS
Hardeep SINGH*, Bruce DUNN, Mark PAYTON
Department of Horticulture and Landscape Architecture,
Oklahoma State University, Stillwater, OK 74078-6027, USA
Received: April 2019; Accepted: June 2019
ABSTRACT
Use of hydroponics is increasing because of its ability to be used for year round vegetable production
using an environmentally sustainable system. Management of solution pH is an important challenge in
hydroponics systems. Our objective was to quantify the effects of various pH modifiers on growth and
nutrient uptake of leafy greens and stability of nutrient solution’s pH. Lettuce, basil, and Swiss chard were
transplanted into an Ebb and flow system, and nutrient solution pH was maintained using three different
pH modifiers (pH Down, lime juice, or vinegar). The nutrient solution’s pH was maintained between 5.5
and 6.5. pH Down resulted in the most stable solution pH and required the least amount of product used
when compared to lime juice and vinegar. The cost of using phosphoric acid or lime juice was greater than
that of using vinegar. Vinegar reduced the yield of all crops in comparison to pH Down. When compared
to pH Down, lime juice reduced the yield of basil and Swiss chard but not that of lettuce. Therefore, growers
can use lime juice as an alternative to pH Down in lettuce production but not for basil and Swiss chard,
while vinegar would not be recommended for any of the crops studied.
Keywords: soilless culture, nutrients, lettuce, basil, Swiss chard, chlorophyll meter
INTRODUCTION
By 2050, the human population is expected to
reach 8.9 billion (USAID 2004), and a major chal-
lenge for the increased population will be maintain-
ing the supply of fresh produce to ensure nutrient-
rich diets. Hydroponic or soilless production could
be an important solution to this problem because of
its higher yields and more nutritious food when
compared to soil production (Skagg 1996). Hydro-
ponics can be defined as a technique of growing
non-aquatic plants without soil in a nutrient solu-
tion with or without soilless substrate (Arancon et
al. 2015). Maintaining an adequate nutrient solu-
tion and pH level are often cited as major obstacles
to hydroponic production (Steiner 1961). Frick and
Mitchell (1993) indicated that pH of a hydroponic
nutrient solution fluctuates because of the unbal-
anced anion and cation exchange reaction with
roots and there is no buffering capacity in hydro-
ponics as in soil.
Plant essential nutrient availability varies with
pH. According to Resh (2004), slightly acidic pH is
optimum for hydroponic production because iron
(Fe), manganese (Mn), calcium (Ca), and magne-
sium (Mg) may form precipitates and become una-
vailable at pH above 7. Islam et al. (1980) reported
that at higher pH, the amount of Fe, Mn, Mg, potas-
sium (K), and Ca increased in the plants, but these
elements were not translocated to the shoot but in-
stead remained stored in the roots. Bugbee (2003)
also reported that availability of K and phosphorus
(P) is slightly reduced in a nutrient solution with
high pH. Dyśko et al. (2008) also reported that the
increase in nutrient solution’s pH led to the decrease
in available P for hydroponic production of tomato
(Solanum lycopersicum L.). Chen et al. (2016) also
reported a difference in nutrient uptake of lettuce
Bereitgestellt von Oklahoma State University - Edmon Low Library | Heruntergeladen 06.11.19 14:13 UTC

32 H. Singh et al.
____________________________________________________________________________________________________________________
with solution pH when using wood vinegar.
Hochmuth (2001) recommended a nutrient solu-
tion’s pH of 5.5–6.5 for greenhouse hydroponic
production, whereas Resh (2004) recommended
a pH of 5.8–6.4. Ahn and Ikeda (2004) also re-
ported a pH of 5–7 as optimum for hydroponic cul-
tivation of Chinese chive (Allium tuberosum Rot-
tler ex Spreng.). Whipker et al. (1996) various
studies examining optimum pH for hydroponic let-
tuce (Lactuca sativa L.) production reported a de-
crease in leaf area, shoot dry weight, leaf length
and width, and stomatal conductance because of
the exposure to a suboptimal solution pH.
There are various chemicals that can be used
to lower the pH of a nutrient solution in hydropon-
ics. Burleigh et al. (2008) recommended the use of
citric acid (lime juice), acetic acid (vinegar), nitric
acid, phosphoric acid, and sulfuric acid for lower-
ing pH of the water used for plant cultivation.
Chen et al. (2016) also reported that pyroligneous
acid (wood vinegar) can be used for hydroponic
cultivation of lettuce at a rate of 0.25 ml·dm-3
while evaluating different strengths of wood vine-
gar as a pH buffer. Frick and Mitchell (1993) com-
pared the use of 2-(N-morpholino)ethanesulfonic
acid (MES) buffer and Amberlite DP-1 (cation-ex-
change resin beads, 16–50 wet mesh, 8.1 mEq·g-1)
for stabilizing the pH of nutrient solution for the
production of mustard (Brassica juncea L.). The
concentration of chemicals used for pH stabiliza-
tion can also affect plant growth. Stahl et al.
(1999) used different concentrations of MES for
hydroponic culture and concluded that plant
growth was affected with increasing concentra-
tions. Stabilizing the pH of a nutrient solution is
necessary for optimum crop productivity in hydro-
ponics (Frick & Mitchell 1993). Identifying
a more economical and readily available product
for reducing the pH of the solution without reduc-
ing the crop yield would be beneficial to growers
(Kirimura & Inden 2005). Therefore, the objective
of this study was to evaluate the use of lime juice,
vinegar, and a commercial pH buffer (pH Down) as
pH modifiers in hydroponics and their effect on
plant growth, chlorophyll content, and nutrient up-
take of leafy greens.
MATERIALS AND METHODS
Plant material and growth conditions
The research was conducted at the Department of
Horticulture and Landscape Architecture Research
Greenhouses in Stillwater, OK, under natural photo-
periods. Temperature was set at 21 °C/18 °C day/night
with a photosynthetic photon flux density (PPFD)
range of 600–1,200 μmol·m-2·s-1 at 1,200 HR.
Seeds of red lettuce ‘Oscarde’, basil (Ocimum basil-
icum L. ‘Citrus’) and Swiss chard (Beta vulgaris L.
‘Magenta Sunset’), were obtained from Johnny’s
Selected Seeds (Winslow, ME). Seeds were sown in
1.5 cm3 rockwool starter cubes (Gordan, Milton, ON)
on 2 February 2017. A styrofoam sheet was used to
support the plants, and 5 cm diameter slots were
drilled into the sheet with a spacing of 28 cm × 28 cm.
Upon obtaining two true leaves (March 6, 2017),
plants were transplanted into 5-cm net pots on Ebb
and flow tables (Gro Master, Maple Park (Virgil), IL).
Plants were randomly assigned to 1 of 30 net pots on
each of 3 tables resulting in 10 replicates of each crop
species per table. Each table was randomly as-
signed one of the three pH modifier treatments:
white vinegar (5% acidity; Wal-Mart Stores Inc.,
Bentonville, AR), lime juice (1.06 g·oz-1 citric
acid; Dr. Pepper Snapple Group, Plano, TX), and
pH Down (General Hydroponics, Santa Rosa, CA).
The lime juice was diluted to reach a pH of 2.5
similar to the other two products. The plants were
harvested 30 days after transplanting. The entire
experiment was repeated three times with planting
also occurring on 20 March and 25 April 2017 and
transplanting occurring on 25 April and 3 June
2017 for the second and third runs, respectively.
Fertilizers and EC
The nutrient solution was designed using a commer-
cially available soluble fertilizer (Peters 5-5.2-21.6,
J.R. Peters Allentown, PA), calcium nitrate (Amer-
ican Plant Products), and local tap water (EC =
0.5 mS·cm-2, pH = 7.8). Initial solutions were pro-
duced using manufacturer recommendations of
147.41 g·dm-3 of Peters and 97.52 g·dm-3 of cal-
cium nitrate. The hydroponics system had
a 141.4 dm3 of tank capacity and was circulated us-
ing a 189 dm3·min.-1 pump (Wayne, Harrison, OH).
Bereitgestellt von Oklahoma State University - Edmon Low Library | Heruntergeladen 06.11.19 14:13 UTC

pH modifiers affect lettuce, basil, and Swiss chard 33
____________________________________________________________________________________________________________________
The EC and pH of the solution were measured every
other day using a pH/EC meter (Hanna Instruments,
Woonsocket, RI). The EC of the nutrient solution
was checked every other day to maintain the EC at
1.5–2.5 ds·m-1 and the pH at 5.5–6.5 by adding fer-
tilizer and pH solution.
Data Collection
At the end of the study, data were collected on fresh
shoot weight and dried shoot and root weight (plants
cut at substrate level and dried for 2 days at
56.6 °C). Three leaves (top, middle, and bottom)
from each plant were scanned using a SPAD-502
chlorophyll meter (SPAD-502, Konica Minolta, Ja-
pan) at the time of harvest. Dried shoot samples
were analyzed for nitrogen content by the Soil, Wa-
ter and Forage Analytical Laboratory (SWFAL) at
Oklahoma State University, using a LECO TruSpec
Carbon and Nitrogen Analyzer (LECO Corporation,
St. Joseph, MI).
Statistics
The experimental design was a split-plot design
with 3 replications of 10 individual replicates per
species per run; Factors were pH-lowering products
(three levels) and species (three levels). Data were
subjected to an analysis of variance (ANOVA) us-
ing PROC MIXED with the LSMEANS statement
and DIFF option within the SAS/STAT software,
version 9.4 (SAS Institute, Cary, NC). Tests of sig-
nificance are reported at the 0.05, 0.001, and 0.0001
levels. Treatment means were separated using
Fisher’s protected least significance difference
(LSD) method. Statistical analyses were conducted
for each crop separately.
RESULTS
Effect of different pH products on nutrient solu-
tion’s pH
Nutrient solution’s pH increased on a near-liner
trend for all three modifiers; however, pH Down ap-
peared to plateau around a pH of 6.7 starting around
25 days (Fig. 1). The pH was in the required range
(5.5–6.5) throughout the growth cycle for treat-
ments using phosphoric acid, whereas for treat-
ments using lime juice and vinegar, the pH reached
7.5 at harvest (Fig. 1). Across the three experimental
runs, the total amount of lime, vinegar, and pH
Down used per run was 6,000 ml; 8,000 ml; and
600 ml, respectively.
Effect of different pH products on growth and
chlorophyll content of lettuce, basil, and Swiss
chard
The fresh weight and dry shoot weight of lettuce
were significantly lower with vinegar than with the
other treatments, while there was no significant dif-
ference between lime juice and phosphoric acid (Ta-
ble 1). There was no significant difference in dry
weight of lettuce roots among the treatment groups.
The SPAD values were lowest for lime juice, and
there was no significant difference between vinegar
and phosphoric acid (Table 1).
The fresh and dry shoot weight of basil was
significantly greater for phosphoric acid than other
pH modifier treatments. The dry root weight of basil
was significantly lower for vinegar than other treat-
ments, whereas no significant difference was ob-
served between lime juice and phosphoric acid. The
SPAD values were significantly greater for phos-
phoric acid than for lime juice or vinegar (Table 2).
The fresh and dry shoot weight of Swiss chard
was greatest for plants grown with phosphoric acid
(Table 3). The dry weight of Swiss chard roots was
lowest using vinegar, whereas no significant differ-
ence was observed between among lime juice and
phosphoric acid. No significant difference was ob-
served for SPAD values among all three treatments
of Swiss chard (Table 3).
Effect of different pH products on nutrient con-
tent of lettuce, basil, and Swiss chard
Nitrogen (N) and K contents for lettuce and basil
were significantly lower for the lime and vinegar
treatments when compared to the phosphoric acid
treatment (Table 4). This corresponded to lower
SPAD values in both lime- and vinegar-treated nu-
trient solution when compared to that treated with
phosphoric acid. There was no significant differ-
ence in the P content among the different treatments
for either lettuce or basil; also, there was no signifi-
cant difference in N or K content among treatments
for Swiss chard (Table 4). Plant micronutrient con-
tent was not affected by pH buffer treatment (data
not presented).
Bereitgestellt von Oklahoma State University - Edmon Low Library | Heruntergeladen 06.11.19 14:13 UTC

34 H. Singh et al.
____________________________________________________________________________________________________________________
Fig. 1. Nutrient solution’s pH before adjustment during the production of lettuce ‘Oscarde’, basil ‘Citrus’, and Swiss
chard ‘Magenta Sunset’
Table 1. Effects of different pH modifiers on ‘Oscarde’ lettuce growth and chlorophyll content after 30 days in Ebb
and flow system (n = 30)
Treatment
Shoot FW (g)***z
Shoot DW (g)**
Root DW (g)NS
SPAD*
pH Down
235.9 ay
7.7 a
0.7 a
22.2 a
Lime juice
210.8 a
7.8 a
0.8 a
20.5 b
Vinegar
116.7 b
5.7 b
0.7 a
22.6 a
z indicates significant at or non-significant (NS) at *p ≤ 0.05, **p ≤ 0.001, or ***p ≤ 0.0001
y LS means within a column followed by same lowercase letter are not significantly different by pairwise comparison in mixed
model (p ≤
0.05)
Table 2. Effects of different pH modifiers on ‘Citrus’ basil growth and chlorophyll content after 30 days in Ebb and
flow system (n = 30)
Treatment
Shoot FW (g)***z
Shoot DW (g)***
Root DW (g)*
SPAD*
pH Down
293.0 ay
24.6 a
4.2 a
26.0 a
Lime juice
213.6 b
19.1 b
4.3 a
24.5 b
Vinegar
151.8 c
13.4 c
3.3 b
23.6 b
Note: see Table 1
Table 3. Effects of different pH modifiers on ‘Magenta Sunset’ Swiss chard growth and chlorophyll content after 30
days in Ebb and flow system (n = 30)
Treatment
Shoot FW (g)**z
Shoot DW (g)**
Root DW (g)*
SPADNS
pH Down
187.7 ay
10.9 a
1.4 a
42.6 a
Lime juice
118.8 b
7.6 b
1.0 a
42.1 a
Vinegar
50.1 c
4.3 c
0.3 b
42.0 a
Note: see Table 1
Table 4. Effects of different pH modifiers on macronutrients element of lettuce ‘Oscarde’, basil ‘Citrus’, and Swiss
chard ‘Magenta Sunset’ (n = 3)
Treatment
Nitrogen (%)
Phosphorus (%)
Potassium (%)
lettuce**z
basil**
Swiss chardNS
lettuceNS
basilNS
Swiss chardNS
lettuce*
basil*
Swiss chardNS
pH Down
4.8 ay
5.2 a
4.3 a
0.7 a
1.3 a
0.4 a
6.2 a
0.8 a
2.1 a
Lime juice
4.4 b
4.2 b
4.5 a
0.7 a
1.3 a
0.4 a
6.0 b
0.6 b
2.2 a
Vinegar
4.8 a
4.2 a
4.1 a
0.7 a
1.3 a
0.5 a
6.1 b
0.7 b
2.1 a
Note: see Table 1
5,3
5,8
6,3
6,8
7,3
7,8
1 8 15 22 29
pH
Days
Lime Vinegar pH Down
Bereitgestellt von Oklahoma State University - Edmon Low Library | Heruntergeladen 06.11.19 14:13 UTC

pH modifiers affect lettuce, basil, and Swiss chard 35
____________________________________________________________________________________________________________________
DISCUSSION
The authors hypothesize that the initial delay
in pH increase may be because the nutrient uptake
was less during the first week because of the small
size of the plant as well as a smaller leaf surface area
for transpiration of water. During the second week,
as the plants grew, more nutrients and water were
taken up. This may have led to an uneven uptake of
anions and cations, which is one of the causes of pH
change in a nutrient solution (Frick & Mitchell
1993). Chen et al. (2016) also reported that pH was
more stable during the first week of the growth cy-
cle but increased thereafter. In contrast to the results
of the present study, Lee and Lee (2006) reported
that because of balanced nutrient uptake during the
growth cycle, the nutrient solution pH remained sta-
ble during hydroponic production of leafy greens.
Because our water pH is alkaline (pH 7.8), adding
water to replenish levels in the tank increased the
pH. According to Sinclair and Eny (1946), juices
such as lemon juice consisting of citric acid can also
be used as an organic buffer to resist changes in the
pH when hydrogen or hydroxyl ions are added. In
this study, citric acid alone was only effective for
a day then the pH increased steadily, whereas with
pH Down, which also contains citric acid, ammo-
nium dihydrogenorthophosphate, and phosphoric
acid, the pH level did not fluctuate as rapidly and
stabilized at the end.
The color of leafy green vegetables is an im-
portant attribute affecting consumer preference (Ali
et al. 2009). Chlorophyll meters can be used to esti-
mate the greenness of leafy green vegetables as Col-
onna et al. (2016) used a SPAD meter to estimate
nitrogen (N) content of leaves as a nondestructive
method. Singh et al. (2019) reported that SPAD val-
ues ranged from 17 to 28 for different cultivars of
lettuce, which corresponded with our values, but
found ‘Oscarde’ to have a greater SPAD value. The
lower SPAD values for basil treated with lime juice
and vinegar corresponded to visual observations of
chlorosis and lower tissue N content. Chen et al.
(2016) found that use of wood vinegar at high con-
centrations (1 ml·dm-3) in hydroponic lettuce pro-
duction may lead to decreased nitrate-N uptake,
photosynthesis rate, and chlorophyll content.
In contrast, the lower SPAD values for lettuce
plants treated with lemon juice did not show evi-
dence of chlorosis, which may be the result of using
a red leafed cultivar.
Generally, inorganic acids such as nitric acid,
sulfuric acid, and phosphoric acid are used for re-
ducing and stabilizing the nutrient solution’s pH in
hydroponic production. This may also affect the nu-
trient composition of the solution as nitric acid may
contribute to the nitrate form of N and sulfuric acid
may contribute to sulfate ions and it would be hard
for a normal grower to calculate the amount of nu-
trients added every time the pH buffer is added
(Chen et al. 2016). Lei et al. (2004) reported hydro-
ponic vegetables to be higher in nitrate concentra-
tion as compared to soil grown vegetables, which is
harmful for human consumption. Greater leaf nitro-
gen and potassium values were observed for ‘Os-
carde’ and ‘Citrus’, which is not unexpected for pH
Down containing additional ammonium and phos-
phates. The additional nitrogen was unexpected for
vinegar, which is derived from ethanol to produce
acetic acid. Kirimura and Inden (2005) reported that
using safe, natural, and less-expensive acidic mate-
rial is more beneficial for hydroponic production as
compared to inorganic acids. This contradicts what
we found, as vinegar and lime juice reduced the plant
growth for ‘Citrus’ and ‘Magenta Sunset’ and re-
duced the plant quality in ‘Oscarde’ compared to pH
Down. The effect of lime juice and vinegar on basil
and Swiss chard may be due to the use of too high of
concentrations to maintain a desirable pH level.
CONCLUSION
From the results of the present experiment, lime
juice or pH Down can be used as a pH modifier for
hydroponic production of ‘Oscarde’ as chlorosis is
not prominent in red cultivars. For hydroponic pro-
duction of basil, only pH Down should be used, be-
cause the use of lime juice and vinegar leads to lower
SPAD values (chlorosis), less N uptake, and reduced
growth. For Swiss chard, pH Down would be recom-
mended for greater growth. For all three species, pH
Down had lower production costs of $2.60 compared
to $8 and $26.40 for vinegar and lime juice, respec-
tively, and had a greater affect at maintaining the pH.
Bereitgestellt von Oklahoma State University - Edmon Low Library | Heruntergeladen 06.11.19 14:13 UTC

36 H. Singh et al.
____________________________________________________________________________________________________________________
Future research should investigate the use of different
organic acids, concentrations, combination with
other buffering compound, or different cultivars of
basil, Swiss chard, and lettuce.
REFERENCES
Ahn D.-H., Ikeda H. 2004. Effects of pH and concentration
of nutrient solution on growth of hydroponically cul-
tured Chinese chive (Allium tuberosum Rottler).
Horticultural Research (Japan) 3: 191–194. DOI:
10.2503/hrj.3.191. [in Japanese with English abstract]
Ali M.B., Khandaker L., Oba S. 2009. Comparative study
on functional components, antioxidant activity and
color parameters of selected colored leafy vegeta-
bles as affected by photoperiods. Journal of Food,
Agriculture and Environment 7: 392–398.
Arancon N.Q., Schaffer N., Converse C.E. 2015. Effects
of coconut husk and sphagnum moss-based media
on growth and yield of romaine and buttercrunch
lettuce (Lactuca sativa) in a non-circulating hydro-
ponics system. Journal of Plant Nutrition 38: 1218–
1230. DOI: 10.1080/01904167.2014.983117.
Bugbee B. 2003. Nutrient management in recirculating
hydroponic culture. Acta Horticulture 648: 99–112.
DOI: 10.17660/actahortic.2004.648.12.
Burleigh M., Roberts E., Wagner D.R. 2008. Acidic so-
lutions adjusting water's pH improves plant growth.
Cactus and Succulent Journal 80: 245–250. DOI:
10.2985/0007-9367(2008)80[245:asawpi]2.0.co;2.
Chen J., Wu J.H., Si H.P., Lin K.Y. 2016. Effects of add-
ing wood vinegar to nutrient solution on the growth,
photosynthesis, and absorption of mineral elements
of hydroponic lettuce. Journal of Plant Nutrition 39:
456–462. DOI: 10.1080/01904167.2014.992539.
Colonna E., Rouphael Y., Barbieri G., De Pascale S.
2016. Nutritional quality of ten leafy vegetables har-
vested at two light intensities. Food Chemistry 199:
702–710. DOI: 10.1016/j.foodchem.2015.12.068.
Dyśko J., Kaniszewski S., Kowalczyk W. 2008. The ef-
fect of nutrient solution pH on phosphorus availa-
bility in soilless culture of tomato. Journal of Ele-
mentology 13: 189–198.
Frick J., Mitchell C.A. 1993. Stabilization of pH in solid-
matrix hydroponic systems. HortScience 28: 981–
984. DOI: 10.21273/hortsci.28.10.981.
Hochmuth G.J. 2001. Fertilizer management for green-
house vegetables. Florida greenhouse vegetable
production handbook, vol. 3. HS787. Florida Coop-
erative Extension Service, University of Florida.
http://edis.ifas.ufl.edu/cv265 [accessed August 5, 2018]
Islam A.K.M.S., Edwards D.G., Asher C.J. 1980. pH op-
tima for crop growth. Results of a flowing solution
culture experiment with six species. Plant and Soil
54: 339–357. DOI: 10.1007/bf02181830.
Kirimura M., Inden H. 2005. Effects of set point of NH4-
N concentration on pH fluctuation in ion concentra-
tion controlled hydroponics in cucumber. Journal
of Science and High Technology in Agriculture (Ja-
pan) 17: 199–204. DOI: 10.2525/shita.17.199. [in
Japanese with English abstract]
Lei J.L., Chen J., Dai D.L., Chen L., Zhou S., Shou W.,
et al. 2004. Study on the hydroponics technique of
leafy vegetables with low-nitrate content. Acta Ag-
riculturae Zhejiangensis 16: 102–104.
Lee Y.C., Lee W.S. 2006. Effect of culture solution for-
mula and the reagent level to the growth in lettuce
and Pak-Choi. Horticulture NCHU 31: 31–40.
Resh H.M. 2004. Hydroponic Food Production, 6th ed.
Newconcept Press, Mahwah, USA, 567 p.
SAS 2013. Base SAS® 9.4 Procedures Guide: Statistical
Procedures, 2nd ed. SAS Institute, USA.
Sinclair W.B., Eny D.M. 1946. Stability of the buffer sys-
tem of lemon juice. Plant Physiology 21: 522–532.
DOI: 10.1104/pp.21.4.522.
Singh H., Dunn B., Payton M. Brandenberger L. 2019.
Fertilizer and cultivar selection of lettuce, basil, and
Swiss chard for hydroponic production. HortTech-
nology 29: 50–56. DOI: 10.21273/horttech04178-18.
Skagg K. 1996. The urban gardener. American Horticul-
turist 75: 9–10.
Steiner A.A. 1961. A universal method for preparing nutri-
ent solutions of a certain desired composition. Plant
and Soil 15: 134–154. DOI: 10.1007/bf01347224.
Stahl R., Grossl P., Bugbee B. 1999. Effect of 2(N-morpho-
lino)ethane)sulfonic acid (MES) on the growth and tis-
sue composition of cucumber. Journal of Plant Nutri-
tion 22: 315–330. DOI: 10.1080/01904169909365629.
USAID 2004. 50 Years of Global Health. Saving Lives and
Building Futures. https://www.usaid.gov/sites/de-
fault/files/documents/1864/USAID_50-Years-of-
Global-Health.pdf [Accessed 5 August, 2018]
Whipker B.E., Bailey D.A., Nelson P.V., Fonteno W.C.,
Hammer P.A. 1996. A novel approach to calculate acid
additions for alkalinity control in greenhouse irrigation
water. Communications in Soil Science and Plant Anal-
ysis 27: 959–976. DOI: 10.1080/00103629609369610.
Bereitgestellt von Oklahoma State University - Edmon Low Library | Heruntergeladen 06.11.19 14:13 UTC