LATEST INSIGHTS INTO WATER
SOILLESS CULTIVATION
AND NUTRIENT
CONTROL
IN
C. de Kreij,
Glasshouse Crops Research Station,
P.O. Box 8, 2670 AA Naaldwijk,
The Netherlands.
Abstract
Transpiration influences transport and translocation of calcium. Nutrition and transpiration
are both important in preventing blossom-end rot. Potassium and high EC have positive
effects on quality of vegetables. For cut flowers low EC promotes quality.
Boron and zinc toxicity can occur when irrigation water is contaminated by these two
elements. Zn is released from galvanized steel. Low pH is favourable for crops or varieties
susceptible to chlorosis, like gerbera, rose, chrysanthemum. In soilless culture, each crop
has its own standard nutrient solution. The composition of the solution is adjusted to
growing period, analytical results of the solution in the root environment, the composition
of the irrigation water, and drainage water (in recirculation).
The area of soilless cultivation in the Netherlands was about 3760 ha in 1992. In the year
2000 cultivation under glass should be in closed, recirculating soilless systems.
1. Requirements of the plant
Water, oxygen and root temperature
Water uptake and transpiration of plants are needed for several reasons. The most
important are the following: transport of nutrients, cooling the plant, and maintaining
turgor pressure for optimal biochemical processes in the cell.
Calcium is transported only by the xylem. In winter time, the vapour pressure deficit in
greenhouses can be so low that Ca deficiency occurs in leaves caused by low transpiration
(Bakker, 1990). Ca deficiency in leaves of tomato leads to low leaf areas and lower
production. Higher Ca levels in the root environment do not prevent Ca deficiency (table
1). Only higher transpiration is effective in preventing Ca deficiency.
In summer time transpiration is so high, that the main xylem stream transports most of the
absorbed Ca to the leaves and not to the fruits. The result is blossom-end rot. Increased air
humidity could diminish blossom-end rot for tomato. The incidence of blossom-end rot in
a wetted climate was 19%, whereas in an unwetted control treatment it was 24% (De
Kreij, 1992).
T a b l e 1.:
Effect of air humidity (h = high, 1 = low) and Ca level in root environment on Ca deficiency
and Ca content in leaves of tomato. Ca deficiency: 0 = no deficiency; 3 = very severe
deficiency.
Ca root
environment
mM
5.7
9.8
13.6
18.7
Ca deficiency
h
i
2.0
1.3
1.3
1.4
Acta Horticulturae 408, 1995
Soilless Cultivation Technology
for Protected Crops
Ca in plant sap
h
i
mM
mM
0.0
0.0
0.0
2.3
5.5
3.5
0.0
8.2
19.6
27.2
35.1
50.7
47
In sweet pepper it was not possible to diminish blossom-end rot by increasing air humidity
(De Kreij and Huys, 1993). On the contrary, Schon (1993) could lower blossom-end rot
of pepper by using antitranspirant to decrease transpiration. This treatment however
caused a significant decline in marketable yield, so it is not recommended for commercial
use.
Oxygen consumption is related to assimilation and root temperature. Oxygen consumption
varies from 1.4-9.1 mg O2 per gram dry root matter per hour (Baas, 1993). Plants create
aerenchym to increase root air diffusion in oxygen deficiency circumstances. It was found
that tomato, rose and gerbera are capable of increasing air porosity from 2-4% under
aerobic to about 8% under anaerobic conditions. Carnation could not increase root air
porosity (Warmenhoven, 1990).
Root zone heating can be very effective. Optimal root temperature is between 17-24 C for
pepper. For flowering pot plants maximum acceptable temperatures in the Botting soil of
26 °C were found. For growing processes maximum is 30 °C (Vogelezang, 1993).
Transpiration differs very much from plant to plant (Sonneveld, 1991; fig. 1).
Macro elements and EC Plants need macro elements in a certain ratio and at a total
concentration of EC 1.5-2.0 dS/m (25 °C) to achieve optimal yield. Lower and higher EC
decrease yield under most circumstances and for most plants. Only in exceptional cases,
like orchids, optimal EC is lower than 1.5 dS/m.
To produce high quality products for most vegetable crops EC should be higher than for
optimal production. Tomato-plant cultivated at high EC produce fruits with longer
shelf-life, better taste and less cracking.
At high EC the fruit is smaller and the skin is firmer (Verkerke et al., 1992; Verkerke and
Gielesen, 1991). However at EC higher than 9 dS/m the fruit's cuticle can get so hard that
it effects the taste in a negative way.
It is possible that increase of EC by NaCl improve shelf-life and taste more than increase
by nutrient elements, because both Na and K improve these quality characteristics.
8
6
4
2
3.2
3.6
4.0
4.4
Water absorption (mm)
48
(previous page) Frequency diagram of the water absorbtion of tomato plants on 32 locations in a
greenhouse during a summerday. A location consisted of lm grown with 2 plants each supplied
with a dripper.
Calcium proved to have a negative effect on taste and shelf-life of tomato. In some cases,
high EC is effective in improving early flowering for tomato or pot plants.
Furthermore, high EC benefits all aspects of quality for cucumber. For sweet pepper it is a
little bit different. High EC improves taste, cuticle cracking, 'white flecks' on the fruits
and 'green spot', but decreases shelf-life.
The vase-life of cut flowers is not influenced by EC for example (De Kreij and Van den
Berg, 1990a, for cymbidium). In other cases low EC improves shelf-life for roses (De
Kreij and Van den Berg, 1990b), and gerbera (De Kreij and Van Os, 1989), and
cymbidium (Van Os and Van Mourik, 1993).
Chloride concentration of 8-13 raM proved to increase Ca uptake, decrease blossom-end
rot and increase gold specks in comparison to 3 mM CI at the same EC and exchanged
against N 0 3 (Nukaya et al„ 1992).
Silicon proved to enhance yield and suppress mildew for cucumber and rose. Addition of
about 1 mM Si to irrigation water (which contained no Si) in rockwool (without release of
Si) increased production with 5% as an average of 10 trials conducted between 1984-1991
(Voogt, 1993). Optimal Si content in the root environment is 0.6 for cucumber and 1.5
mM for rose.
The potassium uptake of tomato-plant in relation to water and elements proved to be very
dependent on the growth period (Voogt, 1993; fig. 2). K uptake increased during the
flowering of the first 10 trusses. This is the period with strong increase in fruit load and a
high demand on K by the fruits. An increase of K-uptake in roses and carnations had been
found for some period after harvest. This increase is caused by the strong growth of the
shoots.
49
UPTAKE
CONCENTRATION
cluster number at anthests
2 4 6 8 10 12 14 16 18 20 22 24 26 28
mmo 1 / t
10
15
20
25
30
35
40
veeknumber
Fig. 2. Uptake concentration of K, Ca and Mg during the growing period of tomato and development of
the cluster number at anthesis
Micro elements
The boron requirement differs strongly among species. Rose has a low demand. Boron is
toxic for certain cultivars, at concentrations of 20 uM in the root environment while for
other crops optimal values are 50-70 uM. Toxicity in rose cultivars is probably strongly
dependant on the type of rootstock which influences B uptake. B toxicity can be observed
as black spots or lines on the margins of older leaves. With leaf analysis, both total and
sap analysis, the toxicity can be distinguished.
The uptake of Zn can be retarded by high P concentrations (Parker et al., 1992; Voogt and
Sonneveld-van Buchem, 1989).
The Cu need is 150 to 200 umol per kg dry matter plant material. This uptake is achieved
when Cu concentrations in the root environment are about 1 uM.
50
Fe and Mn need of crops like gerbera, rose and chrysanthemum, that easily become
chlorotic, is more important than that of crops that stay green like pepper. Regarding
uptake, Fe and Mn behave antagonistically and the absorption is strongly enhanced by low
pH.
High concentrations of Zn can cause enormous problems with Fe uptake and deteriorate
plants; 34 uM Zn caused lower rose yield and root area while 126 uM Zn showed severe
Fe-deficiency chlorosis (Kaminski and Scharpf, 1991)
Proton concentration has enormous effects on root activity, and therefore on the whole
plant. Most crops prefer pH 5-6 in the root environment. Nevertheless, there are
exceptions. Chrysanthemum, susceptible to chlorosis, prefers pH 4 to pH 5 (Van der
Hoeven et al„ 1993; fig. 3). The optimum is perhaps even lower. This, however cannot
be determined from the above mentioned experiment, since pH 4 was the lowest pH
treatment. Differences between chrysanthemum cultivars are such that some cultivars can
only be cultivated in substrates with very low pH. Roses need low pH (table 2), although
in the experiment of Voogt (1992) only pH levels between 5.5 and 7.4 were tested.
Differences between rose cultivars and rootstocks (if grafted) and interaction of cutting
with rootstock can cause differences in optimal pH.
Table 2. Stem fresh weight of rose cultivar Sonia, production and chlorosis index (0 = no
chlorosis; 10 = very severe chlorosis).
pH
5.5
5.9
6.4
6.8
7.0
7.4
Chlorosis index
1.8
1.2
2.0
4.4
6.8
8.3
Production,stem.m
193
168
190
160
141
124
Fresh weight,g.stem
32
32
31
30
25
23
51
day number in 1992
F i g . 3. Influence of pH on stem fresh weight of chrysanthemum in aeroponics at pH 4, 5, 6, and 7 with
Fe-DTPA and at pH 7 with Fe-EDDHA (treatment 7a)
Harmful elements
Irrigation water, fertilizers, substrates, irrigation pipes or gutters and even the plant itself
may be the source of harmful elements. Toxicity occurs only at certain concentrations. If
concentrations are high enough each compound can be toxic, even water.
Dibutyl or diisobutylphtalate plasticisers volatiled from flexible polyvinyl chloride are
highly toxic (Hannay and Millar, 1986).
Na, C1 and B are the most frequently occurring harmful elements of irrigation water. Most
crops are sensitive to Na. Sweet pepper and Cymbidium are very sensitive, gerbera, rose,
cucumber and carnation are intermediate and tomato is more tolerant. In a test with sweet
pepper 12 mM Na in the root environment showed 10% less yield of first grade fruits than
3 mM Na at the same EC. Fruit weight was also depressed. Taste and shelf-life were
unaffected (Post, 1993). Na contents in the root environment should be lower than 3 - 1 5
mM, depending on the crop. Sweet pepper has Na uptake concentrations of 0.1 -0.4 and
tomato 0.8 mM. Tomato accepts high levels. Na can even be used in a positive way to
increase quality.
Phosphate or potassium nitrate fertilizers may contain impurities of F and perchlorates,
respectively. F toxicity mainly occurs on some species like Lilium, Dracaena, and
Cordyline (Straver, 1992). Cucumber proved to get chlorosis on leaf margins and
problems with opening of flowers when Chilean potassium nitrate had been used
52
containing 0.5% perchlorate (Van Uffelen et al., 1989).
Phenolic compounds from wood wastes, used as substrate, are suspected to cause growth
retardation. These compounds probably cause the sharp reduction in growth of Ficus
when self-heated peat was used as a substrate (Wever and Hertogh-Pon, 1993).
Polyurethane foam has caused chlorosis on margins of cucumber (Groente- en
Bloementeeltvereniging Noord-Nederland, 1990). In polyurethane foam the fire retardant
tri(2-chloroethyl)phosphate was used, which is toxic to plants. It is possible that this
fire-retardant caused the problems. Now, polyurethane foam is subjected to biological
tests before using the material (Benoit and Ceustermans, 1992).
Toxicity symptoms on leaf edges and margins, which mostly reflect B excess, were
observed on Impatiens, Chrysalidocarpus and Nephrolepis (Fischer and Meinken, 1993)
grown on expanded clay.
2. Control of water, oxygen and temperature
Increasing root temperature is very easy. Due to high cost, however cooling is more
problematic. Very high root temperatures are caused by very high air temperatures or by
addition of energy in recirculating systess. Energy is added by pumps or by sterilisation
units using heat.
Depth, cm
EC (mS/cm)
Fig. 4. EC in 1:1.5 volume-extract of peat on different layers in a pot plant culture with
ebb and flow irrigation system.
53
Physical parameters, irrigation regime and water absorption by the plant determine oxygen
supply and oxygen diffusion rate. In ebb and flow irrigation systems the air filled pore
space can be too low in the lower part of the substrate. In this irrigation technique there is
only an upward waterflow which is responsible for a fast accumulation of salts in the top
layer of the substrate (fig. 4).
Trickle and sprinkler irrigation techniques are very uneven (Sonneveld, 1991, fig. 5).
Irrigation systems should be more equal; then water supply could be limited. In an uneven
irrigation system the water supply is so abundant that each place gets plenty ofwater. This
means that on certain places more water than required for the transpiration is supplied.
Frequency
5.2
5.6
6.0
6.4
6.8
Water supply (mm)
Fig. 5. Frequency diagram of the water supply on 32 locations in greenhouse. For other
conditions see fig. 1
Physical parameters of substrates vary. Air and water holding capacity of a substrate have
to be assessed together with the irrigation system (type, frequency and capacity), volume
of substrate per area, substrate height, and sometimes, plant species as well. Water
retention curves of rockwool are such that in steady state (no water transport) big
differences in water and air content correspond to small differences in height (Tanaka and
Yasui, 1992; fig. 6).
Volumetric water content in rockwool slabs of 7.5 cm in height can be measured by the
'Watergehaltemeter' (water content meter). This measurement is based on the high
dielectric constant of water and the low one of air and solids. In practice, trickle irrigation
54
frequency is regulated by the predicted transpiration calculated in a model, and on the
measurement of the leachate from one or more representative places in the greenhouse (De
Graaf, 1988). Leachate fraction can be installed. Normal values are 20-30% of the supply.
Rockwool structure changes when it is dried out for some period (fig. 7). Moisture
content was above 80% (vol/vol). After drying out for 10 hours and rewetting water
content was about 75%.
100
••
80
c
o
t-l
60
ao
3
a
a
as
cfl
l-i
Ë
U
<D
1-4
6X)
<L>
Q
40
.0 D
20
0
o
DO
a
40
• : Upper half part
• : Lower half part
60
80
Degree of water saturation in whole rockwool mat (%)
Fig. 6. Degree of water saturation of upper and lower parts in rockwool mat in tomato
culture, size of mat is 30 x 91 x 7.5 cm
Rockwool can be dried out easily when pressure head is lowered. In fig. 8 it is shown that
rockwool is more easily dried out than peat when pressure head is decreased to -31.6 cm.
This factor is also used, when after the cultivation period, rockwool slabs have to be dried
before sterilization. Plugs, which connect water in the slabs with water in the drain
reservoir, suck out water from the slabs (Richardson, 1992).
55
0)
cn
ra
c
a>
o
(i>
Q.
0
10
20
28
48
72
96
120
Tim* (hour)
Fig. 7. Moisture contents (vol/vol) of rockwool slabs. Irrigation was stopped during time
10-20 hour.
100
Peat moss (92kg/m 3 )
C'
50
Granulated rockwool
— 1 8 0 k g / m
3
)
Rockwool mat(95kg/m 3 )
0
. Rockwool mat(70kg/m 3 )
Rockwool mat(60kg/m3)
0
5
10 (hour)
Time elapsed on soil column(pF1.5)
Fig.8. Change in degree of water saturation (v/v) of rockwool and peat moss under
pressure head of -31.6 cm, size of rockwool 10 x 10 x 7.5 cm.
56
3. Control of Nutrients
Fertilization schemes
A nutrient control system has been designed by IKC (1993). In table 3 an example is
given.
Table 3. Example of recipe for nutrient solution for tomato in rockwool with recirculation.
Element
Analytical data
recalculated
to EC = 2.7
(reference EC)
Target
value
Standard
solution
5,.5
3..0
<0..5
7 .0
1..6
1..0
6..5
7..0
3..5
17..0
2..75
1..0
10..75
5. 8
pH
EC, mS/cm 4.2* high
NH, , mM
8..7 high
K 4
2..4*
Na
6.,7
Ca
3..1
Mg
19..6
NO0,.5*
CI 3
3.. 4 low
so
HC0.
1,.94 high
P 3
16 low
Fe, uM
9 .7*
Mn
6 .0
Zn
57..0
B
0..6
Cu
Mo
* not recalculated
Adjustments Corrections recipe
Crop Analy- basic drain
stage tical 100% 0%
result
-2.0
-1..0
-0.,25
+0.75
2..5
0 .7
25..0
5 .0
7 .0
50 .0
0. 7
2..21
0..92
9.,87
1..5
5 .0
1..6
1..0
5 .25
1..25
15..0
10..0
4..0
20,.0
0..75
0 .5
-0.25
+25%
2.,07
2.,5
1..0
18..75
10..0
4.,0
20..0
0. 75
0.,5
Growth stage of crop: flowering of 12th truss and higher. Irrigation water code : A 5.4.1.
That is 5*0.5 = 2.5 mmol/1 HCO3. 4 * 0.25 =1.0 mmol/1 Ca. 1 * 0.25 =0.25 mmol/1 Mg.
Solid fertilizers.
Analytical results
Ph: 5.8
EC (mS/cm) : 4.2
Macro elements (mmol/1')
NH4
K
Na
Ca
Mg
NO3
CI
SO4
HCO3
P
<0.1
12.8
2.4
9.8
4.6
28.7
0.5
5.0
<0.1
2.84
Micro elements (umol/1)
Fe
24
Mn
9.7
Zn
8.8
B
83
Cu
0.9
57
From the recipe in table 3 an A and B tank stock solution (100 * concentrated) can be
calculated (Sonneveld and Voogt, 1991). A more recent development is the use of liquid
fertilizers in separate tanks and injection of the fertilizers directly into the irrigation water
stream. Then, pumps and velocity measurements should be very accurate. Special
measures have to be taken to avoid precipitation of C a S 0 4 and Ca(H2P04)2 in pipes of
the fertilization system. Mistakes have caused real damage to crops, when after
precipitation there was a shortage of water.
Most recently the nutrient control system has been supplemented with Si as a fertilizer for
cucumber and rose, CI as a fertilizer for tomato, K supply dependant on truss
development for tomato, ideas to diminish environmental pollution by nutrients and
fertilization schemes for recirculation.
The control of nutrients is more important when small volumes of substrate per unit of
area are used. Control is more difficult in recirculation than in open systems. Until now
measurements of EC and pH are being done by the grower hisself. Macro and micro
elements are being analyzed in laboratories. These analytical results are being used to
optimize the composition of nutrient solutions. Ion selective electrodes are being fitted for
commercial use in practice (Van den Vlekkert, 1992). Drift and temperature sensitivity still
limit the practical application. To regulate nutrient supply all macro and micro elements
have to be analysed. Since this it not possible in the near future with the Ion Sensitive
Field Effect Transistor (ISFET), analysis on laboratories will be needed.
Target values in the root environment
Target values and desired optimal range of EC, pH, macro and micro elements are being
used to formulate compositions of nutrient solutions. Until now the target values for open
and closed systems are the same. In the future changes have to be made between closed
and open systems. Then, Na should be taken into account and target values of K, Mg and
Ca should be lowered (table 4).
Parameter
EC, dS/m (25°C)
Na, mM
K
Ca
Mg
Open system
5TÔ
2.0
7.0
7.0
3.5
Closed system
570
10.0
5.0
5.0
2.5
Table 4. Target values for nutrients and Na in the root environment for tomato in an open
and a closed system.
Standard nutrient solutions
A standard nutrient solution serves to meet the target values in the root environment. The
amount which is released from the system determines the nutrient solutions to get the
target value. In the root environment a relatively high concentration of bivalent ions should
be found, because uptake of bivalent ions is more difficult than that of monovalant ions.
That means that in an open system more bivalent ions are lost. To compensate this loss,
58
the bivalent ions must be in a higher ratio in the supply of an open system than in a closed
system. For example, for tomato in an open system (with about 25% release of irrigation
water) S 0 4 : N 0 3 ratio on molar basis in the supply should be on average 0.27 : 1,
wherever in a closed system 0.14 : 1. Therefore for most crops there are two standard
nutrient solutions for an open and a closed system, respectively.
Adjustments of standard nutrient solution
Composition of the solution in the root environment can change due to differences in plant
uptake, leaching fraction and pH. Composition has to be measured on regular times, more
often for closed systems than for open systems. Results of the analysis are compared with
the target values and optimal ranges. From this it is determined whether the standard
nutrient solution in the supply has to be changed and to what extent. For closed systems
adjustments are greater than for open systems.
For tomato the standard nutrient solution is adjusted for high K uptake during the period
of flowering of first flower from third until twelfth truss. Adjustments are 1.0 or 3.5 mM
K extra and less Ca and Mg. The first irrigation water before planting has more bivalent
than to monovalent ions, and more Fe than standardly used.
Most irrigation water and all drainage water contain nutrients. For open systems only
irrigation water has to be taken into account, but for closed the ratio of drainage water to
'fresh' irrigation water and both the element contents have to be diminished from the
nutrient solution which is supplied standard.
4. Developments in the Netherlands
The area of soilless cultivation in 1992 is estimated at 3760 ha, including pot plants and
bedding plants on a total area of 9900 ha under glass. Each year the share of soilless
cultivation increases (table 5). The aim is to reach 100% of the area in a closed soilless
culture system in the year 2000.
"Year
Area soilless culture
Flowers
Vegetables
%
%
1990
19
61
1991
21
65
1992
23
68
'Claim agricultural regulation'
1994
30
80
2000
100
100
In 2000: Closed
Table 5.
Fraction of the area in soilless culture from 1990-1992 and the aimed
development
In table 6 some important crops are given with the share of soilless culture. Some cut
flowers are not produced in substrate.
59
Crop
Area 1992
ha
Tomato
1506
Cucumber
854
Pepper
845
Eggplant
83
Rose
891
Gerbera
187
61
Anthurium
233
Carnation
Orchids
188
Chrysanthemum 765
Freesia
307
Soilless
%
92
84
90
99
25
33
100
16
100
0
2
Table 6. Area of some crops and fraction of soilless culture in the year 1992
References
Baas, R. 1993, personal communication. Research Station for Floriculture, Aalsmeer, The
Netherlands.
Bakker, J.C., 1990. Effects of day and night humidity on yield and fruit quality of
glasshouse tomatoes (Lycopersicon esculentum Mill.), Jour, of Hort. Sci. 65 (3),
323-331.
Benoit, F. and N. Ceustermans, 1992. Acquis de la recherche sur les méthodes
écologiques de la culture hors sol en Belgique. PHM Revue Horticole, No. 325,
54-58.
Fischer, P. and E. Meinken, 1993. Blahton-Herkunfte gepruft. Deutscher Gartenbau
47(3), 157-159.
De Graaf, R., 1988. Automation of the water supply of glasshouse crops by means of
calculating the transpiration and measuring the amount of drainage water. Acta
Horticulturae 229: 219-231.
Groente- en Bloementeeltvereniging Noord-Nederland, 1990. Jasarverslag Proeftuin
Noord-Nederland, the Netherlands.
Hannay, J.W. and D.J. Millar, 1986. Phytotoxicity of phtalate plasticisers. 1. Diagnosis
and commercial applications. J. Exp. Bot. 37 (179), 883-897.
Van der Hoeven, A.P., de Kreij, C., Huys, A. and C. Swinkels, 1993. Chrysant. Lage
pH geeft betere groei. Vakblad Bloemisterij 48 (4), 26-27.
IKC, 1993. Bemestlngsadviesbasis Glastuinbouw, Aalsmeer/Naaldwijk, the Netherlands.
Kaminski, R. and H.C. Scharpf, 1991. Zinkschaden an Rosen in Aeroponik
(Wurzelspruhkultur), Gartenbau 38 (7), 42, 44.
Kipp, J.A., and G. Wever, 1993. Wortelmedia. Serie : Informatiereeks no 103.
Glasshouse Crops Research Station, Naaldwijk, the Netherlands.
De Kreij, C. and Th. J.M. van den Berg, 1990a. Effect of electrical conductivity of the
nutrient solution and fertilization regime on spike production and quality of cymbidium.
Sci. Hort. 44, 293-300.
De Kreij, C. and Th. J.M. van den Bera, 1990b. Nutrient uptake, production an quality of
Rosa hybrida in rockwool as affected by electrical conductivity of the nutrient solution.
In: Plant Nutrition-physiology and applications (ed. M.L. van Beusichem), 519-523.
Kluwer.
De Kreij, C. and P.C. van Os, 1989. Production and quality of Gerbera in rockwool as
affected by electrical conductivity of the nutrient solution. In: Proc. 7th Intern Congr.
on Soilless Culture, 225-264.
60
De Kreij, C., 1992. Tomaat. Hoge luchtvochtigheid geeft minder neusrot. Groenten +
Fruit/Glasgroenten. No. 20, 33.
De Kreij, C. and A. Huys, 1993. Paprika. Luchtbevochtiging is zinloos. Groenten +
Fruit/Glasgroenten no. 1,19.
De Kreij, C., C. Sonneveld., M. Warmenhoven and N.A. Straver, 1992. Guide values
for nutrient element contents of vegetables and flowers under glass. Serie:
Voedingsoplossingen glastuinbouw no. 15. Aalsmeer/Naaldwijk, the Netherlands.
Nukaya, A., W. Voogt and C. Sonneveld, 1992. Effects of N03, S04, and CI ratios on
tomatoes grown in recirculating system. Acta Horticulturae 303, 91-98.
Van Os, P.C. and N. van Mourik, 1993. Orchidee produceert beter bij lage EC. Vakblad
Bloemisterij 48(9), 36-37.
Parker, D.R., J.J. Aguilera, and D.N. Thomason, 1992. Zinc-phosphorus interactions in
two cultivars of tomato (Lycopersicon esculentum L.) grown in chelator-buffered
nutrient solutions. Plant Soil 143:163-177.
Post, W., 1993. Paprika. Produktie lijdt onder hoog natriumgehalte Groenten +
Fruit/Glasgroenten. No. 11, 13.
Richardson, F., 1992. New system aimed at root zone control. Grower 117 (22), 12-14.
Schon, M.K., 1993. Effects of foliar antitranspirant or calcium nitrate applications on
yield and blossom-end rot occurence in greenhouse-grown peppers. Jour, of Plant
Nutrition 16 (6), 1137-1149.
Sonneveld, C., 1991. Rockwool as a substrate for greenhouse crops. In: High-Tech and
Micropropagation I (ed. by Y.P.S. Bajaj), Biotechnology in Agriculture and Forestry,
Vol. 17. Chapter II. 4, 285-312. Springer-Verlag.
Sonneveld C. and W. Voogt, 1991. Calculation of nutrient solution for soilless culture.
Serie voedingsoplossingen Glastuinbouw, No. 10. Naaldwijk, the Netherlands.
Straver, N., 1992. Overzicht literatuur/onderzoek met betrekking tot fluor. Internal
publication, Research Station for Floriculture, Aalsmeer, the Netherlands.
Tanaka, K. and H. Yasui, 1992. Studies on practical application of rockwool culture for
fruit vegetables. Series A (Vegetables and ornamental Plants) No. 5, March 1992,
Bulletin of the National Research Institute of Vegetables, Ornamental Plants and Tea,
Japan.
Van Uffelen, J.A.M., Voogt, W. and C.W. van Elderen 1989. Komkommer. Gebruik
Chileense kalisalpeter af te raden. Groenten + Fruit 44 (40), 34-35.
Verkerke, W. and W. Gielesen, 1991. Tomaat. Hoge EC verbetert stevigheid. Groenten +
Fruit/Glasgroenten no. 13, 38-39.
Verkerke, W., Gielesen, W. and R. Engelaan, 1992. Tomaat langer houdbaar door
stevigere schil. Groenten + Fruit/Glasgroenten no. 7, 22-23.
Van den Vlekkert, H.H., 1992. Ion-selective field effects transistors. Acta Horticulturae
304, 113-126.
Voogt, W., 1993. Silicium. In: Plantenvoeding in de Glastuinbouw, chapter 25, 186-196.
Informatiereeks no. 87. Glasshouse Crops Research Station, Naaldwijk, the
Netherlands.
Voogt, W. and H.G.M. Sonneveld-Van Buchem, 1989. Tomaat. Hoge
fosfaatconcentraties hebben negatieve effecten. Groenten + Fruit 44 (35), 38-39.
Voogt, W., 1993. Nutrient uptake of year round tomato crops. Acta Horticulturae,339,
99-112.
Vogelezang, J.V.M., 1993. Bench heating for potplant cultivation. Doctoral thesis,
Agricultural University, Wageningen, the Netherlands.
Warmenhoven, M., 1990. Substraat. Zuurstofgebrek in wortelmilieu heeft grote
gevolgen. Vakblad Bloemisterij 45 (50), 54-55.
Wever, G. and M.H. Hertogh-Pon, 1993. Effects of self-heating on biological, chemical
and physical characteristics of peat. Acta Horticulturae 342, 15-24.
61