July 2015
Adapt2change – Adapt Agricultural Production to climate change and limited water
supplies
ADAPT2CHANGE SUSTAINABLE WATER USE IN GREENHOUSES
TEI of Larisa Lead Partner
Agricultural Research Institute
TEI of Piraeus
Europliroforisi S.A.
Project Partnership
Project Brief Description
• Co-funded by EC – LIFE+ (50%) • Project budget: €2.576.548 • Project duration: 1/09/2010-31/08/2014
Adapt2change – Adapt Agricultural Production to climate change and limited water supply
Guide for sustainable water use in greenhouses
Contents
1. Introduction ............................................................................................................ 3
2. State of the art and current status in hydroponic cultivations .............................. 5
3. Systems of Hydroponic/Soilless culture ................................................................. 9
3.1 Soilless cultures ............................................................................................... 9
3.2 Hydroponic cultures ...................................................................................... 14
4. Adapt2Change system technical description ....................................................... 24
5. Diagnostic testing and preparatory procedures for hydroponic greenhouses .... 34
6. Recent Trends in Salinity Control for Soilless Growing Systems Management ... 41
6.1 Managing physiological processes to control salinity stress ........................ 42
6.2 Practical means to overcome salt accumulation .......................................... 47
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1. Introduction
The word hydroponics has its derivation from the combining of two Greek words,
hydro meaning water and ponos meaning labor, i.e., working water.
Webster’s New World College Dictionary, Fourth Edition, 1999, defines
hydroponics as “the science of growing or the production of plants in nutrient-rich
solutions or moist inert material, instead of soil”; the Random House Webster’s
College Dictionary, 1999, as “the cultivation of plants by placing the roots in liquid
nutrient solutions rather than in soils; soilless growth of plants”; and The Oxford
English Dictionary, 2nd Edition, 1989, as “the process of growing plants without soil,
in beds of sand, gravel, or similar supporting material flooded with nutrient
solutions.”
The most common aspect of all these definitions is that hydroponics means
growing plants without soil, with the sources of nutrients either a nutrient solution
or nutrient-enriched water, and that an inert mechanical root support (sand or
gravel) may or may not be used.
Searching for definitions of hydroponics in various books and articles, the
following were found. Devries (2003) defines hydroponic plant culture as “one in
which all nutrients are supplied to the plant through the irrigation water, with the
growing substrate being soilless (mostly inorganic), and that the plant is grown to
produce flowers or fruits that are harvested for sale.” In addition, Devries (2003)
states, “hydroponics used to be considered a system where there was no growing
media at all, such as the nutrient film technique in vegetables. But today it’s
accepted that a soilless growing medium is often used to support the plant root
system physically and provide for a favorable buffer of solution around the root
system.” Resh (1995) defines hydroponics as “the science of growing plants without
the use of soil, but by use of an inert medium, such as gravel, sand, peat, vermiculite,
pumice, or sawdust, to which is added a nutrient solution containing all the essential
elements needed by the plant for its normal growth and development.” Wignarjah
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(1995) defines hydroponics as “the technique of growing plants without soil, in a
liquid culture.” In an American Vegetable Grower article entitled “Is hydroponics the
answer?” (Anonymous, 1978), hydroponics was defined for the purpose of the
article as “any method which uses a nutrient solution on vegetable plants, growing
with or without artificial soil mediums.” Harris (1977) suggested that a modern
definition of hydroponics would be “the science of growing plants in a medium,
other than soil, using mixtures of the essential plant nutrient elements dissolved in
water.” Jensen (1997) stated that hydroponics “is a technology for growing plants in
nutrient solutions (water containing fertilizers) with or without the use of an artificial
medium (sand, gravel, vermiculite, rockwool, perlite, peat moss, coir, or sawdust) to
provide mechanical support.” Jensen (1997) defined the growing of plants without
media as “liquid hydroponics” and with media as “aggregate hydroponics.” Another
defining aspect of hydroponics is how the nutrient solution system functions,
whether as an “open” system in which the nutrient solution is discarded after
passing through the root mass or medium, or as a “closed” system in which the
nutrient solution, after passing through the root mass or medium, is recovered for
reuse.
Thus, from the sort introduction presented above, it can be seen that soilless or
hydroponic technique and soilless or hydroponic cultivations are a means for
sustainable water and nutrients use in agriculture. Accordingly, this guide aims at
presenting this technique and guide through its correct application and
implementation.
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2. State of the art and current status in hydroponic cultivations
The growing of plants in nutrient-
rich water has been practiced for
centuries. For example, the ancient
Hanging Gardens of Babylon and the
floating gardens of the Aztecs in
Mexico were hydroponic in nature.
The world's rice crops have been
grown in this way from time
immemorial. And also the floating
gardens of the Chinese, as described
by Marco Polo in his famous journal,
are examples of "hydroponic culture".
In the 1800s, the basic concepts for
the hydroponic growing of plants
were established by those
investigating how plants grow
(Steiner, 1985). The soilless culture of plants was then popularized in the 1930s in a
series of publications by a California scientist (Gericke, 1929, 1937, 1940).
During the Second World War, the U.S. Army established large hydroponic
gardens on several islands in the western Pacific to supply fresh vegetables to troops
operating in that area (Eastwood, 1947). Since the 1980s, the hydroponic technique
has become of considerable commercial value for vegetable (Elliott, 1989) and
flower (Fynn and Endres, 1994) production, and as of 1995 there are over 60,000
acres of greenhouse vegetables being grown hydroponically throughout the world,
an acreage that is expected to continue to increase (Jensen, 1995).
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Hydroponics for space
applications — providing a means of
purifying water, maintaining a
balance between oxygen and
carbon dioxide in space
compartments, and supplying food
for astronauts — is being intensively
researched (Knight, 1989;
Schwartzkopf, 1990; Tibbitts, 1991;
Brooks, 1992).
Hydroponic growing in desert areas of the world (Jensen and Tern, 1971) and in
areas such as the polar-regions (Tapia, 1985; Rogan and Finnemore, 1992; Sadler,
1995; Budenheim et al., 1995) or other inhospitable regions will become important
for providing food and/or a mechanism for waste recycling (Budenheim, 1991, 1993).
Actually, hydroponics is only one form of soilless culture. It refers to a technique
in which plant roots are suspended in either a static, continuously aerated nutrient
solution or a continuous flow or mist of nutrient solution. The growing of plants in an
inorganic substance (such as sand, gravel, perlite, rockwool) or in an organic material
(such as sphagnum peat moss, pine bark, or coconut fiber) and periodically watered
with a nutrient solution should be referred to as soilless culture but not necessarily
hydroponic. Some may argue with these definitions, as the common conception of
hydroponics is that plants are grown without soil, with 16 of the 19 required
essential elements provided by means of a nutrient solution that periodically bathes
the roots.
Although the methods of solution delivery and plant support media may vary
considerably among hydroponic/ soilless systems, most have proven to be workable,
resulting in reasonably good plant growth. However, there is a significant difference
between a “working system” and one that is commercially viable. Unfortunately,
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many workable soilless culture systems are not commercially sound. Jensen (1997),
in his overview, stated, “hydroponic culture is an inherently attractive, often
oversimplified technology, which is far easier to promote than to sustain.
Unfortunately, failures far outnumber the successes, due to management
inexperience or lack of scientific and engineering support.” Experience has shown
that hydroponic/soilless growing requires careful attention to details and good
growing skills. Most hydroponic/soilless growing systems are not easy to manage by
the inexperienced and unskilled. Soil growing is more forgiving of errors made by the
grower than are most hydroponic/soilless growing systems, particularly those that
are purely hydroponic.
In 1981, Jensen listed the advantages and disadvantages of the hydroponic
technique for crop production, many of which are still applicable today:
Advantages
• Crops can be grown where no suitable soil exists or where the soil is
contaminated with disease.
• Labor for tilling, cultivating, fumigating, watering, and other traditional practices
is largely eliminated.
• Maximum yields are possible, making the system economically feasible in high-
density and expensive land areas.
• Conservation of water and nutrients is a feature of all systems. This can lead to a
reduction in pollution of land and streams because valuable chemicals need not
be lost.
• Soil borne plant diseases are more readily eradicated in closed systems, which can
be totally flooded with an eradicant.
• More complete control of the environment is generally a feature of the system
(i.e., root environment, timely nutrient feeding or irrigation), and in greenhouse-
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type operations, the light, temperature, humidity, and composition of the air can
be manipulated.
• Water carrying high soluble salts may be used if done with extreme care. If the
soluble salt concentrations in the water supply are over 500 ppm, an open system
of hydroponics may be used if care is given to frequent leaching of the growing
medium to reduce the salt accumulations.
• The amateur horticulturist can adapt a hydroponic system to home and patio-
type gardens, even in high-rise buildings. A hydroponic system can be clean,
lightweight, and mechanized.
Disadvantages
• The original construction cost per acre is great.
• Trained personnel must direct the growing operation. Knowledge of how plants
grow and of the principles of nutrition is important.
• Introduced soil borne diseases and nematodes may be spread quickly to all beds
on the same nutrient tank of a closed system.
• Most available plant varieties adapted to controlled growing conditions will
require research and development.
• The reaction of the plant to good or poor nutrition is unbelievably fast. The
grower must observe the plants every day.
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3. Systems of Hydroponic/Soilless culture
3.1 Soilless cultures
A number of ways to grow plants by means of hydroponic/soilless culture exist.
For the purposes of this guide, the following classification scheme will be followed:
hydroponics is one distinct technique for plant growing where no root-supporting
medium is used, whereas the other systems employ a rooting medium, either
inorganic or organic.
As indicated earlier, growing plants hydroponically is different for systems that
employ a support or rooting medium compared to non-media systems. Management
of the nutrient solution for these two classes of systems is quite different. It is
important, however, to keep in mind not only the differences but also the similarities
between these growing systems, as some of the management procedures can be
successfully transferred, whereas others cannot.
All forms of hydroponic/soilless culture involve growing plants in some kind of a
container — a bed, pot, bag, bucket, enclosed slab, or trough. The volume and
dimensions of the rooting vessel are frequently chosen on the basis of convenience
or availability. Today, growers are placing soilless medium in a free-standing plastic
bag and using it as the growing container, or they are growing directly in the bag that
is used to package and transport a soilless mix or perlite.
What should the volume and dimensions for the rooting vessel be, whether the
vessel is a bag, slab, pot, bucket, trough, or bed, in order to provide adequate space
for normal root growth and development? The answer to that question, as far as
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most hydroponic/soilless growing
systems are concerned, has not been
adequately determined. It is surprising
how little good information is available
on the importance of rooting volume
required by plants and the relationship
that exists between rooting habit,
rooting medium, and container
environment and volume.
Despite the uncertainties about the relationship between rooting vessel size and
plant performance, there are some guidelines that will assist the grower in
determining the rooting volume needed for the crop and system being employed:
• For all containers, the depth should be one-and-a-half to two times the
diameter of the surface area covered by the plant canopy when the plant
reaches its maximum size. For example, if the canopy covers (or will cover) a
surface area 30 cm in diameter, the growing container should be 46 to 61 cm
deep.
• In bed culture systems, increased spacing between plants can, in part,
substitute for a lesser depth. For example, plants with a canopy occupying a
surface area 30 cm in diameter growing in a bed less than 30 cm deep should
be spaced 46 cm from one plant center to another. This ratio of 2 to 3 can be
applied to plants with smaller or larger canopies when growing in bed
systems.
• It is generally accepted that roots of neighboring plants inhibit each other’s
growth. Therefore, close contact and intermingling of roots between
neighboring plants (the result of close spacing or shallow rooting
depth) should be minimized by providing the proper area and
depth required.
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Some feel that the present lack of knowledge about root growth in
varying environments restricts our knowledge of plant growth in general.
The conse quence for the hydroponic/soilless culture grower is that he
or she must experiment with the growing system to determine the rooting
volume required to obtain maximum plant performance. Beginning with the
recommendations given above, plants can be spaced closer together until
a significant change in plant growth and yield appears.
Needless to say, root volume requirement becomes academic when
plants must be widely spaced to allow sufficient light to penetrate the
plant canopy for those plants that are widely branched and/or grow tall.
However, the trend today is grow in the minimum of medium to reduce
cost.
From 1930 to the late 1950s, gravel or sand was commonly used as the
rooting medium in closed recirculating ebb-and-flow commercial soilless
culture systems. For small home hydroponic units, gravel, lava rock,
expanded clay, or Hadite are the materials selected for use as the
rooting medium. For the commercial hydroponic systems of today, perlite
and rockwool are the most commonly used inorganic rooting media
materials.
A wide variety of various organic rooting media materials are used
today, most of which are combinations of various materials, primarily
mixtures containing peat moss and/or composted milled pinebark or peat
moss and composted milled pinebark mixed with inorganic substances,
such as vermic ulite and perlite.
The use of a rooting medium, whether inorganic or organic, poses a
set of challenges. Although the medium itself may be inert, such as gravel,
sand, perlite, or rockwool, it harbors pore spaces that will hold nutrient
solution, which may eventually be absorbed by plant roots; the elements
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move with the solution by mass flow or by diffusion within the solution
and are also reached by root extension (growth). Organic media, such as
peat moss and composted milled pinebark, have similar pore spaces, as
well as a cation/anion exchange capacity that can remove ions from the
solution and hold them for later release into solution. In both types of
media, a precipitate of elements can occur, essentially as a combination
of calcium phosphate and calcium sulfate, which can also entrap other
elements, mainly the micronutrients. Although this precipitate is essentially
insoluble, portions can become soluble, which will then contribute to the
essential element supply being delivered to the plant roots by repeated
passage of the nutrient solution through the rooting medium.
There are two basic systems of nutrient solution use:
• An “open” system in which the nutrient solution is passed through
the rooting vessel and discarded
• A “closed” system in which the nutrient solution is passed through
the rooting vessel and then collected for reuse
Both systems have advantages and disadvantages. The major
disadvantage to the “open” system is its inefficiency due to the loss of
water and unused essential elements, since the flow of the nutrient
solution is greater than that required by the plants. For the “closed”
system, the nutrient solution can be substantially changed when passed
through the rooting vessel, requiring some adjustment in volume
(replacement of lost water) and pH and replenishment of absorbed
essential elements (Hurd et al., 1980). In addition, any disease or other
organisms picked up by the nutrient solution in its passage through the
rooting vessel will be recirculated into the entire system unless removed
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or inactivated by some form of nutrient solution treatment. The controls
and requirements for a recirculating hydroponic system have been
discussed by Wilcox (1991), Schon (1992), and Bugbee (1995).
The nutrient solution is expected to provide both water and the
essential elements needed by the plant in its flow through the rooting
vessel. It is easily and erroneously assumed that these two physiological
requirements, the need for both water and essential elements, occur in
tandem. On warm days when plants are transpiring rapidly, however, only
water may be needed to meet the atmospheric demand, while the
nutrient elements in the nutrient solution may not be required by the
crop in other than their usual amounts. The consequence is that the
need for water is out of phase with the feeding cycle. This juxtaposition of
events poses a major problem, as it is not common to have a water-only
system operating in parallel with the nutrient solution delivery system.
Therefore, increasing the circulation of the nutrient solution to meet the
demand for water may lead to an elemental imbalance and an
undesirable accumulation of unwanted elements.
With automatic control (Bauerle et al., 1988; Berry, 1989; Bauerle,
1990; Edwards, 1994) and an “open” system, it is possible to modify the
nutrient solution composition by adding water into the flowing stream
of nutrient solution passing through the rooting vessel, thereby reducing
the nutrient element concentration. With a “closed” system, a delivery–
collection system would be required to pass water only through the rooting
vessel. Such “engineering” aspects of hydroponic culture have recently
been discussed by Giacomelli (1991).
In all commercial and most other types of hydroponic/soilless culture
systems, the movement of the nutrient solution requires either electrical
power (active) or gravity (passive), or a combination of both. For some
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situations, less dependency on electrical power can be of considerable
advantage. However, with the requirements for greater control over the
composition, application requirements, etc. of the nutrient solution more
widely recommended and applied in commercial systems (Bauerle et al.,
1988; Berry, 1989; Bauerle, 1990; Schon, 1992), the need for
uninterrupted electrical power is becoming essential.
In addition, computer programmed systems are replacing manual
management operations. Sensors are being placed in the growing medium
and nutrient solution storage tanks for regulating the flow and composition
of the nutrient solution, respectively. Measurements such as light intensity
and duration and the temperature of the plant environment are factors
being used to regulate the flow and composition of the nutrient solution.
Therefore, passive systems of nutrient solution flow are becoming
obsolete.
3.2 Hydroponic cultures
True hydroponics is the growing of plants in a nutrient solution without a
rooting medium. Plant roots are either suspended in standing aerated
nutrient solution or in a nutrient solution flowing through a root channel,
or plant roots are sprayed periodically with a nutrient solution. This
definition is quite different from the usually accepted concept of hydroponics,
which has in the past included all forms of hydroponic/soilless growing.
Standing Aerated Nutrient Solution
This is the oldest hydroponic technique, dating back to those early
researchers who, in the mid-1800s, used this method to determine which
elements were essential for plants. Sachs in the 1840s and the other early
investigators grew plants in aerated solutions and observed the effect on
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plant growth with the addition of various substances to the nutrient
solution (Russell, 1950). This technique is still of use for various types of
plant nutrition studies, although some researchers have turned to flowing
and continuous replenishment nutri ent solution procedures.
The requirements for the aerated standing nutrient solution technique are:
a. A suitable rooting vessel
b. A nutrient solution
c. An air tube and pump in order to bubble air continuously into the
nutrient solution.
The bubbling air serves to add O2 to the nutrient solution as well as
stirring it. The commonly used formula is Hoagland’s or some modification
of it as has been designed by Berry (1985).
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The nutrient solution will require periodic replacement, usually every 5
to 10 days, the frequency based on the number of plants and their size as
well as the volume of nutrient solution. Water loss from the nutrient
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solution will need to be replaced daily, using either nutrient-free water
(pure water), or a diluted (1/10th strength) nutrient solution, although
there is the danger that any further additions of nutrient elements could
alter the initial balance among the elements and adversely affect the
plants. It should also be remembered that with each day of use, the pH
and composition of the initial nutrient solution will be altered by root
activity and element uptake, changes that can have an effect on plant
growth. The question becomes “should the pH and elemental content of
the nutrient solution be restored daily to their original levels before
replacement?” In most instances, adjustment other than water loss
replacement is normally recommended.
The aerated standing nutrient solution method of hydroponic growing
has limited commercial application, although lettuce and herbs have
been successfully grown on styrofoam sheets floating on an aerated
nutrient solution. The plants are set in small holes in the styrofoam,
with their roots growing into the nutrient solution. The sheets are lifted
from the solution when the plants are ready to harvest.
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Another reason why this system of growing hydroponically is not well
suited for commercial application is that water and chemical use are
quite high due to the requirement of frequent replacement. In addition, the
composition of the nutrient solution is constantly changing, requiring mon
itoring and adjustment in order to maintain the pH and elemental ion
balance and sufficiency concentration levels during the use period, which
may range from 45 to 65 days. Temperature and root disease control
are additional requirements if this method of growing is going to produce
successful results.
Nutrient Film Technique (NFT)
A significant development in
hydroponics occurred in the
1970s with the introduction of
the nutrient film technique,
frequently referred to as NFT
(Cooper, 1976, 1979ab). Some
have modified the name by
using the word “flow” (Schip
pers, 1979) in place of “film,” as
the plant roots indeed grow in a flow of nutrient solution. When Allen Cooper
first introduced his NFT system of hydro ponic growing (1976), it was heralded
as the hydroponic method of the future. It was, indeed, the first major change
in hydroponic growing technique since the 1930s. At the “Hydroponics
Worldwide: State of the Art in Soilless Crop Production” conference (Savage,
1985a), Cooper and his colleagues discussed their experiences with this method,
which left those in attendance with the belief that the science of hydroponics
had made a major step forward.
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Experience has shown, however, that the NFT method does not solve
the common problems inherent in most hydroponic growing systems. How
ever, this did not deter its rapid acceptance and use in many parts of
the world, particularly in Western Europe and England. NFT has been
widely discussed and tested (Khudheir and Newton, 1983; Hurd, 1985;
Cooper, 1985, 1988; Edwards, 1985; Gerber, 1986; Molyneux, 1988;
Hochmuth, 1991b), but its future continues to be highly questionable unless
better means of disease and nutrient solution control are found. A
change in the design of the trough has been suggested by Cooper (1985),
from the “U” shape to a “W” (called a divided gully system), in which
the plant base sets on the top of the W center with the roots divided
down each side of the W. A capillary mate is placed on the inverted “V”
portion of the “W” to keep the roots moist with nutrient solution. There
are a number of advantages to this redesign of the NFT single-gully system
as initially proposed by Cooper (1976, 1979ab). A portion of the plant
roots — that on the inverted “V”— is in air; a portion of the roots lies
on a moist surface (capillary matting), which provides for better
oxygenation of the rooting system; and the remaining root mass is now
divided into two channels, which should minimize the problems
associated with a large mass of roots in a single channel. It is now
possible to use two different irrigation systems by flowing water or
various types of nutrient solutions down either channel. Unfortu nately,
the NFT channel system has now been made more complicated in
design, and it is uncertain whether this change would significantly
improve plant performance. Cooper (1996) recently published a revision
of his 1976 book on NFT in which he recognized some of the problems
that can occur with this technique of hydroponic growing.
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Simply put, in the NFT system, plant roots are suspended in a trough,
channel, or gully (trough will be the word used from this point on)
through which a nutrient solution passes. The trough containing the plant
roots is set on a slope (usually about 1%) so that the nutrient solution is
introduced at the top of the trough can flow from the top to the lower
end by gravity at a recommended flow rate of 1 L per minute. As the root
mat increases in size, the volume rate down the trough diminishes. As the
nutrient solution flows down the trough, plants at the upper end of the
trough reduce the O2 and/or elemental content of the nutrient solution,
a reduction that can be sufficient to significantly affect growth and
development of plants at the lower end. Furthermore, as the root mat
thickens and becomes denser, the flowing nutrient solution tends to move
over the top and down the outer edge of the root mat, reducing its contact
within the root mass. This interruption in flow results in poor mixing of
the current flowing nutrient solution with water and elements left behind
in the root mat from previous nutrient solution applications. One of the
means for minimizing these effects is to make the trough no longer than 30
feet (9 m) in length. In addition, the trough can also be made wider, which
can be more accommodating for root growth with longer-term crops.
One of the major advantages of NFT is the ease of establishment and the
relative low cost of construction materials. The design of NFT troughs and
materials suitable for making troughs is discussed by Morgan (1999c) and
Smith (2004). A trough can be simply formed by folding a wide strip of
polyethylene film into a pipe or triangular-like shape (Figure 9.3). The
polyethylene film may be either white or black but must be opaque to
keep light out. If light enters the trough, algae growth becomes a serious
problem. The polyethylene sheet is pulled around the plant stem and closed
with pins or clips, forming a lightproof, pipe-like rooting trough. If the
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trough is formed from strips of polyethylene film, it can be discarded after
each crop, thus only necessitating sterilization of the permanent piping and
nutrient solution storage tank.
NFT systems are closed systems, that is, the nutrient solution exiting
the end of the trough is recovered for reuse. Bugbee (1995) discusses the
requirements for the management of recirculating hydroponic growing
systems. The addition of make-up water, the need for reconstituting the
pH and nutrient element content, filtering, and sterilization are
procedures that need to be established. An open system would mean that
the nutrient solution exiting the trough is discarded, which is costly in
terms of water and reagent use as well as posing a problem for proper
disposal (Johnson, 2002c).
Aeroponics
Another promising hydroponic technique for the future was thought to
be aeroponics, which is the distribution of water and essential elements by
means of an aerosol mist bathing the plant roots (Nickols, 2002). One of the
significant advantages of this technique compared to flowing the nutrient
solution past the plant roots is aeration, as the roots are essentially
growing in air. The technique was designed to achieve substantial
economies in the use of both water and essential elements. The critical
aspects of the technique are the character of the aerosol, frequency of
root exposure, and composition of the nutrient solution. Adi Limited
(1982) described an aeroponic system that it said had proven to be highly
successful. The system is computer controlled and requires a special
fogging device, troughs, and an array of sensing devices. Although yields of
crops obtained with this growing system have been reported to be
considerably above those obtained with conventional hydroponic sys
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tems, the initial cost for the Adi system plus operating costs are very
high, bringing into question its commercial viability (Soffer, 1985), although
its value in plant propagation is considerable (Soffer, 1988).
Several methods have employed a spray of the nutrient solution
rather than a fine mist; droplet size and frequency of exposure of the
roots to the nutrient solution are the critical factors. Continuous exposure
of the roots to a fine mist gives better results than intermittent spraying or
misting. In most aeroponic systems, a small reservoir of water is allowed
to remain in the bottom of the rooting vessel so that a portion of the
roots has access to a continuous supply of water. The composition of the
nutrient solution would be adjusted based on the time and frequency of
exposure of the roots to the nutrient solution.
Medium Hydroponic Systems
In the culture systems described in this section, plants are grown in
some type of inorganic rooting medium (Straver, 1996a,b; Morgan, 2003f),
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with the nutrient solution applied by flooding or drip irrigation. Some of
the physical and chemical properties of commonly used inorganic
substrates are given bellow:
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4. Adapt2Change system technical description
Hydroponic Head - Fertigation Mixer:
The Automatic Fertilizer Mixer (Hydroponic head) for the closed hydroponic
system has been installed in both ARI Research Station in Zygi, Cyprus and TEI of
Larissa in Greece. The hydroponic head consists of an automatic mixing fertilizer unit
which is designed to prepare the nutrient solution for the plants.
Photo 1. Overview of the hydroponic head
The system is intended for experimental use, and thus the operation of each input
and output of the controller is determined by the user.
Automatic Hydroponic Head:
The Automatic Hydroponic Head consists of the following parts:
• Waterproof electrical control panel that includes a manual switch, protection unit
for the pump and controller, water level, according to the regulations of the
Electricity Authority of each country.
• A polyethylene mixing tank, where the irrigation nutrient solution is automatically
prepared by mixing the water and fertilizers in the correct proportions.
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A sensor monitors the amount of water in the tank and optimizes the inflow of the
water, according to the level, so as to prevent overflow and to avoid emptying the
tank during irrigation. Furtherore, it stops the pump when the water level reaches a
minimum level.
• A triple purpose, non-corrosive stainless steel (P1) electrical water pump is
installed to provide pressure for irrigation, aspiration and mixing fertilizer nutrient
solution. It contains an electric pressure switch to prevent the operation of the pump
without water (dry run protection). The pump covers irrigation water up at least 4
cubic meters per hour at 2.5-3 bars. A pressure-sustaining valve prevents cavitation
of water at the electric pump.
Photo 2-3. Polyethylene mixing tank, where the irrigation nutrient solution is
automatically prepared and Triple purpose, non-corrosive stainless steel electrical
water pump
• 4 Venturi type Lubricators of suitable capacity, with electrically controlled fertilizer
valves have been provided (the system is able to support up to at least 4 more valves
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for future expansion purposes). The electric valves, located at the entrance of the
Venturi can be either fully open or closed. The controller calculates the fertilizer
quantities that are required to be infused and simultaneously operates the solenoids
of the valves accordingly, or by adjusting the time that the valves remain open (pulse
duration), or by changing the frequency of the openings of the coils.
Photo 4. Ventury
The operator is able to choose their preferred mode. Every Venturi type fertilizing
pipe has:
- A meter / Flow Indicator: A transparent conical tube that has a ball inside.
The ball rises according to the instantaneous flow of fertilizer. It is equipped
with a small valve to regulate the flow rate.
- A fertilizer meter sends pulses to the controller thereby enabling control of
lubrication depending on the volume of the nutrient solution.
• Industrial pH and EC electrodes are installed in a special housing. A configuration
module pH / EC (accuracy of at least one tenth) transmits 4-20 mA signal and is
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linked to the automatic controller with a large LCD, Galvanic isolation and an
optional keyboard for quick and easy calibration.
Photo 5. pH and EC sensors
• A water meter which transmits electrical pulses to measure the water supply
system.
Photo 6. Water meter
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• 8 irrigation stations with a potential for further expansion in the future up to 36
stations. 4 solenoid valves be installed on the system.
Operational Features
The system allows:
1. Direct monitoring or programming from the keyboard and the controller display
and also from the external connection to a remote PC.
2. Irrigation based on the amount of nutrient solution, which is determined by at
least 50 individual programs.
3. Proportional fertigation with 3-8 fertilizer injectors, using up to 10 different
fertilization programs.
4. Precise pH control (to the nearest tenth) and of the Electric Conductivity, on-line,
with full control over the stability of the nutrient solution with a perfect mix of
nutrients in the water.
6. Allows the integration and operation of Drain-Water sampling system-for the
monitoring and recording of pH, EC and the quantity of water runoff for
automatically adjusting programs fertigation according to indications.
7. Automatic start of the irrigation cycles, depending on the programming of time
and adjusted in accordance with measurements of solar radiation.
8. Constant pressure of outlet irrigation, regardless of the variation of the supply
pressure of the water in the mixer.
9. Alarm indication (visual and auditory) in case of water or fertilizer leakage.
10. Automatic reset function of the program after the end of the cause of the alarm.
11. Manual sleep mode through the keyboard controller with automatic or manual
restart.
12. The operation of a dual feeding and mixing system (Mixing Junction): An input for
the fresh water and one for the recycled water. The system is be able to control the
pump and the analog power valve of each input, and monitor the flow of two entries
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from their respective water meters. The mixture’s conductivity (EC) of the fresh and
recycled water should be pre-adjusted to a value slightly below the required EC of
the final nutrient solution to the plants, and the system will automatically adjust the
percentage of fresh and recycled water to achieve the target value. The two streams
of water will be effectively mixed, by passing through a suitable mixing device, so as
to provide a stable EC measurement value.
Transportation and storage of water:
- 2 sunken pumps (P2), provision 3m3/h at 3 bar), to transfer water from the tank
(No. 2) of each greenhouse in plastic pot of 500 L (No.3). The tank has a plastic
container of 100 Liters buried beneath the soil surface and fastened suitably with
concrete. The construction is such so as to prevent rainwater entering the tank and
is suitable for opening and cleaning of the container for inspection and for future
repairs of the pump. The water in the tank is transferred automatically, through a
suitable filter (No. 9) to tank No. 3, when the level rises beyond the desired height.
The pumps is protected with a Dry Run if there is no water in the reservoir.
- 2 pressure pumps (providing 3m3/h at 4 bars) one for the fresh water and one for
the recycled (P3) and (P4), for the water transportation from the containers to the
mixer Fertilizer (No. 1). The pump P3 for the recycled water is suitable for acidic
solutions (water containing fertilizers). The pumps are protected with a Dry Run if
there is no water in the reservoir. (See photo 6).
Irrigation:
- Rockwool (slabs) 1.0 m x20.0 cm x7.5 cm, with a longlife of two years.
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Photo 7. Rockwool (slabs)
- Hydroponics tables of flexible hard type ‘envelope’ plastic (into which the
substrates are installed and at the bottom in order to return the outflow water of
each line to the tank of each greenhouse. Hydroponics tables placed on metal
benches at 40 cm distance from the ground.
Photo 8. Hydroponics tables
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- Application pipelines, PE, 4 bar, diameter 20 mm, with self-aligning emitters’
constant flow, with rubber and lance anti-percolate mechanism.
Photo 9. Application pipelines
Fertigation Strategies
Most of the nutrient solution controlling systems are currently based on EC
(electrical conductivity) control. Typically, in commercial closed-loop substrate
systems with drip irrigation, the fertigation water is automatically prepared by
mixing drainage nutrient solution with raw water and subsequently adding stock
solutions of fertilisers in this mixture, in order to achieve pre-set values of EC and pH.
In this experimental system, the control of nutrition in the crop was based on the
target composition of two nutrient solutions: a) the standard nutrient solution
supplied to the crop and b) the nutrient solution in the root zone. In the open
system, the irrigation solution was simply prepared by adding standard amounts of
fertilizers per liter of irrigation water (Savvas, 2012). In the closed system, the
recycling of nutrient solution was managed by mixing drainage solution with raw
water at a ratio resulting in a target EC followed by the injection of fertilizers on the
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basis of imposed EC values for the mixture and the outgoing solution to the
controlling system. The target EC values were selected according to the concept of
«drainage solution plus raw water» based on literature recommendations (Savvas,
2012).
Savvas, D. (2012): Soilless Cultivations. Hydroponics, Substrates. AgroTypos
Publications, Athens, Greece, pp. 528 (in Greek).
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System Setup:
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5. Diagnostic testing and preparatory procedures for hydroponic greenhouses
Success with any growing system is based to a considerable degree on
the ability of the grower to effectively evaluate and diagnose the condition
of the crop at all times (Roorda and Smilde, 1981; Paterson and Hall, 1981;
1993a). This is particularly true for the hydroponic/soilless culture and
absolutely essential for the hydroponic grower, since all the essential
elements except C, H, and O required by plants are being supplied by
means of a nutrient solution. Errors in making and using the nutrient
solution will affect plant growth, sometimes within a matter of a few
days. Some growers possess a unique ability to sense when things are not
right and take the proper corrective steps before significant crop damage is
done. Most, however, must rely on more obvious and objective measures
to assist them in determining how their growing system is working and
how plants are responding to their manage ment inputs. In the latter case,
no substitute for systematic observations and testing exists. As the
genetic growth and fruit yield potential of a crop is approached, every
management decision becomes increasingly important. Small errors can
have a significant impact; therefore, every task needs to be performed
without error in timing or process. Under such conditions, nutri tional
management is absolutely essential.
Laboratory testing and diagnostic services are readily available in the
United States and Canada (Anon., 1992) as well as other parts of the world.
Samples can be quickly and easily sent to a laboratory from almost
anywhere. Once the laboratory selection has been made, it is important
to obtain from the laboratory its instructions for collecting and shipping
samples before sending them. It also important that the laboratory
selected to do the analytical work is familiar with the type of samples
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being submitted, and if an interpretation is to be made, that those making
the interpretation are skilled analysts.
With the analytical capabilities available today, together with the ease
of quickly transporting samples and analysis results, growers can almost
monitor their plant growing system on a real-time basis. Although a routine
of periodic testing is time consuming and costly, the application of the
results obtained can more than cover the costs in terms of a saved crop
and superior quality production. The grower should get into the habit of
routinely analyzing the water source, nutrient solution, growing media, and
crop. Interpretations and recommendations based on assay results are
designed to assist the grower in order to avoid crop losses and product
quality reductions.
Water Analysis
Water available for making a nutrient solution or for irrigation may not
be of sufficient quality (i.e., free from inorganic as well as organic
substances) to be suitable for use. Pure water is not essential, but the
degree of impurity needs to be determined. Even domestic water
supplies, although safe for drinking, may not be suitable for plant use.
Water from surface ground water sources, ponds, lakes, and rivers is
particularly suspect, while collected rain water and deep-well water are
less so.
For the elements, the presence of Ca and Mg could be considered com
plementary because both elements are essential, whereas the presence
of B and Na, and the anions CO32–, HCO3–, Cl–, F–, and S– could be
considered undesirable if levels are relatively high.
The only way to determine what is in the water is to have it
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assayed. Testing for the presence of organic constituents is a decision that is
based on expected presence. Surface waters may contain disease
organisms and algae, while in agricultural areas, various residues from the
use of herbicides or other pesticides may be in the water. Tomato, for
example, is quite sensitive to many types of organic chemicals;
therefore, their presence in water could make its use undesirable,
particularly for this crop.
Nutrient Solution Analysis
Errors in the preparation of a nutrient solution as well as in the
functioning of dosers (Christian, 2001; Smith 2001f) are not uncommon;
hence the require ment for an analysis to check on the final elemental
concentrations prior to use. Since the elemental composition of the
nutrient solution can be altered considerably in closed recirculating
systems, it is equally important to monitor the composition of the solution
as frequently as practical. A record of the analysis results should be kept
and a track developed to determine how the concentration of each
element changes with each passage through the rooting media. On the basis
of such analyses, change schedules, replenishment needs, and crop
utilization patterns can be determined.
The track establishes what adjustments in the composition of the
nutrient solution are needed to compensate for the “crop effect,” not
only for the current crop but for future crops as well.
In addition, periodic analysis allows the grower to properly
supplement the nutrient solution in order to maintain consistent elemental
levels to ensure good crop growth as well as extend the useful life of the
nutrient solution. Significant economy can be gained by extending the life of
the nutrient solution in terms of both water and chemical use.
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Water and Nutrient Solution Analysis Methods
It is now possible to continuously monitor the nutrient solution with
devices such as specific ion, pH, and conductivity meters. The grower needs
to determine how best to monitor the nutrient solution based on cost and
the requirement of the selected growing system.
Electrical conductivity (EC) is frequently used as a means of determining
elemental replenishment needs in closed recirculating nutrient solution
grow ing systems (see page 106). This technique is useful if previous
knowledge is available as to which elements are likely to change and by
how much. It is far more desirable to do an elemental analysis that
quantifies each individual element and its ratio in the nutrient solution so
that specific adjustments can be made to bring the nutrient solution back
to its original composition.
The analysis of the nutrient solution should include pH and tests to
determine the concentration of the major elements N (i.e., NO3 and NH4),
P, K, Ca, and Mg. Although laboratory analysis is recommended, on-site
analysis is possible with the use of kits and relatively simple analytical
devices (Schippers, 1991; Hershey, 1992b). Although test kit procedures are
available for determination of some of the micronutrients, laboratory
analysis is recommended. However, concentration monitoring of the
micronutrients is not as critical as monitoring of the major elements unless a
micronutrient problem is suspected. For any diagnostic problem, laboratory
analysis is always recommended, including all the essential elements —
both the major elements and micronutrients.
Sampling Procedures
If a sample is to be collected for a laboratory analysis, it is best to
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contact the laboratory beforehand to obtain their recommended sampling
(volume of solution required) and shipping procedures. A directory/registry
of analytical laboratories that specialize in water and nutrient solution
assays has been published for the United States and Canada (Anon., 1992).
Keeping the water and/or nutrient solution sample from being
contaminated is essential; therefore, clean sampling devices and sample
bottles should be used. One of the best sampling/shipping bottles is a
new baby formula bottle. Remove the rubber nipple and tightly seal the
lid after the sample has been drawn. When drawing a water sample or
nutrient solution, run the water or nutrient solution for a few minutes, fill
the bottle, dump, and then fill the bottle again.
Elemental Analysis of the Growth Medium
Elemental analysis of plant growth medium is an important part of the
total evaluation of the elemental status of the medium-crop system. When
coupled with a plant analysis, it allows the grower to determine what
elemental stresses exist and how best to bring them under control. This
analysis may be comprehensive, to determine the concentration present in
the growth medium by element, or more general, measuring the total soluble
salt (EC measurement) content of effluent from the medium or by extraction
of an equilibrium solution. A comprehensive test is more valuable as a
means of pinpointing possible elemental problems than just a determination
of the EC of the effluent or extracted solution.
A test of an inorganic growth medium, such as gravel, sand, perlite, or
rockwool, measures the accumulation of salts that will significantly affect
the elemental composition of the nutrient solution being circulated
through it. Knowing what is accumulating in the growth medium, it then
becomes possible to alter the nutrient solution composition sufficiently to
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utilize the accumulated elements or to begin to make adjustments in the
nutrient solution formula with the idea of reducing the rate of
accumulation while partially utilizing those elements already present in
the medium.
For those using perlite in bags or buckets or rockwool slabs, the recom
mendation today is to periodically draw solution with a syringe from the
bag or bucket or slab for assay. Based on either a complete analysis of this
solution or only its EC, water leaching may be recommended to remove
accumulated salts. In some management schemes, leaching of the growth
medium is performed on a regular basis as a matter of normal routine.
Systems using regularly scheduled leaching should also be subjected to
periodic analysis of the growth medium effluent to confirm that the
leaching schedule is in fact doing the job intended.
For an organic growth medium, such as peat mixtures or composted
milled pinebark, the sampling and assay procedures are quite different.
Monitoring of the medium is not necessary as a matter of routine, but an
assay should be made at its initial use, whenever plant stress appears, or
when a significant change in a cultural practice occurs. Cores of media taken
to the rooting depth or to the bottom of the growing vessel are randomly
collected and composited, and the composite sample sent to the
laboratory for analysis. The various methods of extraction and analysis of
soilless organic media can be found in the laboratory guide by Jones
(2001).
Although the testing procedures are quite different for each growing
medium, the objective of the analysis is the same: determine the pH
and elemental status of the medium for diagnostic evaluation. The elements
present in the growth medium serve as a major contributor toward
meeting the crop requirement. Therefore, one objective for an analysis is to
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determine the level of each of the essential elements in the growing
medium that will contribute toward satisfying the crop requirement.
The other purpose of medium analysis is to track preferential
element accumulation by the medium. In systems where the bulk of the
elemental requirement is supplied by the nutrient solution, growth medium
analysis serves to determine accumulation rates so as to avoid imbalances
and potential toxicities. In such cases, an EC measurement of the effluent
from the medium, or an extraction of it, is not sufficient.
By tracking, the elemental composition of the growth medium can be
followed and adjustments made based on changing concentrations away
from or beyond the sufficiency range. Therefore, these periodic analyses
become the means for regulating the input of the essential elements in order
to prevent deficiencies or excesses from occurring.
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6. Recent Trends in Salinity Control for Soilless Growing Systems Management
Salinity has a big impact on growth and development (Munns, 2002). Salinity
reduces water uptake and causes growth reduction, and salt-specific effects may
occur. New societal demands on both sustainability and the quality of vegetables and
ornamentals produced in protected cultivation, new scientific approaches, and the
need for maximisation of water use efficiency stimulate new developments in soilless
systems and new trends in salinity control studies in greenhouses.
Sustainable horticulture nowadays demands less pesticides and less mineral
pollution, but without loss of yield and product quality. However, greenhouse
growers routinely apply more irrigation water to the crops than the estimated water
consumption (Voogt, 2004) and high variations in irrigation water supply have been
reported for the same crop. The soilless culture system with free drainage is still
popular. In such a system, at least 10–15% of the water and nutrients are lost from
the root environment during low light period and 30–50% during the high light
period to avoid salt accumulation (Sonneveld, 1995). In some European countries
growers have to comply with new regulations, resulting from the European Nitrate
Directive (ND) and the Water Framework Directive (WFD) aiming at the reduction of
the emission of nutrients and PPP’s from greenhouses until 2020 and the regulations
on limitations of drainage discharge from hydroponic systems (Hofman et al., 2013).
To comply with the above regulations and also for economical reasons, re-use of
drained irrigation solution is desirable and has to become a common practice in the
near future. But the requirements for the water quality in closed systems are rather
strict (Voogt and Sonneveld, 1997), and water quality in many greenhouse areas
does not meet these requirements. Recirculation of the nutrient solution will lead to
the accumulation of nutrients and higher salinity, which may require a flushing rate
of 30% or more of the nutrient solution (Stanghellini et al., 1998).
A successful practice of a closed soilless system requires good knowledge of plant
needs for water and nutrients. The application of water and nutrients must follow
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exactly those needs to avoid nutrient imbalances and excessive EC levels in the root
zone. For example, Na+ and Cl are absorbed in low concentrations by the plant. Their
accumulation in the root environment may result in an unbalanced nutrient solution,
and depletion of other nutrients such as K+ (Voogt and Sonneveld, 1997). A constant
nutrient supply will increase the total salt concentration, which may reduce growth
and yield or induce physiological disorders (Sonneveld et al., 1991).
6.1 Managing physiological processes to control salinity stress
Greenhouse climate
Proper greenhouse climate management may alleviate the negative salinity
effects on crop yield. Increasing the air relative humidity (RH), decreasing the air
temperature, and decreasing the solar radiation inside the greenhouse may result in
a lower air vapour pressure deficit (VPD, Katsoulas et al., 2001; Baille et al., 2001).
This may result in a lower crop transpiration (Katsoulas et al., 2001; 2007), a
decreased plant water flow and thus in a reduced upward transport of Na+ via the
xylem to plant leaves (An et al., 2001). Thus the adverse effects of salinity on growth
and crop production can be mitigated. However, high salinity and high humidity both
affect the Ca uptake and distribution in the plant negatively, thus increasing the risk
of Ca deficiency (Sonneveld and Welles, 1988).
Saline water applied during the day, and also in spring and summer cultivation,
causes more serious yield reductions than during the night, or in autumn cultivation
(Van Ieperen, 1996) because lower RH, higher temperatures, and illumination induce
a higher transpiration rate and thus a lower water potential (Johnson et al., 1992). It
has been reported that under saline conditions induced by high Na+ concentration in
the root zone, Na+ accumulation in leaves is lower under high RH conditions
(Romero-Aranda et al., 2002; Backhausen et al., 2005).
Xu et al. (1999) studied the effect of EC (high 4.5 dS m-1 and low 2.3 dS m-1) on
tomato crop production under two greenhouse air humidity regimes, and found that
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photosynthetic capacity was decreased but quantum yield was increased by increase
of root zone salinity. Biomass production of a relatively salt-sensitive tomato cultivar
grown in 80 mM NaCl in nutrient solution declined much less at 70% than at 30%
relative humidity (An et al., 2005). The ameliorative effect is attributed to increases
in stomatal conductance, photosynthetic rate and leaf area, and a decrease in Cl and
Na+ accumulation in the leaves (Xu et al., 1999; An et al., 2002; 2005).
In an experiment with tomato plants grown in a greenhouse and irrigated with
saline nutrient solution at an EC level of 9.5 dS m-1, Li and Stanghellini (2001) found
that fresh fruit yield increased by 8% in a treatment under low transpiration rate as
compared to control plants (at high transpiration rates), but that the dry matter
content in fruits hardly responded to these treatments.
Another environmental factor that can be controlled in the greenhouse to
alleviate the negative salinity effects is the intensity of radiation, since shading can
lower the air and crop temperature resulting in lower VPD. Farag et al. (2006) found
that for cucumber plants grown under moderate saline conditions, shading the
greenhouse increased growth and fruit yield. Sonneveld and Welles (1988) showed
that high EC levels under poor light conditions in winter and autumn did not affect
long term yield in tomato, but that high EC in periods with ample irradiation were
detrimental for yield.
One other way to increase crop salt tolerance is through the increase of the air
CO2 concentration. There are two possible explanations for this stress alleviation: (i)
extra supply of photosynthate may help to offset increased respiration demands and
(ii) the increase in external CO2 levels compensates for the decrease in stomatal
conductance with respect to the CO2 diffusion rates through stomata. Munns et al.
(1999) and Takagi et al. (2009) found that an increase in aerial CO2 concentration
alleviates salt stress effects. This augmented photosynthesis and assimilate
transport, improved the plant water status through stomatal closure, and reduced
oxidative stress, which, in turn, stimulated biomass production.
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Irrigation
Irrigation events in soilless cultivations aim to refill the substrate with water and
nutrients that are absorbed by the crop. They require a certain drainage rate of
about 25%-35% to average the differences in supply and crop uptake between spots
(Schröder and Lieth, 2002; Sonneveld, 1995), but also to prevent EC increase in the
root zone. In soilless systems with free drainage, the drainage fraction should be
maintained at a minimum, since the leaching of nutrients from the root environment
contrasts with the environmental goals to minimize emission of nutrients and plant
protection products (PPP’s). For various reasons, the EC in the root environment is
usually kept higher than the “uptake EC” by the plant. So, as soon as part of the
nutrient solution given trough irrigation is absorbed by the crop, the EC of the
nutrient solution in the root zone increases. Accordingly, when the interval between
irrigation events increases the root zone EC increases. In closed systems, the
drainage percentage is not restricted by environmental concerns and hence the
irrigation frequency may be considerably higher than recommended for free
drainage systems.
Katsoulas et al. (2006) evaluated the effects of two irrigation frequencies on
growth, flower yield, and quality in a rose crop grown on rockwool slabs in a closed
hydroponic system. The higher irrigation frequency increased both the number and
the fresh and dry weight of cut flowers per plant by about 30%, while the
greenhouse water use efficiency was improved. According to Xu et al. (2004) and
Silber et al. (2005), a high irrigation frequency may improve crop performance due to
a higher availability of nutrients, specifically P and Mn. Moreover, high irrigation
frequency is associated with constantly elevated moisture levels in the root zone of
substrate-grown plants. Therefore, the hydraulic conductivity and the water
availability are maintained for longer time at high levels (Raviv et al., 1999; 2002).
The only precaution regarding the application of a frequent irrigation schedule is the
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possible creation of excessive moisture conditions in the root zone that might reduce
oxygen availability (Schröder and Lieth, 2002).
Savvas et al. (2007) applied two irrigation regimes in a pepper crop grown in a
closed soilless system under saline conditions. They found that the low irrigation
frequency imposed a more rapid salt accumulation in the root zone, which was
ascribed to restriction of the volume of drainage solution. They also mention that a
decrease in salt concentration in the root zone and in the drainage solution that
occurs when the irrigation frequency increases, originates from the increase of the
volume of drainage solution. Thus, although the actual salt quantity in the system
may be the same; this increase of the volume of the nutrient solution raises the ratio
of salt dilution in the permanently recycled nutrient solution and results in lower salt
concentration.
Fertilisation level
Salinity may interfere with mineral nutrition acquisition by plants in two ways
(Grattan and Grieve, 1992): (i) the total ionic strength of the solution, regardless of
its composition, can reduce nutrient uptake and translocation; and (ii) uptake
competition with specific ions such as sodium and chloride can reduce uptake of
specific nutrients. These interactions may lead to Na+ induced Ca++ and/or K+
deficiencies (Volkmar et al., 1998) and Cl induced inhibition of NO3 uptake (Xu et al.,
2000). Accordingly, the nutrient supply to the root environment should account for
these competitive uptake phenomena and for the osmotic potential effects of salt
accumulation in closed systems. The interaction between solution concentration and
nutrients has many aspects. For instance, if the EC of the nutrient solution is
increased by nutrients, this results usually to higher K+ uptake and lower Ca++ and
Mg++ uptake, as K+ is easily taken up by the roots. Sonneveld and Voogt (1990)
demonstrated that under high EC values caused by increased nutrients
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concentration, Ca++ absorption is reduced due to antagonism (with K+, Mg++ and
NH4+) and due to reduced osmotic potential of the root system.
Element EC value (dS m-1)
0.75 2.5 5.0
Cation content (mmol kg-1 dry matter)
K 658 953 1080
Ca 856 794 587
Mg 274 161 160
They also suggested that a reduced root development at higher salinity levels
even more reduces the Ca++ uptake. The authors also note that no difference was
found between the effects of EC increase by nutrients or by salts on yield and fruit
quality (except for BER). Navarro et al. (2002) found that the negative effects of
salinity on pepper production and fruit damage by blossom end rot (BER) were more
severe under exposure to chlorides than to sulphates. In contrast, Voogt and
Sonneveld (1997) reported an increase in Ca uptake and specifically in Ca
translocation to fruits under conditions of increased Cl and equivalent reduced NO3-,
resulting in less BER and more gold specks in fruits, which symptom coincide with
high Ca levels in tissue. Savvas and Lenz (2000) studied the effect of the source of
salinity on eggplant production. They increased the EC of nutrient solution from 2.1
up to 4.7 dS m-1 by providing either additional amounts of nutrients or 25 mmol L-1
NaCl. They found that fresh fruit yield was significantly reduced to the same extent
in all salinity treatments. Lycoskoufis et al. (2012) mention that tomato plants grown
under high EC (12.5 dS m-1) imposed by excess amounts of macronutrients showed a
20% less decrease in yield than those grown under NaCl induced salinity. The lower
susceptibility of tomato plants to nutrient-induced salinity in comparison to equally
high EC levels caused by NaCl is ascribed to differences in osmotic pressure in
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combination with the occurrence of specific ion toxicity in the case of NaCl induced
salinity.
6.2 Practical means to overcome salt accumulation
Desalination
Desalination technologies do exist but it is a matter of cost if and when they will
be applied. Stanghellini et al. (2005) mentioned that the fraction of nutrient solution
that is leached is nearly proportional to the ratio between the concentration of the
critical salt of the irrigation solution and the concentration at which the system is
leached. In principle, therefore, the ‘optimal’ EC-ceiling, that balances marginal costs
of water and fertilizers with marginal yield loss can be calculated. They showed that
the optimal EC is very near to the value that ensures maximal yield. That is, there is
no advantage to the grower in maintaining a closed loop when the quality of
irrigation water is poor because in this case the EC will increase in levels higher than
the ‘optimal’ EC-ceiling resulting in yield loss. Therefore, they concluded that closed
systems are financially viable only in two cases: (i) in regions with good quality water
or (ii) with high-value crops that offset the costs of ensuring good water, such as rain
collection or desalination.
Dealing with salinity
All closed loop irrigation systems must support an option for solution discharge
based on a predefined discharge criterion. Therefore, a system could be considered
as ‘closed’ when there is no intention to discharge the drainage nutrient solution.
But for various reasons (e.g. salinity, nutrients, diseases) this is not always possible
and occasionally discharge is needed. Then, ‘semi-closed’ could be considered a
system when for various reasons there is no intention to reuse all the drainage
nutrient solution but for economic or sustainability reasons to re-use part of it.
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Cascade crop systems. Accordingly, for economical and sustainability reasons, the
drainage fraction will have to be optimised in open or semi-closed systems. A
cascade cropping system, which is a combination of crops where a salt sensitive crop
(donor crop) produces exhausted nutrient solution that is reused to feed a more
tolerant species (user crop) could be a sustainable and economical solution (Voogt,
2010). Examples for combinations of sensitive and tolerant crops could be lettuce
(max value of nutrient solution EC of 2.5 dS m-1) and rocket (max EC of 6 dS m-1),
strawberry (max EC of 2 dS m-1) and melon (max EC of 5 dS m-1) and others (Incrocci
and Pardossi, 2001). Incrocci et al. (2003) demonstrated that a cherry tomato may be
grown with depleted nutrient solution that is flushed out from a culture of more salt-
sensitive tomato cultivar, thus reducing the environmental impact that is provoked
by semi-closed soilless systems. Muñoz et al. (2012) calculated the effect on the
environmental impact of a cascade cropping system using LCA. They showed that the
adoption of the cascade crop system reduced environmental impact for climate
change category by 21%, but increased eutrophication category by 10% because of
the yield reduction.
Maximum acceptable salt accumulation levels. To deal with salinity a grower will
have to optimise its nutrient solution in time based on regular analysis of the
nutrient solution and taking into account the lower limits of the different ions in the
solution and the higher acceptable concentrations of the salt ions (Sonneveld, 2000).
For example, a tomato crop with a recommended average EC in the substrate
solution of 4.0 dS m-1 and a minimum total nutrient concentration of about 1.5 dS m-
1 could give space for accumulation of residual salts of 2.5 dS m-1. In case of NaCl as
residual salt, the concentration in the root environment may be about 22 mmol L-1.
Thus, the nutrient levels maintained in the root environment are strongly related to
the salt accumulation allowed. The consequences for the nutrient concentrations for
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a tomato crop grown in substrate are shown bellow in case of accumulation of NaCl
(Sonneveld and Voogt, 2009).
Accumulation of NaCl
Element Without Maximum
K 8.0 4.0
Na 0.0 22.0
Ca 10.0 4.0
Mg 4.5 1.5
NO3 23.0 10.5
Cl 0.0 22.0
H2PO4 1.0 0.5
SO4 6.5 2.0
Obviously the highest efficiencies for water and nutrients will be reached if
accumulation of salts is allowed to the maximum acceptable for the root
environment. This has been demonstrated specifically for Na by Voogt and van Os
(2012), showing that the lowest discharge rates will be reached at the highest
acceptable Na concentrations and discharge is carried out proportional to the Na
input rate.
Monitoring salt concentration. In all cases mentioned above regular monitoring
of the concentration of both salt and nutrient ions is necessary. This could be done
(i) by regular off-line laboratory analysis of the nutrient solution, (ii) use of ion-
selective sensors or (iii) use of model based predictions of ion concentrations.
However, automation of this process requires either on-line measurement or model-
based prediction of the salt concentrations in the drainage solution (Pardossi et al.,
2004; Carmassi et al. 2005).
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On-line ion-selective sensing could help to increase crop yield and decrease water
and nutrient requirements, aid growers in meeting ever tightening environmental
regulations and provide considerable supplemental benefits (Bamsey et al., 2012).
However, there are several practical limitations for on-line use of these sensors since
they need regular calibration, special attention to their maintenance, have certain
time limits on the min and max time needed/allowed to be in contact with the
nutrient solution (Gieling, 2001) and they are currently too expensive for most
growers (Van Os et al., 2008).
Temporal Na+ concentration in the nutrient solution was satisfactorily
simulated by Carmassi et al. (2003) and Kempkes and Stanghellini (2003), based on
the ion mass balance. The last authors used a couple of examples to show how such
a model can be used for determining the best management strategies under various
external conditions. They concluded that while it is true that management under
scarcity requires more skills than are now common among growers in arid regions,
tools can be developed that could warrant economic viability of protected
cultivation also in the regions where sustainability is presently in doubt. Recently,
Voogt et al. (2012) presented the ‘Waterstreams’ model, which was developed to
estimate the total water demand and waste water flows from greenhouse crops and
to optimize between options for water sources, concerning Na accumulation and
nutrient emission. They mention that the model calculations can be used to
determine the total water demand from individual crops to clusters of greenhouses,
as well as to optimize the size of rainwater collection tanks or the required capacity
of additional water sources, using actual, historical or forecasted meteorological
data. Fig. 1 shows some simulation results for the year round discharge from a
tomato crop. It can be seen that while for a tomato crop the total discharge fraction
is about 2%, a rose crop that is more salt sensitive crop requires a total discharge
fraction of about 10%, corresponding to a discharge of 54 kg N ha-1 and 240 kg N ha-
1, respectively.
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Guide for sustainable water use in greenhouses
Simulated Na concentration in the root environment and the required discharge of
drain water to prevent Na accumulation; in case of a dry year for a tomato crop. Left:
condensation water is used as supplemental water source. Right: tap water is used as
supplemental water source. Source: Voogt et al. (2012).
Although the above models may be useful tools for policy makers,
greenhouse designers and scenario studies, their development was targeted to
simulate and not to online control the salt ion concentration. A decision-support-
system (DSS) for management of the drainage solution in semi-closed hydroponic
systems, based on a Na+ mass-balance model (Savvas et al., 2005; 2007, Varlagas et
al., 2010) and measurements of plant water consumption was developed by
Katsoulas et al. (2012). The DSS was tested and evaluated during several
experimental periods and was capable to maintain a predefined level of Na+
concentration in the nutrient solution and minimize nutrient solution drainage and
nitrate emission in tomato crops grown in semi-closed systems.
In Conclusion, it could be noted that for fruit vegetables, high salinity of the
nutrient solution in the root environment induce inhibition of growth and production
at one hand, but it may increase fruit quality aspects on the other hand and thus
offer possibilities for control of produce quality. Therefore the maximum acceptable
salt accumulation will depend on the expected yield and quality responses at one
hand and the desire for the highest water and nutrient use efficiencies. To enable
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growers of regions with low quality water to adopt recycling of the excess irrigation
water, efficient solutions have to be found to control, manage and minimize salt
accumulation. It was shown that the effects of salinity on the crop strongly depend
on the greenhouse climate, the irrigation and drainage management method and
the fertilization level, so the above tools could be used for reduction of the negative
effects of salinity on the crop. Finally, the DSSs with on-line measurement or model-
based prediction of the salt concentrations in the drainage solution for semiclosed
hydroponic systems management seem to be a promising tool for salinity
management in greenhouses.
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Literature cited
An, P., Inanaga, S., Kafkafi, U., Lux, A. and Sugimoto, Y. 2001. Different effect of
humidity on growth and salt tolerance of two soybean cultivars. Biol. Plantar.
44:405-410.
An, P., Inanaga, S., Li, X.J., Eneji, A.E. and Zhu, N.W. 2005. Interactive effects of
salinity and air humidity on two tomato cultivars differing in salt tolerance. J.
Plant Nutr. 28:459-473.
An, P., Inanaga, S., Lux, A., Li, X.J., Ali, M.E.K., Matsui, T. and Sugimoto Y. 2002.
Effects of salinity and relative humidity on two melon cultivars differing in salt
tolerance. Biol. Plantar. 45:409–415.
Backhausen, J.E., Klein, M., Klocke, M., Jung, S. and Scheibe, R. 2005. Salt tolerance
of potato (Solanum tuberosum L. var. Desirée) plants depends on light intensity
and air humidity. Plant Sci. 169(1):229-237.
Baille, A., Kittas, C. and Katsoulas, N. 2001. Influence of whitening on greenhouse
microclimate and crop energy partitioning. Agr. For. Meteorol. 107(4):293-306.
Bamsey, M., Berinstain, A. and Dixon, M. 2012. Development of a potassium-
selective optode for hydroponic nutrient solution monitoring. Anal. Chim. Acta.
737:72-82.
Carmassi, G., Incrocci, L., Maggini, R., Malorgio, F., Tognoni, F. and Pardossi, A. 2005.
Modeling salinity build-up in recirculating nutrient solution culture. J. Plant Nutr.
28:431-445.
Carmassi, G., Incrocci, L., Malorgio, M., Tognoni, F. and Pardossi, A. 2003. A simple
model for salt accumulation in closed loop hydroponics. Acta Hort. 614:149-154.
Farag, A., El-Gizawy, A.M., Abu-Hadid, A.F., El-Behairy, U.A. and Medany, M.A. 2006.
Effect of modified micro-climate in the plastic house on salt tolerance of
cucumber plants. Egypt. J. Hort. 33:45-58.
53
Adapt2change – Adapt Agricultural Production to climate change and limited water supply
Guide for sustainable water use in greenhouses
Gieling, Th.H. 2001. Control of water supply and specific nutrient application in
closed growing systems. PhD thesis Wageningen University, ISBN 90-5808-525-2,
2001.
Grattan, S.R. and Grieve, C.M. 1992. Mineral element acquisition and growth
response of plants grown in saline environments. Agric. Ecosyst. Envir. 38(4):275-
300.
Hofman, G., De Vis, R., Crappé, S., Van de Sande T., Mechant, E., D’Haene, K., Amery,
F., Vandecasteele, B., Willekens, K., De Neve, S. 2013. Benchmark study on
nutrient legislation for horticultural crops in some European countries.
Proceedings NUTRIHORT; Nutrient management, innovative techniques and
nutrient legislation in intensive horticulture for an improved water quality.
September 16-18, 2013, Ghent, Belgium, 211-227.
Incrocci, L. and Pardossi, A. 2001. Cascade cropping systems as a tool for increasing
water use efficiency in protected horticulture. Hortimed project, Deliverable no 9.
Incrocci, L., Pardossi, A., Malorgio, F., Maggini, R. and Campiotti, C.A. 2003. Cascade
cropping system for greenhouse soilless culture. Acta Hort. 609:297-300.
Johnson, R.W., Dixon, M.A. and Lee, D.R. 1992. Water relations of the tomato fruit
during growth. Plant Cell Env. 15:947-953.
Katsoulas, N., Baille, A. and Kittas, C. 2001. Effect of air misting on transpiration and
bulk conductances of a greenhouse rose canopy. Agr. For. Meteorol. 106(3):233-
247.
Katsoulas, N., Kakavikakis, G., Kittas, C., Bartzanas, T. and Savvas, D. 2012.
Performance test of a Na+ concentration model included in a decision support
system for closed hydroponic systems management. Acta Hort. 957:139-145.
Katsoulas, N., Kittas, C., Dimokas, G. and Lykas, Ch. 2006. Effect of irrigation
frequency on rose flower production and quality. Biosyst. Engin. 93:237-244.
54
Adapt2change – Adapt Agricultural Production to climate change and limited water supply
Guide for sustainable water use in greenhouses
Katsoulas, N., Kittas, C., Tsirogiannis, I.L., Kitta, E. and Savvas, D. 2007. Greenhouse
microclimate and soilless pepper crop production and quality as affected by a fog
evaporative cooling system. Trans. ASAΒE 50:1831-1840.
Kempkes, F. and Stanghellini, C. 2003. Modelling salt accumulation in closed system:
a tool for management with irrigation water of poor quality. Acta Hort. 614:143-
148.
Li, Y. and Stanghellini, C. 2001. Analysis of the effect of EC and potential transpiration
on vegetative growth of tomato. Sci. Hortic. 89:9-21.
Lycoskoufis, I., Mavrogianopoulos, G., Savvas, D. and Ntatsi, G. 2012. Impact of
salinity induced by high concentration of NaCl or by high concentration of
nutrients on tomato plants. Acta Hort. 952:689-696.
Munns, R. 2002. Comparative physiology of salt and water stress. Plant, Cell Env.
25:239-250.
Munns, R., Cramer, G. R. and Ball, M.C. 1999. Interactions between rising CO2, soil
salinity and plant growth. In: Luo, Y. and Mooney, H. A. (Eds). Carbon Dioxide and
Environmental Stress. pp. 139–167. Academic Press, San Diego.
Muñoz, P., Paranjpe, A., Montero , J.I. and Antón , A. 2012. Cascade crops: an
alternative solution for increasing sustainability of greenhouse tomato crops in
Mediterranean zone. Acta Hort. 927:801-805.
Navarro, M., Garrido, C., Carvajal, M. and Martinez, V. 2002. Yield and fruit quality of
pepper plants under sulphate and chloride salinity. J. Hort. Sci. & Biot. 77(1):52-
57.
Pardossi, A., Falossi, F., Incrocci, L. and Bellocchi, G. 2004. Empirical models of
macronutrient uptake in melon plants grown in recirculating nutrient solution
culture. J. Plant Nutr. 27:1261-1280.
Raviv, M., Wallach, R., Silber, A. and Bar-Tal, A. 2002. Substrates and their analysis. p.
25 101. In: D. Savvas and H. Passam (Eds). Hydroponic Production of Vegetables
and Ornamentals. Embryo Publ., Athens.
55
Adapt2change – Adapt Agricultural Production to climate change and limited water supply
Guide for sustainable water use in greenhouses
Raviv, M., Wallach, R., Silber, A., Medina, Sh. and Krasnovsky, A. 1999. The effect of
hydraulic characteristics of volcanic materials on yield of roses in soilless culture.
J. Am. Soc. Hort. Sci. 124(2):205–209.
Romero-Aranda, R., Soria, T. and Cuartero, J. 2002. Greenhouse mist improves yield
of tomato plants grown under saline conditions. J. Am. Soc. Hort. Sci. 127:644-
648.
Savvas, D. and Lenz, F. 2000. Effects of NaCl or nutrient-induced salinity on growth,
yield, and composition of eggplants grown in rockwool. Sci. Hort. 84:37-47.
Savvas, D., Kotsiras, A., Meletiou, G., Margariti, S. and Tsirogiannis, I. 2005. Modeling
the relationship between water uptake by cucumber and NaCl accumulation in a
closed hydroponic system. HortSci. 40:802-807.
Savvas, D., Mantzos, N., Barouchas, P., Tsirogiannis, I., Olympios, C. and Passam, H.C.
2007. Modelling salt accumulation by a bean crop grown in a closed hydroponic
system in relation to water uptake. Sci. Hort. 111:311-318.
Schröder, F.G. and Lieth, H.J. 2002. Irrigation control in hydroponics. In: Savvas, D.
and Passam, H.C. (Eds). Hydroponic Production of Vegetables and Ornamentals.
Embryo Publications, Athens, Greece, pp. 263–298.
Silber, A., Bruner, M., Kenig, E., Reshef, G., Zohar, H., Posalski, I., Yehezkel, H.,
Shmuel, D., Cohen, S., Dinar, M., Matan, E., Dinkin, I., Cohen, Y., Karni, L., Aloni, B.
and Assouline, S. 2005. High fertigation frequency and phosphorus level: Effects
on summer-grown bell pepper growth and blossom-end rot incidence. Plant Soil.
270:135-146.
Sonneveld, C. 1995. Fertigation in the greenhouse industry. In: Proceedings of the
Dahlia Greidinger International Symposium on Fertigation. Technion Israel
Institute of Technology, Haifa, Israel, pp. 121-140.
Sonneveld, C. 2000. Effects of salinity on substrate grown vegetables and
ornamentals in greenhouse horticulture. Ph.D. Diss. Wageningen University,
Wageningen, 151 pp.
56
Adapt2change – Adapt Agricultural Production to climate change and limited water supply
Guide for sustainable water use in greenhouses
Sonneveld, C. and Voogt, W. 1990. Response of tomatoes (Lycopersicon esculentum)
to an unequal distribution of nutrients in the root environment. Plant Soil,
124:251-256.
Sonneveld, C., Bos, B. and Voogt, W. 1991. Fertigation in the greenhouse industry in
the Netherlands. FAO Proceedings "Fertigation/Chemigation, 186-194.
Sonneveld, C., Voogt, W., 2009. Plant Nutrition of Greenhouse Crops. Springer
Science, 431 p.
Sonneveld, C. and Welles, G.H.W., 1988. Yield and quality of rockwool grown
tomatoes as affected by variations in EC value and climatic conditions. Plant Soil
111, 37-42.
Stanghellini, C., Kempkes, F., Pardossi, A. and Incrocci, L. 2005. Closed water loop in
greenhouses: effect of water quality and value of produce. Acta Hort. 691:233-
242.
Stanghellini, C., Van Meurs, W.Th.M., Corver, F., Van Dullemen, E. and Simonse, L.
1998. Combined effect of climate and concentration of the nutrient solution on a
greenhouse tomato crop. II: Yield quantity and quality. Acta Hort. 458:231-237.
Takagi, M., El-Shemy, H.A., Sasaki, S., Toyama, S., Kanai, S., Saneoka, H. and Fujita, K.
2009. Elevated CO2 concentration alleviates salinity stress in tomato plant. Acta
Agricult. Scandin. Sect. B Soil & Plant Sci. 59(1):87-96.
Van Ieperen, W. 1996. Effects of different day and night salinity levels on vegetative
growth, yield and quality of tomato. J. Hortic. Sci. 71:99-111.
Van Os, E., Gieling, T.H. and Lieth, H.J. 2008. Technical equipment in soilless
production systems. In: Raviv, M., Lieth, H.J. (Eds). Soilless Culture: Theory and
Practice. Elsevier, Amsterdam, pp. 157-207.
Varlagas, H., Savvas, D., Mouzakis, G., Liotsos, C., Karapanos, I. and Sigrimis, N. 2010.
Modelling uptake of Na+ and Cl by tomato in closed-cycle cultivation systems as
influenced by irrigation water salinity. Agr. Wat. Man. 97:1242-1250.
57
Adapt2change – Adapt Agricultural Production to climate change and limited water supply
Guide for sustainable water use in greenhouses
Volkmar, K.M., Hu, Y. and Steppuhn, H. 1998. Physiological responses of plants to
salinity: A review. Can. J. Plant. Sci., 78:19-27.
Voogt, W. 2004. Nutrient management in soil and soilless culture in the Netherlands:
towards environmental goals. Proc. 529, Intern. Fertil. Soc. York, UK. 27 p.
Voogt, W. 2010. Water and nutrient reuse in closed circuits: The greenhouse
cascade. Floriade Dialogue 2009-2012, June 8, 2010, World Expo, Shanghai
(China).
Voogt, W. and Sonneveld, C. 1997. Nutrient management in closed growing systems
for greenhouse production, in: Goto E. (Ed). Plant production in closed
ecosystems. Kluwer Academic Publishers, pp. 83-102.
Voogt, W. and Van Os, E.A. 2012. Strategies to manage chemical water quality
related problems in closed hydroponic systems. Acta Hort. 927:949-955.
Voogt, W., Swinkels , G.J. and van Os, E. 2012. 'WATERSTREAMS': A model for
estimation of crop water demand, water supply, salt accumulation and discharge
for soilless crops. Acta Hort. 957:123-130.
Xu, G., Hillel, M., Tarchitzky, J. and Kafkafi, U. 2000. Advances in chloride nutrition of
plants. Adv. Agron. 68:97-150.
Xu, G., Levkovitch, I. and Soriano, S. 2004. Integrated effect of irrigation frequency
and phosphorus level on lettuce: P uptake, root growth and yield. Plant Soil.
263:293-304.
Xu, H.L., Wang, R., Gauthier, L. and Gosselin, A. 1999. Tomato leaf photosynthetic
responses to humidity and temperature under salinity and water deficit, Pedosph.
9:105-112.
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