CHANGE SUSTAINABLE WATER USE IN GREENHOUSES€¦ · Guide for sustainable water use in greenhouses...

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

13



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

16



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

20



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

21



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),

22



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:

23



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.

24



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

25



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

26



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

27



• 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

28



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.

29



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

30



- 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

31



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).

32



System Setup:

33



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

34



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

35



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.

36



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

37



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

38



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

39



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.

40



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

41



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

42



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.

43



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

44



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

45



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

46



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.

47



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

48



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.

50



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

51



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.

52



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