Management of the Greenhouse Environment

7/23/2019 Management of the Greenhouse Environment

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Management of the Greenhouse Environment:

Light

Light limits the photosynthetic productivity of all crops (Wilson et al 1992) and is the

most important variable affecting productivity in the greenhouse (Wilson et al 1992,

Papadopoulos and Pararajasingham 199)! "he transpiration rate of any greenhouse cropis the function of three variables# ambient temperature, humidity and light ($tanghellini

and %an &eurs 1992, %an &eurs and $tanghellini 1992)! 'f these three, it is light hich

is usually out of our control as it is received from the sun ($tanghellini and %an &eurs1992, %an &eurs and $tanghellini 1992)! $upplementary lighting does offer opportunity

to increase yield during lo light periods, but is generally considered commercially

unprofitable (Warren et al 1992, Papadopoulos and Pararajasingham 199)! "he other

means for manipulating light are limited to screening or shading ($tanghellini and %an

&eurs 1992) and are employed hen light intensities are too high! oever, there arealso general strategies to help ma*imi+e the crops access to the available light in the

greenhouse!

Properties of light and its measurement

-n order to understand ho to control the environment to ma.e the ma*imum use of theavailable light in the greenhouse, it is important to .no about the properties of light and

ho light is measured! /onsiderable confusion has e*isted regarding the measurement of

light (L-0/' -nc!), hoever it is orthhile for groers to approach the subject!

Light has both ave properties and properties of particles or photons ("illey 199)!

epending on ho light is considered, the measurement of light can reflect either itsave or particle properties! ifferent companies provide a number of different types oflight sensors for use ith computeri+ed environmental control systems! 3s long as the

sensors measure the light available to plants, for practical purposes it is not as important

ho light is measured, as it is for groers to be able to relate these measurements to hothe crop is performing!

Light is a form of radiation produced by the sun, electromagnetic radiation! 3 narrorange of this electromagnetic radiation falls ithin the range of 455 to 55 nanometers

(nm) of avelength! 'ne nanometer being e6ual to 5!555555551 meters! "he portion of

the electromagnetic spectrum hich falls beteen 455 to 55 nm is referred to as the

spectrum of visible light, this is essentially the range of the electromagnetic spectrum thatcan be seen! Plants respond to light in the visible spectrum and use this light to drive

photosynthesis!


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Figure 14. The visible spectrum.

Photosynthetically 3ctive adiation (P3) is defined as radiation in the 455 to 55 nm

aveband! P3 is the general term hich covers both photon terms and energy terms( L-0/' -nc!)! "he rate of flo of radiant (light) energy in the form of an

electromagnetic ave is called the radiant flu*, and the unit used to measure this is the

Watt (W)! "he units of Watts per s6uare meter (W7m8) are used by some light meters andis an e*ample of an instantaneous measurement of P3 (L-0/' -nc!)! 'ther meters

commonly seen in greenhouses ta.e integrated measurements reporting in units of

joules per s6uare centimeter (j7cm8) (L-0/' -nc!)! 3lthough the units seem fairlysimilar, there is no direct conversion beteen the to! Photosynthetic Photon :lu*

ensity (PP:) is another term associated ith P3, but refers to the measurement of

light in terms of photons or particles! -t is also sometimes referred to as ;uantum :lu*

ensity (L-0/' -nc!)! Photosynthetic Photon :lu* ensity is defined as the number ofphotons in the 455 0 55 nm aveband reaching a unit surface per unit of time (L-0/'

-nc!)! "he units of PP: are micromoles per second per s6uare meter (micromol7m8)!

Figure 15.The photosynthetic action spectrum.

3s the scientific community begins to agree on ho best to measure light there may bemore standardi+ation in light sensors and the units used to describe the light radiation

reaching a unit area!


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it is heat gain hich usually calls for modification of the environment as temperatures

rise on the high end of the optimum range for photosynthesis, and ventilation and cooling

begins! Plants also re6uire more ater under increasing light levels!

The light use efficiency of plants

Plants use the light in the 455 to 55 nm range for photosynthesis, but they ma.e betteruse of some avelengths than others! :igure 1=presents the photosynthetic action

spectrum of plants, the relative rate of photosynthesis of plants over the range of P3,

photosynthetically available light! 3ll plants sho a pea. of light use in the red region,appro*imately >=5 nm and a smaller pea. in the blue region at appro*imately 4=5 nm

($alisbury and oss 19?)! Plants are relatively inefficient at using light and are only able

to use about a ma*imum of 22@ of the light absorbed in the 455 to 55 nm region

($alisbury and oss 19?)! Light use efficiency by plants depends not only on thephotosynthetic efficiency of plants, but also on the efficiency of the interception of light

(Wilson et al 1992)!

Maximizing the crop's access to available light"he high cost of greenhouse production re6uires groers to ma*imi+e the use of light

falling on the greenhouse area ( Wilson et al 1992)! Aefore the crops are able to use thelight, it first has to pass through the greenhouse covering, hich does not transmit light

perfectly! "he greenhouse intercepts a percentage of light falling on it alloing a

ma*imum of ?5@ of the light to reach the crop at around noon, ith an overall average of>?@ over the day (Wilson et al 1992)! oever, the greenhouse covering also partially

diffuses or scatters the light coming into the greenhouse so that it is not all moving in one

direction (Wilson et al 1992)! "he implication of this is scattered light tends to reach

more leaves in the canopy than directional light hich thros more shados!

-t is important that the crop be orientated in such a ay that the light transmitted through

the structure is optimi+ed to allo for efficient distribution to the canopy! !B hen compared to greenhousecucumbers and tomatoes at B!4 to 2!B respectively (and et al 199B)!

"he optimum leaf area inde* varies ith the amount of sunlight reaching the crop! Cnderfull sun, the optimum L3- is , at >5@ of full sun the optimum is =, at 2B@ full sunlight,

the optimum is only 1!= ($alisbury and oss 19?)! "his point has application to a

groing and developing crop! -n 3lberta, vegetable crops are seeded in Dovember toecember, the lo light period of the year! Eoung crops have loer leaf area inde*es

hich increase as the crop ages! Cnder this crop cycle, the plants are groing and

increasing their L3- as the light conditions improve! /rop productivity increases ith

L3- up to a certain point because of more efficient light interception, as L3- increases
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/opp2902#figure15http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/opp2902#figure15


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beyond this point no further efficiency increases are reali+ed, and in some cases decreases

occur ($alisbury and oss 19?)!

"here is also a suggestion that an efficient crop canopy must allo some penetration of

P3 belo the uppermost leaves, and the sharing of light by many leaves is a

prere6uisite of high productivity (Papadopoulos and Pararajasingham 199)! Leaves canbe divided into to groups# sun leaves that intercept direct radiation and shade leaves,

that receive scattered radiation (Wilson and Loomis 19>, Papadopoulos and

Pararajasingham 199)! "he structures of these leaves are distinctly different (Wilson andLoomis 19>)!

"he major greenhouse vegetable crops (tomatoes, cucumbers and peppers) are arranged

in either single or double ros (Wilson et al 1992, and et al 199B)! "his arrangement ofthe plants and subse6uent canopy represents an effective compromise beteen

accessibility to or. the crop, and light interception by the crop (and et al 199B)! :or a

greenhouse pepper crop, this canopy provides for light interception e*ceeding 95@ under

overcast s.ies and 94@ for much of the day under clear s.ies (and et al 199B)! "here isa dramatic decrease in interception that occurs around noon, and lasts for about an hour

hen the sun aligns along the a*is of north 0 south aligned crop ros! -nterception falls to=5@ at the gap centers here the remaining light reaches the ground, and the overall

interception of the canopy drops to ?5@ (and et al 199B)!

"he strategies to reduce this light loss ould be to align the ros east0est instead of

north0south, reduced light interception occurring hen the sun aligns ith the ros

ould ta.e place early and late in the day hen the light intensities are already 6uite lo

(and et al 199B)! "he use of hite plastic ground cover can reflect bac. light that haspenetrated the canopy and can result in an overall increase of 9@ over crops ithout

hite plastic ground cover (Wilson et al 1992, and et al 199B)!

"he effect of ro orientation varies ith time of the day, season, latitude and canopy

geometry (Papadopoulos and Pararajasingham 199)! -t has been demonstrated that at

B4F latitude, north0south orientated ros of tall crops, such as tomatoes, cucumbers andpeppers, intercepted more radiation over the groing season than those orientated east0

est (Papadopoulos and Pararajasingham 199)! "his finding as the opposite for crops

gron at =1!BF latitude (Papadopoulos and Pararajasingham 199)! "he majority of

greenhouse vegetable crop production in 3lberta occurs beteen =5F (edcliff) and =BF(Gdmonton) Dorth! "his ould suggest that the optimum ro alignment of tall crops for

ma*imum light interception over the entire season, ould be east0est! oever, in

3lberta, high yielding greenhouse vegetable crops are gron in greenhouses ith north0south aligned ros as ell as in greenhouses ith east0est aligned ros!

3lberta is .non for its sunshine, and the sun is not usually limiting during the summer!-n fact, many vegetable groers apply hiteash shading to the greenhouses during the

high light period of the year because the light intensity and associated solar heat gain can

be too high for optimal crop performance! "he strategies for increasing light interception

by the canopy should focus specifically on the times in year hen light is limiting, for


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3lberta, this is early spring and late fall! When light is limiting, a linear function e*ists

beteen light reduction and decreased groth, ith a 1@ increase in groth occurring

ith a 1@ increase in light (e Honing 19?9, Wilson et al 1992) under light levels up to255 W7m8!

When light levels are limiting, supplementary artificial lighting ill increase plant grothand yield (Papadopoulos and Pararajasingham 199)! "he use of supplemental lighting

has its limits as ell! Csing supplemental lighting to increase the photoperiod to 1> and

25 hours increased the yield of pepper plants hile continuous light decreased yieldscompared to the 25 hour photoperiod (emers and


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Figure 16. High pressure sodium light.

Temperature Management

evelopment and floering of plants relates to both root +one and air temperature (Hhah

and Passam 1992), and control of temperature is an important tool for the control of cropgroth (e Honing 199>)!

Managing air temperatures

"he optimum temperature is determined by the processes involved in the utili+ation of

assimilate products of photosynthesis, ie! distribution of dry matter to shoots, leaves,roots and fruit (e Honing 199>)! :or the control of crop groth, average temperature

over one or several days is more important than the day7night temperature differences

(Aa..er 19?9, e Honing 199>)! "his average temperature is also referred to as the 240hour average temperature or 240hour mean temperature (Aa..er 19?9, Portree 199>)!

%arious greenhouse crops sho a very close relationship beteen groth, yield and the

240hour mean temperature (Aa..er 19?9, Portree 199>)!

With the goal of directing groth and maintaining optimum plant balance for sustained

high yield production, the 240hour mean temperature can be manipulated to direct the

plant to be more generative in groth, or more vegetative in groth! 'ptimumphotosynthesis occurs beteen 21 to 22 F/ (Portree 199>), this temperature serves as the

target for managing temperatures during the day hen photosynthesis occurs! 'ptimum

temperatures for vegetative groth for greenhouse peppers is beteen 21 to 2B F/, iththe optimum temperature for yield about 21 F/ (Aa..er 19?9)! :ruit set, hoever, is

determined by the 240hour mean temperature and the difference in day 0 night

temperatures (Aa..er 19?9), ith the optimum night temperature for floering and fruit

setting at 1> to 1? F/ (Pressman 199?)! "arget 240hour mean temperatures for the maingreenhouse vegetable crops (cucumbers, tomatoes, peppers) can vary from crop to crop

ith differences even beteen cultivars of the same crop!

"he 240hour mean temperature optimums for vegetable crops range beteen 21 to 2B F/,

depending on light intensity! "he general management strategy for directing the groth of

the crop is to raise the 240hour average temperature to push the plants in a generativedirection and to loer the 240hour average temperature to encourage vegetative groth


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(Portree 199>)! 3djustments to the 240hour mean temperature are made usually ithin 1

to 1!= degrees /elsius ith careful attention paid to the crop response!

'ne assumption that is made hen using air temperature as the guide to directing plant

groth is that it represents the actual plant temperature! "he role of temperature in the

optimi+ation of plant performance and yield is ultimately based on the temperature of theplants! Plant temperatures are usually ithin a degree of air temperature, hoever during

the high light periods of the year, plant tissues e*posed to high light can reach 15 to 12 F/

higher than air temperatures! -t is important to be aare of this fact and to use strategiessuch as shading and evaporative cooling to reduce overheating of the plant tissues!

-nfrared thermometers are useful for determining actual leaf temperature!

Precision heat in the canopy

Precision heating of specific areas ithin the crop canopy add another dimension of air

temperature control beyond maintaining optimum temperatures of the entire greenhouse

air mass! Csing heating pipes that can be raised and loered, heat can be applied close to

floers and developing fruit to provide optimum temperatures for ma*imumdevelopment in spite of the day 0 night temperature fluctuations re6uired to signal the

plant to produce more floers! "he rate of fruit development can be enhanced ith littleeffect on overall plant development and floer set (e Honing 199>)! Precise application

of heat in this manner can avoid the problem of lo temperatures to the floers and fruit

hich are .non to disturb floering and fruit set (Aa..er 19?9)! "he functioning ofpepper floers are affected belo 14 F/ , the number of pollen grains per floer are

reduced and fruit set under lo night temperatures are generally deformed (Pressman

199?)! Problems ith lo night temperatures can be sporadic in the greenhouse during

the cold inter months and can occur even if the environmental control system isapparently meeting and maintaining the set optimum temperature targets! "here can be a

number of reasons for this, but the primary reasons are 1) lags in response time beteen

the systems detection of the heating setpoint temperature and hen the operation of thesystem is able to provide the re6uired heat throughout the greenhouse and 2) specific

temperature variations in the greenhouse due to drafts and cold poc.ets!

Managing root zone temperatures

oot +one temperatures are primarily managed to remain in a narro range to ensure

proper root functioning! "arget temperatures for the root +one are 1? to 21 F/! /ontrol of

the root +one temperature is primarily a concern for 3lberta groers in inter, and isobtained through the use of bottom heat systems such as pipe and rail systems! /ontrol is

maintained by monitoring the temperature at the roots and maintaining the pipe at a

temperature that ensures optimum root +one temperatures!

"he use of tempered irrigation ater is also a strategy employed by some groers!

&aintaining arm irrigation ater (25 F/ is optimum) minimi+es the shoc. to the rootsystem associated ith the delivery of cold irrigation ater! -n cases during the inter

months, in the absence of a pipe and rail system, root +one temperatures can drop to 1=

F/ or loer! "he performance of most greenhouse vegetable crops is sub optimal at this

lo root +one temperature! Csing tempered irrigation ater alone is not usually


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successful in raising and maintaining root +one temperatures to optimum levels! "he

reasons for this are to fold# firstly, the volume of ater re6uired for irrigation over the

course of the day during the inter months is too small to allo for the ade6uatesustained arming of the root +one, and secondly, the temperature of the irrigation ater

ould have to be almost hot in order to effect any immediate change in root +one

temperature! oot injury can begin to occur at temperatures in e*cess of 2B F/ in directcontact ith the roots! "he recommendation for irrigation ater temperature is not to

e*ceed 24 0 2= F/! "he purpose of the irrigation system is to optimi+e the delivery of

ater and nutrients to the root systems of the plants, using it for any other purposegenerally compromises the main function of the irrigation system!

$ystems for controlling root +one temperatures are primarily confined to providing heat

during the inter months! uring the hot summer months temperatures in the root +onecan climb to over 2= F/ if the plants are gron in sadust bags or roc.ool slabs, and if

the bags are e*posed to prolonged direct sunlight! 3voiding high root +one temperatures

is accomplished primarily by ensuring an ade6uate crop canopy to shade the root system!

3lso, since larger volumes of ater are applied to the plants during the summer, ensuringthat the irrigation ater is relatively cool, appro*imately 1? F/, (if possible) ill help in

preventing e*cessive root +one temperatures! 'ne important point to .eep in mind ithrespect to irrigation ater temperatures during the summer months is irrigation pipe

e*posed to the direct sun can cause the standing ater in the pipe to reach very high

temperatures, in e*cess of B= F/I -rrigation pipe is often blac. to prevent lightpenetration into the line hich can result in the development of algae and the associated

problems ith clogged drippers! -t is important to monitor irrigation ater temperatures

at the plant dripline, especially during the first part of the irrigation cycle, to ensure that

the temperatures are not too high! 3ll e*posed irrigation pipe should be shaded ithhite plastic or moved out of the direct sunlight if a problem is detected!

Management of the elative !umidity "sing #apour Pressure $eficits

Plants e*change energy ith the environment primarily through the evaporation of ater,

through the process of transpiration (Papada.is et al 1994)! "ranspiration is the only typeof transfer process in the greenhouse that has both a physical and biological basis

(Papada.is et al 1994)! "his plant process is almost e*clusively responsible for the

subtropical climate in the greenhouse (Papada.is et al 1994)! $eventy percent of the light

energy falling on a greenhouse crop goes toards transpiration, the changing of li6uidater to ater vapour (anan 1995), and most of the irrigation ater applied to the crop

is lost through transpiration (Papada.is et al 1994)!

elative humidity () is a measure of the ater vapour content of the air! "he use of

relative humidity to measure the amount of ater in the air is based on the fact that the

ability of the air to hold ater vapour is dependent on the temperature of the air! elativehumidity is defined as the amount of ater vapour in the air compared to the ma*imum

amount of ater vapour the air is able to hold at that temperature ("illey 199, Portree

199>)! "he implication of this is that a given reading of relative humidity reflects

different amounts of ater vapour in the air at different temperatures! :or e*ample air at


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a temperature of 24 F/ at a of ?5@ is actually holding more ater vapour than air at a

temperature of 25 F/ at a of ?5@!

"he use of relative humidity for control of the ater content of the greenhouse air mass

has commonly been approached by maintaining the relative humidity belo threshold

values, one for the day and one for the night ($tanghellini and %an &eurs 1992)! "histype of humidity control as directed at preserving lo humidity ($tanghellini and %an

&eurs 1992), and although humidity levels high enough to favour disease organisms

must be avoided ($tanghellini and %an &eurs 1992), there are more optimal approachesto control the humidity levels in the greenhouse environment! "he sole use of relative

humidity as the basis of controlling greenhouse air ater content does not allo for

optimi+ation of the groing environment, as it does not provide a firm basis for dealing

ith plant processes such as transpiration in a direct manner! (anan 1995)! "he commonpurpose of humidity control is to sustain a minimal rate of transpiration ($tanghellini and

%an &eurs 1992)!

"he transpiration rate of a given greenhouse crop is a function of three in0housevariablesJ temperature, humidity and light ($tanghellini and %an &eurs 1992, %an &eurs

and $tanghellini 1992)! Light is the one variable usually outside the control of mostgreenhouse groers! -f the e*isting natural light levels are accepted, then crop

transpiration is primarily determined by the temperature and humidity in the greenhouse

($tanghellini and %an &eurs 1992)! 3chievement of the optimum transpiration setpointdepends on the management of temperature and humidity ithin the greenhouse! &ore

specifically, at each level of natural light received into the greenhouse, a transpiration

setpoint should allo for the determination of optimal temperature and humidity

setpoints ($tanghellini and %an &eurs 1992)!

"he relationship beteen transpiration and humidity is a.ard to describe, as it is

largely related to the reaction of the stomata to the difference in vapour pressure beteenthe leaves and the air ($tanghellini and %an &eurs 1992)! "he most certain piece of

.noledge about ho stomata behave under increasing vapour pressure difference is it is

dependent on the plant species in 6uestion ($tanghellini and %an &eurs 1992)! oever,even ith the current uncertainties ith understanding the relationships and determining

mechanisms involved, the main point to remember about environmental control of

transpiration is that it is possible ($tanghellini and %an &eurs 1992, %an &eurs and

$tanghellini 1992)!

"he concept of vapour pressure difference or vapour pressure deficit (%P) can be used

to establish setpoints for temperature and relative humidity in combination to optimi+etranspiration under any given light level! %P is one of the important environmental

factors influencing the groth and development of greenhouse crops (Kabri and Aurrage

199), and offers a more accurate characteristic for describing ater saturation of the airthan relative humidity because %P is not temperature dependent (odov et al 199=)!

%apour pressure can be thought of as the concentration, or level of saturation of ater

e*isting as a gas, in the air ("illey 199)! 3s arm air can hold more ater vapour than

cool air, so the vapour pressures of ater in arm air can reach higher values than in cool


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air! "here is a natural movement from areas of high concentration to areas of lo

concentration! ust as heat naturally flos from arm areas to cool areas, so does ater

vapour move from areas of high vapour pressure, or high concentration, to areas of lovapour pressure, or lo concentration! "his is true for any given air temperature! "he

vapour pressure deficit is used to describe the difference in ater vapour concentration

beteen to areas! "he si+e of the difference also indicates the natural dra or forcedriving the ater vapour to move from the area of high concentration to lo

concentration! "he rate of transpiration, or ater vapour loss from a leaf into the air

around the leaf, can be thought of, and managed using the concept of vapour pressuredeficit (%P)! Plants maintained under lo %P had loer transpiration rates hile

plants under high %P can e*perience higher transpiration rates and greater ater stress

(Kabri and Aurrage 199)!

3 .ey point hen considering the concept of %P as it applies to controlling plant

transpiration is the vapour pressure of ater vapour is alays higher inside the leaf than

outside the leaf! &eaning the concentration of ater vapour is alays greater ithin the

leaf than in the greenhouse environment, ith the possible e*ception of having a veryundesirable 155@ relative humidity in the greenhouse environment! "his means the

natural tendency of movement of ater vapour is from ithin the leaf into the greenhouseenvironment! "he rate of movement of ater from ithin the leaf into the greenhouse air,

or transpiration, is governed largely by the difference in the vapour pressure of ater in

the greenhouse air and the vapour pressure ithin the leaf! "he relative humidity of theair ithin the leaf can be considered to alays be 155@ (Papada.is et al 1994), so by

optimi+ing temperature and relative humidity of the greenhouse air, groers can establish

and maintain a certain rate of ater loss from the leaf, a certain transpiration rate! "he

ultimate goal is to establish and maintain the optimum transpiration rate for ma*imumyield! /rop yield is lin.ed to the relative increase or decrease in transpiration, a

simplified relationship relates increase in yield to increase in %P (olliet et al 199B)

"ranspiration is a .ey plant process for cooling the plant, bringing nutrients in from the

root system and for the allocation of resources ithin the plant! "ranspiration rate can

determine the ma*imum efficiency by hich photosynthesis occurs, ho efficientlynutrients are brought into the plant and combined ith the products of photosynthesis,

and ho these resources for groth are distributed throughout the plant! $ince the

principles of %P can be used to control the transpiration rate, there is a range of

optimum %Ps corresponding to optimum transpiration rates for ma*imum sustainedyield (Portree 199>)!

"he measurement of %P is done in terms of pressure, using units such as millibars (mb)or .ilopascals (.Pa) or units of concentration, grams per cubic meter (g7mB)! "he units of

measurement can vary from sensor to sensor, or beteen the various systems used to

control %P! "he optimum range of %P is beteen B to grams7mB(Portree 199>), andregardless of ho %P is measured, maintaining %P in the optimum range can be

obtained by meeting specific corresponding relative humidity and temperature targets!

"able 1 presents the temperature 0 relative humidity combinations re6uired to maintain

the range of optimal %P in the greenhouse environment! -t is important to remember


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that this table only displays the temperature and humidity targets to obtain the range of

optimum %Ps, it does not consider the temperature targets that are optimal for specific

crops! "here is a range of optimal groing temperatures for each crop that ill determinea narroer band of temperature 0 humidity targets for optimi+ing %P!

Table %& elative !umidity and Temperature Targets to btain ptimal #apourPressure $eficitsGram(m)* and millibars +mb,

elative !umidity

"empo/

9=@ 95 @ ?= @ ?5 @ = @ 5 @ >= @ >5 @

gm7mB mb gm7mB mb gm7mB mb gm7mB mb gm7mB mb gm7mB mb gm7mB mb gm7mB mb

1= 5!= 5!> 1!1 1!4 1! 2!2 2!2 2!9 2!? B! B!B 4!B B!9 =!1 4!4 =

1> 5!> 5!? 1!2 1!> 1!? 2!4 2!B B!5 2!9 B!? B!= 4!> 4!1 =!4 4! >

1 5!> 5!? 1!B 1! 1!9 2!= 2!= B!B B!1 4!1 B! 4!9 4!B =!> =!5 >1? !5 5!9 1!B 1! 2!5 2!> 2! B!> B!B 4!B 4!5 =!B 4!> >!1 =!B

19 !5 5!9 1!4 1!? 2!1 2!? 2!9 B!? B!> 4! 4!B =!> =!5 >!> =!

25 !5? 1!5 1!= 2!5 2!2 2!9 B!5 B!9 B!? =!5 4!= =!9 =!B !5 >!1 ?

21 !5? 1!5 1!> 2!1 2!4 B!2 B!B 4!B 4!1 =!4 4!9 >!4 =! != >!= ?

22 !59 1!2 1! 2!2 2!> B!4 B!= 4!> 4!B =! =!2 >!? >!5 !9 >!? ?

2B !59 1!2 1!? 2!4 2! B!> B! 4!9 4!> >!1 =!= !2 >!4 ?!4 !4 9

24 1!5 1!B 2!5 2!> B!5 B!9 B!9 =!1 4!9 >!4 =!? !> >!? ?!9 !? 15

2= 1!5 1!B 2!5 2!> B!5 B!9 4!1 =!4 =!2 >!? >!2 ?!1 !2 9!= ?!2 15

2> 1!1 1!4 2!2 2!9 B!B 4!B 4!4 =!? =!= !2 >!> ?! ! 15!1 ?!? 11

2 1!2 1!> 2!4 B!2 B!> 4! 4! >!2 =!9 !? !1 9!B ?!B 15!9 9!4 12

2? 1!B 1! 2!= B!B B! 4!9 =!5 >!> >!B ?!B != 9!9 ?! 11!4 9!9 1B

29 1!4 1!? 2! B!> 4!1 =!4 =!B !5 >! ?!? ?!5 15!1 9!B 12!2 15!? 14

B5 1!4 1!? 2!? B! 4!2 =!= =! != !1 9!B ?!= 11!2 9!9 1B!5 11!B 14

M'ptimum range B0 grams7mB, B!909!2 mb

"he plants themselves e*ert tremendous influence on the greenhouse climate (Lange and

"antau 199>), transpiration not only serves to add moisture to the environment, but is also

the mechanism by hich plants cool themselves and add heat to the environment(Papada.is et al 1994)! 'ptimi+ation of transpiration rates through management of air

temperature and relative humidity can change over the course of the season! Garly in the

season, hen plants are young and the outside temperatures are cold, both heat andhumidity (from mist systems) can be applied to maintain temperature and humidity

targets! 3s the season progresses and the crop matures, increasing light intensity increases

the transpiration rate and the moisture content of the air! "o maintain optimum rates oftranspiration, venting is employed to reduce the relative humidity in the air! oever,


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under typical summer conditions in 3lberta, particularly in the south, ventilation is almost

e*clusively triggered by high temperature setpoints calling for cooling! Cnder these

conditions, ventilation can occur continuously throughout the daylight period and resultsin very lo relative humidity in the greenhouse! 3s the hot, moist air is vented, it is

replaced by still arm, dry air! $outhern 3lberta is a dry environment ith the relative

humidity of the air in summer routinely falling belo B5@! Cnder these conditions someform of additional cooling, mist systems or pad and fan evaporative cooling, is re6uired

to both reduce the amount of ventilation for cooling as ell as to add moisture to the air!

-arbon $ioxide .upplementation

/arbon dio*ide (/'2) is one of the inputs of photosynthesis and as such /' 2plays an

important role in increasing crop productivity (and 199B, ij.dji. and outer 199B)!'ptimal /'2concentrations for the greenhouse atmosphere fall ith the range of

beteen 55 to 955 ppm (parts per million) (omero03randa et al 199=, "remblay and


13/23

hoever the relationship beteen /'2and light conditions may be relatively loose

("remblay and


14/23

visible injury to the leaves, can reduce groth rates or both (Alom 199?)! "omatoes and

cucumbers are particularly sensitive to air pollutant injury (Portree 199>)! When

considering the effects of greenhouse air pollutants ,it is important to remember that thesepollutants pose significant health ris.s for people or.ing the crops!

/ommon pollutants are often by0products of combustion! 3lthough sources of pollutantscan be outside the greenhouse, a number of sources of pollutants can be found ithin the

greenhouse! Pollutants can be produced by direct0fired heating units, gas supply lines or

carbon dio*ide generators that burn hydrocarbon fuels such as natural gas (Alom 199?)!$ignificant sources of pollutants outside the greenhouse can include industrial plants or

vehicle e*haust (Alom 199?)!

Table 0& Maximum acceptable concentration +ppm, of some noxious gases for

humans and plants

Gas !umans Plants

/arbon i*oide (/'2) =,555 4,=55

/arbon mono*ide (/') 4 155

$ulfur dio*ide ($'2) B!= 5!1

yfrogen sulfide (2$) 15!= 5!51

Gthylene (/24) =!5 5!51

Ditrous o*ide (D') =!5 5!51 to 5!1

Ditrogen dio*ide (D'2) =!5 5!2 to 2!5

Adapted from Portree 16

3ir pollution from sources ithin the greenhouse commonly arise through crac.ed heate*changers on furnaces or incomplete combustion in the furnace or /'2generators!

eaters and generators should be chec.ed at the beginning of the cropping season to

ensure they are operating properly and complete combustion is occurring! "he mostcommon air pollutants resulting from incomplete combustion include nitrogen o*ides,

nitric o*ide (D') and nitrogen dio*ide (D'2), sulfur dio*ide ($'2), ethylene (/24),

propylene (/B>), o+one ('B), carbon mono*ide (/') and hydrogen sulfide (2$)(Portree 199>, Alom 199?)!

$ymptoms of air pollutant injury vary ith the specific gases involved! "he commonsymptoms of sulfur dio*ide injury is characteri+ed by severe leaf burn appearing ithin

24 to B> hours of e*posure to high levels of the gas (Alom 199?)! "here is a distinct linebeteen the affected and unaffected areas on the leaves and young leaves are moresusceptible to injury than mature leaves (Alom 199?)! $ymptoms of D'2injury include

dar.er than normal green leaves ith donard curling leaf margins and dead areas on

the leaves in severe cases (Alom 199?)! Gthylene functions as a plant groth regulator,

involved in seed germination, root development, floer development and leaf abscission($alisbury and oss 19?, Alom 199?)! Gthylene injury can include a reduction in

groth, shortening and thic.ening of stems and tisting of stems, as ell as premature


15/23

leaf and floer drop (Alom 199?)! Propylene injury is similar to ethylene but usually

occurs at concentrations 155 times higher than those for ethylene (Alom 199?)! '+one

injury is characteri+ed by mottling, necrotic flec.ing or bron+ing necrosis of leaves,premature leaf drop and decreased groth (Alom 199?)!

Gro1ing Media

&ost commercial vegetable production greenhouses in 3lberta use some form of

hydroponic culture! "he term hydroponics essentially translates as ater culture! -t isan advanced form of crop culture hich allos for specific control of the delivery of

nutrients to the plants ($alisbury and oss 19?, Weiler and $ailus 199>)! "he term

hydroponics can bring to mind a number of variations on the same theme! ydroponic

groing systems can includeJ substrate culture here the roots are alloed to gro in aninert or semi0inert media# solution culture here the roots are immersed in ponds of

nutrient solution# D:" culture (nutrient film techni6ue) here the roots are contained

such that a thin film of nutrient solution constantly runs by the roots# and aeroponics

here the root systems are suspended ithin an enclosed area and are misted ithnutrient solution (Weiler and $ailus 199>)! 3 general or.ing definition of hydroponic

culture that ould include all of the above systems, is plant culture here the plantsreceive fertili+er nutrients every time they receive ater!

Csing this or.ing definition of hydroponics also leaves room for the inclusion of soil asa groing medium! oever, soil culture is not idely practiced in commercial

vegetable greenhouses in 3lberta! "he main reason for moving out of soil, into soilless

culture, is to escape problems due to soil borne diseases (&aree 1994, Portree 199>) that

can build0up in the soil used year after year! $oilless media such as roc.ool and sadustoffer an initially disease0free groing medium! "here are other advantages of moving the

root system out of the soil and into confined spaces such as sadust bags or roc.ool

slabs! "he main advantages are reali+ed in the improved management of atering andnutrition, topics hich are discussed in more detail in folloing sections!

Media for seeding and propagation

oc.ool plugs are the most common media used for seeding! oc.ool is

manufactured by subjecting roc. mineral materials to very high temperatures and then

spinning the materials into a fibre (Portree 199>)! "he plugs can be s6uare (2 cm * 2 cm

by 4 cm deep) and can come joined together as a roc.ool flat that fit into standard 2?cm * =4 cm plastic seeding flats! 3s the seed germinates and the seedlings are ready for

their first transplanting, the plugs easily separate from each other hen the seedlings are

transplanted into roc.ool bloc.s!

oc. ool bloc.s are typically around 15 cm * 15 cm by ? cm deep, ith a depression

cut into the upper surface to receive the roc.ool plug at the first transplanting! 3s theseedling continues to gro, the root system develops from the roc.ool plug into the

confines of the bloc.! When the seedling is ready for transplanting into the main

production greenhouse at house set, the bottom of the roc.ool bloc. is placed in

direct contact ith the larger volume of groing media used in the production house!


16/23

Gro1ing media for the production greenhouse

"he majority of 3lbertas commercial greenhouse vegetable production is based onsubstrate culture here the plants are gron in sadust or roc.ool! "hese substrates

contain practically nothing in the ay of plant nutrients and serve as a substrate for the

root system to anchor the plant! "he groing media plays a significant role in definingthe environment of the root system and allos for the transfer of ater and nutrients to

the plant! "ypically, for sadust culture, 2 or B plants are gron in 25 to 2= litre hite

plastic bags (hite reflects more light) filled ith spruce and7or pine sadust! oc.oolculture uses appro*imately 1> litres of roc.ool substrate for every 2 to B plants (Portree

199>)! "he sadust bags or roc.ool slabs are placed directly on the hite plastic floor

of the greenhouse!

$adust is less e*pensive than roc.ool in initial cost, hoever standard density

roc.ool slabs can be pasteuri+ed and reused for up to three years (&aree 1994, Portree

199>)! $adust is a aste product of the lumber milling process hich is usually burned,

so the use of sadust as a groing media is an environmentally sound practice! :orsadust culture it is important to use a moderately fine sadust, lumber mills in 3lberta

understand the sadust re6uirements for plant production and ill supply horticulturalgrade sadust if they are made aare that the sadust is to be used for plant culture!

Csing sadust that is too fine ill brea. don over the production season ith resulting

loss of airspace around the roots hich can lead to root death (Aenoit and /eustermans1994, Portree 199>)!

"here is alays some decomposition of the sadust during the groing season (Aenoit

and /eustermans 1994) hich ma.es the product useful for further composting or addingto mineral soils to improve soil 6uality! "hrough the continued action of soil microbes the

sadust residue at the end of the cropping season is returned to the environment in an

ecologically sound manner! "he aste from sadust culture is confined to the plasticbags themselves hich are recovered hen the sadust bags are dumped and can be

recycled here facilities e*ist!

Management of 2rrigation and 3ertilizer 3eed

-n hydroponic crop production systems the application of ater is integrated ith the

application of the fertili+er feed! "he management of fertili+er application to the plants istherefore integrated ith the management of atering! "he management of atering and

nutrition is focused on the optimal delivery of ater and nutrients over the various

groth stages of the plant, through the changing groing environment over theproduction year, in order to ma*imi+e yield!

4ater 5uality

Plants are comprised of ?5 to 95@ ater ($alisbury and oss 19?) and the availability

of ade6uate 6uality ater is very important to successful crop production (Portree 199>,

$tyer and Horans.i 199)! "he 6uality of ater is determined by hat is contained in the

ater at the source# ell, dugout, ton or city ater supply, and the acidity or al.alinity


17/23

of the ater! Water is a solvent, and as such, it can contain or hold a certain 6uantity of

soluble salts in solution! :ertili+ers, by their nature, are soluble salts, and groers

dissolve fertili+ers in ater to obtain nutrient solutions in order to provide the plants ithade6uate nutrition! Prior to using any source of ater for crop production it is important

to have it tested for 6uality! Water 6uality tests determine the amount of various salts

commonly associated ith ater 6uality concerns! "he ma*imum desirableconcentrations, in parts per million (ppm), for specific salt ions in ater for greenhouse

crop production are presented in table B! Parts per million are one unit of measurement of

the amount of dissolved ions, or salt in ater, and are also used to measure the level ofdissolved fertili+er salts in nutrient solutions! "he level of nutrients as dissolved ions in

ater can also be reported in milligrams7Litre of solution! "here is a direct relationship

beteen milligrams7Litre (mg7L) and ppm, here 1 mg7L N 1 ppm! 3nother common unit

of measure for dissolved fertili+er salts is the millimole (m&), the concept of millimolesand the relationship beteen millimoles and ppm is e*plained in the special topic section!

1 mmho7cm N 1 m$7cm N 1555 microsiemens7cm

Figure 17. The relationship bet!een common units of measurement for electricalconductivity "#.$.%

Table )& The maximum desirable concentrations6 in parts per million +ppm,6 for

specific salt ions in 1ater for greenhouse crop production&

ElementMaximum desirable

+ppm,

Ditrogen (D'B0 D) =

Phosphorus (2P'40 P) =

Potassium (HO) =

/alcium (/aOO) 125

&agnesium (&gOO) 2=

/hloride (/l0) 155

$ulphate ($'400) 255

Aicarbonate (/'B0) >5

$odium (DaOO) B5

-ron (:eOOO) =

Aoron (A) 5!=

Kinc (Kn

OO

) 5!=&anganese (&nOO) 1!5

/opper (/uOO) 5!2

&olybdenum (&o) 5!52

:luoride (:0) 1


18/23

p =

G!/! 1

4ater 5uality tests 1ill also report the p!6 the acidity or al7alinity of the 1ater&

nce the source of 1ater has been determined as suitable for greenhouse crop

production it is also important to have the 1ater tested routinely to ensure that

any fluctuations in 5uality that may occur does not compromise crop

production&

Glectrical conductivity of ater

4ater 5uality analyses also report the electrical conductivity or E&-& of the

1ater& The ability of 1ater to conduct an electrical current is dependent of the

amount of ions or salts dissolved in the 1ater& The greater the amount of

dissolved salts in the 1ater6 the more readily the 1ater 1ill conduct electricity&Electrical conductivity is an indirect measurement of the level of salts in the

1ater and can be a useful tool for both determining the general suitability of

1ater for crop production6 and for the ongoing monitoring of the fertilizer feed

solution& "sing electrical conductivity as a measure to maintain E&-& targets in

the nutrient solution and the root zone can be used as a management tool for

ma7ing decisions regarding the delivery of fertilizer solution to the plants&

Electrical conductivity is measured and reported using a number of

measurement units including millimhos per centimeter +mmhos(cm,6

millisiemens per centimeter +m.(cm, or microsiemens per centimeter& 4ater

suitable for greenhouse crop production should not have a E&-& in excess of %&8mmhos(cm&

p

The relative acidity and al7alinity of the 1ater is expressed as p! +.tyer and

9orans7i %;,6 and is measured on a scale from 8 to %%,& The p! scale is a logarithmic scale6 meaning that

every increase of one number ie& < to ?6 represents a ten times increase in

al7alinity& -onversely6 every single number decrease6 ie& ? to


19/23

The optimum p! of a feed solution6 1ith respect to the availability of nutrients

to plants6 falls 1ithin the range of ?&? to @&8 +Portree %@,& The p! of a solution

can be adAusted through the use of acids such as phosphoric or nitric acid6 or

potassium bicarbonate6 depending on 1hich direction the feed solution needs to

be adAusted& 4hen acids or bases are used to adAust the p! of the feed solution6

the nutrients added by the acidB nitrogen6 phosphorus6 must be accounted for1hen the feed solution is calculated& Most 1ater supplies in /lberta are basic in

p! and re5uire the use of acid for p! correction&

The amount of acid re5uired to adAust the p! is usually dependent on the

bicarbonate +!-)C, level in the 1ater& The amount of bicarbonate in the 1ater

supply can be determined by a 1ater analysis6 and is reported in ppms& / good

target p! for nutrient feed solution is ?&>6 and as a general rule this p!

corresponds to a bicarbonate level of about @8 ppm& 2f the incoming 1ater has6

for example6 a p! of >&% and a bicarbonate level reported at 08; ppm6 08; ppm C

@8 ppm D %&% to ?&>&

/pplication of 3ertilizer and 4ater

Water and fertili+er are delivered simultaneously to the crop via the nutrient solution, and

the amounts of ater and fertili+er delivered varies ith the changing re6uirements of theplants! "he plants re6uirements change as they develop from seedlings to mature plants

and in accordance ith the day to day changes in the groing environment! -n order to

manage the delivery of nutrients and ater to the plant, it is important to have a ay of

determining the crops re6uirements for fertili+er and ater!

Figure 1&. Typical fertili'er feed system !ith t!o fertili'er stoc( tan(s and computeri'edcontrol of pH and #.$.

:eed monitoring stations are established throughout the crop, one or to stations perevery 5!4 hectare (1 acre) of greenhouse area are usually sufficient, but having one

monitoring station for every atering +one of the greenhouse is a good idea! "he

purpose of the monitoring station is to measure the volume of feed delivered to theindividual plants, and to determine the volume of feed solution leachate, or over0drain

that is floing past the plants and out of the root +one over the course of the day! "he


20/23

G!/! and p of the feed solution is ta.en on a daily basis, as is the G!/! and p of the

leachate!

aily monitoring the percentage of feed solution volume floing through the root +oneenvironment, the sadust bags, or roc.ool slabs etc!, is used to adjust the volume of

feed solution delivered to the plants! "he G!/! of the leachate is used to ma.e adjustmentson the feed solution G!/! &onitoring the p of the feed and leachate helps to ensure that

the correct p is being fed to the crop and gives an indication of hat is happening in theroot +one ith respect to p! 'ptimum feed p is appro*imately =!?, and this p

optimum also applies to the root environment as ell!


21/23

is, the amount of salts in the root +one increases! "he general rule for managing the level

of salts in the root +one is that the root +one G!/! should not be greater than 1!5 mmho

above the feed G!/!

"he design of the feed solution is based on delivering ade6uate nutrition to the plants, and

these feed programs usually have an G!/! 2!= mmhos (this is largely dependent on theG!/! of the irrigation ater)! With the optimum feed solution G!/! at appro*imately 2!= 0

B!5 mmhos, the salt levels in the root +one should be maintained at around B!= 0 4!5mmhos! Garly in the crop cycle, the salt levels in the root +one can be maintained at the

proper target fairly easily by increasing the volume of nutrient solution delivered to the

plant to ensure a = to 15@ over0drain! 3s the season progresses and the ater has beenincreased so that the upper limit of B5@ over0drain has been reached, and the G!/! of the

over0drain continues to climb above the target of B!= 0 4!5 mmhos, the G!/! of the

solution can be dropped! "he reduction in feed solution G!/! is accomplished in stagesith gradual, incremental reductions in feed G!/! in the order of 5!2 mmhos every 2 to B

days! -t is never advised to apply straight ater to the plants in order to loer the root

+one G!/!, since the rapid reduction in root +one G!/! and increased p can reduce theperformance of the crop and compromise the health of the roots (&aree 1994)!

uring periods hen the plants are in a rapid stage of groth, the G!/! in the root +one

can be belo that of the feed solution! :or e*ample, the feed can be at 2!= mmhos hile

the leachate G!/! may be a 2!5 mmhos! "his is an indicator that the plants re6uire morenutrients and the feed G!/! should be increased in increments in the order of 5!2 mmhos

until the G!/! in the root +one begins to approach the upper target limit of 4!5 mmhos!

Ay varying the volume and G!/! of nutrient solution delivered to the plants, in accordance

to the leachate over0drain and G!/! targets, it is possible to optimi+e the delivery of

ade6uate ater and nutrients to the crop ithout over atering and over fertili+ing!3pplying too much or too little ater can compromise the health and performance of the

crop!

"he delivery of ater to the plants occurs over the course of the entire day! Watering canbe scheduled by using a time cloc. or in more sophisticated systems the atering events

can be triggered by the amount of incoming light received by the greenhouse! -n general,

the greater the ability to control the delivery of ater, the greater the ability to ma*imi+ecrop performance!

3 starting point for atering the crop early in the crop cycle ould be to apply ater

every half hour from one half hour after sunrise to appro*imately one hour before sunset!

"he amount of ater re6uired to meet the over0drain target is divided amongst theaterings based on the duration of the individual aterings! :or e*ample if a 45 second

atering delivers 155 ml of ater, then 15 atering events are re6uired to deliver one

litre of ater! When more than a litre of ater is re6uired in one day the duration of theindividual atering events can be increased, or the number of atering events can be

increased or both!


22/23

variation of the fre6uency and duration of the atering events over the course of the day,

then it is possible to increase the fre6uency and7or duration of the atering events during

the high light period of the day ithout necessarily increasing the duration of the earlymorning or late afternoon atering events!

Watering fre6uency can be used to help direct the vegetative7generative balance of theplant! :or any given volume of ater that is delivered to the plants, the more fre6uent the

aterings throughout the day, the more the plant ill be directed to gro vegetatively!"he longer the duration beteen aterings, the stronger the generative signal sent to the

plant! :re6uent atering during the summer months in 3lberta can help balance plants

that are overly generative due to the intense sunlight, high temperatures and lo relativehumidity!

When the concept of percent over0drain is discussed, it is preferable to obtain the

majority of the over0drain during the high light period of the day! "he first of the over0

drain should start to occur at 15J55 am and the greater part of the daily over0drain target

should be reached by 2J55 to BJ55 p!m! aving the capability of varying the duration ofthe atering events over the course of the day allos for more nutrient feed being

delivered to the plants beteen 15J55 am and 2J55 0 BJ55 p!m!

"he use of over0drain targets is one ay to ensure the plants are receiving ade6uate aterthroughout the day! 3nother strong indicator of hether or not the plants have received

ade6uate ater during the previous day is hether the groing points, or the tops of the

plants have a light green color early in the morning! 'ver the course of the day hen theplant is under transpiration stress, the color of the plants ill progress from a light green

to a dar.er blue0green! -f the plants have received ade6uate ater throughout the previous

day, the light green color ill return overnight as the plant recovers and improves its

ater status! -f the plants remain a dar.er bluish0green in the early morning, the amountof ater delivered the previous day as inade6uate! Csually, this means that the over0

drain target for the previous day have not been met and the amount of nutrient solutiondelivered to the plants has to be increased!

uring the summer months, under continuous periods of intense light, the plants may not

have recovered their ater status overnight even hen the daily over0drain targets have

been met! "he plants begin the day a dar. blue0green in color, an indication that they arealready under ater stress, even though the day has just begun! Cnder these

circumstances the overdrain targets for the day could be increased, but there is the

associated ris. of over0atering and decreasing root health and performance! -n these

cases it is advisable to consider one or to night aterings, one at appro*imately 15J55p!m! or one at 2J55 am or both! Csually the night atering events are the same length of

time as the minimum atering duration applied during the day! Dight atering can also

help increase the rate of fruit development, but there is an associated ris. of fruit splittingif too much ater is applied at night! "he night aterings should not be continued

indefinitely and the decision to use night atering events and to continue ith night

atering has to be based on the assessed needs of the crop!


23/23

"he management of the feed solution, and its delivery to the crop has to be relatively

fle*ible to meet the changing needs of the crop! With e*perience, groers gain more

confidence and s.ill in meeting and anticipating the changing needs of the cropthroughout the crop cycle and through periods of fluctuating light levels! "he general

information presented in this section serves as a starting point and by folloing the

principles of over0drain management, G!/! and p monitoring and correction, asuccessful strategy for delivery of ater and nutrients can be established!

3s ith many things there is no one right ay to apply ater and nutrients to the crop!

"he use of leaching, although ensuring that salt levels do not accumulate to high levels in

the root +one, does result in some aste of fertili+er solution as runoff! "here arestrategies that can be employed to minimi+e the aste associated ith leaching!

/ollection and recirculation of the leachate, ith an associated partial sterili+ation, or

biofiltration of the nutrient solution is one approach (Portree 199>, Dg and van der )!

Documents

Management of the Greenhouse Environment