Management of the Greenhouse Environment

This section looks at how the tools that growers have at their disposal to control the environment, are manipulated with respect to the important environmental influences on plant growth and development, for the actual optimization of the greenhouse environment. As stated in "Concepts Involved in the Optimization of the Greenhouse Environment for Crop Production," the primary goal of optimization of the greenhouse environment is to maximize the photosynthetic process in the crop. The strategy used to maximize photosynthesis is through the management of transpiration. Therefore, on-going modifications are made to the greenhouse environment to manage the transpiration of the crop to match the maximum rate of photosynthesis.

Growth can be defined as an increase in biomass (Papadopoulos and Pararajasingham 1997). The increase in size of a plant or other organism can also be considered as the fundamental definition of growth (Salisbury and Ross 1978). The growth of plants is associated with changes in the numbers of plant organs occurring through the initiation of new leaves, stems and fruit, abortion of leaves and fruit, and physiological development of numbers from one age class to the next (Jones et al 1989). Managing growth and development of an entire crop for maximum production involves the manipulation of temperature and humidity to obtain not only the maximum rate of photosynthesis under the given light conditions, but also the optimum balance of vegetative and generative growth of plants for sustained production and high yields (Portree 1996). This implies that growers can direct the results of photosynthesis, the production of assimilates, sugars and starches, towards both vegetative and generative in a balance.

Generative growth is the growth associated with fruit production. For maximum fruit production to occur, the plant has to be provided both with the appropriate cues to trigger the setting of fruit and the cues to maintain adequate levels of stem and leaf development. The balance is achieved when the assimilates from photosynthesis are directed towards maintaining the production of the new leaves and stems required to support the continued production of fruit. The appropriate cues are provided through the manipulation of the environment, and are subject to change depending on the behavior of the crop. Careful attention must be paid to the signals given by the plant, the indicators of which direction the plant is primarily headed, vegetative or generative, and how corrective action is applied through further manipulation of the environment to maintain high production.

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 1997). The transpiration rate of any greenhouse crop is the function of three variables; ambient temperature, humidity and light (Stanghellini and Van Meurs 1992, Van Meurs and Stanghellini 1992). Of these three, it is light which is usually out of our control as it is received from the sun (Stanghellini and Van Meurs 1992, Van Meurs and Stanghellini 1992). Supplementary lighting does offer opportunity to increase yield during low light periods, but is generally considered commercially unprofitable (Warren et al 1992, Papadopoulos and Pararajasingham 1997). The other means for manipulating light are limited to screening or shading (Stanghellini and Van Meurs 1992) and are employed when light intensities are too high. However, there are also general strategies to help maximize the crop's access to the available light in the greenhouse.

Properties of light and its measurement
In order to understand how to control the environment to make the maximum use of the available light in the greenhouse, it is important to know about the properties of light and how light is measured. Considerable confusion has existed regarding the measurement of light (LI-COR Inc.), however it is worthwhile for growers to approach the subject.

Light has both wave properties and properties of particles or photons (Tilley 1979). Depending on how light is considered, the measurement of light can reflect either its wave or particle properties. Different companies provide a number of different types of light sensors for use with computerized environmental control systems. As long as the sensors measure the light available to plants, for practical purposes it is not as important how light is measured, as it is for growers to be able to relate these measurements to how the crop is performing.

Light is a form of radiation produced by the sun, electromagnetic radiation. A narrow range of this electromagnetic radiation falls within the range of 400 to 700 nanometers (nm) of wavelength. One nanometer being equal to 0.000000001 meters. The portion of the electromagnetic spectrum which falls between 400 to 700 nm is referred to as the spectrum of visible light, this is essentially the range of the electromagnetic spectrum that can be seen. Plants respond to light in the visible spectrum and use this light to drive photosynthesis.



Figure 14. The visible spectrum.


Photosynthetically Active Radiation (PAR) is defined as radiation in the 400 to 700 nm waveband. PAR is the general term which covers both photon terms and energy terms ( LI-COR Inc.). The rate of flow of radiant (light) energy in the form of an electromagnetic wave is called the radiant flux, and the unit used to measure this is the Watt (W). The units of Watts per square meter (W/m²) are used by some light meters and is an example of an "instantaneous" measurement of PAR (LI-COR Inc.). Other meters commonly seen in greenhouses take "integrated" measurements reporting in units of joules per square centimeter (j/cm²) (LI-COR Inc.). Although the units seem fairly similar, there is no direct conversion between the two. Photosynthetic Photon Flux Density (PPFD) is another term associated with PAR, but refers to the measurement of light in terms of photons or particles. It is also sometimes referred to as Quantum Flux Density (LI-COR Inc.). Photosynthetic Photon Flux Density is defined as the number of photons in the 400 - 700 nm waveband reaching a unit surface per unit of time (LI-COR Inc.). The units of PPFD are micromoles per second per square meter (micromol/m²).


Figure 15. The photosynthetic action spectrum.

As the scientific community begins to agree on how best to measure light there may be more standardization in light sensors and the units used to describe the light radiation reaching a unit area. Greenhouse growers will still be left with the task of making day-to-day meaning of the light readings with respect to control of the overall environment. Generally speaking, the more intense the light, the higher the rate of photosynthesis and transpiration (increased humidity), as well as solar heat gain in the greenhouse. Of these, it is heat gain which 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 require more water under increasing light levels.

The light use efficiency of plants
Plants use the light in the 400 to 700 nm range for photosynthesis, but they make better use of some wavelengths than others. Figure 15 presents the photosynthetic action spectrum of plants, the relative rate of photosynthesis of plants over the range of PAR, photosynthetically available light. All plants show a peak of light use in the red region, approximately 650 nm and a smaller peak in the blue region at approximately 450 nm (Salisbury and Ross 1978). Plants are relatively inefficient at using light and are only able to use about a maximum of 22% of the light absorbed in the 400 to 700 nm region (Salisbury and Ross 1978). Light use efficiency by plants depends not only on the photosynthetic 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
The high cost of greenhouse production requires growers to maximize the use of light falling on the greenhouse area ( Wilson et al 1992). Before the crops are able to use the light, it first has to pass through the greenhouse covering, which does not transmit light perfectly. The greenhouse intercepts a percentage of light falling on it allowing a maximum of 80% of the light to reach the crop at around noon, with an overall average of 68% over the day (Wilson et al 1992). However, 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). The implication of this is scattered light tends to reach more leaves in the canopy than directional light which throws more shadows.

It is important that the crop be orientated in such a way that the light transmitted through the structure is optimized to allow for efficient distribution to the canopy. Greenhouse vegetable crops have a vertical structure in the greenhouse, so light filters down through "layers" of leaves before a smaller percentage actually reaches the floor. Leaf area index (LAI) is widely used to indicate the ratio of the area of leaves over the area of ground which the leaves cover (Salisbury and Ross 1978). Leaf area indexes of up to 8 are common for many mature crop communities, depending on species and planting density (Salisbury and Ross 1978). Mature canopies of greenhouse sweet peppers have a relatively high leaf area index of approximately 6.3 when compared to greenhouse cucumbers and tomatoes at 3.4 to 2.3 respectively (Hand et al 1993).

The optimum leaf area index varies with the amount of sunlight reaching the crop. Under full sun, the optimum LAI is 7, at 60% of full sun the optimum is 5, at 23% full sunlight, the optimum is only 1.5 (Salisbury and Ross 1978). This point has application to a growing and developing crop. In Alberta, vegetable crops are seeded in November to December, the low light period of the year. Young crops have lower leaf area indexes which increase as the crop ages. Under this crop cycle, the plants are growing and increasing their LAI as the light conditions improve. Crop productivity increases with LAI up to a certain point because of more efficient light interception, as LAI increases beyond this point no further efficiency increases are realized, and in some cases decreases occur (Salisbury and Ross 1978).

There is also a suggestion that an efficient crop canopy must allow some penetration of PAR below the uppermost leaves, and the sharing of light by many leaves is a prerequisite of high productivity (Papadopoulos and Pararajasingham 1997). Leaves can be divided into two groups; sun leaves that intercept direct radiation and shade leaves, that receive scattered radiation (Wilson and Loomis 1967, Papadopoulos and Pararajasingham 1997). The structures of these leaves are distinctly different (Wilson and Loomis 1967).

The major greenhouse vegetable crops (tomatoes, cucumbers and peppers) are arranged in either single or double rows (Wilson et al 1992, Hand et al 1993). This arrangement of the plants and subsequent canopy represents an effective compromise between accessibility to work the crop, and light interception by the crop (Hand et al 1993). For a greenhouse pepper crop, this canopy provides for light interception exceeding 90% under overcast skies and 94% for much of the day under clear skies (Hand et al 1993). There is a dramatic decrease in interception that occurs around noon, and lasts for about an hour when the sun aligns along the axis of north - south aligned crop rows. Interception falls to 50% at the gap centers where the remaining light reaches the ground, and the overall interception of the canopy drops to 80% (Hand et al 1993).

The strategies to reduce this light loss would be to align the rows east-west instead of north-south, reduced light interception occurring when the sun aligns with the rows would take place early and late in the day when the light intensities are already quite low (Hand et al 1993). The use of white plastic ground cover can reflect back light that has penetrated the canopy and can result in an overall increase of 9% over crops without white plastic ground cover (Wilson et al 1992, Hand et al 1993).

The effect of row orientation varies with time of the day, season, latitude and canopy geometry (Papadopoulos and Pararajasingham 1997). It has been demonstrated that at 34° latitude, north-south orientated rows of tall crops, such as tomatoes, cucumbers and peppers, intercepted more radiation over the growing season than those orientated east-west (Papadopoulos and Pararajasingham 1997). This finding was the opposite for crops grown at 51.3° latitude (Papadopoulos and Pararajasingham 1997). The majority of greenhouse vegetable crop production in Alberta occurs between 50° (Redcliff) and 53° (Edmonton) North. This would suggest that the optimum row alignment of tall crops for maximum light interception over the entire season, would be east-west. However, in Alberta, high yielding greenhouse vegetable crops are grown in greenhouses with north-south aligned rows as well as in greenhouses with east-west aligned rows.

Alberta is known for its sunshine, and the sun is not usually limiting during the summer. In fact, many vegetable growers apply whitewash 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. The strategies for increasing light interception by the canopy should focus specifically on the times in year when light is limiting, for Alberta, this is early spring and late fall. When light is limiting, a linear function exists between light reduction and decreased growth, with a 1% increase in growth occurring with a 1% increase in light (De Koning 1989, Wilson et al 1992) under light levels up to 200 W/m².

When light levels are limiting, supplementary artificial lighting will increase plant growth and yield (Papadopoulos and Pararajasingham 1997). The use of supplemental lighting has its limits as well. Using supplemental lighting to increase the photoperiod to 16 and 20 hours increased the yield of pepper plants while continuous light decreased yields compared to the 20 hour photoperiod (Demers and Gosselin 1998). The economics of artificial light supplementation generally do not warrant the use of supplementary light on a greenhouse vegetable crop in full production. However, supplementary lighting of seedling vegetable plants prior to transplanting into the production greenhouse is recommended for those growers growing their own plants from seed.

Light is generally limiting in Alberta when greenhouse vegetable seedlings are started in November to December. Using supplemental lighting for seedling transplant production when natural light is limiting resulted in increased weight of tomato and pepper transplants grown under supplemental light compared to control transplants grown under natural light (Demers et al 1991, Fierro et al 1994). Young plants exposed to supplemental light also were ready for transplanting 1 to 2 weeks earlier than plants grown under natural light (Demers et al 1991). When supplemental lighting was combined with carbon dioxide supplementation at 900 ppm, not only did the weight of the transplants increase, but total yield of the tomato crop was also higher by 10% over the control plants (Fierro et al 1994). It is recommended that supplementary lighting be used for production of vegetable transplant production in Alberta during the low light period of the year. This translates to about 4 to 7 weeks of lighting depending on the crop. Greenhouse sweet peppers are transplanted into the production greenhouse at 6 to 7 weeks of age. The amount of light required varies with crop but ranges between approximately 120 - 180 W/m², coming from 400 W lights. A typical arrangement of lights for the seedling/transplant nursery would be to have the lights in rows 1.8 m (6 ft) off the floor, spaced at 2.7 m (9 ft.) along the rows with 3.6 m (12 ft) between the rows of lights .

Natural light levels vary throughout the province with areas in southern Alberta at 50° latitude receiving 13% more light annually than areas around Edmonton at 53° latitude (Mirza 1990). Strategies to optimize the use of available light for commercial greenhouse production involve a number of crop management variables. Row orientation, plant density, plant training and pruning, maintaining optimum growing temperatures and relative humidity levels, CO₂ supplementation, and even light supplementation, all play a role. All the variables must be optimized for a given light level for a given crop, and none of these variables are independent from one another. How a grower manipulates one variable, affects the others.


Figure 16. High pressure sodium light.

Temperature Management

Development and flowering of plants relates to both root zone and air temperature (Khah and Passam 1992), and control of temperature is an important tool for the control of crop growth (De Koning 1996).

Managing air temperatures
The optimum temperature is determined by the processes involved in the utilization of assimilate products of photosynthesis, ie. distribution of dry matter to shoots, leaves, roots and fruit (De Koning 1996). For the control of crop growth, average temperature over one or several days is more important than the day/night temperature differences (Bakker 1989, De Koning 1996). This average temperature is also referred to as the 24-hour average temperature or 24-hour mean temperature (Bakker 1989, Portree 1996). Various greenhouse crops show a very close relationship between growth, yield and the 24-hour mean temperature (Bakker 1989, Portree 1996).

With the goal of directing growth and maintaining optimum plant balance for sustained high yield production, the 24-hour mean temperature can be manipulated to direct the plant to be more generative in growth, or more vegetative in growth. Optimum photosynthesis occurs between 21 to 22 °C (Portree 1996), this temperature serves as the target for managing temperatures during the day when photosynthesis occurs. Optimum temperatures for vegetative growth for greenhouse peppers is between 21 to 23 °C, with the optimum temperature for yield about 21 °C (Bakker 1989). Fruit set, however, is determined by the 24-hour mean temperature and the difference in day - night temperatures (Bakker 1989), with the optimum night temperature for flowering and fruit setting at 16 to 18 °C (Pressman 1998). Target 24-hour mean temperatures for the main greenhouse vegetable crops (cucumbers, tomatoes, peppers) can vary from crop to crop with differences even between cultivars of the same crop.

The 24-hour mean temperature optimums for vegetable crops range between 21 to 23 °C, depending on light intensity. The general management strategy for directing the growth of the crop is to raise the 24-hour average temperature to push the plants in a generative direction and to lower the 24-hour average temperature to encourage vegetative growth (Portree 1996). Adjustments to the 24-hour mean temperature are made usually within 1 to 1.5 degrees Celsius with careful attention paid to the crop response.

One assumption that is made when using air temperature as the guide to directing plant growth is that it represents the actual plant temperature. The role of temperature in the optimization of plant performance and yield is ultimately based on the temperature of the plants. Plant temperatures are usually within a degree of air temperature, however during the high light periods of the year, plant tissues exposed to high light can reach 10 to 12 °C higher than air temperatures. It is important to be aware of this fact and to use strategies such as shading and evaporative cooling to reduce overheating of the plant tissues. Infrared thermometers are useful for determining actual leaf temperature.

Precision heat in the canopy
Precision heating of specific areas within the crop canopy add another dimension of air temperature control beyond maintaining optimum temperatures of the entire greenhouse air mass. Using heating pipes that can be raised and lowered, heat can be applied close to flowers and developing fruit to provide optimum temperatures for maximum development in spite of the day - night temperature fluctuations required to signal the plant to produce more flowers. The rate of fruit development can be enhanced with little effect on overall plant development and flower set (De Koning 1996). Precise application of heat in this manner can avoid the problem of low temperatures to the flowers and fruit which are known to disturb flowering and fruit set (Bakker 1989). The functioning of pepper flowers are affected below 14 °C , the number of pollen grains per flower are reduced and fruit set under low night temperatures are generally deformed (Pressman 1998). Problems with low night temperatures can be sporadic in the greenhouse during the cold winter months and can occur even if the environmental control system is apparently meeting and maintaining the set optimum temperature targets. There can be a number of reasons for this, but the primary reasons are 1) lags in response time between the system's detection of the heating setpoint temperature and when the operation of the system is able to provide the required heat throughout the greenhouse and 2) specific temperature variations in the greenhouse due to drafts and "cold pockets".

Managing root zone temperatures
Root zone temperatures are primarily managed to remain in a narrow range to ensure proper root functioning. Target temperatures for the root zone are 18 to 21 °C. Control of the root zone temperature is primarily a concern for Alberta growers in winter, and is obtained through the use of bottom heat systems such as pipe and rail systems. Control is maintained by monitoring the temperature at the roots and maintaining the pipe at a temperature that ensures optimum root zone temperatures.

The use of tempered irrigation water is also a strategy employed by some growers. Maintaining warm irrigation water (20 °C is optimum) minimizes the shock to the root system associated with the delivery of cold irrigation water. In cases during the winter months, in the absence of a pipe and rail system, root zone temperatures can drop to 15 °C or lower. The performance of most greenhouse vegetable crops is sub optimal at this low root zone temperature. Using tempered irrigation water alone is not usually successful in raising and maintaining root zone temperatures to optimum levels. The reasons for this are two fold; firstly, the volume of water required for irrigation over the course of the day during the winter months is too small to allow for the adequate sustained warming of the root zone, and secondly, the temperature of the irrigation water would have to be almost hot in order to effect any immediate change in root zone temperature. Root injury can begin to occur at temperatures in excess of 23 °C in direct contact with the roots. The recommendation for irrigation water temperature is not to exceed 24 - 25 °C. The purpose of the irrigation system is to optimize the delivery of water and nutrients to the root systems of the plants, using it for any other purpose generally compromises the main function of the irrigation system.

Systems for controlling root zone temperatures are primarily confined to providing heat during the winter months. During the hot summer months temperatures in the root zone can climb to over 25 °C if the plants are grown in sawdust bags or rockwool slabs, and if the bags are exposed to prolonged direct sunlight. Avoiding high root zone temperatures is accomplished primarily by ensuring an adequate crop canopy to shade the root system. Also, since larger volumes of water are applied to the plants during the summer, ensuring that the irrigation water is relatively cool, approximately 18 °C, (if possible) will help in preventing excessive root zone temperatures. One important point to keep in mind with respect to irrigation water temperatures during the summer months is irrigation pipe exposed to the direct sun can cause the standing water in the pipe to reach very high temperatures, in excess of 35 °C! Irrigation pipe is often black to prevent light penetration into the line which can result in the development of algae and the associated problems with clogged drippers. It is important to monitor irrigation water temperatures at the plant dripline, especially during the first part of the irrigation cycle, to ensure that the temperatures are not too high. All exposed irrigation pipe should be shaded with white plastic or moved out of the direct sunlight if a problem is detected.

Management of the Relative Humidity Using Vapour Pressure Deficits

Plants exchange energy with the environment primarily through the evaporation of water, through the process of transpiration (Papadakis et al 1994). Transpiration is the only type of transfer process in the greenhouse that has both a physical and biological basis (Papadakis et al 1994). This plant process is almost exclusively responsible for the subtropical climate in the greenhouse (Papadakis et al 1994). Seventy percent of the light energy falling on a greenhouse crop goes towards transpiration, the changing of liquid water to water vapour (Hanan 1990), and most of the irrigation water applied to the crop is lost through transpiration (Papadakis et al 1994).

Relative humidity (RH) is a measure of the water vapour content of the air. The use of relative humidity to measure the amount of water in the air is based on the fact that the ability of the air to hold water vapour is dependent on the temperature of the air. Relative humidity is defined as the amount of water vapour in the air compared to the maximum amount of water vapour the air is able to hold at that temperature (Tilley 1979, Portree 1996). The implication of this is that a given reading of relative humidity reflects different amounts of water vapour in the air at different temperatures. For example air at a temperature of 24 °C at a RH of 80% is actually holding more water vapour than air at a temperature of 20 °C at a RH of 80%.

The use of relative humidity for control of the water content of the greenhouse air mass has commonly been approached by maintaining the relative humidity below threshold values, one for the day and one for the night (Stanghellini and Van Meurs 1992). This type of humidity control was directed at preserving low humidity (Stanghellini and Van Meurs 1992), and although humidity levels high enough to favour disease organisms must be avoided (Stanghellini and Van Meurs 1992), there are more optimal approaches to control the humidity levels in the greenhouse environment. The sole use of relative humidity as the basis of controlling greenhouse air water content does not allow for optimization of the growing environment, as it does not provide a firm basis for dealing with plant processes such as transpiration in a direct manner. (Hanan 1990). The common purpose of humidity control is to sustain a minimal rate of transpiration (Stanghellini and Van Meurs 1992).

The transpiration rate of a given greenhouse crop is a function of three in-house variables: temperature, humidity and light (Stanghellini and Van Meurs 1992, Van Meurs and Stanghellini 1992). Light is the one variable usually outside the control of most greenhouse growers. If the existing natural light levels are accepted, then crop transpiration is primarily determined by the temperature and humidity in the greenhouse (Stanghellini and Van Meurs 1992). Achievement of the optimum "transpiration setpoint" depends on the management of temperature and humidity within the greenhouse. More specifically, at each level of natural light received into the greenhouse, a transpiration setpoint should allow for the determination of optimal temperature and humidity setpoints (Stanghellini and Van Meurs 1992).

The relationship between transpiration and humidity is awkward to describe, as it is largely related to the reaction of the stomata to the difference in vapour pressure between the leaves and the air (Stanghellini and Van Meurs 1992). The most certain piece of knowledge about how stomata behave under increasing vapour pressure difference is it is dependent on the plant species in question (Stanghellini and Van Meurs 1992). However, even with the current uncertainties with understanding the relationships and determining mechanisms involved, the main point to remember about environmental control of transpiration is that it is possible (Stanghellini and Van Meurs 1992, Van Meurs and Stanghellini 1992).

The concept of vapour pressure difference or vapour pressure deficit (VPD) can be used to establish setpoints for temperature and relative humidity in combination to optimize transpiration under any given light level. VPD is one of the important environmental factors influencing the growth and development of greenhouse crops (Zabri and Burrage 1997), and offers a more accurate characteristic for describing water saturation of the air than relative humidity because VPD is not temperature dependent (Rodov et al 1995). Vapour pressure can be thought of as the concentration, or level of saturation of water existing as a gas, in the air (Tilley 1979). As warm air can hold more water vapour than cool air, so the vapour pressures of water in warm air can reach higher values than in cool air. There is a natural movement from areas of high concentration to areas of low concentration. Just as heat naturally flows from warm areas to cool areas, so does water vapour move from areas of high vapour pressure, or high concentration, to areas of low vapour pressure, or low concentration. This is true for any given air temperature. The vapour pressure deficit is used to describe the difference in water vapour concentration between two areas. The size of the difference also indicates the natural "draw" or force driving the water vapour to move from the area of high concentration to low concentration. The rate of transpiration, or water vapour loss from a leaf into the air around the leaf, can be thought of, and managed using the concept of vapour pressure deficit (VPD). Plants maintained under low VPD had lower transpiration rates while plants under high VPD can experience higher transpiration rates and greater water stress (Zabri and Burrage 1997).

A key point when considering the concept of VPD as it applies to controlling plant transpiration is the vapour pressure of water vapour is always higher inside the leaf than outside the leaf. Meaning the concentration of water vapour is always greater within the leaf than in the greenhouse environment, with the possible exception of having a very undesirable 100% relative humidity in the greenhouse environment. This means the natural tendency of movement of water vapour is from within the leaf into the greenhouse environment. The rate of movement of water from within the leaf into the greenhouse air, or transpiration, is governed largely by the difference in the vapour pressure of water in the greenhouse air and the vapour pressure within the leaf. The relative humidity of the air within the leaf can be considered to always be 100% (Papadakis et al 1994), so by optimizing temperature and relative humidity of the greenhouse air, growers can establish and maintain a certain rate of water loss from the leaf, a certain transpiration rate. The ultimate goal is to establish and maintain the optimum transpiration rate for maximum yield. Crop yield is linked to the relative increase or decrease in transpiration, a simplified relationship relates increase in yield to increase in VPD (Jolliet et al 1993)

Transpiration is a key plant process for cooling the plant, bringing nutrients in from the root system and for the allocation of resources within the plant. Transpiration rate can determine the maximum efficiency by which photosynthesis occurs, how efficiently nutrients are brought into the plant and combined with the products of photosynthesis, and how these resources for growth are distributed throughout the plant. Since the principles of VPD can be used to control the transpiration rate, there is a range of optimum VPDs corresponding to optimum transpiration rates for maximum sustained yield (Portree 1996).

The measurement of VPD is done in terms of pressure, using units such as millibars (mb) or kilopascals (kPa) or units of concentration, grams per cubic meter (g/m³). The units of measurement can vary from sensor to sensor, or between the various systems used to control VPD. The optimum range of VPD is between 3 to 7 grams/m³ (Portree 1996), and regardless of how VPD is measured, maintaining VPD in the optimum range can be obtained by meeting specific corresponding relative humidity and temperature targets. Table 1 presents the temperature - relative humidity combinations required to maintain the range of optimal VPD in the greenhouse environment. It is important to remember that this table only displays the temperature and humidity targets to obtain the range of optimum VPDs, it does not consider the temperature targets that are optimal for specific crops. There is a range of optimal growing temperatures for each crop that will determine a narrower band of temperature - humidity targets for optimizing VPD.

Table 1. Relative Humidity and Temperature Targets to Obtain Optimal Vapour Pressure Deficits Gram/m³* and millibars (mb)

Element	Maximum desirable (ppm)
Nitrogen (NO3 - N)	5
Phosphorus (H2PO4 - P)	5
Potassium (K+)	5
Calcium (Ca++)	120
Magnesium (Mg++)	25
Chloride (Cl-)	100
Sulphate (SO4--)	200
Bicarbonate (HCO3-)	60
Sodium (Na++)	30
Iron (Fe+++)	5
Boron (B)	0.5
Zinc (Zn++)	0.5
Manganese (Mn++)	1.0
Copper (Cu++)	0.2
Molybdenum (Mo)	0.02
Fluoride (F-)	1
pH	75
E.C.	1

Element	Symbol	Available as	Symbol
Macronutrients
Nitrogen	N	Nitrate ion Ammonium ion	NO3- NH4+
Phosphorus	P	Monovalent phosphate ion Divalent phosphate ion	H2PO4- HPO4-2
Potassium	K	Potassium	K+
Calcium	Ca	Calcium ion	Ca+2
Magnesium	Mg	Magnesium ion	Mg+2
Sulfur	S	Divalent sulfate ion	SO4-2
Chlorine	Cl	Chloride ion	Cl-
Micronutrients
Iron	Fe	Ferrous ion Ferric ion	Fe-2 Fe-3
Manganese	Mn	Manganous ion	Mn+2
Boron	B	Boric acid	H3BO4
Copper	Cu	Cupric ion chelate Cuprous ion chelate	Cu+2 Cu+
Zinc	Zn	Zinc ion	Zn+2
Molybdenum	Mo	Molybdate ion	MoO4-

1. The volume of water that will be used to make the feed solution.

2. The types of fertilizers that are available, and the relative amounts of each nutrient present in the fertilizer.

Crop	Nutrient Targets (ppm)			Nutrient Ratio
	N	K	Ca	N	K	Ca
cucumber	20 0	30 0	173	1.00	1.51	0.86
pepper	21 4	31 8	200	1.00	1.48	0.93

Moles and millimoles in the greenhouse; Just another couple of rodents? - Just when you thought you had all your rodent problems under control, some greenhouse vegetable growers have been concerned about millimoles and moles. Not to worry, these growers are not referring to four legged moles. Rather they are using another unit of measure to discuss fertilizer feed targets and root zone targets.

So, what exactly is a millimole? A millimole is one thousandth of a mole, and a mole is defined as the amount of a substance of a system which contains as many elementary entities as there are atoms in exactly 12 grams of 12 C (Carbon 12). Now, you were probably expecting that a definition would help clarify the situation, isn't that what definitions are supposed to do? The concept of the mole has come out of stoichiometry, that branch of chemistry which studies the quantities of reactants and products in chemical reactions.

Now a lot of chemists and physicists have argued for a long time over how to measure the masses of individual elements (some of those same elements that growers feed their crops in fertilizer feed solutions) and in 1961 they settled on using the mole. A good way to understand what a mole is and why to use it is to related it to the concept of a dozen. We understand that a dozen is twelve of something, be it cucumbers, eggs or whatever. A mole is 6.02 x 10²³ of some entity, and chemists usually refer to actual molecules of a substance when they talk about moles, although you could have a mole of eggs or a mole of cucumbers. You would be quite the grower to grow a mole of cucumbers, tomatoes or peppers. The number 6.02 x 10²³ , which in long hand is 602 000 000 000 000 000 000 000, is called Avogadro's number after the nineteenth century chemist who did some pioneering work on gases and was largely ignored for his trouble. The lesson here is that if you do something great and are not feeling appreciated for the greatness, someone, far into the future may name a big number after you.

Moles do relate to parts per million (ppm), they are both ways to measure how much of a given nutrient we are dealing with in a fertilizer feed sample, leachate or tissue sample. The difference is that ppm is a measure of mass (e.g. 1 ppm = 1 milligram/litre) and moles measure amounts. One mole of any substance contains Avogadro's number of entities or basic units. Those entities, as mentioned earlier, can be atoms or molecules or whatever you want. When we talk about one mole of nitrate nitrogen, NO₃, we are referring to 6.02 x 10²³ molecules of NO₃, because the basic NO₃ entity is made up of one atom of nitrogen (N) and three atoms of oxygen (O). If we are talking about a mole of iron, Fe, we are talking about atoms, because the basic entity of iron is the iron atom.

All atoms and molecules have different basic weights, some being heavier than other. If we talk about 1 ppm of NO₃ versus 1 ppm of Fe, we are talking about the same mass of each, i.e., 1 milligram/litre. However, there will be a different number of basic entities or moles of NO₃ and Fe in a solution which contains 1 ppm each of NO₃ and Fe.

Now, we are getting close to being able to convert ppm to moles or millimoles, but we will first consider the concept of atomic and molecular weights. The atomic weights of all the elements can be found on the periodic table, that handy chart that we carried with us throughout all our chemistry classes. The atomic weights of the elements are given in grams per mole. The molecular weight of oxygen is 16 grams/mole, this means that 6.02 x 10²³ atoms of oxygen weights 16 grams. One mole of nitrogen weighs 14 grams. By combining all the atoms which make up molecules we can arrive at the molecular weights. Therefore, the molecular weight of NO₃ would equal 14 + 3(16) grams/mole or 62 grams/mole. One last thing to remember is that moles are related to millimoles the same way that grams are related to milligrams. So if moles are related in the range of grams, millimoles are in the range of milligrams.

We know that 1 ppm is equal to 1 milligram/litre, so to convert ppm to millimoles you divide ppm by the molecular weight of the element you are working with. For example:

• 1 ppm of NO₃ = 1 mg/litre

1 mg/ litre of NO₃ / 62 mg/mole = 0.016 millimoles of NO₃ in one litre

• 1 ppm of Fe = 1 mg/litre

1 mg/litre of Fe / 56 mg/millimole = 0.018 millimoles of Fe in one litre.

• 1 ppm of magnesium (Mg) = 1 mg/litre

1 mg/litre of Mg / 24 mg/millimole = 0.041 millimoles of Mg in one litre.

• To convert millimoles to ppm:
  
  ppm = millimoles/litre x molecular weight (mg/millimole)
  Example:
  ppm NO₃ = 0.016 millimoles of NO₃ in one litre x 62 mg/millimole
  
  = 1 ppm NO₃

Calculating the required amounts of the various fertilizers is dependent on the volume of water to be used. This is determined by the volume of the stock tank (e.g. 200 litres) multiplied by the injection ratio (e.g. 100:1 or 200:1). For example, using a 200 litre fertilizer concentrate stock tank, and a 200:1 injection ratio, the volume of water that will be used to calculate the amount of fertilizer to add will be:

Having the weight of the acid, it is now possible to determine the amount of phosphorus contributed to the pH-adjusted water by 85% phosphoric acid. One more piece of information is required, phosphoric acid contains 32 % available phosphorus. This is also referred to as the fertilizer grade of the acid. Now, using the following formula:

(Formula 1) * from Mirza and Younus, 1994

ppm =	grams of acid x grade of acid x 10
	litres of water

the amount of phosphorus (in ppm) contributed by 7,857 grams of 85% phosphoric acid

=	9,080 grams x 32% x 10
	40,000
=	73 ppm of phosphorus, actual "P".

This same sequence of calculations can be used to determine the amount of nitrogen contributed if 67% nitric acid was used. In this example 49 ppm of nitrogen would be contributed if 67% nitric acid was used.

Determining the required amounts of the various fertilizers necessary to meet the feed targets
For the purposes of this discussion of designing a fertilizer program, only component fertilizers will be considered, a list of the common component fertilizers for greenhouse crop production is presented in table 9. The fertilizers are identified by their chemical name, and their fertilizer number designation, ie. 0-53-35 for monopotassium phosphate. The "grade" of the fertilizer with respect to the different nutrients supplied by the fertilizer, is also provided. It is important to know that the three number designation of the fertilizer represents the percentages or grade of nitrogen (N), phosphorus (P) and potassium (K), in that order, that is present in the fertilizer. However, it is very important to note when the percentages for phosphorus and potassium are used, the number on the bag represents the percentages of phosphate (P₂O₅) and potash (K₂O) and not actual phosphorus and potassium. Phosphate is only 43% actual phosphorus and potash is only 83% actual potassium. For this reason, monopotassium phosphate, 0-53-35, is listed as containing 23% phosphorus (53% x 0.43 = 23%), and 29% potassium (35% x. 0.83 = 29%).

Table 8. Fertilizer "upgrades" of phosphoric and nitric acid

grams of fertilizer required =	ppm desired x litres of water
	grade of fertilizer x 10

ppm =	grams of fertilizer x grade of fertilizer x 10
	litres of water

grams of calcium nitrate required =	180 ppm x (200 x 200) litres of water
	19% x 10
=	180 ppm x 40,000 litres of water
	190
=	37,894 grams
=	37.9 kilograms

ppm of nitrogen =	37,894 grams x 15.5% x 10
	40,000 litres
=	146.8 = 147 ppm of nitrogen

Tank A		Tank B
calcium nitrate		potassium nitrate (one half the total amount)
potassium nitrate (one half the total amount)		magnesium sulfate
iron chelate		monopotassium phosphate
		potassium sulfate
		manganese chelate
		zinc chelate
		copper chelate
		sodium molybdate
		boric acid

Daily monitoring the percentage of feed solution volume flowing through the root zone environment, the sawdust bags, or rockwool slabs etc., is used to adjust the volume of feed solution delivered to the plants. The E.C. of the leachate is used to make adjustments on the feed solution E.C. Monitoring the pH of the feed and leachate helps to ensure that the correct pH is being fed to the crop and gives an indication of what is happening in the root zone with respect to pH. Optimum feed pH is approximately 5.8, and this pH optimum also applies to the root environment as well. Generally the activity of the roots tends to raise the pH in the root environment and feeding at a lower pH can help counteract this rise in pH.

Figure 19. General schematic of a fertilizer feed monitoring station.

It is not recommended to feed at a pH of lower than 5.5 when attempting to lower the pH in the root zone. In addition to feeding at a pH of 5.5, the use of ammonium nitrate at 2 to 5 ppm of ammonium nitrogen (NH₄ - N) will help to lower the pH of the root zone due to the acidifying effect of this fertilizer.

A schematic of a typical feed monitoring station is presented in figure 19. In addition to monitoring the feed and leachate, recording the leachate percentage, feed and leachate E.C. and pH can be used as a tool to chart the performance of the crop with respect to these recorded values over time, and in relationship to other parameters including the amount and intensity of available light.

The amount of nutrient solution delivered to the plant on a daily basis can be determined by the percentage over-drain or leachate that is recovered from the plants over the course of the day. Leaching, or allowing a certain percentage of nutrient solution applied to the crop, to pass through the root system, allows for a flushing of the root zone to avoid the accumulation of salts. Generally, when the plants are young, a percentage leachate of 5 to 10% is a good target. As the plants develop, the amount of water required to attain this over-drain target increases. As the season progresses and the light levels increase and the plants mature and begin to bear fruit, the over-drain targets increase to 20 to 30%. Generally these higher over-drain targets apply as the high light period of the year begins, usually in June. As the percentage over-drain decreases, the leachate E.C. increases, that is, the amount of salts in the root zone increases. The general rule for managing the level of salts in the root zone is that the root zone E.C. should not be greater than 1.0 mmho above the feed E.C.

The design of the feed solution is based on delivering adequate nutrition to the plants, and these feed programs usually have an E.C. 2.5 mmhos (this is largely dependent on the E.C. of the irrigation water). With the optimum feed solution E.C. at approximately 2.5 - 3.0 mmhos, the salt levels in the root zone should be maintained at around 3.5 - 4.0 mmhos. Early in the crop cycle, the salt levels in the root zone can be maintained at the proper target fairly easily by increasing the volume of nutrient solution delivered to the plant to ensure a 5 to 10% over-drain. As the season progresses and the water has been increased so that the upper limit of 30% over-drain has been reached, and the E.C. of the over-drain continues to climb above the target of 3.5 - 4.0 mmhos, the E.C. of the solution can be dropped. The reduction in feed solution E.C. is accomplished in stages with gradual, incremental reductions in feed E.C. in the order of 0.2 mmhos every 2 to 3 days. It is never advised to apply straight water to the plants in order to lower the root zone E.C., since the rapid reduction in root zone E.C. and increased pH can reduce the performance of the crop and compromise the health of the roots (Maree 1994).

During periods when the plants are in a rapid stage of growth, the E.C. in the root zone can be below that of the feed solution. For example, the feed can be at 2.5 mmhos while the leachate E.C. may be a 2.0 mmhos. This is an indicator that the plants require more nutrients and the feed E.C. should be increased in increments in the order of 0.2 mmhos until the E.C. in the root zone begins to approach the upper target limit of 4.0 mmhos.

By varying the volume and E.C. of nutrient solution delivered to the plants, in accordance to the leachate over-drain and E.C. targets, it is possible to optimize the delivery of adequate water and nutrients to the crop without over watering and over fertilizing. Applying too much or too little water can compromise the health and performance of the crop.

The delivery of water to the plants occurs over the course of the entire day. Watering can be scheduled by using a time clock or in more sophisticated systems the watering events can be triggered by the amount of incoming light received by the greenhouse. In general, the greater the ability to control the delivery of water, the greater the ability to maximize crop performance.

A starting point for watering the crop early in the crop cycle would be to apply water every half hour from one half hour after sunrise to approximately one hour before sunset. The amount of water required to meet the over-drain target is divided amongst the waterings based on the duration of the individual waterings. For example if a 40 second watering delivers 100 ml of water, then 10 watering events are required to deliver one litre of water. When more than a litre of water is required in one day the duration of the individual watering events can be increased, or the number of watering events can be increased or both. Generally, as the crop matures, it is better to increase the frequency of watering events than the duration of each event. If the watering system allows the variation of the frequency and duration of the watering events over the course of the day, then it is possible to increase the frequency and/or duration of the watering events during the high light period of the day without necessarily increasing the duration of the early morning or late afternoon watering events.

Watering frequency can be used to help direct the vegetative/generative balance of the plant. For any given volume of water that is delivered to the plants, the more frequent the waterings throughout the day, the more the plant will be directed to grow vegetatively. The longer the duration between waterings, the stronger the generative signal sent to the plant. Frequent watering during the summer months in Alberta can help balance plants that are overly generative due to the intense sunlight, high temperatures and low relative humidity.

When the concept of percent over-drain is discussed, it is preferable to obtain the majority of the over-drain during the high light period of the day. The first of the over-drain should start to occur at 10:00 am and the greater part of the daily over-drain target should be reached by 2:00 to 3:00 p.m. Having the capability of varying the duration of the watering events over the course of the day allows for more nutrient feed being delivered to the plants between 10:00 am and 2:00 - 3:00 p.m.

The use of over-drain targets is one way to ensure the plants are receiving adequate water throughout the day. Another strong indicator of whether or not the plants have received adequate water during the previous day is whether the growing points, or the tops of the plants have a light green color early in the morning. Over the course of the day when the plant is under transpiration stress, the color of the plants will progress from a light green to a darker blue-green. If the plants have received adequate water throughout the previous day, the light green color will return overnight as the plant recovers and improves its water status. If the plants remain a darker bluish-green in the early morning, the amount of water delivered the previous day was inadequate. Usually, this means that the over-drain target for the previous day have not been met and the amount of nutrient solution delivered to the plants has to be increased.

During the summer months, under continuous periods of intense light, the plants may not have recovered their water status overnight even when the daily over-drain targets have been met. The plants begin the day a dark blue-green in color, an indication that they are already under water stress, even though the day has just begun. Under these circumstances the overdrain targets for the day could be increased, but there is the associated risk of over-watering and decreasing root health and performance. In these cases it is advisable to consider one or two night waterings, one at approximately 10:00 p.m. or one at 2:00 am or both. Usually the night watering events are the same length of time as the minimum watering duration applied during the day. Night watering can also help increase the rate of fruit development, but there is an associated risk of fruit splitting if too much water is applied at night. The night waterings should not be continued indefinitely and the decision to use night watering events and to continue with night watering has to be based on the assessed needs of the crop.

The management of the feed solution, and its delivery to the crop has to be relatively flexible to meet the changing needs of the crop. With experience, growers gain more confidence and skill in meeting and anticipating the changing needs of the crop throughout the crop cycle and through periods of fluctuating light levels. The general information presented in this section serves as a starting point and by following the principles of over-drain management, E.C. and pH monitoring and correction, a successful strategy for delivery of water and nutrients can be established.

As with many things there is no one "right" way to apply water and nutrients to the crop. The use of leaching, although ensuring that salt levels do not accumulate to high levels in the root zone, does result in some "waste" of fertilizer solution as runoff. There are strategies that can be employed to minimize the waste associated with leaching. Collection and recirculation of the leachate, with an associated partial sterilization, or biofiltration of the nutrient solution is one approach (Portree 1996, Ng and van der Gulik 1999). The sterilization or biofiltration steps are required in order to minimize the disease risk associated with recycling nutrient solutions. Some estimates place the fertilizer cost savings at between 30 to 40% when recirculation is used (Portree 1996). In addition to being economical, recycling nutrient solutions is an environmentally sound practice (Zekki et al 1996).

There is a limit to how long nutrient solutions can be recirculated, prolonged recycling of the same solution can negatively affect growth and yield. This is primarily associated with the accumulation of sulfate ions in the solution (Zekki et al 1996). In addition to sulfates, chlorides and bicarbonates also have a tendency to accumulate and can influence crop growth (Zekki et al 1996). The progressive accumulation of sulfates in recirculating solutions require occasional "refreshing" of the solution whereby the solution would have to be allowed to leave the greenhouse as waste (Zekki et al 1996).

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