I. Greenhouse vegetable farming:
Greenhouse vegetable farming is a highly controlled form of agriculture based on the production of vegetables under a transparent cover material all year round by creating environmental conditions favorable for plants growth.
II. Greenhouse sustainability:
Greenhouse production is augmenting in response to the population increase, water shortage, climate change and consumer demands for high quality products all the year.
The world population is continuing to grow and our resources are slowly but surely being depleted. In the future we will need double the amount of food that we need now, but it will have to be produced with half of the resources we use now, so we have to be able to do more using less and in a better way.
Vegetables produced in greenhouse are grown sustainably with the greatest care and attention. The grower has the ability to control different factors such as temperature, ventilation, relative humidity, energy and water so that nothing is wasted.
Nowadays crop protection products are hardly ever used in greenhouses. Harmful insects are controlled by releasing predatory mites or predatory wasps to the crops.
All of these sustainable solutions ensure that plants can grow as well as possible with the least possible side effects. More can be produced using the fewer possible resources in such way that the quality of vegetables is improved.
III. Advantages and disadvantages of greenhouse vegetable farming:
• Sustainable crop production
• Controlled environment for plant production
• Reduces pesticides and other chemicals use
• Protects plants from adverse environmental conditions
• Protects plants from biotic and abiotic adversity
• Conserves water
• Reduces labor
• Facilitates harvesting
• Allows production of crops all year round
• Increases the yield
• Increases the quality
• High installation cost
• High maintenance cost
• High technical knowledge
• Requires regular inspection
IV. Principle of greenhouse effect:
The greenhouse effect occurs when the solar radiation particularly the visible light enters the greenhouse through the cover material. Part of this solar radiation is reflected back outside the greenhouse and part of it is absorbed by plants and soil. As a result the temperature inside the greenhouse rises.
V. Greenhouse shapes:
• Even span greenhouse: a greenhouse where both sides of the roof are of equal length
• Uneven span greenhouse: a greenhouse where both sides of the roof are of unequal length, one side of the roof is longer than the other side. The longer side faces south, exploiting the suns heat to the maximum
• Lean-to greenhouse: a greenhouse attached to a building or other greenhouses. The roof slopes to one direction facing south
• Quonset greenhouse (tunnel): a semicircular greenhouse that lacks of height near the side walls
• Gothic arch greenhouse: a greenhouse that has a pointed arch frame resting on side walls
• Round arch greenhouse: a greenhouse that has a rounded arc frame resting on side walls
• Ridge and furrow greenhouse: a series of even span greenhouses connected together
• Sawtooth greenhouse: a greenhouse where multiple lean-to greenhouses are connected together
VI. Greenhouse structure:
1) Frame materials:
Frame materials must be strong with a light weight and must allow even distribution of light to the plants.
• Wood: is easy to construct, has a low installation cost, but it has a limited life duration and it requires high maintenance
• Steel: is durable, resistant to loads, has a low cost but may rust due to oxidation
• Aluminum: is resistant to loads, has a long life duration, does not rust but it is expensive
2) Covering materials:
There are many types of covering materials such as glass, plastic films (polyethylene film, polyvinyl chloride film, ethylvinyl acetate film, polyvinyl fluoride film, polyester film, and ethylene tetrafluoroethylene film) and rigid plastic (rigid polyvinyl chloride, acrylic sheets, methyl polymetracrycil, fiberglass reinforced plastic panels and polycarbonate sheets). A good cover material should be strong, light in weight, transparent with good transmissivity, resistant to heat, to UV, to scratch, to hailstone damage and to weather damage.
VII. Greenhouse location:
• Climate: optimally the greenhouse should be constructed in an area characterized by a high light intensity, a mild temperature in winter, and a moderate atmospheric humidity
• Topography: the greenhouse should be constructed on an area with a gentle slope, this will allow water to drain systematically without any mechanical method
• Soil type: the greenhouse should be installed on a well-drained soil, rich in nutrient and free from insects and diseases
• Wind direction: the greenhouse should be constructed on a site protected from strong winds
• Orientation: the greenhouse should be oriented either toward the east, the southwest or the west
• Water: the greenhouse should be located near a water source, water should be always available
• Accessibility: the greenhouse should be located in a place that is in proximity to markets for easy transportation
VIII. Greenhouse soil culture:
1) Seedling production facilities:
a) Germination chamber:
It is a chamber used to facilitate the seed germination process by providing optimal environmental conditions. It is equipped with a heating/cooling system for temperature control and a misting/fogging system for humidification. Air circulation is important because it ensures a uniform temperature and humidity in the chamber. Once the germination process has finished, the seedlings should be removed outside the germination chamber.
b) Tray support system:
Trays should be placed on benches to avoid the direct contact between the roots and the soil, eliminating the risk of disease transmission to the plants.
Stages of filling and sowing trays:
• Tray cleaning
• Tray filling with growing medium
• Removing the surplus of growing medium by hand or by brush
• Pressing to firm the growing medium
• Sowing the seeds
• Covering the seeds with the growing medium
• Again removing the surplus of growing medium by hand or by brush
2) Factors affecting seeds germination:
a) Seed quality:
Good quality seeds play an important role in increasing production, as they provide vigorous growth, good germination and rapid emergence. The seeds used should be viable, certified, and free from diseases.
Components of seed quality are:
• Genetic quality: cultivar purity and longevity
• Physical quality: analytical purity, moisture content, size and appearance
• Physiological quality: germination capacity, viability, vigor, vitality and dormancy
• Pathological quality: health
The quantity of light required during germination process depends on seeds type. Some seeds require high light level, some seeds are indifferent to light and other seeds require low light level.
Water is essential for seed germination and it must be uniformly distributed. It is important never to let the surface of the substrate dry out until the germination is completed.
Temperature should be stable and regularly monitored for good seed germination.
Humidity is helpful during the germination process.
3) Growing medium:
A growing medium is a porous material that provides functions essential for plant growth:
• Physical support to the plants
• Water, oxygen and nutrient to the roots
The growing medium should have the following desired characteristics:
• Good water holding capacity
• Good air capacity
• Stable structure
• Slow decomposition
• Pathogens and weed seeds free
• Easy handling
• Low cost
a) Physical properties of a growing medium:
Plants need macropores and micropores for their growth. A good growing medium is characterized by the total porosity which is a combination of different particle sizes that gives good water holding capacity as well as good air capacity.
Bulk density is defined as the dry mass of the substrate per unit of substrate volume in a dry state. Excessive bulk density indicates compaction. Bulk density and porosity are indirectly related, porosity decreases when bulk density increases.
Oxygen is necessary for good healthy roots. It is supplied through the larger macropores, which allow the dissipation of carbon dioxide during respiration. A good growing medium is characterized by a high percentage of macropores.
Water holding capacity:
Water holding capacity is the maximum water content that can be retained by a particular containerized substrate which is allowed to drain after saturation.
b) Chemical properties of a growing medium:
The pH indicates the acidity or alkalinity of a solution and it is measured by means of a pH meter which is a portable instruments. The pH also plays a role in determining the nutrient availability for plants. pH in the root zone should be between 5.5-6.5.
The growing medium should contain all essential nutrients, macronutrients (nitrogen, phosphorus, sulfur, potassium, calcium and magnesium) and micronutrients (manganese, zinc, copper, boron and molybdenum).
Cation exchange capacity:
CEC is the ability of growing medium to exchange cation for charged nutrient ions that help in growth and development.
c) Types of growing medium:
• Growing medium with organic ingredients: peat moss, compost, coconut noir, rice hulls, sawdust and bark
• Growing medium with inorganic ingredients: vermiculite, perlite, pumice and cinder, and sand
Fertilization begins when the set of true leaves appears. Fertilization plays an important role in increasing crop production. Fertilizers found in solid or liquid forms are added to provide plants with macronutrients and micronutrients essential for their growth. Nitrogen, phosphorus and potassium are the most essential nutrients that the plants may need. Nitrogen helps leaf of young plants to grow. Phosphorus and potassium promote fruiting and flowering, and guarantee robust stem growth and water movement. Other nutrients may also be added depending on plants nutrient requirement.
Seedlings should be hardened prior to transplanting. Hardening is to slowly acclimate seedlings to the new environment they will be grown in by reducing water supply, reducing fertilization, reducing temperature and by gradually exposing seedlings to outside climate condition (light, moderate wind, etc).
Care is needed when extracting the seedlings from the trays to minimize root disturbance. Seedlings can be transplanted to soil either by hand or by machine. The soil should be disinfected in order to be free from pests, pathogens and weeds. Soil solarization is a nonchemical soil disinfection method that is based on trapping solar radiation by covering the wetted soil with a transparent polyethylene sheet for a month or more. Soil preparation begins by ploughing or turning the soil and smoothing its surface. The soil is then watered, a wet soil has the ability to conduct heat making soil organisms vulnerable. Then a plastic sheet is laid on the soil to be treated. After the soil solarization is completed, the plastic sheet is removed.
IX. Greenhouse soilless culture:
1) Soilless culture definition:
Soilless culture is the technique of growing plants using mineral nutrient solution, either in a substrate that provide mechanical support to the plant roots or directly in an aquatic based environment.
2) Closed and open soilless system:
a) Closed system:
In a closed system the drained nutrient solution released from the root environment after its supply to the crop is collected, replenished with nutrients and water and reused.
b) Open system:
In an open system the drained nutrient solution released from the root environment after its supply to the crop is not reused nor recirculated but it is thrown out of the system, thus an adequate run- off must be maintained in order to keep nutrient balance in the root zone.
3) Water culture:
a) Deep water culture:
Plant roots are suspended directly into the nutrient solution. The system consists of a bucket covered with a thin layer of sand placed on a net and cloth, to support the plants. The main drawback of the system is that due to the limited air-water exchange area, hypoxic conditions may occur at the root level. This problem can be solved by using air pumps to oxygenate the nutrient solution.
b) Floating system:
Plants are grown on a raft of expanded plastic floating in tank filled with nutrient solution. The tank is covered with a polyethylene film that helps to hold the nutrient solution. A Styrofoam sheet is added to create a floating platform. The nutrient solution is oxygenated by the use of pumps that play a role in driving part of the solution into a pipe connected to a Venturi tube to insufflate air. To avoid roots damage, the airflow should not be very strong.
c) Nutrient film technique:
Plant roots lie in channels that contains a very thin layer of nutrient solution. Channels are installed on a slope that is provided by the use of adjustable benches so the required elevation can be obtained. The nutrient solution is applied at the higher end and flows down through the channels to keep the roots totally wet, then it is drained to a large pipe located at the lower end of the channels to return the solution to the cistern and to recirculate it. Good aeration of the roots is provided by the thin layer of the nutrient solution, as the roots are continuously exposed to the air especially on their upper surface.
d) Deep flow technique:
Plants are grown on a polystyrene trays that float in channels containing nutrient solution with 50- 150 mm depth.
Plants are grown on trays and their roots are suspended in air. The nutrient solution is constantly sprayed or fogged on the fully exposed roots of the plants.
4) Substrate culture:
a) Chemically active substrate:
Plants are grown in pots, containers, channels or bags filled with chemically active substrate. There are 2 types of chemically active substrate: inorganic substrate (zeolite, vermiculite, and tuff) and organic substrate (peat, coir, sawdust).
b) Chemically inactive substrate:
Plants are grown in pots, containers, channels or bags filled with chemically inactive substrate (sand, rockwool, perlite, pumice, expanded clay, etc).
Fertigation is to obtain an irrigation solution that results from the combination of fertilization and irrigation. The irrigation solution also called nutrient solution is the result of the dilution of a stock solution to a lower concentration and then injected into the irrigation water. Fertigation system requires pressure regulators, filters, tanks for stock and for acid solution, fertilizer injection devices, pH and EC measuring tools and a water/solution delivering system.
Three tanks are used to feed cultivated plants. Two of them are used to separate fertilizers that can interact, the possible combination is a tank containing calcium fertilizer and another tank containing phosphate and sulphate fertilizers to avoid precipitation of calcium phosphate or calcium sulphate, and the third one contains an inorganic acid used to control pH of the nutrient solution, to wash the irrigation system and to avoid clogging of the nutrient solution emitters.
6) Nutrient solution:
A nutrient solution is a diluted water solution containing all essential nutrients, macronutrients (nitrogen, phosphorus, sulfur, potassium, calcium and magnesium) and micronutrients (manganese, zinc, copper, boron and molybdenum), in form of inorganic ions or soluble inorganic compounds, with the exception of iron, which is a micronutrient present in form of an organic chelate.
Nutrient solution characteristics:
• Electrical conductivity
• Macronutrient concentrations
• Micronutrient concentrations
7) Monitoring and adjusting the pH of nutrient solution in the root zone:
It is very important to monitor and to adjust the pH of the nutrient solution regularly because a too high or a too low pH has a negative effect on plants, it prevents them from getting minerals that they need for their growth. If the pH is too high there is phosphorus, iron, manganese, copper and zinc deficiency and if the pH is too low there is calcium, magnesium and potassium deficiency. A solution with a pH ranging between 5.5-5.8 is delivered to the crop to maintain the pH in the root zone between 5.5-6.5. The pH of the nutrient solution may change in the root zone due to selective ion uptake by the plants. If anion uptake exceeds cation uptake, HCO3- and/or OH- are excreted in the root zone thus the pH in the external solution increases. An acid is added (nitric acid, phosphoric acid, sulphuric acid, citric acid or acetic acid) to reduce the pH of the nutrient solution. If cation uptake exceeds anion uptake, H+ is excreted in the root zone thus the pH in the external solution decreases. A base KOH, KHCO3 or K2CO3 may be added to increase the pH of the nutrient solution.
8) Monitoring and adjusting the EC of nutrient solution in the root zone:
The EC indicates the total salt concentration in the nutrient solution and it is measured by means of an EC meter which is a portable instruments. It is very important to monitor and to adjust the EC of the nutrient solution regularly. For an accurate EC measuring, standard solutions should be used to calibrate the EC meter. The portable meter should be stored in a cool dry place not in the greenhouse. An increase of the EC in the root zone is a frequent problem that growers can face and it may cause an excessive top growth or may cause damage to the root tips.
A high EC can be controlled by:
• Using a good quality water containing low amount of NaCl, Ca and Mg
• Balancing the composition of the supplied nutrient solution (EC, nutrient ratios)
• Adjusting the target EC in the nutrient solution supplied to the crop by taking into consideration the EC of the drainage solution and its composition
• Increasing the irrigation frequency
• Using a correct irrigation scheduling
• Using rain water instead of irrigation water to wash out salts from substrates
9) Nutrient solution recycling in closed system:
The recycling of nutrient solution saves and conserves water but it may result in recycling of pathogens (Pythium sp., Phytophtora sp., Fusarium sp., Verticilium sp., etc) that may infect plants via the recycled nutrient solution and in the long term it may result in accumulation of Na and Cl which may cause a problem if the concentration of these ions in the solution supplied to the crop is not very low.
10) Nutrient solution disinfection:
The disinfection of the drainage solution in closed system can be done through several methods:
• Pasteurization using heat treatment
• UV radiation (ultraviolet radiation)
• Slow sand filtration
• Micro-membrane filtration
• Active hydrogen peroxide
• Ozonation (O3)
• Chlorine application
X. Greenhouse irrigation systems:
1) Irrigation delivery systems:
a) Overhead systems:
Water or nutrient solution is applied directly to the shoot, above and around the foliage of the plants.
Different overhead systems that can be used:
• Sprinkler Systems
• Mist and Fog Systems
• Boom Systems
• Low installation cost
• Applicable in large areas
• Have a cooling effect
• Water is wasted due to unused runoff
• Risk of diseases
• Risk of residue on leaves and flowers
• Inefficient water use
• The surrounding area of the plant is also wetted
b) Surface systems:
Water or nutrient solution is delivered slowly to the roots either on the substrate surface or directly to the root zone.
Different surface systems that can be used:
• Hand watering
• Drip irrigation
• Each plant is irrigated individually
• Efficient water use
• Less runoff
• Less evaporation
• Emitter clogging
• Difficulty in evaluating system operation and application uniformity
• Substrate/application rate interaction
• Maintenance requirement
• Smaller wetting pattern
2) Irrigation decision:
Irrigation decision is based on the irrigation frequency and the duration of the irrigation event. Irrigation frequency is linked to the water use of the plant and to availability of the water pressure in the supply lines. Duration of irrigation event is linked to the amount of water that is needed to be supplied to the plant and to the amount of leachate that is needed.
XI. Greenhouse cooling systems:
• Natural ventilation through roof and sides openings, the cool air enters from the side openings and the warm air exits from the roof openings
• Forced ventilation through fan and pad evaporative cooling system. The system is composed of a wet pad, ventilators, water tanks and pumps. The air enters through the pads and exits through the fans.
• Shading: through white washing or screen
XII. Greenhouse heating systems:
• Hot water heating system
• Steam heating system
• Electrical heating system
• Solar radiation heating system