Optimizing greenhouse lighting to increase crop yield

Light is one of the most important factors influencing the rate of plant growth and the size of the harvest, and it is the energy source for the process of photosynthesis. However, it can be not only a benefit but also a detriment. Everything depends on the intensity and duration of lighting (day length), as well as the angle of incidence of the rays.

Different crops require different amounts of light. Depending on this, all plants are generally divided into light-loving and those not demanding of lighting. The first group includes such vegetable crops as bell peppers, lettuce, cucumbers, tomatoes, etc. They must be kept in conditions of continuous lighting for at least 10 hours a day. Representatives of the second group are dill, parsley, onions, etc.

Orient the structure correctly relative to the cardinal directions. During the season, wash the glass one or two times and clean the film of contaminants. Position the plants so that, if possible, opaque structural elements do not shade them. Inside the greenhouse, paint everything possible with white paint.

Good light transmission is a feature of polyethylene film and polyester covering reinforced with fiberglass. The transparency of glass depends on its quality (presence of foreign inclusions), the degree of contamination—which is greater the higher the dust content of the air—and the duration of the glass's use.

With average dust levels, the amount of light passing into the greenhouse decreases by 15—20% after one month of operation, and by 40% after two. Yet a decrease in light intensity of 1% reduces the harvest by 1% as well. This is why in the autumn and very early spring, the glass of greenhouses needs to be washed.

In order to ensure normal conditions for crops grown in protected ground, natural lighting can be supplemented with artificial lighting. But as is known, a fairly high level of humidity is noted inside a hotbed or greenhouse. Therefore, when installing electrical wiring, observe maximum caution.

An outdoor cable installed in a protected ground structure should be secured inside the premises and then led outside. For a lean-to greenhouse, the cable is connected to the central electrical wiring. When led outside, the cable is buried in the soil or secured to wooden poles. In the first case, it will be necessary to dig a trench at least 70—80 cm deep.

In order to prevent accidental damage to the cable, for example during construction work on the site, it is recommended to place slats or tiles over the wire. The laid cable should pass in areas on the plot adjacent to the hotbed or greenhouse that are removed from drainage ditches.

The cable attached to wooden poles is run through the air and suspended from high-thickness wire secured between individual supports. It is not recommended to run the cable near trees, as branches can damage the cable insulation during windy weather.

In the interior greenhouse space or hotbed, it is necessary to place an electrical panel for controlling the operation of the power supply system and devices.

The panel contains sockets and fuses with individual switches. You can use the sockets after the cable connection work is completed. To light the structure, it is best to use devices equipped with fuses and rubber plugs.

In vegetable growing, several types of lighting lamps are used: fluorescent, mercury, and sodium. Fluorescent lamps provide white and pink light. Mercury lamps are generally more powerful. Gold-orange light is provided by sodium lamps—the best of those listed; their use shortens the period for obtaining tomato or cucumber transplants by 10—14 days.

Fluorescent lamps are also used to set up artificial lighting inside the greenhouse space and hotbed. Their main advantage is that they do not heat up during operation and, therefore, are not capable of changing the air temperature or drying it out. Lamps of this type can be placed in close proximity to plants without harming them. Among all varieties of fluorescent lamps, LDTs and LD type lamps are most often used for lighting hotbeds and greenhouses.

Various methods of artificial lighting can be used for rapid seed germination and obtaining transplants. After all, the daylight hours in early spring are short, and additional lighting is simply necessary for normal plant growth. It is best to construct frames equipped with several fluorescent lamps. They are placed 1—1.2 m above the shelves with trays in which the seeds have been sown.

To create additional lighting inside a greenhouse or hotbed, light-reflecting screens are often installed. They are positioned so that their front side faces the window. As such reflectors, one can use a mirror or a sheet of galvanized iron.

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One of the most important conditions for obtaining a good harvest is maintaining an optimal temperature for the plants. The colder it is outside, the harder it is to maintain the required temperature in protected ground structures. Greenhouses, hotbeds and film covers are made of materials that have high thermal conductivity, so it is quite difficult to keep the necessary temperature in them during cold snaps.

It should be remembered that in a greenhouse with good lighting, the temperature should be higher than in those periods when there is little light (for example, at night).

The influence of air and soil temperature on plants

Maintaining the correct temperature balance in a greenhouse directly affects yield and product quality. Optimal air temperature for most vegetable crops is 16–25 °C during the day, and at night it should be reduced by 4–8 °C. If this rule is neglected, the plants will begin to uncontrollably grow green mass to the detriment of fruiting.

Soil temperature is critically important for the functioning of the root system. At values that are too low (6–10 °C, and for cucurbits — below 12 °C) or excessively high (20–28 °C, for cucurbits — above 30 °C), the roots stop absorbing water. This leads to physiological drought and wilting of plantings even with moist soil.

The uptake of nutrients also depends on soil warming. If the soil temperature drops below 10 °C, the absorption of nutrients, especially phosphorus, is blocked. As a result, signs of phosphorus starvation appear on the plants, even if fertilizers have been applied in sufficient quantities.

• Optimal daytime air temperature — 16–25 °C
• Nighttime air temperature reduction — by 4–8 °C
• Optimal soil temperature — 14–26 °C
• Minimum soil temperature for cucurbits — 12 °C
• Water temperature for emergency soil warming — 25–30 °C

For rapid development of the root system, the soil temperature, unlike the air temperature, must remain stable around the clock. The optimal range for substrate warming is from 14 to 26 °C. To maintain these indicators, agronomists use biological fuel (self-heating substrates) or drench the soil with warm water at a temperature of 25–30 °C.

Greenhouse heating methods and planting dates

Heating systems help protect heat-loving crops from spring frosts and sudden temperature changes. In practical vegetable growing, three main methods are used: natural (solar), technical, and biological. The simplest of them is natural heating, which works due to the greenhouse effect, when sunlight penetrates through the film or glass and heats the internal space.

To improve the thermal insulation of a film greenhouse, lay the polyethylene film in two layers with an air gap of up to 5 cm. Double covering reduces the level of natural lighting, so artificial supplementary lighting must be provided inside the structure. Such an approach allows for a significant shift in the timing of vegetable cultivation.

• Cold-hardy crops: from the second week of April to the last week of September.
• Heat-loving crops: from the second week of May to mid-September.

Among technical means of heating, water, gas, furnace, and electric systems are the most common. With water heating, it is better to move the boiler to the greenhouse vestibule, using firewood, coal, peat, or gas for the furnace. Furnace heating is installed according to the classic scheme: a firebox, a long horizontal flue along the greenhouse, and a vertical exhaust pipe.

Biological heating and natural ventilation

For greenhouse heating, electrical energy is rarely used. Most often, electric air heaters, tubular heaters, and portable fan heaters serve as a supplement to the main heating or act as emergency heating during the spring period. The main source of heat in hotbeds and greenhouses can be biological heating.

It is based on the natural decomposition of organic matter, which releases heat and saturates the air with carbon dioxide necessary for plants. As biofuel, they use manure (the most popular option), as well as cattle, pig, and sheep manure, compost, household waste, wood bark, and sawdust.

• Temperature of biofuel during decomposition — 60–70 °C
• Area limit for intake vents — 20% of the greenhouse area
• Spacing of vents in large greenhouses — every 2 m
• Critical time for ventilation failure in hot weather — 1–2 hours

In addition to heat, crops require a constant supply of fresh air. Due to the greenhouse effect, the temperature inside protected ground rises sharply, which suppresses plants. In an unventilated room, pests and pathogens also spread quickly. Regular ventilation solves this problem by maintaining optimal humidity and temperature regime.

Fresh air intake is organized through vents in walls, the roof, and doorways. The total area of opening sections should not exceed 20% of the entire structure area. In small greenhouses, vents are placed on the sides of the roof, one on each side. In large structures, they are installed at 2-meter intervals or in a continuous row on both sides of the roof.

Ventilation also performs the function of hardening transplants, increasing their resistance to diseases and temperature fluctuations. Before transplanting plants into open ground, it is recommended to keep ventilation transoms open around the clock. At the same time, it is important to avoid drafts, icy air, and strong winds, which can destroy young plantings.

Ventilation automation: choosing equipment

In industrial greenhouse complexes, microclimate parameters are controlled by automation: sensors track temperature, humidity, and light levels, while a computer manages fans, transoms, and irrigation. For small farms and garden plots, such equipment is too expensive. Here, simpler autonomous devices for automatic vent opening are used.

Such automatic devices optimize the temperature regime and are divided into two main types:

• Electric automatic devices. Consist of a fan and a thermal relay. They are distinguished by high sensitivity, unlimited power, and convenient adjustment.
• Bimetallic automatic devices. Consist of connected plates of two materials with different thermal expansion. Upon heating, the plate bends and opens the transom; upon cooling, it straightens.

Both options have their operational limitations. Bimetallic devices are cheap and autonomous but have low power: they cannot cope with a heavy, warped, or moisture-swollen frame. Electric systems are more powerful and precise but completely depend on a stable power supply.

Hydraulic. Based on the property of liquids to expand when heated. They represent a sealed system filled with low-boiling (freons) or other liquids with a high coefficient of thermal expansion: water, oil, diesel fuel, etc. The system contains a corrugated brass tube (bellows), which changes length when filled with liquid, or a hydraulic cylinder with an extendable rod.

• Advantage — autonomy and undoubted high system reliability due to the simplicity of the design, as well as very high power and durability.
• Disadvantage — relative high cost due to the use of complex production technologies, which is fully compensated by high benefits.

A hydraulic automatic device can be designed independently. The device shown in Figure 30 has a weight of about 3 kg and a rod force of about 100 kg. Such an apparatus can open a glass frame with an area of about 2 m? or a film frame with an area of about 5 m?, i.e., ventilate a greenhouse with an area of 10-20 m?. In larger greenhouses, several such automated ones can be made

Fig. 30. Hydraulic automatic device for greenhouse ventilation

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Fresh vegetables contain 80—90% water. Almost all vegetables grown under glass and film are moisture-loving. A plant's water consumption depends on its species, cultivar, developmental stage, light intensity, soil and air temperature, soil salinity, soil and air humidity, and the state of the plant.

In the early stages of development, a plant requires much less moisture than during the period of intensive formation of the fruit part (head of lettuce, cucumber fruits, etc.). During the day, as the temperature increases, the plant "drinks" more.

Excessive nutrient content in the soil leads to its salinization, which hinders water uptake. In saline soil, a plant wilts and dies. However, a deficiency in nutrients, especially potassium, also leads to a disturbance in water balance.

The lower the air humidity under glass or film, the faster water evaporates from the plant surface, and consequently, the more water the plant needs to absorb. A vegetable grower must monitor the humidity in the greenhouse or under cover: if air humidity drops sharply, the plant may experience a water deficit even if there is sufficient soil moisture in the ground.

In soil that is too dry, a plant wilts; in soil that is too wet, its roots rot. When moistening the soil, one must strike a happy medium. Soil moisture is of great importance for the progress of microbiological processes in the soil, the decomposition of organic matter, and its conversion into forms easily assimilable by plants.

Since protected ground structures essentially protect plants from precipitation, all the plants' water requirements must be met through artificial irrigation. It would seem that nothing could be simpler than watering plants grown under cover. As it turns out, this also requires certain skills. Watering crops planted in pots should be done with particular care.

Watering potted crops must be performed in such a way that the water reaches the root system. To do this, the pot should be filled with water to the brim. The frequency of irrigation can be determined by several criteria:

• temperature regime inside the room;
• dormancy period or active growth period of the crop;
• soil type;
• vessel volume.

Watering of plants characterized by rapid growth and highly branched roots should be carried out twice a day. At the same time, the temperature of the water should be taken into account. When watered with water that is too cold, the plant may become diseased and die. In winter, plants should be watered no more than twice a week, and in some cases even less often.

You can tell that a plant needs watering by the following signs: wilting of leaves and the top of the stem; a change in the color of the soil mixture (the saturated color is replaced by a pale one). Plants need watering if the soil has dried to a depth of more than 5 cm.

For watering vegetable and leafy crops cultivated in greenhouses on beds, it is best to use hoses or watering cans with spray nozzles. In this case, complete soil moistening can be achieved. To determine whether the plants are sufficiently moistened, it is recommended to dig a small hole (up to 15 cm deep). If the subsoil layers lying at a depth of up to 15 cm turn out to be dry, then the watering will have to be repeated.

It is worth noting that experienced vegetable growers prefer to water plants grown in greenhouses using special automatic irrigation devices. The most common system is considered to be drip irrigation. What exactly is drip irrigation? It is economical water consumption, preservation and improvement of soil structure, saving the owner's labor and time, savings in root growth, and the absence of water stress in the plant. All this is true with proper drip irrigation. But there are also disadvantages that are rarely talked about, although they negate all the advantages.

The basic principle of drip irrigation is that water is supplied constantly or in small portions directly under each plant. The soil never dries out, and the plant positions its roots in the zone of optimal moisture, "saving" on growing deeply penetrating roots. The water is always warm because it arrives slowly and has time to heat up in the pipes. Plants never experience a moisture deficit, and neither do the soil microflora and fauna. In the United Arab Emirates, this method is used to water everything from petunias in flower beds to palm trees on the beach.

A correctly designed drip irrigation system delivers moisture directly to the plant roots, reducing water consumption and preserving soil structure. For assembling the distribution network, it is better to use opaque material, such as black polyethylene. This will prevent the development of algae inside the hoses, which quickly clog the emitters. Installation is carried out according to a pre-planned planting scheme, taking into account the length of the beds and the plant placement layout.

• System slope — 5 cm per 1 m
• Tube length — no more than 6–8 m
• Tube diameter — 10–15 mm
• Nozzle hole diameter — 1–2 mm
• Height above soil — 10–20 cm
• Mulch layer thickness — 3–5 cm

1. Cut the tubes to the required length and connect them into a single system using fittings.
2. Secure the pipeline to support pegs, maintaining the necessary slope for uniform water distribution.
3. Make holes in the marked places of the tubes and insert nozzles made from plastic parts of medical IV sets into them.
4. Turn on the water at working pressure, check the joints for leaks, and level the water flow rate from the nozzles by changing the inclination of the tubes.
5. Plant the transplants strictly opposite the working emitters, either individually or in small groups.
6. Cover the entire surface of the bed with mulching material immediately after completing the planting work.

Batch irrigation: how to avoid uneven water distribution

In drip irrigation practice, a common problem occurs: with a slow flow rate, water only discharges from the first holes, failing to reach the end of the hose. If the pressure is increased, water consumption rises above the application rate, leading to over-saturation of the root zone. Automated batch dosing using a DIY siphon helps solve this problem.

To create the dispenser, an ordinary 2-liter plastic bottle is used, which is mounted in an inverted position. Water from the storage tank must flow into it very slowly through a capillary tube with a diameter of 1–3 mm. A drain tube with a diameter of 5–10 mm is hermetically installed into the bottle cap and bent into a loop inside the container.

The highest point of the loop bend should be located at the very bottom of the bottle, and the open end of the tube — in the neck, almost touching the cap. As the bottle fills, the water reaches the bend of the loop, after which the siphon effect is triggered. The entire accumulated portion of water is quickly discharged into the irrigation network, distributing along the entire length of the tubes.