Control of greenhouse environment means control of temperature, light, air composition and nature of root media. Precise control of different parameters of greenhouse environment results in better timing of crops, higher quality crops, disease control to maximize economic returns and conservation of energy that optimizes energy inputs.
A. Temperature control: The thermostat can be coupled to water circulating pump or exhaust fan for controlling the temperature inside the greenhouse. Bimetallic strip (differential expansion) or thin metal tube filled with liquid or gas (movement of tube due to change in volume of gas or liquid) acts as sensors and activates a mechanical switch. Generally, temperature may be controlled by following methods:
1.Ventilation
a. Natural convection: A temperature difference is set between greenhouse temperature and ambient temperature and causes natural movement of air through roof vent provided in the roof.
b. Forced convection: If the rate of heating of room temperature becomes higher than the rate of heat removal through roof vents then heat removal is possible only through forced convection in which fan is provided in greenhouse. The rate of heat removal depends on capacity of fan and its rpm.
a. Fan and Pad cooling system Most common summer cooling system in greenhouses. A pad composed of excelsior (wood shreds) or cellulose material is placed vertically along one side of the greenhouse and exhaust fan on opposite side. Warm outside air is drawn in through the pad. Supplied water in the pad by the process of evaporation absorbs heat from the greenhouse and produces cooling effect. Khus-Khus grass mats can also be used as cooling pads
b. Fog cooling: Fog or sprinklers can be used to cool green houses and maintain humidity but it is costlier than pad fan cooling. A high-pressure pumping apparatus generates fog containing water droplets with a mean size of less than 10 microns using suitable nozzles. These droplets are sufficiently small to stay suspended in air while they are evaporating and utilize the heat of greenhouse air. Fog is dispersed throughout the greenhouse, cooling the air everywhere. As the system does not wet foliage there is less scope of disease and pest attack. Both types of summer cooling systems can reduce the greenhouse air temperature well below outside temperature. The fan and pad cooling system can lower the temperature of incoming air by 80% and fog system 100% of difference between dry and wet bulb temperature respectively. Due to bigger droplet size of water in fan and pad system complete evaporation not taking place but fog system will have complete evaporation because of minute size water droplets. A maximum night temperature of 13 to 15.5° C and a day temperature of 24° C are generally set to start the heaters and fans; respectively.
B. Relative humidity control: Humidistat coupled to water circulating pump or exhaust fan to control the RH inside the fan and pad greenhouse. With the evaporative pad cooling system lowering the dry bulb temperature will generally rise the RH by 70-80%. This is usually sufficient for crops such as carnation and chrysanthemum. The RH in Non-ventilated (NV) Greenhouse can be increased by providing foggers.
C. Light intensity control: In certain areas where natural illumination is absent or very low, illumination for plants may be provided by artificial sources. Incandescent bulbs generate excessive heat and are unsatisfactory in most instances. Fluorescent tubes are useful as the sole source of light for African violets, gloxinias and many foliage plants which grow satisfactorily at low light intensities. Excessive light intensity destroys chlorophyll even though the synthesis of this green pigment in many plants is dependent upon light.
D. CO2 control: The present, more sophisticated CO2 generator control systems are based on CO2 sensors. These sensors continually monitor the CO2 level in the greenhouse and a single sensor can be connected to several greenhouses by sampling tubes and air samples drawn by a pump. The signal from the sensor is used to control the CO2 generator so that a constant CO2 level can be maintained. Information from the single sensor with multiple sampling tubes is received by a computer, which in turn controls CO2 generators in each greenhouse.
E. Controlling Light in Greenhouses:
1.Light Quality: In commercial greenhouse production, light quality is important when selecting a light source for supplemental photosynthetic lighting or photoperiod control. A broad emission spectrum within the 400 to 700 nm range is desirable especially when adding light to increase photosynthetic rate. Light sources being used to extend day length and create artificially long days must provide sufficient light in the red range in order to affect the phytochrome photoreceptor. Reducing the far-red light and increasing the blue light experienced by the plant results in shorter, darker-colored and stronger plant. Light quality can also affect the development of certain foliar diseases such as Botrytis. Greenhouse glazings have been developed with additives or pigments that filter certain wavelengths of light and allow for a shift in the relative ratios of wavelengths of light entering the greenhouse.
2.Light Quantity: The Light level might need to be increased or decreased to maintain optimal levels. Different plant species have different optimal light levels. However, for a given species, plant spacing, nutritional level and plant age can affect the optimal light level. For example, the optimal light level for a tomato seedling is lower than that for a well established and actively growing tomato. A range of 32.3 to 86.1 k lux is required by crops like cucurbits, capsicum, brinjal and sweet potato, while cabbage and potato require 21.5 to 86.1klux.
Greenhouse shading methods: Two methods are commonly used to reduce light levels in greenhouses.
Ø The first is the application of a shading compound to the glazing. Retractable
shade systems are being installed in many new greenhouses. These systems are placed in the gables of the greenhouses and are controlled by a computer that is in turn connected to a photometer (Light meter). A desired light level can be programmed into a computer and the shade automatically pulled when light levels exceed the desired level. The shade will automatically be retracted when light levels fall below the desired level.
Stages of evolution of environmental control systems: Stages of evolution of environmental systems are manual controls, thermostats, step controllers microprocessor and computers.
i) Analog control: In this system proportioning thermostats or electric sensors are used to gather temperature information. Analog controls are costlier than thermostats but offer better performance.
ii) Computerized environment control: The amplifiers and logic circuit analogs have now been replaced by computerized environmental system, which involves microprocessor, which gathers information on a variety of sensors like temperature, humidity, light intensity, wind directions etc. to provide more precision. Although more costly than thermostats or analogs but computer controls offer significant energy and labour saving and increases production efficiency in propagation. The deviations from the present levels of temperature and humidity can trigger alarms by the computer.
RAKESH KUMAR PATTNAIK
Asst. Prof. Horticulture
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