Irrigating Greenhouse Crops
Greenhouse Irrigation Systems
Boom Irrigation Systems
Alternatives to fixed micro-irrigation systems are mobile watering booms (Figure 16.2). A boom irrigation system consists of a pipe that delivers water to nozzles attached along its length. The spray bar usually spans the entire width of the greenhouse, from column post to column post in gutter-connected ranges or from sidewall to sidewall in freestanding structures. It can irrigate the entire width or just portions to match the bench layout or aisle configuration. In some cases, the nozzles generate a spray similar to a sprinkler system, while in other cases, water is allowed to flow in a stream to plants below. In the latter case, the plants need to be positioned very precisely under the path of the boom, and the substrate needs to have adequate bulk density and consistency so that the water stream does not wash it out of the pot or knock plants over. The water boom can be turned on and off automatically, as well as the quantity of water delivered as it passes over different blocks of plants on its way down the bay.
Advantages and Disadvantages
Boom irrigation systems allow for uniform overhead irrigation and customized delivery to each plant. Less water is needed because the system can be operated to provide the optimum amount of water for the crop. The use of boom irrigation systems also improves the efficiency of space use in the greenhouse.
Crop Use
With a selection of spray nozzles and speeds, these systems can water a wide range of plants. Boom sprayers are most often used for the production of plugs or seedlings where good water and fertilizer control is vital for efficient production.
Timing of Irrigations
Most growers water when the crop approaches stress, applying a sufficient quantity of water or fertilizer to cause 10 to 15 percent of the applied fluid to pass out the bottom of the trays. This is an open system. A few growers practice what is commonly called zero-leach watering, which is considered a closed-irrigation system. They water much more frequently during the day than the former group.
Boom Configuration
The boom is attached at its center point to a carriage that rides along rails, often suspended from a single or double overhead rail system attached to the trusses or greenhouse frame. As the name implies, single-rail booms run on a single rail centered in the bay. They're low-cost and are an excellent irrigation solution for bays from 21 to 28 feet (6.4–8.5m) wide and up to 200 feet (61m) long. One disadvantage is that they are not very fast and can't support watering over more than one row of crops, which usually extends the full width of the bay. Double-rail booms run on two rails mounted about 3 feet (1m) apart in the center of the bay and are often used in greenhouses. They are ideal for bays from 21 to 42 feet (6.4–12.8m) wide and up to 300 feet (91m).
Nozzles
By using multiple nozzles, these systems can be used for not only irrigation but also applying liquid fertilizers and fungicides by selecting the nozzle that provides the prescribed application rate and droplet size. A range of nozzles is available, and these broadly fall into three categories: hollow cone, solid cone, and fan nozzles. Some systems are available with more than one boom. These can be fitted with nozzles with different application rates for misting, feeding, or pest control.
Methods of Boom Control
Several methods are available to control a boom irrigation system. Control of such systems can be by manual operation, especially if intervention is needed to start and stop the irrigation. Alternatively, a time clock or cycle timer can be used that activates the drive unit at predetermined times. In more sophisticated systems, they can be programmed to make repeated passes over the same area or to shut on and off while passing over different areas in their path.
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