Irrigating Greenhouse Crops
Greenhouse Irrigation Systems
The irrigation system you select will depend a great deal on the range of plants the grower intends to produce, the size and complexity of the operation, the quality of the water, and how much water is available and when it is available. Small greenhouses and those growing a variety of species may prefer hand-watering for irrigation, whereas large greenhouse operations growing only a few species may use sophisticated semi-automatic systems that can be programmed to water an entire range of greenhouse crops, such as with overhead booms, micro-sprinklers, or flood floor irrigation. The three types of irrigation systems used in greenhouse production systems are overhead systems (which apply irrigation solution to the substrate surface), subsurface systems (which apply water to the base of the root zone and relies on capillary action to bring water into the root zone from below), and hydroponics systems that require growing plants in water.
Overhead Irrigation Systems
The most common form of irrigation found in greenhouse operations is overhead irrigation by applying water to the media surface by hand watering with a hose, micro-irrigation, overhead sprinklers, or booms. In overhead irrigation systems, the nutrient solution is applied to the upper surface of the substrate in a bench or pot, and any excess solution applied can drain from the bottom of the container out to the environment. Overhead irrigation systems, also referred to as open irrigation systems, can prevent fertilizer salt accumulation in the medium and can be installed with relatively little expense.
Subsurface Irrigation Systems
Subirrigation systems bring irrigation solution into the root zone from below (Figure 16.4). In subirrigation, a water-containing structure is flooded until the water level contacts the medium. Once contact is made, capillary action (the attraction of water molecules for one another and other surfaces) moves water up through the medium and throughout the container. Growing medium pore space and medium type (e.g., sphagnum peat) are the primary factors dictating saturation height and speed. Subirrigation is widely used in the European greenhouse industry. In the United States, however, many greenhouse growers have indicated that high initial investment costs and a lack of technical production information impede the adoption of this technology. Despite relatively high initial investment costs, subirrigation systems are attractive to growers because they work well for practically all potted crops and for most bedding plants. Watering is highly uniform, labor for watering is minimal, and flood floors (and benches, to a slightly lesser degree) allow great flexibility in crop spacing, choice of containers, and efficient use of space. Foliage does not get wet, so there are fewer problems with the spread of foliar diseases, reducing fungicide use. Pots of different sizes can be grown in the same watering zone since capillary action naturally regulates the uptake. Full recirculation of the irrigation water means zero runoff, and growers are under increasing pressure to reduce or eliminate irrigation water discharge.
Advantages and Disadvantages
The most commonly cited advantage is the savings in labor needed for watering the plants—a single person can water thousands of plants by operating the flooding system manually or with the help of a computer. Watering is highly uniform; labor for watering is minimal; and flood floors (and benches, to a slightly lesser degree) allow great flexibility in crop spacing, choice of containers, and efficient use of space. Many growers report more uniform plant growth and less foliar disease with subirrigation. The increase in plant uniformity may be the result of more even and complete moistening of the growth medium and better distribution of nutrients absorbed by capillary flow.
Media
The physical characteristics of media used with subirrigation systems are important. Very light and open mixes do not work well because the reduced capillary pores do not move water to the upper regions of the media very well.
Water Storage Tanks
Irrigation water in subirrigation systems is most effectively stored in closed storage tanks. This provides a means to better control water quality as it protects water from outside contaminants in addition to minimizing loss of water due to evaporation. Filling a group of benches or a floor requires that large amounts of water be supplied in a short time, commonly ranging from 50 gallons (189L) per minute to 1,500 gallons (5,678L) per minute. Most plants take in 5 to 25 percent of the water delivered to the zone. The remaining water drains back to the storage tanks to be filtered and recycled to another zone. Tanks can be installed above or below grade, with below-grade tanks usually having a higher installation cost but having several advantages.
Irrigation Scheduling
To maximize performance in a subirrigation system, the medium should be allowed to dry out to the point just before stress to the plants occurs. This will vary greatly depending on the type of media used and its moisture-holding capabilities. Many growers irrigate by the Growers Eye approach.
Types of Containers
Pots with holes that extend up the sides work well. Pots with holes on the bottom work best only if there is a ridge on the bottom of the pot to elevate it off the bench floor.
Closed Irrigation Systems
In a closed irrigation system (e.g., capillary mat, ebb-and-flow, flooded floor), the nutrient solution is recirculated. Nutrients are not allowed to leach into the ground. Water is pumped from a storage tank and delivered to the plant root zone; when the irrigation cycle is completed, the water returns to the reservoir tank. Typically, water is held in the until the medium is brought to field capacity; however, a range (that is, low to high cost) of equipment is available to facilitate and fine-tune this process.
Hydroponics Systems
Hydroponic systems refer to both true water culture systems as well as production in soilless substrates using relatively inert growing media. The benefits of hydroponics include precise control over root-zone conditions, such as pH, electrical conductivity (EC, a measure of fertilizer strength), and, in some cases, temperature and dissolved oxygen. This precise control can allow for optimizing plant performance, as well as the ability to grow food crops where field soil conditions are poor. While in-ground production in greenhouses is possible, it can become increasingly difficult over time due to the development of root diseases or the accumulation of fertilizer salts. The three most common hydroponic systems in use today are NFT, the floating raft system, and the aeroponic system.
Advantages and Disadvantages
Some of the benefits of hydroponic systems include more efficient use of water in the greenhouse. Plants require enough water to grow well, but conventional methods cause excessive waste. When compared to other growing methods, hydroponic systems are considered more water-efficient since water can be reused. They use 10 times less water than normal soil-based systems. Hydroponic systems help you grow your fresh fruits and vegetables without taking up as much space.
Crops Suitable for Hydroponics Systems
There are a number of different crops that can be produced in greenhouses and other controlled environments. However, in terms of production systems, for simplicity, they can be grouped into three main categories: leafy crops (lettuce, greens, and culinary herbs), vine crops (tomatoes, peppers, cucumbers, and eggplants), and small fruit crops (strawberries and brambles).
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