(Netherlands), Argus Control Systems (Canada), Heliospectra AB (Sweden), and Scotts Miracle Gro (United States). Leading hydroponic crop producers include AeroFarms (United States), Terra Tech Corp (United States), Triton Foodworks (India), and Emirates Hydroponic Farm (United Arab Emirates). The hydroponic industry is expected to be worth US$22.2 billion by 2028 (Intrado 2021). TYPES OF HYDROPONIC SYSTEMS There are a variety of hydroponic systems, from simple to sophisticated. There are numerous approaches to hydroponics—including aeroponics, fogponics, aquaponics, dryponics, and others. Some of these approaches are open and some are closed, but each follows the same principle: plants grow, without soil, in a circulating nutrient-rich water system (Wootton-Beard 2019). Among the different hydroponic approaches are various systems, including wick, deep water culture, ebb and flow, drip method, NFT, aquaponics, and aeroponics. These systems share many features but fundamentally differ in how they manage the nutrient solution. The most popular systems are deep water culture, drip method, and NFT (Resh 1995). The grower chooses the system depending on the type of plant and the facility’s limitations, whether a lack of growing space or materials (Jensen 1997). Figure 5.1 describes the different systems and provides examples of how each is set up. Hydroponic systems can generally be delineated into open and closed systems. Open systems, also known as “run-to-waste systems,” do not reuse water. The nutrient solution flows through the system only once and is discarded (Jensen 1997; Nederhoff and Stanghellini 2010). Open systems provide two primary advantages over closed systems: (1) they do not require nutrient solution maintenance, and (2) they reduce the risk of acquiring infectious plant pathogens (Jones 2016). Despite these advantages, open systems are known to be wasteful of water and nutrients (Nederhoff and Stanghellini 2010), which may not be appropriate for arid, water-scarce regions. By contrast, closed systems recirculate the nutrient solution for an unspecified length of time (Lykas et al. 2006). These systems add water and nutrients as necessary instead of replacing the entire solution after each use (Jensen 1997; Nederhoff and Stanghellini 2010). The nutrient solution is regularly monitored and adjusted to maintain proper nutrient ratios. As a result, closed hydroponic systems use 20 to 40 percent less water and nutrients than open hydroponic systems, but closed systems require more monitoring and maintenance. This need arises because ions accumulate as the nutrient solution recirculates (Lykas et al. 2006). And recirculation requires reservoirs and pumping systems that must be monitored and maintained. This infrastructure can be susceptible to failure if it is not managed well (Nederhoff and Stanghellini 2010). Wick techniques are open systems that comprise raised garden beds sitting above a water reservoir. This is the most common hydroponic technique and the easiest to set up and maintain (Wootton-Beard 2019). In areas with Understanding Hydroponics
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