<p><span xml:lang="en-us">The production systems found in modern greenhouses can be divided into two categories: <strong>soilless</strong> and soil-based. While soil-based production can be found in older operations, the current trend is for growers to optimize root conditions for crops by designing and implementing systems that allow greater control <strong>of </strong>the vital <strong>root zone</strong> variables than is possible with soil. This generally involves using little or no soil other than, perhaps, sand or gravel. </span></p>
<p><span xml:lang="en-us">In general, hydroponic systems are <strong>soilless</strong> production systems.<span>  </span>In this chapter the two terms will be used interchangeably. The public generally thinks of hydroponic systems as being water culture systems, without media; however, most hydroponic systems in commercial production use some sort of substrate to create a matrix that forms the <strong>root zone</strong>.<span>  </span>The only thing that all hydroponic systems have in common is that no soil is used.<span>  </span>Conversely, all containerized <strong>soilless</strong> production systems (e.g., production <strong>of </strong>potted flowering plant<strong>s</strong>) in use today are hydroponic production systems in that they create an artificial <strong>root zone</strong> that attempts to optimize water availability for the roots. Such production systems consist of<strong>:</strong> (1) the <strong>root zone</strong>, (2) the aerial part of the plants, (3) an irrigation system for <strong>the supply of</strong> <strong>a </strong>nutrient solution to the <strong>root zone</strong>, and (4) a drainage system for dealing with run-off from the <strong>root zone</strong>.<span>  </span>While this chapter focus<strong>es on</strong> the irrigation system, it should be noted that this is tightly linked to the exact nature of the <strong>root zone</strong> and the physio-chemical environment that is maintained there.</span></p>
<p><span xml:lang="en-us">While the substrate in such <strong>a </strong>system is not consumed by the plants, some of the gas and liquids are. In hydroponic systems the liquids consist of water in which ions and gases are dissolved.<span>  </span>This liquid is termed the nutrient solution; it is the primary function of the irrigation system to deliver this solution to the plants in an optimal manner. The practice of delivering fertilizer ions along with water is termed fertigation. The formulation of such solutions is handled elsewhere in this book (<strong>Chapter 5</strong>).</span></p>
<p><span xml:lang="en-us">While the term <strong>“</strong>nutrient solution<strong>”</strong> refers to the irrigation solution that is applied to the <strong>root zone</strong>, once this liquid reaches the <strong>root zone</strong> and mingles with liquids that are already present, the result is called the <strong>“</strong>substrate solution<strong>”</strong>. This solution is continually changing as the plants and microbes use portions of it and discharge compounds into it. It is, in fact, this dynamically-changing liquid that must be managed and optimized through various horticultural practices.</span></p>
<p><span xml:lang="en-us">Many types of irrigation systems have been developed and a lot of substrates<strong>,</strong> like gravel, rockwool, sand, perlite, expanded clay or synthetic materials<strong>,</strong> are used. </span></p>
<p><strong><span xml:lang="en-us">I</span></strong><span xml:lang="en-us">rrigation management in hydroponics is more sensitive<strong> than in</strong> soil-based systems. Compared to <strong>the </strong>soil, hydroponic systems typically have <strong>a </strong>much smaller root zone volume, greater water-holding capacity, more available water, lower moisture tension, greater hydraulic conductivity, and <strong>a </strong>higher dissolved oxygen concentration <strong>in</strong> the irrigation solution. Typically soil contains a multitude of <strong>substances </strong>that act <strong>as a</strong> buffer <strong>to </strong>chemical changes. In some hydroponics systems the volume of the <strong>root zone</strong> is very small, e.g. 14 l m^-2 in rockwool compared with 500 l m^-2 in soil <strong>as pointed out by Sonneveld, 1981</strong>. In the same way<strong>,</strong> the water and nutrient capacity <strong>are</strong> drastically reduced. Consequently fertigation is <strong>probably the</strong> most important management factor through which growers can control plant growth, yield and quality <strong>in hydroponics</strong> (Schroeder, 1994; Bar‑Yosef, 1999; Wever et al., 2000).</span></p>
<p><span xml:lang="en-us">Hydroponics generally requires on-demand irrigation. Plant water requirements can be determined either directly or indirectly. The most common is <strong>an</strong> indirect way <strong>in which</strong> plant status is correlated with the environment (temperature, humidity, <strong>vapour</strong> pressure deficit or, radiation). Direct measurements with sensors (e.g. <strong>a</strong> stem flow gauge) <strong>are </strong>now sufficiently developed and capable of successfully monitoring the plant. Future plant monitoring and control systems will combine plant features as well as its environment and provide closed–loop control (Giacomelli, 1998).</span></p>
<p><span xml:lang="en-us">Providing the optimal amount of nutrient solution, while maintaining an ideal level of oxygen in the root zone, requires a suitable irrigation system. Such a system should operate efficiently and dependably and deliver a uniform amount of liquid to all plants so as to obtain uniformly high-quality products. The irrigation system includes distribution components which interact with the substrate and environment. The irrigation system and <strong>its </strong>control minimize over‑watering and water stress of plants. However, optimization has not been achieved <strong>yet</strong> due to the inaccuracy of control systems (sensors), irrigation systems, or interactions between <strong>the </strong>plant and <strong>its </strong>environment.<span>  </span></span></p>
<p><span xml:lang="en-us">The objective of this chapter is to provide principles and basic knowledge of irrigation systems and their control with respect to crop growth and production. <strong>T</strong>he basic strategy of irrigation, components of irrigation systems, methods for optimization of irrigation systems, and methods of controlling the system <strong>are discussed</strong>. </span></p>