Soilless crop cultivation has become a preferred practice in the greenhouse industry (Van Os and Benoit, 1999); the most widely used soilless system is growing crops on rockwool (Sonneveld, 1991). In comparison with the common soil-based production system, the soilless methods have increased the productivity significantly. However, further yield increases have been difficult as a result of several limiting factors such as unsatisfactory spatial root development, rapid root collapse, and occasional disease incidence. Improved water management could alleviate these problems, to some extent, because it would improve water–air distribution in the growing medium, thereby improving plant health and productivity. Irrigation in soilless cultivation is indeed fertigation, in which plants are supplied with complete nutrient solution rather than water. As explained by Warren and Bilderback (2004), irrigation scheduling is the process of determining how much water (or nutrient solution) to apply (i.e., irrigation volume) and timing (when to apply). The goal of irrigation scheduling is to control the water status of the crop for a targeted level of plant performance. The targeted performance level is largely situational; it could be optimizing irrigation input for maximizing yield or economic return or increasing water use efficiency.
The root zone oxygen level has immediate effects on root formation (Gislerod, 1983; Soffer and Burger, 1988) and growth (Soffer et al., 1991), water and nutrient uptake, and many other metabolic activities (Morard et al., 2000). Hence, it is one of the principal determinants of water and nutrient use efficiency as well as plant growth and yield. Although the soilless growing media including rockwool are considered well-aerated in comparison with soil, events of oxygen deficiency are common (Allaire et al., 1996). This is because greenhouse crops grown on rockwool (or other soilless media) generally have higher growth rate and hence increased rate of root respiration (Raviv et al., 2004), thereby demanding more oxygen. Furthermore, in satisfying the increased water demand of a rapidly growing crop, it becomes harder to have enough air in the medium, because the water and air are nearly mutually exclusive in the pore spaces of the medium. Thus, ensuring an adequate oxygen supply to the roots is of paramount importance, and it is a difficult task even when the greenhouse crops are grown on rockwool or other carefully chosen well-aerated media (Soffer et al., 1991). The key consideration to ensure an adequate root zone oxygen level is to maintain a proper balance between water and air distribution in the slab by regulating the water content through supplying the right amount of water (or nutrient solution) at the right time, i.e., the development of an appropriate irrigation/fertigation control scheme for a particular crop under certain conditions. If developed and adopted, such an appropriate irrigation/fertigation control scheme would eventually result in further yield increases.
Intensive greenhouse crop production in rockwool and other soilless media usually involves sufficient water discharge aimed at preventing localized water shortage or salinity buildup in some slabs. The required leaching factor (i.e., the ratio of leached water to irrigation water) under these conditions may be 25% to 50% depending on the electrical conductivity (EC) of the feed solution and climatic conditions (Kläring, 2001; Van Os, 1995). One tomato plant is estimated to require 250 L of water during the growing season (Uronen, 1995). If the plant density is 2.5 plants/m2, the water requirement is 625 L·m2. Considering the required leaching factor, the amount of water applied in practice is 781 to 938 L·m2 of growing area. Because greenhouse crop production is concentrated in certain areas, the discharge of drained water carrying unused nutrients from greenhouses is of some environmental concern (Santamaria et al., 2003). As an example, typically, up to 50% of the applied nitrogen is leached out of the medium in the form of nitrates (McAvoy, 1994). This causes a common nitrate concentration in the drained water in the range of 600 to 900 ppm (McAvoy, 1994). High-nitrate water is unacceptable for human consumption. Soilless growing systems are considered potential contributors of nitrates to both under- and above-groundwater reservoirs in many horticulturally intensive countries (Van Os, 1995). Supplying fertigation to a crop at the correct time and in the correct amount will reduce the discharge of nutrients to the environment and will improve water management.
Substantial research effort has already been devoted to establishing optimum schedules in drip-irrigated greenhouse crops (Bar-Tal et al., 2001; Jovicich et al., 2003; Papadopoulos and Tan, 1991). These studies often compared varying durations or frequencies of irrigation, on the basis of preset time intervals, in which the supply of water to the growing media was not fully matched with the amount of water used by the crop. We need to develop irrigation strategies that would optimize water and oxygen supply to the roots, which would ensure better root growth and function thereby ensuring better plant growth, increased yield, and improved quality. The physical properties of rockwool determine the critical water contents that ensure simultaneous optimal air and water supply in the root zone. Ideally, irrigation should then compensate for any amount of water that evaporated from the medium and transpired by the plants. Thus, irrigation control strategies based on the slab water content or on the extent of water lost from the slab (determined by balances) seemed to be promising.
The overall objective of this research was to study the effects of advanced irrigation control strategies on root growth, water use efficiency, yield and fruit quality of tomato, thereby developing proper irrigation schedules for rockwool-grown tomato crops.
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