Water is essential for agricultural crop production, but globally, the availability of water of acceptable quality is increasingly becoming a limited resource. Irrigated agriculture is one of the largest yet most inefficient users of this resource. As population increases, competition for fresh water between civil uses and agriculture will only increase. Therefore, agricultural producers will need to adapt to using alternative irrigation sources such as municipal reclaimed water that may contain elevated levels of impurities such as Na, and also adopt new irrigation and cultivation practices that maintain high productivity but with a greater WUE.
Greenhouse crop production is an intensive form of agriculture with high crop production rates and equally high fertilizer and water consumption. Greenhouse systems also enable producers to use sophisticated irrigation techniques to maximize water and fertilizer use efficiency. The most efficient systems deliver water directly to each plant and capture and reuse runoff. In closed subirrigation systems such as those used in controlled-environment agricultural production for example, water is taken up by capillary action through the base of the production container, and unused water is drained back into a reservoir for reuse. Water management within these systems further affects WUE and crop quality. In these subirrigation systems, the production surface, typically impervious benches or floors, is flooded up to a depth of 2 cm. Subirrigation management reduces fertilizer use by 30% to 40% (Strefeler, 1991). A modified subirrigation watering system was developed by Geremia Greenhouse (Wallingford, CT) in collaboration with TrueLeaf Technologies (Petaluma, CA). This system allows growers to limit the duration of subirrigation to a short-cycle (Gent and McAvoy, 2011), and thus, limit the degree of medium saturation. This method of watering has been referred to as short-cycle subirrigation management or partial-saturation irrigation (Gent and McAvoy, 2011). Short-cycle subirrigation is a process where the GWC of the medium is regulated by limiting the contact time between the container and the irrigation solution. In this management system, medium saturation does not reach full capacity at each irrigation event. Containers under short-cycle subirrigation management may take in 25% less water than those under long-cycle subirrigation, and the incidence of root rot disease is reduced significantly compared with crops produced under irrigation management practices that allow the potting medium to approach full saturation (Elmer et al., 2012). Under short-cycle subirrigation, growers have more control over crop growth, especially stem elongation, so that ornamental plants develop the compact stature most desired by the industry and accumulate less biomass (Gent and McAvoy, 2011).
The physico-chemical properties of a growing medium, determined by the material composition of the medium, have major effects on plant growth. These factors include water-holding capacity, cation-exchange capacity, pH, and EC. Most commercial growing media used in greenhouse production contain sphagnum moss, vermiculite, perlite, or bark in various proportions. More recently, other amendments such as coconut coir, rice hulls, and wood fibers have been tested for efficacy as media amendments. Coir-amended mixes have higher moisture-holding capacity than peat-based mixes (Stamps and Evans, 1997).
As no leaching occurs in subirrigation, excess salts from fertilizer and contaminants in the irrigation water such as salinity accumulate in the potting medium resulting in elevated EC (Argo and Biernbaum, 1996; van Iersel, 2000), a phenomenon that is similar to salt crusting observed in arid regions. The accumulation of salts can be a concern when raw water quality is low or when an alternate source of irrigation water carries unwanted ions. How such circumstances might affect greenhouse crop performance under short-cycle subirrigation, where the water available to the plant is restricted, has not been reported. Therefore, the focus of this research was to determine the effect of NaCl on the growth and nutrient composition of zinnia grown under different subirrigation management regimes.
Zinnia elegans, a species previously identified as salinity sensitive (Villarino and Mattson, 2011), was selected as the test crop in this study.
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