Conventional overhead watering uses excess water to ensure complete coverage of the entire crop to the point of effective saturation of the root medium or substrate. Under certain conditions, up to 75% of the water and fertilizer applied by overhead irrigation may be wasted or leached from potted plants (Yelanich and Beirnbaum, 1994). Even with drippers that deliver water only to the substrate, overwatering by 10% to 30% is recommended to prevent salt buildup (Argo and Biernbaum, 1995; Mastalerz, 1977). The runoff of water, fertilizer, and pesticides resulting from these irrigation methods is a potential risk to the quality of the environment in proximity to a greenhouse operation.
Subirrigation, and in particular ebb and flow, systems are more efficient than overhead watering (Dole et al., 1994; Morvant et al., 2001) and also more efficient in terms of nutrient use (Haley and Reed, 2004; Purvis et al., 2000; Zheng et al., 2004). The benefits of ebb and flow, as it is currently applied, is that no water is lost to the environment and very little is lost to evaporation. In ebb and flow systems, the bench or floor holding potted plants is flooded to a depth of 2 to 10 cm. Capillary action in the substrate takes up water or nutrient solution through the base of the pots (Nelson, 1998). Then the solution is drained from the bench or floor to a reservoir. In most commercial ebb and flow systems, the duration of watering is relatively long, 20 to 30 min, and the substrate may take up water to 90% or more of effective water-holding capacity. It should be noted that subirrigation results in a lower effective water-holding capacity than when pots are watered from overhead (Elliot, 1992) as a result of the limitation of capillary action compared with percolation for watering the medium. Geremia Greenhouse (Wallingford, CT), in collaboration with True Leaf (Petaluma, CA), developed a method for rapid ebb and flow watering for production of potted ornamental plants that can complete an ebb and flow cycle in as short as 4 min. This short cycle restricts water uptake. From here on we refer to this short cycle irrigation process as the partial saturation ebb and flow watering (PSEFW).
There are numerous benefits that could derive from PSEFW. Water stress may be used to control plant growth and quality. A constant low volumetric water content can control both biomass and height growth of ornamental plants (Burnett et al., 2005; Van Iersel et al., 2004), although the extent of restriction of height, rather than biomass, growth depends on plant species. If the substrate is only partially saturated, the oxygen content is higher than with full saturation, which can benefit the metabolic activity of roots (Nemati et al., 2002). Partial saturation may alter the microenvironment in which both beneficial and disease-causing microbes grow. In contrast to PSEFW, pots watered to their effective water-holding capacity tend to leak some of the water back out onto the floor. Pathogens can spread from pot to pot through the irrigation water (George et al., 1990; Hong and Moorman, 2005; Stanghellini et al., 2000).
There are some other characteristics of ebb and flow irrigation systems that could cause problems. Because there is no leaching of the medium with subirrigation, nutrients tend to accumulate. In particular, a vertical gradient of nutrients develops in the substrate with a high electrical conductivity (EC) in the upper-most layer, which may affect the distribution of roots (Morvant et al., 2001). This can inhibit growth of plants unless the fertilizer supply is adjusted to maintain EC in the medium within a target range (Kang and van Iersel, 2009). A dryer substrate would also exacerbate any deleterious effect of high EC on plant growth as a result of an excessive fertilizer supply.
To determine the benefits and drawbacks to partial saturation ebb and flow watering, we compared PSEFW with a control treatment of slow-cycle ebb and flow watering applied to grow geranium and chrysanthemum in pots on a flooded floor. We measured water uptake per irrigation cycle and water and nutrient content of the root medium after repeated irrigation cycles. We also measured the vertical gradient and distribution of water and nutrients in the substrate. We determined biomass and height growth and tissue nutrient composition as a function of watering treatment and location of pots across a flooded floor.
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