Chapter 14
Ebb-and-flow benches (also called ebb-and-flood benches) combine an elevated benching system with a closed subirrigation system (See Figure 14.7). It provides for individual watering and fertigation of a number of different plants on different spacing without individual feed lines going to each plant and without overhead watering. Commercial ebb-and-flow systems are used chiefly for plants that are on the production tables for a relatively short period of time such as seedlings and the production of potted plants in the floriculture industry. As with other closed irrigation systems, the nutrient solution is recirculated and EC, pH and often temperature levels are adjusted at the main nutrient reservoir.
The ebb-and-flow system is one of the simplest hydroponic systems to set up and use, but it still takes some experience and effort to master. The ebb-and-flow system costs are usually very low in comparison to many other substrate systems, because it doesn't require any high tech, expensive components. The ebb-and-flow system is well-used because it can use many different types of growing media, expanded clay pellets, perlite, sphagnum moss, and rockwool. The disadvantage with an ebb-and-flow system is the high relative humidity that can build up in the canopy of plants.
Ebb-and-flow benches may be designed as stationary benches or as rolling benches, like Dutch trays. Either approach can work just fine, so the choice depends largely on personal preference and the desire to automate other production practices. The primary characteristic of an ebb-and-flow bench is the tray that makes up the bench surface. The tray may be made of hardened plastic or aluminum. The length and width of the tray varies depending upon the desired bench dimensions.
At one end of the tray there is an inlet and an outlet. The inlet allows fertilizer solution (or water) to be pumped into the tray. As fertilizer solution is pumped into the tray, it first floods the deepest of the channels. When the deepest channels are flooded, the solution floods the next level of channels. After both sets of channels are flooded, the solution continues to rise above the bottom surface of the bench. In a production situation, containers with plants would be placed on the tray surface. As the solution floods the tray and rises up around the bottom of the containers, the solution would come into contact with the root substrate inside of the containers and the fertilizer solution would move up and into the substrate in the container by capillary action.
Dutch trays are placed on steel tracks that serve to not only support the trays, but the trays (which have small wheel-like structures underneath) roll on the metal support structure (See Figure 14.8). This allows trays to be rolled together to maximize space usage efficiency, but the track system also serves as a transportation system around the greenhouse facility. When the plant material needs to be moved to another location in the facility, the trays may be rolled on the steel tracks to a major aisle or walkway.
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