Ebb and flow

Ebb and flow (also called ebb and flood and flood drain) are two phases of the tide or any similar movement of water. The ebb is the outgoing phase, when the tide drains away from the shore; and the flow is the incoming phase when water rises again. The terms are also common in figurative use.

Ebb and Flow is a form of hydroponics that is known for its simplicity, reliability of operation and low initial investment cost. Pots are filled with an inert medium which does not function like soil or contribute nutrition to the plants but which anchors the roots and functions as a temporary reserve of water and solvent mineral nutrients. The hydroponic solution alternately floods the system and is allowed to ebb away.

Under this system a water-tight growing bed, containing either clean gravel or coarse sand as the rooting medium, is periodically flooded for a short period (5 to 10 minutes) with a nutrient solution pumped from a supply tank. By placing the nutrient solution supply tank below the growing bed, the nutrient solution can drain back by gravity. This hydroponic growing system is little used today other than for hobby-type systems. The method is inefficient in its use of water and plant nutrient reagents. Root disease occurrence and nutrient element insufficiencies can occur with repeated use of the nutrient solution. Because it is a "closed” system, the re-circulated nutrient solution will require reconstitution, filtering, and sterilization. Within the growing period, the nutrient solution may require replacement. The rooting medium will require washing to remove root debris and accumulated precipitates as well as sterilization before reuse.

The fundamental principle of hydroponics relies on fertilized and aerated water which provides both nutrition and oxygen to a plant's root zone. It often involves relatively sophisticated mechanization processes which can be daunting to casual hobbyists. Nutrient solutions must usually be below the temperature at which pathogen growth can begin, yet not so cool that root activity is suppressed. Active aeration of the fertilizer solution is common, since root systems themselves remove oxygen, creating conditions which also can promote pathogenic bacteria and water-borne molds.

E&F utilizes the fact that the solution is not left in constant contact with the roots of plants, to avoid the need for oxygenating or chilling of the solution. Instead it relies on characteristics of root function to provide passive oxygenation at a high level which tends to suppress pathogen growth.

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