ebb and flow

Ebb and Flow (also called Ebb 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 the water rises again. The terms are also common in figurative use.

In Hydroponics

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 (table), pots that are placed in the table will hold the medium. The medium can consist of material that the root system can reach the level ebb reaches. A medium can consist of lava rock, Rockwool cubes, and fiber. A mixture of these different types of medium can absorb the moister more evenly and efficiently. this 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 can be very effective for any size plant depending on space.

The method is inefficient in its use of water and plant nutrient reagents. Root disease occurrence and nutrient element insufficiency can occur without repeated use of the solution without proper oxygenation on the tables that are flooded. Because it is a “closed” system, the re-circulated nutrient solution will require monitoring every time the system flows back the reservoir for pH and PPM. 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

Principles of Operation

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.

Simplicity is maintained through usage of a single, two-directional path for the solution. Water flows in and out using the same tube. When the pump has raised water into the tray, briefly submerging the roots, the pump is rendered inactive using a switch, typically a timer, and the water flows back down the same tube. This eliminates the need for more than one sealed fitting and reduces overall complexity of the system.

Ebb and flow systems come on according to the water-holding capacity of the medium in which the roots sit. Highly water-retentive media can require watering only once a day, while others require two to as many as six floodings, with each “flood” stage only lasting a few minutes. The time it takes to flood the roots is not a critical parameter, which means that pumps are often moderate in capacity and can be small for systems sustaining indoor plants. This makes the method popular with amateur and urban gardeners. Gravity acts as drain pump, and aeration is accomplished through thin-filming and positive displacement of air as it is forced out of the root zone by water.

Aeration in Ebb and Flood Systems

Aeration of an ebb and flood system is an important aspect of its operation. Automatic displacement eliminates air which has been de-oxygenated by the roots as the water rises to its highest flood stage. When the pump turns off, gravity pulls the water downwards, which re-exposes the space around the roots to the air.

The film of water left around the roots during ebb has a high surface-to-mass ratio, which means that even as the roots absorb oxygen, its high surface area facilitates re-oxygenation, which can sustain the roots as long as their surfaces remain damp. The high oxygen content of water filmed in this way suppresses the most harmful lifeforms, keeping the root zones disease-free. In other types of hydroponics this function must be performed by cooling the solution to protect it from pythium, a form of water mold responsible for a condition called ‘root rot’, in which the outer cells of the roots die, turn brown and slough off when handled. The need for supplementary oxygenation using air pumps is also eliminated, which increases reliability and reduces complexity.

Ebb and flow hydroponic systems are also quiet while using less power than other hydroponic systems, which means that they can be used in environments where acoustic signature and excessive plumbing is objectionable, such as residential or classroom applications where space is at a premium.


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