Ebb and flow
Ebb and Flow or Flood and Drain is a form of hydroponics that is known for its simplicity, reliability of operation, and low initial investment, while providing the advantages of hydroponics. Also called 'E&F', it is a system of arranging pots filled with inert media which do not function like soil, contributing nutrition to the plants; rather, the medium anchors the roots and functions as a temporary reserve of water and solvent mineral nutrients as the hydroponic solution is alternately flooded and allowed to ebb.
Principles of Operation
The fundamental principle of hydroponics relies on fertilized & aerated water which provides both nutrition and oxygen to a plant's root zone, often involving relatively sophisticated mechanization which can be daunting to casual hobbyists. Nutrient solutions must usually be chilled 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 relying 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, bidirectional path for the solution : water flows in, and out, 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 it was pumped up in; eliminating the need for more than one sealed fitting and reducing overall complexity of the system.
Ebb and Flood systems come on according to the water holding capacity of the medium the roots sit in; highly water retentive media can require watering only once a day; while others require twice to as many as six; with each "Flood" stage only lasting a few minutes. The time it takes to flood the roots is not a critical parameter; therefore pumps are often moderate in capacity, and can be small for systems sustaining indoor garden plants; making the method popular with amateur and urban gardeners. Gravity itself 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 back off, gravity pulling the water back downward re-exposes the space around the roots to atmospheric pressure, which re-fills the voids in the medium.
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 this way suppresses most harmful lifeforms keeping the root zones disease free; a function that must be performed by cooling the solution in other types of hydroponics, to protect it from pythium, a form of water mold responsible for a condition called 'root rot'; wherein the outer cells of the roots die, turn brown, and slough off when handled. 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.
Drawbacks to E&F systems
Ebb and Flood systems are flexible with few practical drawbacks. Though typically known for compact cultivation of plants having smaller stature, it has been used for growing large plants, using buckets ranging in size from 1 gallon to 5 gallons, making use of high volume pumps such as those in large aquariums, decorative fountains, and koi ponds.
There are facets to these systems that present some labor investment in large scale applications: primarily management of media between uses, such as washing and sterilization; which can be done by dumping into the tray and filling with a sterilizing solution such as hydrogen peroxide or chlorine solution; temporarily plugging the drain, with hand removal of root fragments. Larger containers require transferring the media to a suitable surface after sterilization to permit removal of leftover plant material.
A second drawback is that the roots tend to grow together; meaning removal of harvested or damaged plants can be somewhat problematic in plants having well developed root systems. Commercial crops harvested at one time are somewhat immune to concerns related to that aspect of the system but, in the event of pathogenic invasion the problem can quickly spread, as all the roots share the same flood source.
Finally, due to conditions wherein roots sit in undrained water trapped by the body of the roots themselves, some E&F systems are not immune to root rot as a well designed system would be; and in tables where plants larger than optimal for the system this can create the need for modifications such as screens or beds of medium sized gravel to prevent standing water. Tilting the tray is one way to achieve better drain characteristics. In bucket E&F this problem can be dealt with in a similar manner, ensuring good drainage through using medium of adequate size and ensuring that drainage of the container between flood cycles is complete.
Hydrogen peroxide is also added to nutrient solutions when there is suspicion that the anaerobic pythium root rot mold has begun to proliferate on the roots' surfaces. The oxygen liberated from the hydrogen peroxide is destructive to single celled organisms and is administered in dosages which vary with the concentration of the peroxide; typically several tablespoons of 3.5% solution per gallon or more; the temporary raising of oxygenation is only minimally damaging to roots while eradicating the water born mold can significantly increase yield or even save a crop's viability.
Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "Ebb and flow". Image Credit.
