Ebb and flow 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. 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, containing either clean gravel or coarse sand as the rooting medium, is
periodically flooded for a short period 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.
= 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 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. 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 flow 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. These are
primarily management of media between uses, such as washing and sterilization.
This 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.
Also, most ebb and flow systems use a recycling reservoir to flood the table.
Over a period of time the pH of the nutrient solution may fluctuate to a
range which is unhealthy for the plant. If the pH is not corrected, various
problems may occur, including but not limited to poor nutrient absorption and
leaf cannibalization. As the name implies, leaf cannibalization occurs as
the plant takes nutrients from one part of the plant and uses those nutrients in
a different part of the plant. Leaf cannibalization appears as yellow or
brown spots on leaves. During Flower pH rises quite a bit each
day. It is best to adjust first thing and last thing each day. Also, during
Flower nutrients and water absorption increases, while root exudate gets
carried back to the reservoir. This causes ppms to increase significantly.
Proper control requires routine checking and replacing with fresh nutrients ~ 5
days to avoid toxicity. Since the plant(s) is being fed several
times a day, lower ppm nutrients are sufficient. Pushing with higher ppms can
cause the plant to burn up from the inside, especially when significant
water evaporation/usage causes the remaining nutrient concentration to
increase beyond 1500 ppms. Choosing a medium like lava rock is
ideal for flood and drain in that it drains quickly, and due to its rough
texture, it traps small amounts of oxygen and nutrients which keeps the
root zone moist between feedings. Also, because it drains quickly, the number of
feedings can be increased to roughly every 45–60 minutes during lights on,
producing explosive growth. Should the grower opt for this method, the nutrient
ppm should be kept below 800, better still 600.
Poor drainage, or incomplete drainage, may cause a condition wherein dense
roots are exposed to stagnant water which is trapped by the root mass. Root
rot and fungal growth are the most common result of stagnant water. Some
E&F systems are not as immune to root rot as a well-designed system would be.
In tables where plants are 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
or more of 3.5% peroxide solution per gallon of water are used. The temporary
rise in the oxygen level is only minimally damaging to roots, while
eradicating the water-borne mold can significantly increase yield or even
save a crop's viability. References