The following is from an article by Larry Brooks, published in Growing Edge,
Fall, 1990. This comment is particularly relevant:

  "The plants are supported above the flowing nutrient. The roots hang down through an
  air gap in which nutrient is sprayed, then into the moving nutrient solution below the air gap.
  The nutrient sprayed through the air gap is not so much intended to feed the plant, but rather
  to infuse oxygen into the nutrient solution wherein the feeder roots remain constantly
  submerged. It is these submerged roots in oxygen rich nutrient that provides most of the
  nutrition and oxygen for the plant."

   The Seattle Hydroponics aero-drip system is similar, but instead of spraying the roots,
water is supplied to the top of the medium. I recommend grow-rocks for the medium,
because it drains fast. I don't recommend emitters because they constrict the opening,
which tends to lead to clogs. The higher flow-rate means you want a fast-draining
medium like grow-rocks. Rockwool doesn't drain fast enough for this system,
and is not re-usable (grow-rocks can easily be re-used).

   Larry's system requires spraying the roots, and the sprayer tends to clog,
isn't easily visible or easy to work on. The sprayed water doesn't penetrate the
medium very well, so the plant basket has to be very small. Larry's system
relies upon long roots growing into the nutrient solution, however to get roots
this long may take a while.

   My system enables a larger plant basket, so there is plenty of root space.
This means your plant won't fall over. The water is passively irrigated by a
large surface area (each bucket stores some water) and water trickling
through the air.


Aero-drip hydroponics
By Larry Brooks

   The aero-hydroponic method was developed in Israel in the early 1980’s. Dr. Hillel Soffer,
senior researcher at the Volcani Institute at Ein Gedi developed the aero-hydroponic method
to overcome the challenges presented by the hot, arid conditions at Ein Gedi. The discoveries
that followed the development of aero-hydroponics offer great benefits to all hydroponic
growers.

   During a two-year period from 1986 to 1988, Dr. Soffer performed extensive research using
the aero-hydroponic method at the University of California at Davis, where he had received his
Ph.D. in the early 1970s. The specific area of research was in quantifying the effect of various
levels of dissolved oxygen on root growth, especially in the propagation of plants from
cuttings.

   The findings of his research were published in the Journal of the American Society for
Horticultural Science, and HortScience. Both studies were co-authored with David Burger at
UCD. In addition, Dr. Soffer presented his findings at the annual conference of the Hydroponic
Society of America in 1988.
   Except for the papers mentioned above, very little has been printed up to now on the aero-
hydroponic method. The method was patented internationally, though few licenses for the
production of equipment have been granted. Without aggressive commercial support, the
aero-hydroponic method has remained largely a research tool, known mostly to university
researchers.In the meantime, the rockwool method was becoming available internationally
following 12 years of exhaustive research and a strong marketing program with lots of
investment in advertising, production and distribution, first in Europe and later in Japan.
   Following nearly a decade of rockwool use, the Dutch contacted Dr. Soffer to request
permission to develop and use the aero-hydroponic method. The reason for the sudden
interest of the Dutch in the aero-hydroponic method has implications for the development of
hydroponic cultivation worldwide.
   For rockwool cultivation to work efficiently in most commercial operations it is preferred to a
use a non-recirculating nutrient solution. Nutrient solution is sent on a one-way trip through
the rockwool and is then discarded. The real cost advantage of rockwool cultivation over other
hydroponic methods was that the nutrient did not have to be recaptured and recirculated,
reducing the system complexity of reservoirs, plumbing, pumps and pH and conductivity
controllers.

   The once-through nutrient system also reduces the problem of nutrient solution
becoming imbalanced due to erratic uptake of minerals by rapidly growing plants; plus the
build up of dissolved minerals from slowly dissolving rockwool.
   The discharge of enormous amounts of spent nutrient solution has become a major problem
in Holland, contaminating surface and ground waters. Consequently, the Dutch government
has prohibited the dumping of nutrients resulting in renewed interest in recirculating systems
such as aero-hydroponics.
   As leaders in both horticulture and commercial hydroponics, the Dutch have recognized the
value of a method, which enables rapid and trouble-free cultivation and eliminates the
problems of disposing of spent nutrients and exhausted media.

   Aero-hydroponics is not a simple method to understand. The equipment required is
somewhat more complicated than other hydroponic methods, but there is a great advantage in
that once an aero-hydroponic system is set up, it will run almost indefinitely without additional
investment in such disposable components as growing media and non-recirculating nutrients.
   What is most surprising about aero-hydroponics is not how it works, but why plants grow
better. The key is dissolved oxygen at the root boundary zone.

   The essence of Dr. Soffer’s work at UCD was in quantifying root growth in proportion to
dissolved oxygen. Only the green parts of the plant can form oxygen from carbon dioxide -
roots require a supply of oxygen for metabolism and growth. Plant growth in oxygen deficient
conditions, such as those found in many soils, is limited. Dr. Soffer found the enhanced
oxygen at the root zone produced enhanced growth.

   In aero-hydroponics, the nutrient solution is sprayed through the air in order to infuse the
nutrient with dissolved oxygen. The method differs from classic aeroponics in that most of the
plant’s roots are not suspended in air and fed by a spray of nutrient solution; rather, the
majority of the roots are submerged in oxygen-infused nutrient which is in constant motion in
order t maintain high levels of dissolved oxygen at the root boundary zone where oxygen and
nutrients are taken in by the plant.

   The result is a propagating tool of unsurpassed performance. Dr. Soffer was successful in
propagating plant varieties at UCD that had never been propagated before. He took particular
delight in propagating varieties of conifers and even pistachio trees (pistachio cuttings
required 90 days to generate roots). Moreover, he found that cuttings could be rooted aero-
hydroponically in purified water without using rooting hormones such as IBA or NAA. This is
because plant tissue already contains the natural rooting hormone IAA (Indole Acetic Acid).

   Aero-hydroponic systems can be built using quite a variety of materials and in numerous
design configurations. The Ein Gedi “Mini Unit” which was used at UCD for dissolved oxygen
studies is a stand-alone module which supports four plants in 10 liters of nutrient solution.
An electric motor mounted on the top of the unit spins a nutrient sprayer, which lifts nutrient
solution and sprays it onto the “aerial roots.” Additionally, the rotation causes the nutrient
within the unit to stir, moving it constantly over the submerged roots.

   Large-scale aero-hydroponic systems follow the design of the commercial installation at Ein
Gedi. These commercial systems consist of “canals” or growing chambers with plant sites on
top. A pump provides the pressure to drive a system of sprayers to supply the aerial roots,
while the submerged roots hang into the flowing nutrient in the canal.

   Both of these systems share fundamental characteristics, which define the aero-hydroponic
method. The plants are supported above the flowing nutrient. The roots hang down through an
air gap in which nutrient is sprayed, then into the moving nutrient solution below the air gap.
   The nutrient sprayed through the air gap is not so much intended to feed the plant, but rather
to infuse oxygen into the nutrient solution wherein the feeder roots remain constantly
submerged. It is these submerged roots in oxygen rich nutrient that provides most of the
nutrition and oxygen for the plant.