Critical Analysis: Aeroponics

 Introduction:

Aeroponics is one of the more recent forms of hobby and commercial hydroponic crop production, rising in popularity throughout the 1970’s and 1980’s, though it originated in the 1920’s. Its own roots lie in plant scientists’ early experiments on root structure and growth, as it provided the perfect way to examine intact roots. Be that as it may, it seems to have been NASA’s adoption of the technology in the 1990’s that helped bring aeroponics into the foreground of public view and elucidate the fundamentals of how it works.

Like any other hydroponic method, aeroponics involves growing plants without soil using a clean system and a nutrient solution. Instead of aerating the solution to increase dissolved oxygen (DO) like in NFT and DFT systems, the solution is misted over the plants’ roots, which hang or sit in an air chamber that always drains completely and is perfectly dark and humid. The gentle irrigation that keeps all root hairs intact as well as the capacity for extreme precision irrigation make aeroponic systems some of the most high performance hydroponic systems out there.

Using Aeroponic Systems:

Aeroponic systems can be designed and configured in various ways depending on the target crops and their respective root zone volume requirements and life cycle durations. These configurations include horizontally oriented chambers that can be stacked vertically, single plant systems for larger crops, or even a-frames, pyramids, towers, barrels etc. Any aeroponic system, whether in a greenhouse or an indoor farm, must be set-up so that the plant canopy can receive consistent lighting throughout, either by natural lighting or by the use of supplemental or sole source artificial lighting.

With such a powerful and precise tool, growers are able to develop irrigation strategies that incorporate irrigation durations and frequencies down to the order of seconds and minutes. This allows for a purposeful manipulation of the effects of irrigation on crop physiology; in altering the duration in between irrigations, a grower can elicit various degrees of controlled drought stress, which can produce positive outcomes on crop yield, quality, and physiology under different circumstances. Many crops respond to deficit irrigation strategies with such responses as increased biomass or dry weight production, increased secondary metabolite production, and even accelerated ripening/maturation.

Aeroponics systems are usually very crop/application specific. The primary consideration in any aeroponic system design is that, as root systems grow, they don’t affect the irrigation’s ability to irrigate/reach them fully and consistently. For horizontally arranged aeroponics systems, this dictates the minimum viable depth as well as the number and placement of nozzles, as you don’t want roots to reach the bottom surface of the system, where they may experience stagnation, or block the nozzles from spraying their intended targets. Aeroponics for vertical farming, therefore, wastes a significant amount of vertical space, even when growing leafy greens, and this effect is only exacerbated in larger crops with more extensive root systems.

Regardless of their format, aeroponic systems fall into two basic categories: low pressure and high pressure aeroponic systems. Some hydroponic tower systems previously touted as aeroponics use virtually no pressure to irrigate plants, simply allowing water to drip onto the roots in series after having been pumped to the top of the system, and these have now been dubbed 3D NFT systems since they irrigate continually and result in a vertically flowing nutrient film over the plants’ roots. Low pressure aeroponic (LPA) systems operate at pressures below 50 psi and result in a droplet size usually greater than 80 um. Droplets of this size fall quickly, and when they come into contact with roots, they weigh down root hairs and proceed to drip down and off the root systems. Irrigation precision is therefore more limited in these systems and they typically irrigate for a minute to 5 minutes before shutting off for 5 to 15 minutes to ensure proper oxygenation. Because of this, large scale LPA systems must have drainage manifolds to avoid water accumulation at the bottom, and small scale LPA systems (most often used for rooting stem cuttings for transplant into soilless potting substrates) simply have the main reservoir situated at the bottom of the chamber. As a general rule with a given nozzle, the higher the pressure applied, the smaller the droplet size that will be produced, and surfactants can be used to reduce the solution’s surface tension and decrease droplet size at lower pressures. Moreover, as the spray angle increases, the droplet size decreases as well.

High pressure aeroponic (HPA) systems operate at pressures over 50 psi (commonly around 100 psi) and thereby result in a droplet size in the range of 50 um or smaller. Droplets of this size stay suspended in air longer and can travel farther before settling. When these come into contact with root hairs, they get trapped between them, maximizing root aeration and irrigation efficiency. In these cases, very high precision can be achieved, and irrigation is typically applied for only a few seconds before shutting off for however seconds or minutes are required for producing a particular physiological effect. These systems can be operated without drainage manifolds, as they can be run without significant accumulation of solution in the chamber.

One final consideration when using aeroponic systems is the tendency of spray nozzles to clog; due to the very small restrictions through which the hydroponic solution must be pumped, debris as well as precipitated salt crystals can obstruct the flow and result in localized dry outs and plant wilting. Therefore, the operator must always ensure that filtration of particulate contaminants occurs before the solution reaches the chambers and that high quality soluble salt fertilizers are used in the system. Root zone biologicals, or beneficial microbes, are discouraged when using hydroponic systems, as they will either contribute to nozzle clogging or become damaged/sheared when passing through the nozzles. This issue results in restrictions unique to aeroponic systems as well as the need to inspect nozzles often and/or put flow sensors in place to detect clogs before crop damage can occur.

What Aeroponics Looks Like in Nature:

In nature, aeroponics is most akin to the mineral and nitrate rich water of a slow flowing river or stream falling over a cliff or ledge as a waterfall. In mid-air, the nutrient solution becomes a mist that continually wets the waiting roots of plants on the walls of the moist cliff face or cave, delivering everything they need while keeping them aerated due to the fact that they are never submerged.

Advantages of Aeroponics:

-Irrigation can be extremely precise.

-Systems are generally lightweight.

-There is no need for active aeration to raise solution dissolved oxygen concentration.

-It is not necessary to pump large volumes of water for irrigation.

-System is highly efficient with water and fertilizer usage.

Disadvantages of Aeroponics:

-Incur the highest cost of all hydroponic systems.

-Require more monitoring and maintenance than other hydroponic systems.

-Aeroponics requires the highest parts count of all hydroponic systems.

-Formats are generally single purpose and not easily adaptable for vastly different crop types.

-Fine filtration required even at irrigation manifold to avoid particulates that might clog nozzles.

Conclusions and Future Directions:

Undoubtedly, aeroponics offers the best precision, efficiency, and performance that hydroponic systems have to offer. This level of control, however, is patently unnecessary in the vast majority of commercial plant production scenarios, where low CAPEX is critical and where reducing maintenance and increasing versatility is key. That means that the risks outweigh the benefits in most real world situations, and few farms are liable to take this approach in the future, especially since those who preceded them in this path are having a hard time turning a profit and pivoting into new crops with their existing systems. Aeroponics was, at one point, poised to take over the indoor cannabis cultivation scene, but as competition increases and cannabis commodity prices fall (while food crop prices climb), operators in that space are having to be just as cost and maintenance conscious as anyone else.

That being said, aeroponics will always be an important tool to plant scientists for many of the experiments they conduct concerning root zone physiology and the effects of irrigation strategies on plant biology. It will continue to be popular among hobbyists who want to achieve maximum performance out of relatively expensive small scale systems. Furthermore, organizations like NASA will continue to deploy aeroponics exclusively as a best option for high budget extraterrestrial cultivation systems that prioritize reducing system weight, pumping power and volumetric flow while eliminating the drain manifold.

Many of the current limitations of aeroponic systems may be overcome with further research into fogponics, wherein no pump is used to move water around and no pressure is built up to operate the nozzles. Using ultrasonic atomizers to vaporize the nutrient solution, there is zero risk of clogging and no need to position nozzles all over the system. Instead, the fog generated can be treated like a gas and can be moved through the system using fans, which require relatively lower power inputs. Although current atomizers are vulnerable to salts in the solution they are vaporizing and may experience short life spans in a hydroponic system, advancements in ultrasonic atomizers for the specific purpose of plant production will likely help overcome these obstacles and create new opportunities for low energy hydroponic crop cultivation.