An noteworthy alternative to hydroponics.

Many green leafy plants can live with their roots in nothing but air with occasional nutrient mist!
  • Aeroponics answers the question:
    "Could there be a horticultural medium even farther removed from natural soil than hydroponics already is?"
  • And perhaps more interestingly,
    "Would such a medium actually offer advantages for cultivation, or would it be a mere curiosity?"

Where to file this oddity?

In the field of horticulture, there exists a spectrum of growing media upon or within which plants can be cultivated. At the natural end of this spectrum stands natural organic soil, naturally obtained. In the other direction, picture a series of progressively more synthetic growing media, from commercial "potting soil" (which is often soil in name only), through synthetic media such as rockwool, cocopeat, perlite, vermiculite, etc., and onward to hydroponics.

Hydroponic cultivation is often thought of as the farthest one could go in the direction of synthetic horticulture -- eliminating the (solid) growing medium entirely and simply growing plants with their roots suspended in a carefully controlled nutrient solution. Aeroponic cultivation takes this one step further: plants are grown with their roots suspended in air, misted at regular intervals with a nutrient solution.

Is that really different enough to merit a noun of its own?

Yes. The liquid medium in hydroponic systems often hosts damaging competitor organisms such as fungi, bacteria, and water molds, and these can be quite difficult to control. In fact, most low-tech and small-scale hydroponic growers typically have little sense of how much trouble they may actually have from these competitors unless or until they suddenly lose an entire crop to root disease. By reducing the amount of time the plant roots stay wholly submerged in a liquid medium, aeroponics limits the access of predatory water-borne organisms, very effectively limiting their success.

So plants can live with their roots in just air?

In a way, it is remarkably close to how most green plants actually grow in most common soils. It is only during heavy rain or flood that the soil is saturated to the surface; the rest of the time the water table is a good distance below the surface. What goes on in that upper region beyond soil surface and water table? Roots interact with water and nutrients and oxygen, yet while moist, they are not fully wet. Most plants grow a large number of 'feeder' roots in this upper region as it is where the soil usually contains the most nutrients, meanwhile sending a taproot or other water-seeking roots farther down to maintain a source of water. In fact, while it is common knowledge that green plants take in CO² through their leaves and breathe out oxygen in return, few realize that the same plants actually intake oxygen through their feeder roots, and if those roots were submerged that action would be restricted. Thus pure hydroponics, in less than the most skilled of hands, is better suited to producing lots of algae than producing root-bearing green plants. In practice, most hydroponic setups only soak the roots at intervals, but due to the presence of a solid growing medium, the roots often stay wetter than what is ideal much of the time.

The Temperature Problem

One of the most difficult factors to control in a hydroponic setup is the temperature of the water/nutrient solution. Obtaining maximum exposure (within reason) to sunlight or artificial light is usually a primary goal; but intense light from any source is unavoidably accompanied by intense heat. Of course solutions for this exist, but they pose problems in cost, complexity and logistics serious enough to make any grower reconsider operating in an enclosed space -- or at least to force a reconsideration of just how close to theoretical maxima of production one truly wishes to operate, and how much labor and investment it is actually worth.

Why does solution temperature matter? Dissolved oxygen, plus those competitor organisms again. At the temperature of a natural stream that is cold to the touch, water naturally contains a great deal of dissolved oxygen. As noted above, oxygen is important to the development and productivity of a plant's feeder roots (comprising the majority of the roots of most green plants). As water temperature approaches and exceeds 70°F/20°C, its ability to hold oxygen available to plants (and fish) drops dramatically to only a small fraction of that of colder water. Furthermore, warmer water supports exponentially greater cell growth in the water molds and other organisms that prey on plant roots.

Thus, a common syndrome observed in hydroponic cultivation goes as follows. The lighting system (or sunlight, in greenhouses), plus the ambient air temperature, eventually bring the nutrient solution to an unhealthily high temperature: for many green plants 70°F/20°C is a warning sign, 75°F/24°C causes real stress, and 80°F/27°C can only be tolerated for a short time without causing widespread damage from which it is difficult to recover. Once a plant, and an entire growing system, is infected with water molds (e.g. pythium spp.) it can be difficult to restore to health even after a complete change of solution or a full system cleaning.

How does aeroponics solve this?

The main advantage of aeroponics is that the nutrient solution spends the vast majority of its time in a reservoir, only periodically being pumped up and exposed to the relatively warm growing environment. Thus it can be much more easily maintained at a temperature that discourages fungi (etc.) and keeps dissolved oxygen high, without resorting to expensive and high-maintenance solutions such as direct refrigeration.

But that also describes hydroponics. What's the difference?

Good point. In a hydroponic system, every time the nutrient solution is introduced into the growing area, it interacts with the thermal mass of the solid growing medium, picking up quite a bit of heat. When air temperature is high, as it usually will be with intense lighting*, it takes very few watering cycles for the reservoir temperature to approximate the air temperature. In aeroponics, this heat transfer is minimized by the relative lack of medium, to such a degree that the reservoir can feasibly be maintained at a temperature close to the nightly air temperature, or to ground temperature, instead.

*Of course lighting cooling systems are in widespread use, but they increase the complexity and expense of the setup, and everything a grower can do to decouple air temperature from water temperature tends to be to great advantage.

The Clean advantage

Accumulation of mineral and organic debris around the roots is another primary difficulty of hydroponics. It is quite difficult to remove such accumulation even with various flushing techniques. If the accumulation were anything like a natural buildup of soil-like material, it would be ok; unfortunately what tends to accumulate is more akin to various types of salt encrustation and algae, both of which are unfriendly to root growth. In aeroponics, by contrast, the dearth of solid media gives no site for such accumulation of unwanted material, and what little does stick to the roots themselves is easily washed off by occasionally running clean water through the system.

So with no solid medium whatsoever, how is aeroponics actually done?

In practice, at least a small amount of solid medium is used, to provide a necessary minimum of toehold or ballast for roots to infiltrate in order to support the plant and keep the stems and leaves from simply falling over. This base medium could be a small cube of rockwool, a yogurt-cup-sized perforated container of lava rocks (real or synthetic), or other inventions. This base medium is suspended in a larger container of any sort which allows the roots to grow out of the medium and dangle directly in air, allows the roots to be misted or sprayed with nutrient solution without spraying everywhere, and collects the solution for return to a central reservoir.

What of the actual plumbing and parts involved?

The wide variety of possibilities here is definitely beyond the scope of a short writeup. Aeroponics tends to be one of the most experimental branches of synthetic horticulture, and does not lend itself nearly as well to retail sales of preconfigured systems as does hydroponics. But therein lies the lure of discovery and the thrill of creating a working system out of parts designed for other uses.

Timing, quantity, delivery, and constitution of nutrient solution

All of these are application-dependent and can only be very generally brushed upon here. One must experiment to determine optimal parameters for a given installation. Suffice it to say that roots should never dry out, and that temperature, pH, and solution strength should be kept under control.

Any reasons not in favor?

All synthetic horticulture systems pose potential maintenance problems that can be quite difficult, foremost among which may be that of salt and debris buildup. It can be a lot of work to prevent such buildup from clogging drains and distribution lines and fouling orifices that emit controlled amounts of solution. Growers have learned to minimize these troubles with a wide range of tricks beyond the scope of this writeup (but as a small example, consider the practice of flushing a system with pure water at intervals... this is not nearly sufficient but it's a start). As difficult as this can be with ordinary hydroponics, the potential for equipment clogging is at least as great with aeroponics. Roots grow best in mist, not hard spray, and this requires fine mist emitters; these are even more easily fouled by mineral or organic buildup than hydroponic drip orifices can be.

Worth the trouble?

Well, is horticulture of any synthetic flavor worth it? Certainly, for pure adventure and experimentation; possibly for profit; and often for the pure satisfaction of seeing organisms simply grow as fast as they can. It helps if you have a tendency toward obsession with twiddling variables. Those looking to simply repeat a given formula do not tend to derive nearly as much satisfaction from it as do the born experimenters and mad scientists. With all that being said, aeroponics is definitely worth looking into if you've done some hydroponic work and want to expand your repertoire. Ultimately, if you want to take your chosen species as far as it can go, the thing to do is invent your own combination of hydro and aero techniques as well as experimenting with the entire spectrum from organic to synthetic nutrients.

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