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The aeroponic cultivation

Soil-less agricultural production methods, such as hydroponics and even more so aeroponics, allow significant savings in water and inputs, including pesticides, thus being compatible with the growing sustainability needs of the primary sector to reduce environmental impact and effectively tackle climate change

by Valeria Tadini
January 2024 | Back

The need (and sometimes the urgency) to meet the growing demand for food and to improve its quality, but also the urgency to reduce environmental impact and cope with climate change, is forcing the gradual transition from traditional cultivation to soilless agricultural production methods. The implementation of these techniques is hardly new: as early as the 19th century, hydroponic cultivation was introduced, with plants grown in controlled environments on inert substrates and with liquid nutrient solutions or with the latter alone. Precise adjustments in temperature, humidity, and lighting allow for optimized crop development with a tangible reduction in water and fertilizer consumption compared to traditional methods.


At the beginning, it was hydroponics...

The first commercial crops grown on a substrate made around 1930 and then refined later, required careful choice and management of materials in order to create and maintain favorable conditions for plant growth. Both organic and inorganic substrates were used for the purpose of ensuring adequate root aeration, facilitating oxygen uptake, and maintaining the best chemical stability of the nutrient solution. Compared with traditional growing methods, hydroponic ones include different approaches with unique characteristics and advantages but also specific disadvantages. In 1929, "Deep Water Culture" was introduced, which took advantage of static tanks containing sand and a nutrient solution but carried a risk of root hypoxia. "Deep Recirculating Water Culture," on the other hand, involved forced insufflation of air and recirculation of the nutrient solution, with the root system partially or totally exposed to air. As the name suggests, the 1965-introduced "Nutrient Film Technique" (NFT) aimed to create a thin film of nutrient solution flowing in conduits placed on a slope, thus providing efficient control of mineral nutrition. Undeniable advantages, such as the absence of substrate and the ease of restarting with new crop cycles, were countered in NFT by vulnerability to electrical blackouts and less water, nutrient, and thermal inertia. A few years later, in 1976, the "Floating System" was introduced: the planting system was simple as it used floating plants above aqueous nutrient solution, but at the same time required careful management to ward off root hypoxia, algae development and excessive atmospheric moisture.


... later followed by aeroponics

Introduced in 1973, particularly to solve hypoxia problems, and later refined in 1985, aeroponics is now an evolved and sustainable methodology. Compared with hydroponics, which still introduced the technique of growing without soil, aeroponics takes a more sophisticated approach, developing the ways by which plants absorb water, nutrients and oxygen. In fact, the roots are suspended in the air instead of submerged in water, which allows for maximum oxygen exposure for faster and more vigorous growth. In addition, in the aeroponic system,  the supply of water and nutrients is carried out by controlled misting, directed directly at the root system of the plants, and delivered by a fertigation unit, including tanks, pumps, pipes and distribution nozzles. Activated by sensors that monitor environmental conditions, the recirculation pump makes the nutrient solution flow in the desired amount, the correct composition of which is maintained with the use of physical filters and the implementation of disinfection methods.   Specifically, it is important to keep the temperature between 18 and 25 °C during the day and 15 to 18 °C at night, with a humidity of about 60 to 70 percent. LED elements provide artificial lighting required for photosynthesis, while a constant pH check ensures proper nutrient concentration. Regular maintenance is essential for the appropriate functioning of aeroponic establishments, especially in terms of nozzle cleaning, while continuous monitoring of the state of the plants is essential to identify stress or disease early on.

Plant-wise, a watertight gutter is planned, with an internal pipe equipped with nozzles, which spray the nutrient solution onto the roots of the plants intermittently, with cycles of 15-40 seconds and breaks of 3-30 minutes, depending on the evapotranspiration conditions and the vegetative stage of the crop. Flat or "rooftop" (A-system) panels are mounted above the gutter to increase the crop density of leafy vegetables.

Aeroponic greenhouses are usually made of honeycomb polycarbonate or polyethylene to allow optimal passage of sunlight necessary for photosynthesis; they commonly have a pavilion shape depending on the specific needs of the crop. There are no openings to the outside environment, a factor that prevents contamination while improving thermal efficiency.   Greenhouse shading can also optimize plant development by reducing infrared radiation and thus helping to control temperature extremes. The selection of the most suitable shading system (which can minimize irradiation intensity by 10 to 90 percent) depends on the characteristics of the outdoor radiation and the needs of the crops (salad greens, for example, require more shading than solanaceous plants. Solutions range from the implementation of special paints to the use of automated moving sheets. An interesting opportunity is offered by the use of photovoltaic modules, which, in addition to reducing the plants' exposure to light, produce electricity that can be used directly on the farm itself or can be marketed.

The choice of the most suitable nozzles for spraying is essential, as they must ensure the most appropriate droplet size of nutrient solution (between 20 and 100 μm).   Smaller spray tip diameters create small droplets that saturate the air, while larger droplets promote direct root contact. Careful control of air humidity is also critical. In particular, in aeroponic cultivation, reference is made to what is known as the "Vapour Pressure Deficit" (VDP), which is the difference, at a given temperature, between the amount of water vapor actually present in the air and the amount the air could contain at saturation. It is, therefore, a parameter that considers both temperature and water vapor pressure simultaneously: high VDP corresponds to low relative humidity. 

The nutrient solution is circulated through the channels by an electric pump; the unabsorbed portion is collected, filtered, disinfected, and reused, making recirculation crucial for monitoring and correcting pH and conductivity. Also critical for the best effectiveness of photosynthesis is the control of carbon dioxide concentration. Careful control of air temperature is also necessary in the cold season, particularly at night. To ensure even heat distribution and prevent temperature swings, we often use unit heaters, radiant bodies and substrate heating systems, taking care to avoid excessive air movement to minimize heat loss. Conversely, in the summer period, up to about 27°C the temperature can be managed by ventilation (natural or mechanical) and shading. Still, for higher values, it is necessary to install specific cooling systems, such as fan pads, fogging, or real air conditioning establishments, for example, based on vapor compression.


Advantages, challenges and future prospects of aeroponics

Compared with conventional cultivation, water savings are as high as 95 percent, while fertilizer savings range from 80 to 90 percent due to the optimization of root oxygenation and the closed-loop process, which allows the recovery and re-injection of unabsorbed water. In addition, the lack of communication between airborne roots eliminates the risk of contamination between plants and, with it, the use of phytosanitary treatments. In addition, with aeroponics, it is possible to produce on a zero-mile basis, growing close to consumption centers. It is also a valuable technique for cloning plants that are difficult to propagate vegetatively, especially useful for species intended for medicinal uses or high market demand. In addition, thanks to progressive automation and the use of artificial intelligence, aeroponic establishments are now able to "learn" from historical data, thus adapting to changing environmental conditions and optimizing performance over time. 

One point of particular note is the high initial establishment and subsequent maintenance costs, which, however, can be recovered as productivity increases and particularly as inputs are reduced. The high power consumption for handling the nutrient solution is another factor to consider, as is vulnerability to power outages and the need to carefully manage air flows. If production cycles exceed 4-5 months and the canals are shallow, a root mat may also form at the bottom, which hinders the proper outflow of the sprayed nutrient solution, causing possible root hypoxia. This problem can be solved by modulating the amount of nutrient solution in relation to evapotranspiration conditions and the stage of crop development. 

In Italy, several companies market components for aeroponic cultivation: among others, important players in the sector are Agricontrol, Netafim and Idromeccanica Lucchini. The latter, in particular, also provides solutions aimed at managing water resources. More generally, in addition to plant engineering, companies in the sector provide consulting and design services to optimize the entire process of aeroponic cultivation. Hydroinvent of Gallarate (VA), on the other hand, deals with the design and construction of establishments for above-ground, hydroponic and aeroponic cultivation.


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