by Dr Lynette Morgan
Aeroponics is a growing method where the plant roots are suspended in the air with a fine mist of nutrient solution applied either continually or intermittently over the root surface. While we tend to think of aeroponics as a recent development in the hydroponics field, it has actually been in use since the 1940's, although largely as a research tool rather than as an economically feasible method of crop production. In the last decade however there has been the development of a number of aeroponics systems both for use commercially and as small 'hobbyist' systems. The reasons for the interest in aeroponic technology stem from the fact that using traditional hydroponics systems (media, NFT and flood and drain), has often made controlling conditions in the root zone difficult, particularly where growers are battling a tropical climate. And for this reason much of the large scale commercial development of aeroponics has occurred in countries such as Singapore where temperate crops such as lettuce and other salad greens can be successfully grown using aeroponics technology where other hydroponic methods failed.
For the smaller hydroponic grower, aeroponics offers other major advantages: Oxygen starvation, stagnation and water logging and over watering are common problems in hobbyist hydroponic systems and many plants have been lost to these causes without the grower even being aware of what is happening. It can be difficult to supply the plant's root system with sufficient oxygen, particularly in warmer conditions where the plant's oxygen requirement is much higher and less dissolved oxygen is held by the nutrient solution. Aeroponic methods ensure the plants not only have sufficient water, nutrients and of course oxygen, but the temperature inside the root chamber can be easily controlled, ensuring temperature stress of the plant does not limit growth. For this reason, the use of a well designed and maintained aeroponic system can give huge increases in growth rate and plant survival when compared to many of the standard NFT or media based hydroponic systems.
How Does Aeroponics Work
With an aeroponic system the young plants can be either raised as seedlings using especially designed lattice pots or cuttings can be placed directly into the aeroponic system for rapid root formation. Lattice pots allow the root system to develop down into the aeroponic chamber or channel where it is regularly misted with nutrient. There is a high success rate with plant cuttings which are rooted in aeroponics - in fact this method has been extensively used as a research tool into root development on many difficult to propagate plant species. The base of the cutting is supplied with high levels of oxygen and moisture in a humid environment which prevents desiccation and accelerates root formation.
Once the young plant has been established into the aeroponics system, the root system rapidly develops in the chamber or channel. What is important at this stage is that the optimum size of the droplets is maintained within the system for maximum efficiency. There is a huge range of aeroponic nozzles so selection of a droplet size range which best suits the plant and system used is fairly easy. Spray droplets less than 30 microns tend to remain in the air as a 'fog' and are not readily absorbed by the roots. The ideal droplet size range for most plant species is 20 - 100 microns. Within this range the smaller droplets saturate the air, maintaining humidity levels within the growth chamber, the larger droplets 30 - 100 microns make the most contact with the roots, while any droplets over 100 microns tend to fall out of the air before containing any roots.
Misting Frequency and Nutrient Reservoir
Aeroponic systems may mist the root system continuously, or intermittently and both methods work well, since water logging and oxygen starvation are not a problem in aeroponics. The major advantage of intermittent aeroponics systems is the saving in running cost, since the pump is only on for a short period of time, but the roots are still contained within the nutrient, moisture and oxygen rich environment between mistings. And since aeroponics systems do require larger pumps with greater energy requirements than other hydroponic systems, this saving is an important feature. Always look for systems that either link light levels to misting frequency or have the ability to program in a large number of misting cycles per 24 hours. As a general rule, a misting cycle of 1 -2 minutes of misting followed by 5 minutes off will ensure the root system does not dry out under most conditions.
Aeroponics systems are further divided into those which have a separate nutrient reservoir and pump the nutrient up into the root chambers and those with an all in one' contained chamber and nutrient tank. The simpler aeroponic systems spray the nutrient up from the reservoir in the bottom of the root chamber, where it drips back down after misting the root system. By the time the plants are mature, the root system has often grown down into the nutrient solution stored in the base of the chamber and blockages can occur. Larger aeroponic systems return the nutrient after misting to a separate nutrient reservoir.
The Importance of Filtration, Light and Pressure
Obviously if a mist of the correct droplet size is to be produced and delivered to the plants, a certain amount of pressure is going to be required - so larger pumps are required in aeroponics systems than would be used in NFT. In order to maintain this pressure, filters need to be kept clean, nozzles free from salts, debris and algae and pumps well maintained. Another important aspect is to ensure the aeroponic chamber is light-proof, growth of algae soon results in blocked nozzles and needs to be avoided. Both inline filters and pump filters assist with removing any particles form the nutrient which might block nozzles, and having sufficient pressure will also assist in keeping nozzles clear and function correctly.
Nutrients
With aeroponic systems the same inorganic nutrients and methods of EC and pH measurement can be used as in other hydroponic systems. However, extra care needs to be taken with certain 'additives' - particularly those which are organic in origin. These sometimes contain tiny particles of suspended solid matter which doesn't cause problems in NFT or media systems, but can block the aeroponic nozzles or jets.
