Air-Assist Spraying of Crops
Much has been said and done over the past thirty years to improve the efficiency of air-blast spraying. Improved fans, oscillators, baffles, even electrostatics, have all contributed to better coverage and penetration especially in the orchard and tree-crop areas, where air-blast machines are the industry staple.
However, when it comes to spraying either horizontally or vertically down, as in the case of row-crops, vegetables and nursery stock, air blast sprayers generally lack the ability of delivering good coverage to the undersides of the leaves and many times are not effective in treating infestations.
Hence, they can be relegated to maintenance work such an application of nutritional and preventive fungicides and insecticides, but when eradication is needed, in come the old style boom sprayers!.
But take heart, several spray professionals (including yours truly), have been working over the years to develop systems that would delivery the spray material to the heart of the plant canopies and attain the coverage that could allow us to use air-blast machines to control infestations, especially in difficult to penetrate crops.
The Concept of Air-Assist Spraying Goes Way Back
The concept of Air-Assist spraying in which the droplet pattern is sprayed into an air-stream directed at the crop goes back before the development of the Air-Curtain (Degania/FMC) type boom sprayer. This machine was effective for open and simple canopy crops such as cereals, wheat, barley, corn, even cotton, where the cloud of spray could drop down into the crop and, through turbulence, cover most of the plant surfaces.
However, when you got into vegetables: cucurbits, melons, tomatoes, soy, beans and other tight canopies, the coverage just wasn’t there. The same held true for the “cannon” sprayers that delivered their pattern laterally and could not get the droplets to penetrate the canopies, much less cover the undersides of the leaves.
Cannon sprayers produced relatively small volumes of air at high speed, which reduced the inertia of the blast and therefore curtailed its ability to displace the air that was already in the way (between the sprayer and the target) in order for the spray material to reach its target. Consequently, forward speeds had to be lowered to increase the sprayer’s reach.
Growers were buying tractors with “crawler gears” and putting out higher volumes of spray than they had originally intended, in order to get the job done. And an extra added headache to all this was the considerable amount of uncontrollable pesticide drift that these machines generated.
Cannon Sprayers – Relatively Efficient Portion of Delivery
The cannon sprayers did have a relatively efficient portion of their delivery: those machines that had supplementary “horns” under the main cannon designed to reach the crops close to the sprayer, were getting good coverage on the plants sprayed by those “horns”.
The reason was that the “horns” were aimed into the crops at an angle and the air pattern coming out of them was relatively close to the targets and directed straight at them. Thus the spray was penetrating the foliage and covering most of it on both sides.
Three years ago, the writer developed a “directed spray” system for use on melons, by modifying a sleeve boom air sprayer into a duct boom sprayer with drops that ended in horns that were oriented directly at the crop. These horns held the nozzles so that the spray was injected into the air stream and the horns directed it onto the crop.
By orienting the horns at a 30 degree downward and 30 degree inwards (towards the crop in the bed) angles, the coverage obtained was far superior to anything attained previously with other systems.
This technique proved that the air assisted spray had to be aimed directly at the canopy and relatively concentrated so as to be able to penetrate the foliage and, once inside, create enough turbulence so that the droplets in the pattern reached all plant surfaces.
The Concept of “Directed Spray”
The concept of “directed spray”, long in use with standard boom/nozzle sprayers, has now been developed by several sprayer designers and manufacturers, including AirTech Sprayers of Winter Haven, Florida and is based on the use of directed “horns” that come off an “air-boom” which is a cylindrical duct that is held horizontally by the boom.