The Science of Spray Application - Agriculture

Perhaps the single most important factor in spray performance is the droplet size of the pesticide you are using. If the droplets are either too small or too large, efficacy is reduced. The right size droplet can mean a huge difference to your bottom line. Let’s take a look at the reasons behind this.

The small droplets shown here are below 150 microns in diameter. Small droplets have a low terminal velocity. Their low kinetic energy causes them to decrease speed faster and float longer. Smaller droplets cover more surface area on the plant and have greater droplet density. Without some form of air assistance, however, they can evaporate or drift away from the target, resulting in poorer performance for you and potentially damaging crops in your neighbor’s field.

The larger droplets that are shown here are above 300 microns and have a high terminal velocity. Their relatively greater mass and higher speed make it easier to manage their trajectory and to reach the target. However, these droplet sizes hit the plant at higher speeds and with greater momentum, which means that they tend to have undesirable aspects of bounce, shatter, and runoff. This reduces their retention on leaf surfaces. If the leaf retains fewer droplets, then the pesticide’s performance is significantly reduced.

One large droplet of 400 microns contains the same volume as 8x 200 micron droplets. Spraying coarse droplets means you often need to increase your water volume to create a sufficient number of droplets.

Large droplets are ideal for spraying pre-emergent herbicides onto bare soil, but they are not suitable for use when spraying fungicides and insecticides where contact is important for product efficacy.