Production Practices and Quality Assessment of Food Crops. Vol. 1

fruit orchards is described as 1000–2000 L/ha, whereas in the citrus industry Beattie
et al. (1989) describes medium volume as 2000–7000 L/ha.
Low volume or concentrate spray application involves applying less water volume
per hectare, but uses nozzles which produce a greater number of droplets with a
smaller mean size. The application of fine droplets is critical, since this results in
an even distribution of sprays with minimal runoff or waste.
4.3. Droplet size
Spray Technology in Perennial Tree Crops 89
Table 1. Classification of sprays according to volume per unit area for deciduous tree crops.
Spray volume Volume applied Classification Common droplet
(L/ha) sizes (microns)
High (HV) 2000+ Dilute < 30 to > 500
Semi low/medium 1000–2000 Semi-concentrate –
Low (LV) 0100–1000 Concentrate 0100–200
Very-low (VLV) 0030–100 – 000–
Ultralow (ULV) < 30 High-concentrate 0030–60
Bals (1969) stated that ‘The efficiency of a spraying machine is inversely proportional
to the range of droplets it emits whilst the suitability for a specific problem
depends on the actual size of droplets emitted.’ The relationship between the size
of the spray droplet and its volume (and thus the chemical it contains) is cubic,
e.g. a 200 µm droplet contains eight times more spray liquid than a 100 µm droplet.
There are a number of terms used to describe droplets, for example diameter, surface
area and volume (mass). For orchard sprayers the volume median diameter (VMD)
is most frequently used. This term indicates the droplet diameter which divides
the spray into two equal portions, i.e. half the volume of the spray will be less
than the VMD and half will be greater. While this term is useful in describing a
spray pattern it does not provide information on the uniformity of the spray, or on
the upper and lower limits of the droplet spectrum.
The traditional air-blast sprayer using conventional hydraulic nozzles produces
a wide droplet spectrum, with droplets ranging from < 30 µm up to > 500 µm.
Although gravity and wind influence the distribution and deposition characteristics
of droplets, those over 300 µm usually fall straight to the ground. Even if
intercepted by foliage, droplets of this size are unlikely to be retained as they run
or bounce off foliage. As well as being biologically ineffective, these droplets also
contain most of the spray volume. Droplets in the 100–300 µm range sediment under
the force of gravity, with some displacement with wind, but are far more biologically
effective and better retained by foliage. Droplets in the 30–120 µm range
are susceptible to drift and are deposited by a mixture of sedimentation and impaction
processes (wind, turbulence and gravity). Droplets under 30 µm (aerosols) are carried
almost entirely by wind and have insufficient kinetic energy to impact on foliage.
Agricultural engineering has now advanced to the stage where almost any droplet
specification can be produced. However, the droplet size requirements of many target