12
Calibrating Small Sprayers
The spray volume that a backpack or hand-held
sprayer will apply per acre can be determined by field
testing the sprayer on a portion of an acre. The size of
the test area commonly used is 1/100 of an acre. It is
important that the test area surface is similar to the
surface to be sprayed, so the walking speed will
remain the same.
Step 1: Establish a test run distance to spray
1/100
acre (40.5 m
2
)
according to the swath width
of the sprayer.
Swath width
Test run length
0.5
metres
81.0
metres
1.0
metres
40.5
metres
1.5
metres
27.0
metres
2.0
metres
20.2
metres
Step 2: At a comfortable walking speed, spray the
test area and measure the volume of water used
(
repeat 2 or 3 times to obtain an average). This is
the amount applied to 1/100 acre.
Example: 2 litres
Step 3: Multiply the figure arrived at in Step 2 by
100
to get the spray volume per acre.
Example: 2 L x 100 = 200 L/acre
Step 4: Determine the amount of pesticide to add
per tank load. Divide the volume applied per acre
by tank capacity to determine the number of fills
required to spray an acre.
Example: 200 L/acre ÷ 20 L/tank = 10 fills
Divide the chemical rate per acre by the number of
tank loads required to spray an acre to determine
the amount of product to add per tank load.
Example: 1 L/acre ÷ 10 fills = 0.1 L/tank
Preparation and Application
of Pesticides
Tank Mixtures
Tank mixtures are two or more separate pesticides
mixed in the sprayer tank, as opposed to a mixture
formulated by the manufacturer. Tank mixing is often
done to reduce the number of applications of
pesticide. Mixing a grassy herbicide with a broadleaf
herbicide is one of the most common mixtures.
Another reason for mixing herbicides is to include
more than one mode of action on the weeds to
combat the development of resistance in the target
pest. Lately, there have been a lot of pesticide
products put together as co-packs. That is when the
tank mix partners are placed in individual containers
in the same box.
Adjuvants (surfactants, wetting
agents, spreaders, etc.)
Adjuvants are added to a pesticide to enhance
application and/or performance. The most common
adjuvants used in pesticides are surfactants. If
adjuvants are required, use only those products
named and recommended on the label. Failure to do
so could result in the following:
•
crop injury
•
reduced pest control
•
invalidation of pesticide warranty
Surfactants facilitate and enhance the emulsifying,
dispersing, wetting, spreading, sticking, penetrating
or other surface-modifying properties of liquids to
bring about enhanced pesticidal action. Because these
chemicals produce physical changes at the surface of
liquids, surfactants are often referred to as surface-
active agents.
Surfactants are generally classified into two major
groups based on how they react in water: ionic or
non-ionic. Ionic surfactants break down into two
entities when mixed in water, – a positively charged
ion (cation) and negatively charged ion (anion). An
example is ammonium sulphate (2 NH
4
+
+ SO
4
–
).