Spray to protect your crops
spray 1
Compiled by J Fuls (Pr Eng)
Last month we discussed what you should do to prepare for calibration. This month we look at the actual calibration process and the steps you must take to ensure that your sprayer delivers the correct quantity of chemicals. We thank the ARC Institute for Agricultural Engineering in South Africa, who made this article available to the readers of ProAgri Zambia.
On the label of the chemicals one finds how much chemicals must be applied onto each hectare of land. Because the chemicals in the container are very strong and concentrated, we have to mix it with water to spray with the sprayer. If we now know how many tanksfull will be used to cover one hectare with the sprayer, we can calculate how much chemicals should be added to a full tank of water every time we re-fill the sprayer tank.
To calculate this is very simple:
If the label says you should apply six litres of chemicals per hectare:
And if we empty four knapsack sprayers over one hectare (Let us suppose the field is one hectare big):
Then we must add one and a half litres of chemicals to a full tank, every time we refill the sprayer:
The formula is:
(Litres per hectare needed) ÷ (Number of tanks per hectare) = Litres per tank 6 ÷ 4 = 1,5 ℓ/tank Now here is one way to determine how many times the tank will have to be filled to spray one hectare:
a. Flush the tank with clean water
and make sure it is clean.
b. Fill the tank with clean water
and practice in the field. Make sure
everything is working well.
c. Mark off a length of 50 metres
in the field, using your measuring
wheel or tape measure.
d. Stand the tank on a level spot
and fill the tank with clean water
up to a clear mark inside the tank.
e. Spray up and down this 50-metre
strip in the field, to cover a total of 100 metres.
f. Stand the sprayer on the same
spot as previously, and re-fill it to
the same mark, using the measuring jug or measuring bottles.
Write down how much water had to be added to refill to the mark. Suppose you had to add 1,25 litres of water to re-fill the tank. Now for every 100 metres you walk, the sprayer will deliver 1,25 litres of water.
g. Calculate the distance you will have to walk until the tank is empty: On the side of your sprayer tank, you will find marks to indicate how much water is in the tank. Read next to the full mark how much water it can take.
Suppose this is a 16 litre sprayer. The distance you will walk is 16 ÷ 1,25 x 100 = 1 280 metres. This means that the tank will be empty after having sprayed 1 280 metres.
h. Calculate the total distance walked when spraying an area of one hectare. Remember that one
hectare is a land size of 100 metres
long and 100 metres wide. The row spacing you plant may be 0,9 metre. The number of rows over one
hectare will then be 100 ÷ 0,9 = 111
rows.
Now remember that we decided to go up and down on one row when spraying,
to get the required overlap for good coverage.
For each one hectare sprayed, the total distance walked will then be 2 x 100 x 111 = 22 200 metres.
i. Calculate the number of refillings needed for each one hectare sprayed:
If a full tank lasts for 1 280 metres, and the total distance walked while spraying one hectare is 22 200 metres, the tank will have to be re-filled 22 200 ÷ 1 280 = 17,3 times. (Do not worry about the decimal ,3. It simply means we will need a little more than 17 re-fillings for each hectare sprayed.)
j. Calculate the amount of chemicals needed for each re-filling:
Remember the formula at the start of this section:
(Litres per hectare needed) ÷ (Number of tanks per hectare) = Litres per tank If the litres per hectare needed is 6, and the number of tanks per hectare is 17, we can calculate the quantity of chemicals to add to each tank refilling to be: 6 ÷ 17,3 = 0,346 litres, or 346 millilitres. (1 000 millilitres make
up one litre.) One can combine all the calculations into one formula.
Just remember the following:
• We do the test spray over a total distance of 100 metres.
• We measure the test re-filling in litres, not millilitres.
• When spraying, we walk two times over each row.
• The chemicals to be applied per hectare is specified in litres per hectare
• The row width is to be measured in metres
• The tank volume must be given in litres
The formula now looks like this:
The quantity of chemicals to add to each tank re-fill is 5 x (chemicals per hectare) x (row width) x (tank volume) ÷ test volume = Millilitres
In our example above, we had:
• Chemicals per hectare specified on the chemical container: 6 litres/ hectare
• Row width: 0,9 metres
• Sprayer tank volume: 16 litres
• Over the 100-metre test, 1,25 litres were needed to re-fill the tank to the same mark
The quantity of chemicals to mix into each tank refill is:
5 x 6 x 0,9 x 16 ÷ 1,25 = 346 millilitres.
When mixing in the chemicals, use the 500 ml measuring bottle, fill it to just below the 375 ml mark and add this to the spray tank. Put on your face mask to protect your eyes, for the chemicals may splash.
Just remember to rinse the measuring bottle three times into the sprayer tank
before putting it away in a safe place, out of reach of children.
Photo: ohioline.osu.edu
Next month we shall look at safety while spraying.
Published with acknowledgement to the ARC Institute for Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.
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