How much pesticide?

Sprayer math can be intimidating, but it’s very important. It ensures you are applying the correct rate to your crop. When combined with a calibrated sprayer, it also ensures you don’t run out of spray mix before the job is done, or have to deal with a lot of left overs.

There are many ways to perform sprayer math, and you need only look to local pesticide safety courses, industrial catalogues, and extension resource centres for examples. If you’re already comfortable with your current method, don’t mix and match with others. Sprayer math is a series of related calculations that employ constants to keep the units straight. It’s all or none.

Here are some standard situations sprayer operators face all the time. Grab your calculator or favourite smart phone app – it’s math time!

Step 1- How large is the area you need to spray?

Multiply the length of the area you plan to spray times the width. If you are using metres, then divide the product by 10,000, which is the number of m2 in a hectare (ha). For feet and acres, divide by 43,560 which is the number of ft2 in an acre (ac):

spray-area-math

 

 

Step 2- How much product is needed to spray the area?

If you measure your area in hectares, use the rate shown on the label. In Canada, it will be written in Litres or kilograms per hectare (L/ha or kg/ha). If you measure your area in acres, you’ll have to convert by multiplying by a constant: 0.4.

product-per-area

 

 

Now multiply the area you want to spray (step 1) by the rate (step 2).

product-per-area2

 

 

Step 3- How far can you go on a full tank?

You know your sprayer output (determined through calibration) so you divide that into your tank size. Watch your units:

full-tank-distance

 

 

Step 4- How much pesticide per tank? 

Multiply the area that can be sprayed per tank (Step 3) by the pesticide rate (Step 2). Watch your units:

pesticide-per-tank

 

 

 

Step 5- How much area is left to spray?

Just subtract what you’ve already sprayed from the total area.

area-left-to-spray

 

 

Step 6- How much pesticide in the last, partially-full tank?

Multiply the area you have left to spray (Step 5) by the pesticide rate (Step 2). Watch your units:

pesticide-partially-full-tank

 

 


Step 7- How much spray mix will I need for the partial tank to finish spraying the total area?

Multiply the area you have left to spray (Step 5) by the sprayer output (determined through calibration). Watch your units:

spray-mix-for-total-area

 

 

Sample problems

Time to test your knowledge. Let’s suppose you want to apply a product rate of 3 L/ha to your blueberries. You calibrate your sprayer and determine your output to be 50 L/ha. Your tank holds 400 L of spray mix. Your planting is 500 m long and 200 m wide.

Q1- How large is the area you need to spray?

area-to-spray

Q2- How much product is needed to spray the area?

product-to-spray-the-area

Q3- How much area can be sprayed on one tank?

area-on-full-tank

Q4- How much product should be added to a full tank?

product-needed-full-tank

Q5- After the tank is empty, how much area is left to spray?

area-left

Q6- How much product to add to the last, partially full tank?

product-partially-full-tank

Q7- How much spray mix will be needed to finish spraying?

spray-mix-to-finish-spraying