- Author: Shelley Stone-Schmidt
Why Are Calculations Important?
Using just the right amount of pesticide and fertilizer minimizes the potential for personal, property, or environmental damage and maximizes consumers' return on the dollar. This publication will assist the homeowner in selecting products and calculating application rates for pesticides and lawn and garden fertilizers commonly used around the home.
How Do I Calculate The Area To Be Treated?
Typically, use rates for lawn care products are stated per 1000 square feet or per acre. Rates for flowers, shrubs, and vegetables often are given per 100 square feet. To determine how much pesticide or fertilizer is needed to do a job, first calculate the size of the area to be treated.
Square or Rectangular Area
The area of a square or rectangle is determined by multiplying the length times the width:Area = L x W A rectangular area measuring 80 ft by 60 ft equals 4800 square feet: Area = 80 ft x 60 ft = 480 = 4800ft ²
The area of a circle is determined by using the equation r2,or 3.14 x the squared radius of the circle: Area = 3.14 x r2 A circular flower bed measuring 40 ft in diameter would have a radius of 20 ft, so: Area= 3.14 x 20² = 3.14 x (20 ft x 20 ft) = 1,256 ft ²
Irregularly shaped flower beds, etc., usually can be divided into smaller, geometric areas (square, rectangle, circle, triangle) to facilitate total area calculation. Simply calculate the area for each individual shape and add them together. Conversely, it may be necessary to compute the total square footage of structures within the area which will not be treated—buildings, patios, sidewalks, ponds, etc.—and subtract it from the overall area.
How Much Do I Need To Mix?
Read the product label for specific instructions on the amount to mix. Some sample calculations follow.
|=8 pints||=2 pints||=16 fluid ounces|
|=4 quarts||=32 ounces||
|= 8 fl oz||
|=32 ounces||= 16 tbsp|
A product label says to use 3 ounces per 1000 square feet. How much product will you need for a 4800-square-foot area?
4800 ft² ÷ 1000 ft² = 4.8 (units of 1000 ft²)
4.8 x 3 oz = 14.4 oz of product
A product label prescribes 2 pounds per acre. How much is needed for a4800 square foot area? Remember that one acre equals 43560 square feet.
4800 ft2 43,560 ft2 per acre= 0.1 acre
0.1 acre x 2 lb = 0.2 lb of product
0.2 lb x 16 oz = 3.2 oz
You need to spread a crabgrass herbicide over a 2400-square-foot area, and you purchase a 25-pound bag of the herbicide. The label states that the contents of the bag will cover 5,000 square feet. How much of the bag will be needed?
2400 ft2 ÷ 5000ft2 per bag = 0.48 bag
0.48 bag x 25 lb per bag = 12 lb
You need to apply a grub control insecticide over 4800 square feet. The label instructions say to apply 1 gallon of water and 4 teaspoons of insecticide for each 200 square feet. How much water and how much insecticide is needed?
4800 ft² ÷ 200 ft² per gal = 24 gal
24 gal x 4 tsp = 96 tsp insecticide per 24 gal water
48 teaspoons equal 1 cup, so convert the 96 tsp to cups, as follows:
96 tsp ÷ 48 tsp = 2 C insecticide per 24 gal of water
If math is just not your thing, and you have access to a computer, there is a wonderful fertilizer calculator provided by the Master Gardener Program in San Diego, CA. that will solve the problem. The Fertilizer calculator has entries for N, P, K. values, Rate of Application, Area of Application and Cups and Pounds conversions. http:http://www.mastergardenerssandiego.org/resources/fertilizer.php
The guidelines presented here are basic, providing the homeowner conversions of liquid pesticides and dry fertilizer rates that are most often given in pounds or ounces per square foot. However, the MG may encounter more complex conversion when working with products such as: wettable powders, emulsion concentrates, dry and water based products and soluble solutions. For a more detailed conversion charts see The University of Georgia's http://extension.uga.edu/publications/detail.cfm?number=B931 for further details.
•Pesticide labels are legal documents.•If you don't read and follow label directions carefully, you are at fault—and liable—when problems result.
Rates and Application
Mix dilutions thoroughly. Choose a calm, cool day to make applications to the lawn or garden in order to avoid movement (drift) of the material onto non target areas. This may prevent damaging a neighbor's property and reduce your potential risk. Follow label rates, and remember: More is NOT better!Granular Application Suggestions
•Determine the area to be treated.•Know the amount of product needed to treat the area.•Set your granular spreader at a low setting and walk quickly over the area. The low setting and brisk pace will necessitate covering the area twice, thus guarding against depletion of the measured product before the total area is covered.•Apply in a uniform pattern until the required amount of the product is used up.•Wear rubber gloves and boots to minimize exposure.
•Set your granular spreader at a low setting and walk quickly over the area. The low setting and brisk pace will necessitate covering the area twice, thus guarding against depletion of the measured product before the total area is covered.
•Apply in a uniform pattern until the required amount of the product is used up.
•Wear rubber gloves and boots to minimize exposure.
•Follow all label instructions carefully.
•Wash your hands, forearms, and face with soap and water after mixing, loading, applying, or otherwise handling pesticides and fertilizers.
•Measuring cups and spoons used for pesticides and fertilizers should never be reused for other purposes.
•Clothing worn during mixing, loading, and application should be washed separate from the family laundry.
•Post a KEEP OFF sign on treated areas, outdoors, until sprays have dried.
•Clean application equipment immediately after use, prior to storage. Sprayers should be rinsed by filling with water, shaking well, and pouring the rinsate onto the actual application site. Repeat at least once, preferably twice.
•Do not apply clippings from recently treated grass as mulch around flowers beds or vegetable gardens.
•Store unused pesticides safely OUT OF REACH OF CHILDREN.
Equipment and Clothing
•Read the label
•Use chemical-resistant gloves—never cotton or leather.l before mixing and applying pesticides.
•Wear long pants and sturdy shoes, not sneakers or sandals.
•Wear a long-sleeved shirt.
•Wear safety goggles to protect the eyes.
•Read the label to determine how long you should wait after applying a pesticide before harvesting the crop for human consumption.
•Always wash treated fruits and vegetables before eating.
•Never apply pesticides to crops not specified on the label.
Wilford, Fred, Coordinator Purdue Pesticide Program, and Martin, Andrew, Specialist Purdue Pesticide Program. March 2003,
Pesticide Safety and Calibration Math for the Homeowner.