# Spray Mix Calculations

##### H. Willard Downs and William W. Casady

Department of Agricultural Engineering

##### Fred Fishel

Department of Agronomy

Liquid pesticide sprayers must apply the proper amount of a carefully mixed spray solution to be effective in controlling weed and insect pests. This publication describes procedures for determining how much pesticide to mix in the tank so the right amount of pesticide will be applied per acre.

Pesticides formulated to be applied as sprays are sold both as liquids and as dry materials such as wettable powders. Calculations for mixing liquids are different from calculations for dry materials. This two-part guide provides specific instructions for mixing both liquid and dry pesticides.

## Liquid pesticides

**Step 1Determine the recommended application rate**

Read the label. The recommended range of application rates for the specific formulation is given on the label. The selected pesticide rate should be based on soil, target pest and crop conditions.

**Caution**

Normally, the label will list pesticide application rates per acre in quarts or pints. However, some labels may list the application rate in pounds of active ingredient. If so, continue with Step 2. If the label refers to quarts, pints, or other volume measurements, go directly to Step 4.

**Step 2Determine the concentration of active ingredient**

Read the label. The label will show the amount of active ingredient in each gallon of pesticide formulation. This amount is normally shown as pounds of active ingredient per gallon (gal).

**Example**

acid equivalent = 4 pounds per gallon

**Step 3Calculate the volume of pesticide product to apply per acre**

If the label gives the pesticide application rate in volume units such as quarts or pints, then the amount was found in Step 1. However, if the rate is shown as pounds of active ingredient per acre, then it is necessary to calculate the volume of pesticide to apply per acre. This volume can be found by dividing the pesticide application rate (Step 1) by the number of pounds of active ingredient per gallon (Step 2).

Gallons of pesticide per acre = application rate (pounds per acre) ÷ concentration or acid equivalent

**Example**

Suppose you want to apply 1.5 pounds of 2,4-D per acre and the 2,4-D contains 4 pounds of active ingredient per gallon.

Gallons of 2,4-D per acre = 1.5 pounds per acre ÷ 4 pounds per gallon = 0.375 gallon per acre or 3/8 gallon per acre

You may find it useful to convert gallons per acre to pints per acre for measuring purposes.

Pints of 2,4-D per acre = 3/8 gallon per acre × 8 pints per gallon = 3 pints per acre

**Step 4Calculate the number of acres sprayed by a full tank of the spray mixture**

**Note**

If you use a sprayer with two or more tanks remember to consider the total volume of all tanks and to divide all ingredients proportionally among the tanks. All references to "tank" in the following material refer to the combined capacity of all tanks.

The number of acres sprayed by a full tank is found by dividing the tank capacity by the sprayer application rate, which was found during calibration

Acres per tank = Total tank capacity (galllon per tank) ÷ Application rate (gallons per acre)

**Example**

Your spray tank holds 400 gallons and your sprayer application rate is 20 gallons per acre.

Acres per tank = 400 gallon per tank ÷ 20 gallon per acre = 20 acres per tank

Small fields can be sprayed with partially filled tanks. The pesticide and carrier (water) are added to the tank until the tank is filled to the correct level. The correct volume of spray is the sprayer application rate multiplied by the number of acres.

**Example**

You want to spray a 12-acre field and your sprayer applies 20 gallons per acre.

Gallons of spray mixture = application rate (gallons per acre) × area to spray (acres)

Therefore, put (20 × 12 =) 240 gallons of pesticide and carrier in the tank.

**Step 5Calculate the volume of pesticide to mix in the tank**

The volume of pesticide added to the tank is the number of acres per tank (Step 4) multiplied by the volume of pesticide per acre (Step 3).

Volume of pesticide per tank = (acres per tank) × volume of pesticide per acre (gallons)

**Example**

You want to spray a full 400-gallon tank.

Gallons of 2,4-D per tank = 20 acres per tank (*Step 4*) × 0.375 gallon per acre (*Step 3*) = 7.5 gallons per tank

If you want to spray the small 12-acre field, the amount of 2,4-D added to the tank before bringing the volume up to 240 gallons would be:

Gallons of 2,4-D = 12 acres × 0.375 gallon per acre = 4.5 gallon

**Note**

Partially fill the spray tank with water before adding pesticides.

## Dry pesticides

Some pesticides are formulated and sold as powders and water dispersible granules for mixing with water. These dry formulations are recommended in units of weight per acre. The amount of active ingredient in these products is shown in percent.

**Step 1**

Determine the recommended rate of application.

Read the label.The recommended range of application rates is given on the label. Be sure the rate you use is the right rate for your soil, target pest, and crop conditions.

**Caution**

The rate can be shown in pounds of active ingredient **or** pounds of product. If the rate is shown as pounds of active ingredient, continue with Step 2. If the rate is shown as pounds of product, go directly to Step 4.

**Step 2Determine the concentration of active ingredient in the pesticide formulation**

Read the label. The label will list the percentage of active ingredient.

**Example**

atrazine: 80 percent

**Step 3**

Calculate the weight of pesticide product to apply per acre. The weight of *pesticide product* to apply per acre is the pesticide application rate (pounds per acre) divided by the percent concentration.

Pounds of pesticide product per acre = (application rate × 100) ÷ percent active ingredient

**Example**

You want to apply 1.5 pounds of atrazine and the label shows atrazine: 80 percent.

Pounds of atrazine formulation per acre = (1.5 pounds of atrazine per acre (*Step 1*) × 100) ÷ 80 (*Step 3*) = 1.875 pounds per acre

**Step 4**

Calculate number of acres sprayed by each full tank. Follow the procedure provided in Step 4 for liquid pesticides.

**Step 5**

Calculate the weight of pesticide to mix in the tank.The weight of pesticide added to the tank is the number of acres per tank (Step 4) multiplied by the weight of pesticide per acre (Step 3).

Weight of pesticide per tank = (acres per tank) × application rate (pounds of product per acre)

**Example**

You want to spray a full 400-gallon tank of spray.

Pounds of atrazine product per tank = 20 acres (*Step 4*) × 1.875 pounds of product per acre (*Step 3*) = 37.5 pounds per tank

If you want to spray the small 12-acre field, the weight of atrazine product added to the tank before bringing the volume up to 240 gallons would be:

Pounds of atrazine product = 12 acres × 1.875 pounds of product per acre = 22.5 pounds

**Note**

Partially fill the spray tank with water before adding pesticides.