Preparing Spray Mixes

A very important part of chemical application is the proper preparation and mixing of the chemical and the carrier (usually water). The application rates of most chemicals are given in terms of pounds of active ingredient to be applied per acre. The concentration for a liquid chemical is usually printed on the label. A typical material might contain 4 lb of active ingredient per gallon. Wettable powders (WPs), on the other hand, are specified as a certain percent strength, such as a 50% or an 80% WP, which means that 50% or 80% of the weight of the material in the container is the active ingredient. The rest is inert material (carrier).

Problem: You need to apply 2.0 lb of active ingredient of an 80% WP in a 30.0 gal/ac dilution. The sprayer has a 100.0-gal tank, how many pounds of WP are required to mix 100.0-gal of spray?

Solution: To obtain 2.0 lb of active ingredient, 2.5 lb of powder would be required (80% of 2.5 = 2.0). If the solution is to be applied at 30 gal/ac, mix 2.5 lb of 80% WP in each 30 gal of water. The tank holds 100 gal or 3.33 units of 30 gal. Because 2.5 lb of WP should be added to each 30 gal of water, you should add 3.33 x 2.5

or 8.33 lb of WP to each 100 gal. Or by ratio: 2.5 lb WP W lb WP


To mix the spray, add 8.33 lb of WP to a partially filled tank, mix it thoroughly, and add water to make 100 gal of mixture. During spraying this mixture must be continuously agitated to prevent the WP from settling.

Preparing a mix using liquid chemicals can be accomplished with the same procedures.

Problem: A liquid concentrate contains 2.0 lb of active ingredient per 5.0 gal. The desired rate is 1.0 lb of active ingredient per acre at a rate of 20.0 gal of chemical and water (solution) per acre. If the sprayer tank holds 180.0 gal, how much water and how much concentrate should be used for each tank full?

Solution: If the liquid contains 2.0 lb of active ingredient per 5.0 gal, then 2.5 gal would contain 1.0 lb of active ingredient. Thus, for each acre 2.5 gal is mixed with 17.5 gal of water (20-2.5) to get 20 gal of spray. By ratio:

2.5 gal concentrate V gal concentrate 20.0 gal solutiion 180.0 gal solution 2.5 x 180.0

V = volume in gallons and 180 gal - 22.5 gal = 157.5 gal. Thus, for every tank 22.5 gal of chemical is mixed with 157.5 gal of water.

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