12.3 Active Ingredient & Finished Spray Dilution Mathematics

12.3 Active Ingredient & Finished Spray Dilution Mathematics

Once equipment is properly calibrated and treatment areas are accurately measured, applicators must calculate the precise quantity of pesticide product required to fill spray tanks or mix active chemical solutions. Mixing errors result in illegal pesticide strengths, crop toxicity, pest control failure, or hazardous chemical waste.

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Product Calculations: Per Acre vs. Per 1,000 Square Feet

Agricultural pesticide labels specify product rates per acre (e.g., $2.0\text{ quarts/acre}$), while turfgrass, ornamental, and structural labels state rates per $1,000\text{ square feet}$ (e.g., $1.5\text{ fluid ounces per 1,000 sq ft}$).

Converting Acre Rates to 1,000 Sq Ft Rates:

Since $1\text{ acre} = 43,560\text{ sq ft}$, there are $43.56$ units of $1,000\text{ sq ft}$ in an acre.

$\text{Rate per 1,000 sq ft} = \frac{\text{Rate per Acre}}{43.56}$

Standard Liquid Volume Units:

$\mathbf{1\text{ Gallon} = 4\text{ Quarts} = 8\text{ Pints} = 128\text{ Fluid Ounces}}$ $\mathbf{1\text{ Quart} = 2\text{ Pints} = 32\text{ Fluid Ounces}}$ $\mathbf{1\text{ Pint} = 16\text{ Fluid Ounces}}$

Worked Example 3.1 (Acre to 1,000 Sq Ft Conversion): A turf herbicide label recommends applying $2.5\text{ quarts}$ of liquid product per acre. An applicator needs to treat a commercial lawn measuring $15,000\text{ sq ft}$.

1. Convert quarts per acre to fluid ounces per acre: $2.5\text{ qt} \times 32\text{ fl oz/qt} = 80\text{ fl oz/acre}$.
2. Calculate fluid ounces per $1,000\text{ sq ft}$: $\frac{80\text{ fl oz}}{43.56} = 1.8365\text{ fl oz per 1,000 sq ft}$.
3. Calculate product for $15,000\text{ sq ft}$: $1.8365 \times 15 = 27.55\text{ fluid ounces}$ of product.

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Active Ingredient (a.i.) Calculations

Some pesticide labels state application recommendations in terms of pounds of active ingredient (lbs a.i.) per acre rather than formulated product. Applicators must convert lbs a.i. into the physical amount of liquid or dry commercial product.

1. Liquid Formulations (EC, SC, F)

Liquid pesticide product names indicate the pounds of active ingredient contained per gallon of concentrate. For example:

• 4EC = Emulsifiable Concentrate containing $4.0\text{ lbs a.i. per gallon}$.
• 2.5EC = Emulsifiable Concentrate containing $2.5\text{ lbs a.i. per gallon}$.

$\mathbf{\text{Gallons of Liquid Product Needed} = \frac{\text{Lbs of Active Ingredient Recommended}}{\text{Lbs a.i. per Gallon of Product}}}$

Worked Example 3.2 (Liquid Active Ingredient): A field extension guide recommends applying $1.5\text{ lbs a.i. per acre}$ of an insecticide. The applicator has a container of 3EC ($3.0\text{ lbs a.i./gallon}$).

1. Calculate gallons product per acre: $\frac{1.5\text{ lbs a.i.}}{3.0\text{ lbs a.i./gal}} = 0.5\text{ gallons product per acre}$.
2. Convert to quarts: $0.5\text{ gal} \times 4\text{ qt/gal} = 2.0\text{ quarts (or 64 fl oz) per acre}$.

2. Dry Formulations (WP, WDG, DF, G)

Dry pesticide product names indicate the percentage of active ingredient by weight. For example:

• 80WP = Wettable Powder containing $80%\text{ active ingredient}$ ($0.80\text{ decimal}$). drop balance ($20%$) is inert carrier.
• 50WDG = Water Dispersible Granule containing $50%\text{ active ingredient}$ ($0.50\text{ decimal}$).

$\mathbf{\text{Pounds of Dry Product Needed} = \frac{\text{Lbs of Active Ingredient Recommended}}{\text{Decimal Percentage of a.i. in Product}}}$

Worked Example 3.3 (Dry Active Ingredient): An orchard recommendation calls for $3.0\text{ lbs a.i. per acre}$ using a 75WP wettable powder ($75%\text{ a.i. = 0.75}$).

1. Calculate dry product pounds: $\frac{3.0\text{ lbs a.i.}}{0.75} = 4.0\text{ pounds of 75WP product per acre}$.

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Tank Load & Tank Capacity Mathematics

When preparing a spray rig, applicators must calculate how many acres a full tank will cover and how much pesticide product to add to the carrier water.

Step-by-Step Tank Load Calculations:

1. Acres per Tankful: Divide tank liquid capacity (gallons) by sprayer calibration output (GPA). $\text{Acres per Tank} = \frac{\text{Tank Capacity in Gallons}}{\text{Sprayer Calibration Output in GPA}}$
2. Total Product per Tankful: Multiply total acres per tank by the product rate per acre. $\text{Product per Tank} = \text{Acres per Tank} \times \text{Product Rate per Acre}$

Worked Example 3.4 (Full Tank Load): A boom sprayer with a $400\text{ gallon}$ tank is calibrated at $25\text{ GPA}$. The herbicide label requires $2.0\text{ quarts}$ of formulated product per acre.

1. Calculate acres covered per tankful: $\frac{400\text{ gallons}}{25\text{ GPA}} = 16\text{ acres per tankful}$.
2. Calculate herbicide to add to full tank: $16\text{ acres} \times 2.0\text{ quarts/acre} = 32\text{ quarts} = 8.0\text{ gallons}$ of herbicide.

Partial Tank Loads

If an applicator has $6.0\text{ acres}$ remaining to treat at the end of the day, calculate the partial water and product required:

• Water needed: $6.0\text{ acres} \times 25\text{ GPA} = 150\text{ gallons}$ total spray mix.
• Product needed: $6.0\text{ acres} \times 2.0\text{ quarts/acre} = 12\text{ quarts}$ of herbicide.

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Finished Spray Dilution Percentage Math

Certain high-volume sprayers, orchard mist blowers, or structural spot treatments state rates as a percentage concentration of active ingredient in the finished spray solution (e.g., a $0.5%\text{ a.i. spray mix}$).

Water weighs $8.34\text{ pounds per gallon}$.

$\mathbf{\text{Lbs Dry Product for Percentage Mix} = \frac{\text{Gallons of Finished Spray} \times 8.34\text{ lbs/gal} \times \text{Target \% a.i.}}{\text{Decimal \% a.i. in Dry Product}}}$

Worked Example 3.5 (Percentage Dilution): An applicator needs to prepare $100\text{ gallons}$ of a $0.5%\text{ active ingredient}$ spray solution using an 80WP dry powder ($80%\text{ a.i. = 0.80}$).

1. Calculate total weight of 100 gallons of water mix: $100\text{ gal} \times 8.34\text{ lbs/gal} = 834\text{ lbs}$.
2. Calculate weight of active ingredient required: $834\text{ lbs} \times 0.005 = 4.17\text{ lbs a.i.}$
3. Calculate pounds of 80WP product to add: $\frac{4.17\text{ lbs a.i.}}{0.80} = 5.21\text{ pounds of 80WP product}$.

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Dilution & Tank Mixing Reference Table