4.5 Pesticide Formulations and Dilution
Key Takeaways
- A formulation is the active ingredient plus carriers, solvents, surfactants, and other inerts; the same active ingredient in two formulations can differ in mixing order, PPE, drift behavior, and skin absorption.
- Emulsifiable concentrates penetrate skin and equipment hoses readily and often require more PPE than a dry flowable containing the same active ingredient.
- Dry formulations — wettable powders, water-dispersible granules, dry flowables, soluble powders — reduce splash and dermal risk at mixing but raise inhalation risk and abrasive nozzle wear.
- Water-soluble packets and closed mixing systems are engineering controls that some labels credit with reduced PPE; the reduction applies only when the label states it.
- Percent active ingredient and pounds of active ingredient per gallon are what make two formulations comparable; a rate written for one concentration is never transferable to another.
4.5 Pesticide Formulations and Dilution
Quick Answer: A formulation is the active ingredient combined with carriers, solvents, emulsifiers, surfactants, and other inert ingredients so it can be handled and applied. 40 CFR 171.103(c)(5) makes types of formulations, compatibility, synergism, persistence, toxicity, and dilution procedures a Core competency. The practical point: the same active ingredient in a different formulation is a different product on the label, at the tank, and on your skin.
Why Formulation Is a Legal Question, Not Just a Convenience
Two containers on a shelf both say "2,4-D." One is an amine salt, one is an ester. One lists a 12-hour reentry interval and gloves; the other lists 48 hours, gloves, coveralls, and a chemical-resistant apron. One is prohibited in five Iowa county areas under IAC 21—45.27 because of vapor drift, and the other is not. Reading only the active ingredient name gets an applicator into all three problems at once.
Liquid Formulations
| Code | Name | How it behaves | Watch out for |
|---|---|---|---|
| EC | Emulsifiable concentrate | Oil-soluble active in a petroleum solvent with emulsifiers; forms a milky emulsion in water | Highest dermal absorption of the common formulations; solvents degrade rubber hoses, gaskets, and some gloves; more likely to injure foliage; often the heaviest PPE |
| SL / S | Soluble liquid / solution | Active dissolves completely in water; no agitation needed to keep it in suspension | Splash exposure to concentrate; usually no formulation-specific residue advantage |
| F / L / SC | Flowable / liquid / suspension concentrate | Finely ground solid suspended in liquid | Settles — needs constant agitation; can cake in a tank left overnight |
| ULV | Ultra-low volume | Near-undiluted active applied at very low volume | Extremely concentrated; specialized equipment only; high drift potential |
| A / AS | Aerosol | Pressurized ready-to-use | Inhalation exposure; pressurized container hazards; never store near heat |
Dry Formulations
| Code | Name | How it behaves | Watch out for |
|---|---|---|---|
| WP | Wettable powder | Fine powder suspended, not dissolved, in water | Inhalation hazard when pouring; abrasive to nozzles and pumps; requires constant agitation |
| WDG / DF | Water-dispersible granule / dry flowable | Granules that disperse into a suspension | Much less dust than a WP with the same handling benefits — a genuine safety upgrade |
| SP | Soluble powder | Dissolves fully in water | Inhalation risk while pouring; fewer suspension problems once dissolved |
| G | Granule | Applied dry, not mixed | Low dermal and inhalation risk; spreader calibration, not sprayer calibration; can be picked up by birds |
| D | Dust | Ready-to-use fine powder | Very high drift and inhalation potential; rarely the right choice outdoors |
| B | Bait | Active mixed with a food attractant | Non-target and pet ingestion risk; placement matters more than rate |
| WSP | Water-soluble packet | Pre-measured packet dropped whole into the tank | Never open or handle a wet packet with bare hands; keep the outer container dry |
The Safety Trade-Off Every Applicator Should Internalize
Dry formulations generally present lower dermal risk and higher inhalation risk at mixing; liquids reverse that. Emulsifiable concentrates are the outlier: their solvents carry the active ingredient through skin faster than water-based formulations do, which is why the same active ingredient may demand a chemical-resistant apron in EC form and not in DF form.
Engineering controls change the calculation. Closed mixing and loading systems, water-soluble packets, and enclosed cabs remove the handler from the concentrate. Some labels credit these controls with reduced PPE — but only when the label says so. If the label is silent, the full PPE applies even inside a closed system.
Compatibility, Synergism, and Persistence
- Compatibility is whether two products can physically share a tank without flaking, gelling, separating, or heating. When a label is silent on a partner, the jar test in Section 4.3 is the answer.
- Antagonism is a biological loss of activity in the mix — hard water tying up glyphosate is the classic Iowa example, corrected with ammonium sulfate.
- Synergism is the opposite: a combination produces more effect than the sum of the parts, which can mean better control or unexpected crop injury and unexpected toxicity to the applicator.
- Persistence is how long the active ingredient stays biologically active. Long persistence buys residual weed control but raises plant-back interval, carryover injury, and leaching risk; short persistence forces repeat applications. Neither is inherently better — the label and the site decide.
Dilution and Formulation Math
Percent active ingredient (% ai) and pounds of active ingredient per gallon (lb ai/gal) are what make two formulations comparable. Section 6.3 works the full calculations; the rule to carry into the exam is that a rate written for one concentration never transfers to another.
Worked comparison: a label calls for 1.0 lb ai per acre.
- With a 4 lb/gal EC, you need 1.0 ÷ 4 = 0.25 gal/A, which is 1 quart per acre.
- With a 2 lb/gal SL, you need 1.0 ÷ 2 = 0.5 gal/A, or 2 quarts per acre — twice the volume for the same active ingredient.
- With an 80% WP, you need 1.0 ÷ 0.80 = 1.25 lb of product per acre.
For dilution to a target concentration, use C1 x V1 = C2 x V2. To make 100 gallons of 1% spray from a 50% concentrate: V1 = (1 x 100) ÷ 50 = 2 gallons of concentrate plus 98 gallons of carrier.
Reading the Formulation Off the Label
The formulation is usually in the product name itself — "Herbicide 4EC," "Fungicide 50WP," "Insecticide 2SC." Confirm it against the ingredient statement, which gives each active ingredient by name and percentage by weight plus the total percentage of inert ingredients. Two products from the same manufacturer with nearly identical brand names can be different formulations at different concentrations. The ingredient statement, not the brand, is the source of truth.
Two products contain the same active ingredient: one is a 4 lb/gal emulsifiable concentrate, the other an 80% dry flowable. Which statement is correct?
A label calls for 1.0 lb of active ingredient per acre. How much of an 80% wettable powder is needed per acre?
An applicator is using a closed mixing system with a product whose label says nothing about reduced personal protective equipment. What PPE applies?