Chapter 13: Mixing and Loading Pesticides

Protecting yourself

The PPE rule is simple: wear what the label lists. If the label gives no specific mixing-and-loading instructions, then at a minimum wear chemical-resistant gloves and protective eyewear. One point trips people up: you must wear PPE even when handling products designed to reduce exposure, like water-soluble packets or closed-handling systems, because studies show that handling these without PPE still exposes you to pesticide residues.

Protecting water

Protecting water is a high priority, because an accident during mixing and loading can contaminate wells, ground and surface water, and municipal drinking water. You guard against that three ways: spill containment, setback requirements, and avoiding back-siphoning.

Spill containment means mixing and loading over a spill containment surface — a mixing and loading pad — whether you're making a 5-gallon backpack batch or a 500-gallon tank. Depending on the amount, this is sometimes required by federal or state law, so know your state's rules. Even a small operation can contain spills: for a backpack, set it inside a plastic bin while filling. Any secondary containment controls spills, protects the environment, and makes cleanup faster and easier.

Setbacks keep you a safe distance from wells and surface water. Some states set specific distances, but regardless of regulation, take extra care near wells — poorly constructed, improperly capped, or abandoned wells can allow surface water carrying pollutants to enter groundwater directly, and wells are sometimes located right in or near treated areas. On top of local rules, pesticide labels may specify their own setbacks, and because the label is the law, you must follow them (one label example prohibits mixing or loading within 50 feet of streams, lakes, or wells).

Back-siphoning — also called backflow — is the reverse flow of liquids into a fill hose, and it's dangerous because it can suck tank contents, pesticides included, back into your water supply. The simplest prevention is an air gap: keep space between the discharge end of the water line and the pesticide solution in the tank, and make that air gap at least twice the diameter of the discharge pipe. You can also install a backflow prevention device or check valve. But no device replaces attention — even with an air gap or check valve, never leave equipment unattended while filling.

Water chemistry and pH

Most of the time you're diluting a concentrate with water, and that water isn't neutral or uniform. pH measures the acidity or alkalinity of water, on a scale that runs from 0 to 14, with 7 being neutral; values from 1 to 6 are acidic, and anything above 7 is alkaline. This matters because some pesticides lose their effectiveness when mixed with alkaline, high-pH water — pH can hurt both stability and performance. If your mix water is the wrong pH, you may need to add buffering agents or other compounds to reach the target, following the label. And be careful with water from open sources like ponds and streams: it can carry sediments that clog equipment, alter the chemistry of the mix, or bind the pesticide and make it less effective.

Adding pesticide and emptying containers

When you add pesticide, handle the containers carefully. Open bags with a sharp knife, then clean that knife and don't use it for anything else. Pour liquids with the container held well below eye level to keep splashes off your face and out of your eyes, and use a pump to draw concentrate from a large container. If a container isn't empty when you're done measuring, close it immediately and return it to storage — or, if you're working at the application site, store the containers securely until you can bring them back.

Emptying containers fully is both good practice and good economics. To make a container completely empty, rinse plastic or metal containers or cut open paper ones. Doing this saves money by using all the product, protects the environment, meets federal and state regulations, and turns the container into a solid waste rather than hazardous waste — even if it held a hazardous pesticide. The key habit is to triple rinse as soon as you empty a container, before residue dries and becomes hard to remove. Don't leave triple-rinsed containers at the application site, where someone who doesn't know better might reuse them; instead, puncture them to render them unusable. Proper disposal of pesticides and their containers is your responsibility. As an alternative to triple rinsing, you can pressure rinse — a special nozzle pierces the container and blasts residues out at high pressure, which you rinse directly into your spray tank while wearing eye protection and other PPE.

Tank mixing and compatibility

You'll sometimes combine a pesticide with adjuvants, or mix two or more pesticides to hit a range of pests in one pass, which saves time and fuel — but mixes can misbehave. The governing rule is caution: never assume you can mix two or more pesticides unless at least one of the product labels specifies the combination. Two pesticides are compatible when they can be mixed and applied together without reducing effectiveness or changing their physical or chemical properties, and incompatible when combining them yields an un-sprayable mixture, reduced effectiveness, or increased toxicity. Incompatibility comes in two types — physical and chemical — and the difference is worth memorizing. Physical incompatibility is usually easy to see: the mixture doesn't blend, so solids settle out or it separates into layers. Chemical incompatibility is the sneaky one: the products physically mix but the effectiveness of one or both is reduced or destroyed, and you can't tell just by looking, so you may not find out until it's too late — and it can also raise the mixture's toxicity or phytotoxicity.

Mix order and the jar test

Even when labels say two products can be mixed, problems can still arise from mixing in the wrong order or using an untested adjuvant, because the wrong order can affect water pH and how well ingredients dissolve. The defense is to read the label or manufacturer information to confirm both that products can be mixed and what the proper order is — and to run a small jar test. A jar test uses the same proportions and the same water as your full tank, just scaled down, and it's valuable because it identifies and avoids incompatible mixtures that cause clogging, separation, and damage to spray systems. Wear your label-required PPE, and if the test mixes well, rinse the jar and add it to the larger mixture.

When no label order is available, a simplified mixing order applies: shake the liquid containers; fill the tank halfway with clean water or carrier (and never pour concentrate into an empty tank); add dry formulations first — wettable powders, water-dispersible granules, and water-soluble packets — after pre-slurrying them; agitate until everything forms a solution; then add emulsifiable concentrates and microencapsulated formulations; then liquids, flowables, and soluble concentrates; then adjuvants such as surfactants and crop oil concentrates last; and finally top off with the required amount of water.

The jar test procedure itself is specific: mix the products in a quart jar (some labels suggest two), label each jar clearly so you know what's in it, then shake and let it stand for 10 to 30 minutes. If the mixture curdles or separates, shake again; if it still won't disperse after that second shake, add a compatibility agent and shake again; and if it still won't mix, try a different compatibility agent or decide not to use the tank mix at all. If it mixes easily without an agent, you don't need one — but you must still constantly agitate the mixture in your spray tank.

How to study this chapter

1. Anchor the core risk — you handle the pesticide most concentrated — and that "mixing and loading" includes rinsing, cleaning, and disposal, not just the pour.
2. Know the PPE minimum (gloves + eyewear) and that it applies even to water-soluble packets.
3. Lock in the three water protections — containment, setbacks, back-siphoning — and the two hard numbers: air gap ≥ twice the discharge-pipe diameter, and never leave filling equipment unattended.
4. Memorize the pH scale (0–14, 7 neutral, 1–6 acidic, >7 alkaline) and that alkaline water can reduce effectiveness.
5. For containers: triple rinse immediately, don't leave them at the site, puncture them, and remember pressure rinse goes into the spray tank.
6. Nail the compatibility rule — never assume unless a label specifies — and the physical (you can SEE it) vs. chemical (you CAN'T) incompatibility distinction.
7. Drill the mixing order (shake → half-fill water → dry first → agitate → EC/microencapsulated → liquids/flowables → adjuvants last → top off) and that you never pour concentrate into an empty tank.
8. Memorize the jar test specifics: quart jar, 10–30 minutes, add a compatibility agent if it won't disperse, and constantly agitate in the tank if it mixes easily.