4.3 Mixing, Loading & Tank-Mix Compatibility Testing

Mixing, Loading & Tank-Mix Compatibility Testing

The processes of mixing and loading pesticides are widely recognized as the most hazardous activities an applicator performs. During these steps, you are handling the pesticide in its most concentrated, toxic form. A splash, spill, or inhalation exposure at this stage poses a significantly higher risk than exposure to the diluted spray mixture. Furthermore, improper mixing procedures can lead to dangerous chemical reactions, ruined equipment, and environmental contamination.

Safety Precautions During Mixing and Loading

Personal Protective Equipment (PPE)

Always wear the PPE specified on the product label for mixing and loading. This often requires more protection than the application itself. Even if not explicitly required, wearing chemical-resistant gloves, a chemical-resistant apron, and protective eyewear (goggles or a face shield) is a crucial best practice to protect against splashes.

Protecting the Water Supply

When filling a spray tank with a water hose, there is a severe risk of back-siphoning. If the water pressure drops suddenly (e.g., a fire truck taps into the water main), the pesticide mixture in the tank can be sucked back through the hose and into the public water supply or a private well. To prevent this, you must use one of two methods:

1. Air Gap: Keep the end of the water hose well above the surface of the liquid in the tank at all times. The air gap should be at least twice the diameter of the pipe/hose.
2. Anti-Siphoning Device (Backflow Preventer): Install a mechanical check valve on the water line or hydrant that automatically closes if water pressure drops, physically blocking the reverse flow of liquid.

Safe Mixing Location

Mix and load pesticides outdoors or in a well-ventilated, well-lit area. Ideally, use a dedicated mixing/loading pad made of an impermeable surface (like sealed concrete) with a containment berm and a sump to catch and recover any spills. Never mix near open water, wells, or storm drains.

Tank-Mixing Multiple Pesticides

Applicators frequently combine multiple pesticides, fertilizers, and adjuvants in a single tank to save time and application costs. This is called a tank mix. However, mixing chemicals randomly can lead to disaster.

Incompatibility

When products do not mix safely or effectively, they are incompatible.

• Physical Incompatibility: This occurs when the products fail to stay uniformly mixed. They might separate into layers, form a thick gel, or create clumps and sludge ("cottage cheese") that can ruin pumps and permanently plug screens and nozzles. Physical incompatibility is often caused by mixing formulations in the wrong order or using cold water.
• Chemical Incompatibility: This is invisible. The products mix fine physically, but a chemical reaction occurs that alters the active ingredients. This can lead to antagonism (the chemicals neutralize each other, reducing effectiveness) or synergism (the mixture becomes hyper-toxic, severely damaging the crop or posing a severe health hazard).

The Jar Test for Physical Compatibility

Before mixing a new combination of products in a large expensive spray tank, you must conduct a jar test to check for physical compatibility.

1. Wear PPE. Obtain a clean, clear quart glass jar.
2. Use water from the exact same source you will use for the application. Add 1 pint of carrier (water or liquid fertilizer) to the jar.
3. Add the proportional amounts of the pesticides and adjuvants in the correct sequence (see the WALE method below). Typically, 1 teaspoon of pesticide per pint of water represents a rate of 1 pint per 100 gallons of water.
4. Shake or stir the jar vigorously after each addition.
5. Let the jar sit undisturbed for 15 to 30 minutes.
6. Inspect the jar. If the mixture remains uniform, or if it separates slightly but readily remixes with gentle agitation, the products are physically compatible. If you see clumping, heat generation, thick scum, or permanent separation, the mix is physically incompatible and should not be used.

Note: The jar test only proves physical compatibility; it does not guarantee chemical compatibility.

The Tank-Mixing Order: The WALES Method

To prevent physical incompatibility, you must add products to the tank in a specific sequence. Unless the product labels dictate otherwise, industry standard dictates the WALES (or WALE) method:

1. W - Water and Water conditioners: Fill the clean spray tank 1/4 to 1/2 full with the carrier (water or liquid fertilizer). Start the agitation system. Add any utility adjuvants like buffers, acidifiers, or compatibility agents first, so they condition the water before pesticides are added.
2. A - Agitation and Dry Formulations: Keep agitation running. Add dry, suspended formulations. The general order for dry products is: Water-Dispersible Granules (WDG), then Dry Flowables (DF), then Wettable Powders (WP). Allow these products adequate time to hydrate and disperse completely before moving to the next step.
3. L - Liquid and Flowable Formulations: Add liquid formulations that form suspensions, such as Flowables (F), Liquids (L), and Microencapsulated (ME) products.
4. E - Emulsifiable Concentrates (EC): Add Emulsifiable Concentrates (EC) and Solutions (S). These are added late in the process because their petroleum solvents can sometimes interfere with the dispersion of dry formulations if added too early.
5. S - Surfactants and Solutions: Finally, add remaining liquid solutions and activator adjuvants like non-ionic surfactants (NIS), crop oil concentrates (COC), or drift retardants. Finish filling the tank with water to the final desired volume.

By strictly following the WALES sequence, ensuring adequate agitation, and performing a jar test for unknown mixtures, applicators can avoid the costly and dangerous consequences of tank-mix incompatibility.