7.3 Adjuvants, Compatibility & Tank Mixing

7.3 Adjuvants, Compatibility & Tank Mixing

Adjuvants: Function, Chemistry, and Selection

An adjuvant is any substance added to a pesticide spray tank (or incorporated into a commercial formulation) to modify the physical properties of the spray solution or enhance biological performance. Adjuvants possess no pesticidal properties of their own.

+-------------------------------------------------------------------------+
|                        COMMON SPRAY ADJUVANTS                           |
|                                                                         |
|  SURFACTANTS (NIS)                 STICKERS & DEPOSITION AIDS           |
|  - Lowers droplet surface tension  - Adheres residues to waxy foliage  |
|  - Flattens droplets / spreads     - Resists rain wash-off & weathering |
|                                                                         |
|  BUFFERS & ACIDIFIERS              PENETRANTS (COC / MSO)               |
|  - Neutralizes alkaline water      - Dissolves plant cuticle wax        |
|  - Prevents alkaline hydrolysis    - Accelerates systemic uptake        |
+-------------------------------------------------------------------------+

1. Surfactants (Surface Active Agents):

   • Water droplets possess high internal surface tension, causing them to bead up and bounce off waxy leaf cuticles. Surfactants disrupt cohesive hydrogen bonding in water, flattening droplets and drastically expanding foliar surface contact.
   • Non-Ionic Surfactants (NIS): Carry no electrical charge. Chemically stable, universal compatibility, non-phytotoxic; the most common surfactant prescribed on agricultural and turf labels.
   • Anionic Surfactants (Negative Charge) & Cationic Surfactants (Positive Charge): Rarely used alone; cationic surfactants can cause severe plant phytotoxicity.

2. Stickers and Spreaders:

   • Spreaders: Enlarge the wetted surface area on the target leaf.
   • Stickers: Polymeric compounds that adhere pesticide residues firmly to foliage, resisting wash-off from rain, dew, or overhead irrigation (increasing "rainfastness") and slowing photolytic UV degradation.

3. Penetrants, Crop Oil Concentrates (COC), and Methylated Seed Oils (MSO):

   • Contain refined petroleum or vegetable oils designed to partially dissolve or soften the waxy cutin layer of weed foliage, accelerating the internal absorption of systemic herbicides.

4. Buffers and Acidifiers:

   • Groundwater and municipal water supplies in many areas exhibit alkaline pH levels (pH > 7.5). High pH causes Alkaline Hydrolysis—a chemical reaction that breaks the molecular bonds of organophosphates, carbamates, and synthetic pyrethroids, destroying their insecticidal efficacy within hours inside the spray tank.
   • Acidifiers and buffering agents lower and stabilize the spray tank pH between 5.5 and 6.5, preserving chemical stability.

5. Defoamers and Drift Retardants:

   • Defoamers: Suppress air entrainment and eliminate foam caused by vigorous tank agitation.
   • Drift Retardants (Deposition Aids): High-molecular-weight polymers that increase spray viscosity, eliminating driftable "fines" (droplets <105 microns) and ensuring droplets deposit on target foliage.

Tank Mixing Order: The WALES / DLABS Sequence

When combining multiple pesticides, fertilizers, and adjuvants in a single spray tank, introducing components out of sequence can result in catastrophic physical incompatibility (e.g., coagulation, curdling, gelatinous sludge, or unmixable oil-clay clumping). Applicators must follow the standardized WALES (or DLABS) addition sequence:

+-------------------------------------------------------------------------+
|                   THE STANDARDIZED "WALES" TANK MIX ORDER               |
|                                                                         |
|  [1] W - WATER: Fill tank 1/4 to 1/2 with clean carrier; start AGITATION|
|  [2] A - AGITATE & ADD DRY PRODUCTS: Add Wettable Powders (WP, WDG, DF) |
|  [3] L - LIQUID FLOWABLES: Add Flowables and Suspensions (SC, F, ME)   |
|  [4] E - EMULSIFIABLE CONCENTRATES: Add ECs (oil-based formulations)    |
|  [5] S - SURFACTANTS & SOLUTIONS: Add water-soluble liquids & adjuvants |
|      -> TOP OFF tank with remaining water to final target volume        |
+-------------------------------------------------------------------------+

[!IMPORTANT] Why Dry Formulations Must Go First: Dry dispersibles (WP, WDG, DF) require free water molecules to thoroughly hydrate and disperse. If an emulsifiable concentrate (EC) is added to the tank before the dry powders, the oil and petroleum solvents will coat the dry clay particles, repelling water and forming sticky, insoluble clumps ("grease balls") that permanently choke suction strainers and plumbing.

Testing Physical Compatibility: The Jar Test

Never mix commercial quantities of unfamiliar tank mixtures without first conducting a small-scale Jar Compatibility Test:

1. Measure one quart of water from the exact water source (well, municipal, or pond) to be used in the field into a clean, clear glass jar.
2. Calculate proportional amounts of each pesticide and adjuvant based on recommended field application rates (typically 1 teaspoon of liquid or dry product per quart represents 1 pint or pound per 100 gallons).
3. Add components to the jar following the exact WALES sequence, capping and inverting the jar 10 times after each addition.
4. Let the capped jar stand undisturbed for 15 to 30 minutes.
5. Inspect the Mixture:
   • Compatible: If the mixture remains uniformly dispersed, suspended, or easily redisperses with gentle swirling, the tank mix is physically compatible.
   • Incompatible: If the mixture curdles, forms flakes, precipitates into sludge, separates into distinct layers that resist re-blending, or generates noticeable heat (an exothermic chemical reaction), the mixture is incompatible and must never be loaded into the sprayer.