3.3 Adjuvants, Compatibility & Tank Mixing

3.3 Adjuvants, Compatibility & Tank Mixing

Core Principle: An adjuvant is any substance added to a pesticide spray mixture—or incorporated during manufacturing—to modify the physical properties of the spray solution, enhance pest control efficacy, improve droplet deposition, or correct water quality deficiencies. Adding the wrong adjuvant, or mixing products in the wrong sequence, can cause chemical deactivation, severe crop injury (phytotoxicity), or form insoluble tank sludges that ruin spray equipment.

Adjuvants are broadly divided into two major functional categories:

1. Activator Adjuvants: Directly enhance the biological performance, leaf penetration, or target uptake of the active ingredient (e.g., surfactants, crop oils, methylated seed oils, nitrogen fertilizer additives).
2. Utility Modifiers (Spray Handling Adjuvants): Modify the physical characteristics of the spray solution without directly altering pesticidal activity, facilitating mixing, application, and drift management (e.g., drift control agents, water conditioners, pH buffers, antifoaming agents, compatibility agents).

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Major Classes of Adjuvants

1. Surfactants (Surface Active Agents)

Water molecules have high surface tension due to cohesive hydrogen bonding, causing spray droplets to form tight spherical beads. When a water droplet strikes a plant leaf with a waxy, hairy, or water-repellent cuticle, the droplet tends to bounce off, shatter, or bead up into small spheres with minimal contact area.

Surfactant Mechanism: A surfactant molecule possesses a dual nature—a hydrophilic ("water-loving") polar head and a lipophilic ("oil-loving") non-polar tail. When added to the tank, surfactant molecules align at the droplet surface, drastically reducing water surface tension. This allows the droplet to flatten, spread out into a thin liquid sheet over the leaf, and maintain intimate contact with the target tissue.

• Non-Ionic Surfactants (NIS): Contain no electrical charge (neutral). NIS is the most widely recommended and versatile surfactant class because it does not react with dissolved hard water minerals (calcium, magnesium) or charged pesticide molecules. It provides excellent spreading, improves droplet retention, and carries a low risk of plant injury.
• Anionic Surfactants: Possess a negative electrical charge. Typically blended into specialized industrial cleaners or soil wetting agents; can precipitate out if mixed with cationic compounds or hard water.
• Cationic Surfactants: Possess a positive electrical charge. Highly phytotoxic (causes severe leaf burning) and rarely used on desirable crops, but utilized in non-selective industrial vegetation knockdown.
• Organosilicone Surfactants ("Super-Spreaders"): Advanced synthetic silicon-based surfactants that reduce surface tension to exceptionally low levels. They allow the spray liquid to spread over massive surface areas and enter the microscopic pores (stomata) on the underside of leaves (a process called stomatal flooding).

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2. Oil Concentrates (COC and MSO)

Oil adjuvants are specifically designed to penetrate the thick, waxy epicuticular wax layers that protect mature or drought-hardened weed leaves.

• Crop Oil Concentrates (COC): Contain 80% to 85% petroleum-based paraffinic oil combined with 15% to 20% non-ionic surfactant. COCs soften and dissolve leaf cuticles, increasing the absorption rate of systemic post-emergence herbicides.
• Methylated Seed Oils (MSO): Contain chemically modified (esterified) vegetable oils (usually soybean or canola oil) blended with surfactants. MSOs provide superior cuticle penetration compared to COCs, making them the premier choice for controlling tough, drought-stressed, hairy weeds. However, MSOs increase the risk of crop leaf injury during hot, humid weather.

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Utility Modifiers & Water Chemistry

Drift Control Agents (Thickeners & Viscosity Modifiers)

Drift control agents (such as polyacrylamide polymers or guar gums) increase the viscosity (thickness) of the spray solution. This increases the Volume Median Diameter (VMD) of the spray droplets, significantly reducing the percentage of fine, driftable "fog" droplets (droplets smaller than 105 microns) that are carried away by wind.

Caution: Adding excessive drift retardant can cause the spray stream to form elastic "strings" or distort fan spray patterns, resulting in uneven application streaks across the field.

Water pH, Buffers & Alkaline Hydrolysis

The water used as a pesticide carrier varies widely in pH and mineral content across Indiana. Water with a pH greater than 7.0 (alkaline water) presents a severe hazard to chemical efficacy known as alkaline hydrolysis.

• The Mechanism of Alkaline Hydrolysis: Hydroxide ions ($OH^-$) in alkaline water attack and break down the ester and phosphate bonds in pesticide molecules, converting the active ingredient into inactive degradation products.
• Affected Pesticide Classes: Organophosphate and carbamate insecticides are especially vulnerable. For example, some organophosphates with a field half-life of 20 days in slightly acidic water (pH 6.0) degrade by 50% in less than 30 minutes in alkaline water (pH 9.0).
• Buffers and Acidifiers: Buffering adjuvants lower the pH of alkaline water and chemically stabilize it within the optimal range of pH 5.0 to 6.5, protecting the active ingredient from rapid chemical breakdown in the spray tank.

Antifoaming Agents (Defoamers)

Vigorous tank agitation of spray mixtures containing surfactants or flowables often generates dense surface foam. Foam can spill out of the tank fill neck, cause pump cavitation, and prevent complete filling. Antifoaming agents (typically silicone-based emulsions like dimethylpolysiloxane) break surface foam bubbles within seconds.

Compatibility Agents

When liquid fertilizers (such as 28% Urea Ammonium Nitrate / UAN) are mixed with emulsifiable concentrates or liquid flowables, the high salt concentration can cause the pesticide to separate, curdle, or form an un-sprayable sludge. A compatibility agent stabilizes the mixture, allowing uniform suspension.

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Pesticide Compatibility & The Jar Test

Tank mixing involves combining two or more pesticide products, or pesticides with fertilizers and adjuvants, into a single spray tank. Tank mixes save labor, fuel, and equipment hours, but applicators must ensure the products are compatible.

Types of Incompatibility

1. Physical Incompatibility: The products cannot physically mix or remain dispersed together. Symptoms include:
   • Formation of curds, gel, flakes, or mayonnaise-like sludge.
   • Phase separation (layering or oil floating on top).
   • Dense crystalline precipitation at the bottom of the tank that clogs screens, lines, and nozzle tips.
2. Chemical Incompatibility: The products mix physically without visual flaws, but a chemical reaction alters their biological activity:
   • Antagonism: The pest control efficacy of one or both chemicals is reduced (e.g., mixing a broadleaf herbicide with a post-emergence grass herbicide can severely suppress grass weed control).
   • Synergism / Phytotoxicity: The mixture becomes excessively toxic to the treated crop, causing severe, unintended foliage burn, chlorosis, stunting, or crop death.

Performing the Standard Compatibility Jar Test

Unless the pesticide labels explicitly state that a specific mixture is tested and approved, applicators must perform a small-scale Jar Test prior to mixing full commercial loads.

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                 STEP-BY-STEP COMPATIBILITY JAR TEST
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1. SAFETY FIRST: Put on all PPE required by the product labels (gloves,
   chemical goggles, apron) and work in a well-ventilated area.
2. CARRIER VOLUME: Fill a clean 1-quart glass jar with 1 pint (16 oz)
   of the exact carrier water or liquid fertilizer to be used in the field.
3. PROPORTIONATE DOSING: Add products in proportionate amounts reflecting
   field application rates (typically 1 teaspoon per pint represents ~1 lb
   or 1 pint of product per 100 gallons of carrier).
4. MIXING ORDER: Follow the standard tank mixing sequence (W-A-L-E):
   a. Water conditioners & Compatibility agents
   b. Dry products (WSP, WP, WDG, DF) - shake and allow to disperse
   c. Liquid flowables & suspensions (F, SC)
   d. Emulsifiable concentrates (EC) and Microencapsulated (ME)
   e. Solutions (S, SL) and Soluble liquids
   f. Surfactants, Crop oils, and remaining Adjuvants
5. SHAKE & EVALUATE: Cap the jar tightly and invert 10 to 15 times.
   - Inspect immediately.
   - Let the jar stand undisturbed for 15 to 30 minutes.
6. INTERPRETATION:
   - PASS: The mixture remains uniform, forms a stable emulsion/suspension,
     or easily re-disperses with gentle swirling.
   - FAIL: The mixture forms curds, sludge, flakes, oily separation, dense
     sediment, or generates HEAT (indicating an exothermic chemical reaction).
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The Standard Tank Mixing Sequence (W-A-L-E / D-A-L-E-S)

The physical order in which pesticide products are added to the spray tank is critical. Adding oil-based products before dry powders coats the dry particles in oil, preventing water from reaching the wetting agents and creating massive lumps of un-dispersed sludge that permanently clog spray plumbing.

To ensure proper dispersion and suspension, applicators must follow the industry-standard W-A-L-E (or expanded D-A-L-E-S) mixing sequence:

                         THE W-A-L-E / D-A-L-E-S SEQUENCE

  1. WATER / CARRIER
     Fill tank 1/4 to 1/2 full with water (or fertilizer) and START AGITATION.

  2. WATER CONDITIONERS & UTILITY MODIFIERS
     Add water-conditioners, pH buffers, compatibility agents, and antifoam.

  3. W / D — DRY FORMULATIONS
     Add Water-Soluble Packets (WSP) FIRST and allow PVA film to dissolve fully.
     Then add Wettable Powders (WP), Water-Dispersible Granules (WDG), and Dry Flowables (DF).
     Allow full dispersion across the water carrier BEFORE adding liquids.

  4. A / A — AGITATION & LIQUID FLOWABLES
     Maintain continuous agitation.
     Add Liquid Flowables (F), Suspension Concentrates (SC), and Micro-slurries.

  5. L / L — LIQUIDS (EMULSIFIABLE CONCENTRATES)
     Add Emulsifiable Concentrates (EC) and Microencapsulated (ME/CS) formulations.

  6. E / S — SOLUTIONS & SURFACTANTS
     Add Solutions (S), Soluble Liquids (SL), Surfactants (NIS), and Crop Oils (COC/MSO).
     Add drift retardants last.

  7. TOP OFF TANK
     Add remaining water to reach final target volume under continuous agitation.

Critical Takeaway: Always allow dry formulations (WSP, WP, WDG) to become completely wetted and dispersed throughout the water carrier before adding emulsifiable concentrates (EC) or crop oils. Introducing oil too early encapsulates dry particles, leading to immediate tank curdling and filter blockage.