3.3 Adjuvants, Tank Mixing Procedures & Compatibility Testing

Adjuvants, Tank Mixing Procedures & Compatibility Testing

Adjuvant Classifications: Activator vs. Utility Modifiers

An adjuvant is any substance added to a pesticide spray tank, formulation, or carrier to modify biological performance or alter physical application characteristics. Adjuvants do not possess standalone pesticidal properties and are not registered as pesticides under FIFRA Section 3. However, their use is governed by pesticide label directions; if a label mandates or prohibits an adjuvant, that directive carries legal force.

Adjuvants are broadly categorized into Activator Adjuvants (enhancing biological efficacy and cuticular penetration) and Utility Adjuvants (optimizing physical mixing and target delivery).

Activator Adjuvants

• Surfactants (Surface Active Agents): Pure water droplets have high surface tension (~72 dynes/cm), causing them to bead up and roll off waxy cuticles. Surfactants are amphiphilic molecules with a hydrophilic head and lipophilic tail that lower water surface tension below 30 dynes/cm, flattening droplets to maximize foliar coverage.
  • Non-Ionic Surfactants (NIS): Carry no electrical charge. NIS is the standard adjuvant for post-emergence herbicides and fungicides, chemically inert and fully compatible with hard water (applied at 0.125% to 0.5% v/v).
  • Anionic Surfactants: Possess a negative charge; prone to binding hard water cations, rarely used alone in crop spraying.
  • Cationic Surfactants: Possess a positive charge; phytotoxic to plant membranes and restricted to non-crop vegetation clearing.
• Crop Oil Concentrates (COC): Contain 80% to 85% petroleum paraffinic oil and 15% to 20% non-ionic emulsifier. COCs soften and penetrate waxy weed cuticles in hot, dry conditions, but increase crop phytotoxicity under warm, humid conditions (> 85°F).
• Methylated Seed Oils (MSO): Chemically esterified vegetable oils (soybean or canola) providing aggressive wax-penetration on drought-stressed weeds, with elevated crop injury risk under crop stress.
• Penetrants and Organosilicones: Super-wetters that reduce surface tension below 22 dynes/cm, enabling stomatal infiltration.

Utility Adjuvants

• Drift Reduction Agents (DRAs): Polyacrylamide polymers that increase viscosity and reduce fine driftable droplets (< 105 microns). Warning: Shear-sensitive; excessive pump recirculation shreds polymer chains, destroying drift retardation.
• Water Conditioners & Ammonium Sulfate (AMS): North Dakota carrier water contains high dissolved calcium ($Ca^{2+}$), magnesium ($Mg^{2+}$), and iron ($Fe^{3+}$).
  • Hard Water Antagonism: Positively charged $Ca^{2+}$ and $Mg^{2+}$ bind negatively charged weak-acid herbicides (glyphosate, glufosinate, 2,4-D, dicamba), forming unabsorbable complexes that cannot cross leaf cuticles.
  • Label override: read the label before reaching for AMS. The 2026 over-the-top dicamba labels for Engenia, Stryax, and Tavium prohibit tank-mixing ammonium sulfate, because AMS lowers spray-solution pH and sharply increases dicamba volatility; those labels require a qualified volatility reduction agent instead. A label prohibition always defeats a general agronomic practice.
  • Remediation Protocol: Where the label allows it, AMS must be dissolved in tank water BEFORE adding weak-acid herbicides. Sulfate anions ($SO_4^{2-}$) precipitate $Ca^{2+}$ and $Mg^{2+}$ into inert sulfates, while ammonium cations ($NH_4^+$) pair with herbicide molecules to accelerate cellular uptake.
• Anti-Foaming / Defoaming Agents: Silicon-based dimethylpolysiloxane compounds that suppress foam from surfactant agitation.
• Buffering and Acidifying Agents: Lower and stabilize pH to 5.0–6.5, preventing alkaline hydrolysis where high pH water (> 7.5) rapidly degrades organophosphate and carbamate insecticides.

Tank Mixing Principles & Legal Permissions

Tank mixing combines two or more pesticides, fertilizers, or adjuvants in a single tank for simultaneous application.

• Agronomic Benefits: Reduces labor, equipment wear, and fuel costs; broadens pest control spectrum; and manages resistance by delivering multiple Modes of Action (MoA) simultaneously.
• Legal Permissions Under FIFRA: Tank mixing is legal under FIFRA unless explicitly barred by product labels. Applicators must follow the most restrictive directions, rate limits, buffer zones, PPE mandates, and Pre-Harvest Intervals (PHIs) among all products in the mix.

Physical vs. Chemical Incompatibility

Because chemical incompatibility frequently produces no visible precipitate, applicators must never assume a mix is safe simply because it sprays smoothly.

The Jar Test: Step-by-Step Bench Compatibility Protocol

The Jar Test is a bench-scale physical compatibility test conducted before loading field sprayers, verifying that formulations mix uniformly without clumping or separating.

Equipment and Proportionality Calculations

1. Use a clean, transparent 1-quart glass jar.
2. Use water from the exact source intended for field filling, at the exact temperature expected during application. Cold well water (40–45°F) severely worsens physical incompatibility.
3. Proportionality for 25 GPA: 1 lb dry formulation = 1.5 teaspoons; 1 pint liquid = 0.5 teaspoon; 1 quart liquid = 1 teaspoon.

Step-by-Step Jar Test Sequence

1. Fill a clean 1-quart glass jar half-full (1 pint) with actual field water at application temperature.
2. Add water conditioners (AMS) and anti-foaming agents; invert to dissolve.
3. Add proportionate dry formulations (WP, WDG, DF); shake gently and allow 3 minutes for complete hydration.
4. Add liquid flowables and suspensions (F, SC, ME); invert 5 times.
5. Add emulsifiable concentrates (EC); invert 5 times.
6. Add soluble liquids (SL, S) and surfactants (NIS, COC, MSO); invert 5 times.
7. Top off with remaining water to 1 quart; invert vigorously 10 times.
8. Evaluation: Check for heat generation (exothermic chemical reaction indicates incompatibility). Let stand 15 to 30 minutes. Inspect for phase separation, oily scum, curdling, or sediment. Invert 2 to 3 times: if settled particles readily redisperse, the mix is physically compatible. Persistent sludge or curdling confirms incompatibility.

The Universal W-A-L-E-S Tank Mixing Sequence

The universal standard for sequencing pesticide formulations into the spray tank is the W-A-L-E-S protocol:

W -> Water carrier and Water conditioners
A -> Agitate thoroughly
L -> Liquid flowables and suspension concentrates
E -> Emulsifiable concentrates
S -> Soluble liquids, Solutions, and Surfactants

Detailed Operational Execution

• Step 1: Water & Conditioners (Fill 1/2 to 3/4 Full): Fill tank 1/2 to 3/4 full with clean water and start continuous agitation. Add water conditioners (AMS) and anti-foam first, allowing AMS to neutralize hard water cations before herbicides enter.
• Step 2: W = Wettable Powders and Dry Flowables (WP, WDG, DF): Add dry products slowly into the hopper. Pre-slurrying dry powders prevents clumping.
• Step 3: A = Agitate Thoroughly: Pause 3 to 5 minutes with full agitation. Dry granules must fully hydrate and disperse before adding liquids.
• Step 4: L = Liquid Flowables and Suspensions (F, SC, ME): Pour liquid flowables slowly into the tank vortex.
• Step 5: E = Emulsifiable Concentrates (EC): Add petroleum-solvent EC products to form a uniform milky emulsion.
• Step 6: S = Soluble Liquids and Surfactants (SL, S, SP, NIS, COC, MSO): Add water-soluble chemicals and activator adjuvants last.
• Step 7: Final Top-Off: Add remaining water to calibrated volume and maintain continuous agitation.

Critical Tank Mixing Hazards

• The Oil-Before-Dry Catastrophe: Never add oil-based products (ECs, COCs, MSOs) before dry formulations (WPs, WDGs). Oil encapsulates dry particles, blocking water hydration and collapsing the mixture into an unresolvable curdled sludge ("cottage cheese") that clogs pumps, lines, and nozzles.
• Cold Water Shock: Early spring water in North Dakota below 45°F retards chemical dissolution. Pre-slurry dry products in warm water and extend agitation.
• Liquid Fertilizer Carriers: High salt concentrations in liquid fertilizers (28% UAN) destabilize emulsifiers; always perform a dedicated jar test with compatibility agents.