6.3 Adjuvants, Compatibility & the Jar Test
Key Takeaways
- Adjuvants are added to a spray mixture to improve performance, and they include surfactants, crop oils, drift-reduction agents, water conditioners, buffers, and defoamers.
- The label controls: some labels require a specific adjuvant type, some prohibit adjuvants, and adding one against label direction is a use inconsistent with labeling.
- Physical incompatibility produces visible sludge, curdling, layering, or gels that plug screens and nozzles, while chemical incompatibility silently reduces efficacy or causes phytotoxicity.
- Synergism means combined activity exceeds the sum of the parts and antagonism means it falls short; both are label and stewardship issues.
- A jar test scaled to the tank ratio, mixed in the intended order, and observed for at least 15 to 30 minutes predicts most physical incompatibility before a full tank is ruined.
6.3 Adjuvants, Compatibility & the Jar Test
Federal core competency area 5 requires knowledge of compatibility, synergism, persistence, and dilution procedures. In the field those abstractions become one question: will this tank mix work, or will it plug my screens and cost me a day?
Adjuvant Families
An adjuvant is any substance added to a spray mixture to improve performance or handling. Some are built into the formulation; others are added to the tank.
| Adjuvant | What it does | When it is used |
|---|---|---|
| Nonionic surfactant (NIS) | Reduces surface tension so droplets spread and wet waxy leaves instead of beading off | The most common turf and ornamental additive; typically 0.25 percent v/v |
| Crop oil concentrate (COC) | Oil plus emulsifier; softens the leaf cuticle and slows droplet drying to improve penetration | Post-emergence herbicides on hard-to-wet or drought-stressed weeds |
| Methylated seed oil (MSO) | More aggressive penetration than COC | Difficult weeds; higher crop injury risk |
| Organosilicone surfactant | Extreme spreading and stomatal flooding | Very low rates; can increase runoff if overdosed |
| Sticker / extender | Improves rainfastness and slows weathering | Fungicides on foliage before forecast rain |
| Drift-reduction agent (DRA) | Increases droplet viscosity, shifting the spectrum coarser | Applications near sensitive sites; often required on dicamba and 2,4-D labels |
| Water conditioner (AMS or equivalent) | Sequesters hard-water cations that tie up weak-acid herbicides | Glyphosate and other weak-acid herbicides in hard water |
| Buffer / acidifier | Adjusts spray solution pH so alkaline hydrolysis does not degrade the active ingredient | Organophosphates and carbamates in alkaline water |
| Defoamer | Suppresses foam that prevents complete tank filling | Vigorously agitated tanks |
| Compatibility agent | Keeps pesticide-fertilizer mixes emulsified | Pesticide-liquid fertilizer combinations |
| Colorant / marking dye | Marks treated area to prevent skips and overlaps | Turf and right-of-way work |
The label controls. Some labels require a specific adjuvant type and rate; some prohibit adjuvants outright; some make the adjuvant optional. Adding an adjuvant the label prohibits is a use inconsistent with the labeling and therefore a violation of FIFRA Section 12(a)(2)(G) and ORC 921.24(A).
Compatibility
Physical incompatibility is visible. Symptoms include:
- Curdling, flocculation, or precipitate - solids that plug screens, strainers, and nozzle orifices
- Separation or layering - oil rising, solids settling to the bottom
- Gelling or thickening - the mix will not pump
- Excessive foam - the tank cannot be filled accurately
- Crystals forming after the mix stands - the most damaging kind, because it appears after the sprayer sits over lunch or overnight
Chemical incompatibility is invisible. The mixture looks fine, sprays fine, and then either fails to control the pest or injures the treated plant. Causes include alkaline hydrolysis in high-pH water, adsorption of an active ingredient onto another product's carrier, and direct chemical reaction between actives.
Synergism occurs when the combined activity of two materials exceeds the sum of their individual activities. Antagonism is the opposite - a common example is a grass herbicide's activity being reduced when tank-mixed with certain broadleaf herbicides. Both are why labels contain specific tank-mix instructions and why a label that is silent about a partner is not an endorsement of it.
The Jar Test
Do the jar test before the first full tank of any unfamiliar combination.
Procedure
- Put on eye protection and chemical-resistant gloves. You are handling concentrates.
- Scale the ratio. Use a clear quart jar and add the same proportion of each product that the full tank will contain. A convenient conversion: for a tank applying 20 gallons of finished spray per acre, 1 teaspoon of product per quart of water approximates 1 pint per acre at that carrier volume - but the safest method is simply to compute the exact proportion for your tank and scale it down.
- Fill the jar about half full with water from the same source you will use in the sprayer. Water source matters: hardness, pH, and sediment all change the result.
- Add products in the same order you will use in the tank, shaking or stirring for 5 to 10 seconds after each addition. Use the standard sequence: water conditioners first, then dry formulations (WP, WDG, DF), then flowables and suspensions, then solutions, then emulsifiable concentrates, then surfactants and oils last.
- Cap and invert the jar 10 to 15 times.
- Let it stand 15 to 30 minutes, then invert again and inspect.
Reading the result
| Observation | Interpretation | Action |
|---|---|---|
| Uniform appearance; any settling redisperses with a few inversions | Compatible under continuous agitation | Proceed, keep agitation running |
| Fine, easily redispersed sediment | Marginal | Proceed only with vigorous continuous agitation; do not let the tank stand |
| Curds, gels, clumps, oily layer that will not redisperse, or crystals | Incompatible | Do not mix. Apply separately, change the adjuvant, or change the water source |
| Heavy persistent foam | Handling problem | Add a defoamer or fill more slowly with reduced turbulence |
After the test
Discard jar-test contents legally - into the spray tank for a labeled use if the mixture was compatible, or as pesticide waste if not. Never pour a failed jar test down a drain, into a ditch, or onto bare ground.
Water Quality: The Overlooked Variable
- Hardness. Calcium and magnesium tie up weak-acid herbicides. A water conditioner added before the herbicide prevents the reaction.
- pH. Many organophosphate and carbamate insecticides hydrolyze rapidly above pH 8. Buffer the water first, then add the pesticide.
- Sediment and organic matter. Suspended clay adsorbs some actives, notably paraquat-type and glyphosate chemistry; filter pond water before use.
- Temperature. Very cold water dissolves dry formulations poorly and increases the chance of undissolved material plugging screens.
An applicator plans a new three-way tank mix and performs a jar test. After 20 minutes the mixture shows an oily layer on top that does not redisperse after inverting the jar. What should the applicator do?
Why should a water conditioner such as ammonium sulfate be added to the spray tank before a weak-acid herbicide?
A pesticide label states that no adjuvant may be added to the spray solution, but a technician adds a nonionic surfactant to improve wetting. What is the legal status of that application?