9.2 Tank Mix Compatibility & Mixing Order
Key Takeaways
- Always run a jar test before mixing unfamiliar products: combine them in a clear jar at the same proportions as the field mix, agitate, and watch for separation, gels, flakes, precipitate, oiling out or heating.
- Mixing order matters because it controls dispersion: a common workable sequence is water, compatibility agent, dry flowables and water-dispersible granules, wettable powders, then flowables and suspensions, then water-soluble products, then emulsifiable concentrates, then surfactants and oils, adding each with agitation running.
- Water quality drives failures independently of product compatibility - hard-water cations tie up glyphosate and some other weak-acid herbicides, and alkaline water hydrolyzes several organophosphates in the tank.
- Antagonism means the mixture performs worse than the components alone - the classic case is a broadleaf herbicide reducing the grass activity of an ACCase inhibitor - while a physically stable mix can still be agronomically wrong.
- The label controls: a prohibited tank mix is a violation, and in Arkansas glyphosate may not be tank mixed with in-crop dicamba during the April 16 through June 30 window.
9.2 Tank Mix Compatibility & Mixing Order
Core Concept: A tank mix can fail three different ways, and they need different fixes. Physical incompatibility produces a mess in the tank - gels, curds, separation, plugged screens. Chemical degradation destroys the active ingredient in the tank without any visible sign. Biological antagonism leaves a perfect-looking tank that simply does not control the pest. A jar test catches the first. Water testing catches the second. Reading the label and knowing the chemistry catches the third.
The Jar Test
Run it any time you are mixing products you have not combined before, or when you change water sources.
- Use a clear quart jar with a lid.
- Add water in the same proportion the field mix will use - if the field rate is 15 gallons per acre with 1 quart of product per acre, scale accordingly.
- Add the products in the intended mixing order, shaking after each addition.
- Include the adjuvants you intend to use, at the same proportions.
- Let the jar stand 15 to 30 minutes.
- Inspect.
| What you see | Meaning |
|---|---|
| Uniform suspension or emulsion | Physically compatible; proceed |
| Separation into layers that re-mix on shaking | Marginal - may be usable with continuous agitation |
| Gel, curd, flakes, sludge or precipitate | Incompatible. Do not mix - this will plug screens and nozzles |
| Oiling out - free oil on the surface that will not re-disperse | Incompatible |
| Heat or gas generation | Stop. A chemical reaction is occurring |
A compatibility agent may resolve a marginal physical incompatibility. It does not fix chemical degradation or antagonism.
Mixing Order
Mixing order exists so each formulation gets a chance to disperse in water before the next one competes for it. A widely used sequence, with agitation running throughout:
1. Fill tank half to two-thirds with clean water; start agitation
2. Compatibility agent, if used
3. Water-dispersible granules (WDG) / dry flowables (DF)
4. Wettable powders (WP)
5. Water-soluble bags (WSP) - add whole, allow to dissolve fully
6. Flowables and suspension concentrates (F, SC)
7. Water-soluble concentrates / solutions (SL)
8. Emulsifiable concentrates (EC)
9. Surfactants, crop oils and drift retardants
10. Top off with the remaining water; keep agitating until sprayed out
Two mnemonics are common - "WAMLEGS" and "dry before liquid, and oils last" - but the label always outranks a mnemonic. If a product label specifies its own order, follow it.
Practical rules that go with the sequence:
- Never add a dry formulation to a nearly empty tank. It will cake on the bottom.
- Pre-slurry wettable powders in a bucket where the label directs it.
- Add water-soluble packets whole - never cut them open.
- Keep agitation running from the first addition until the tank is empty; a mix that separated overnight cannot be salvaged by agitating in the morning.
- Do not let a mixed tank stand. Many mixes are stable for hours, not days.
Water Quality: The Invisible Failure
Physical compatibility can be perfect while the mix is chemically dying.
| Water problem | Effect | Fix |
|---|---|---|
| Hardness - calcium, magnesium, iron, sodium cations | Bind weak-acid herbicides such as glyphosate and some others, forming poorly absorbed salts. Efficacy drops with no visible change | Ammonium sulfate where the label permits, added before the herbicide |
| High pH (alkaline) | Alkaline hydrolysis degrades many organophosphate and carbamate insecticides and some fungicides, sometimes within hours in the tank | Buffering agent to lower pH where the label permits; mix and spray promptly |
| Low pH (acidic) | Degrades a smaller set of products; can also affect adjuvant performance | Buffer as the label directs |
| Suspended sediment and organic matter | Adsorbs active ingredient, particularly cationic herbicides such as paraquat and diquat; plugs screens | Use clean water; filter |
| Very cold water | Slows dissolution of dry formulations, causing lumps | Allow more agitation time |
Test your water. A single water analysis for pH and hardness costs almost nothing and explains a great many "the product did not work" complaints.
Antagonism Versus Synergism
- Antagonism: the mixture performs worse than the components would alone. The best-known agronomic example is a broadleaf herbicide reducing the grass activity of an ACCase inhibitor applied in the same tank - the tank is perfectly stable and the grass control is poor. Sequential application, or a label-specified adjuvant and rate adjustment, is the fix.
- Synergism: the mixture performs better than the sum of the components. Useful when it is intended and label-supported, and a resistance-management asset when it comes from combining effective modes of action.
- Additive: performance equals the sum of the parts, which is the usual and expected result.
A tank mix that is physically fine and chemically stable can still be the wrong decision agronomically. Check the label's tank mix section, and where a label directs a sequential rather than a tank-mix application, follow it.
When the Label or the State Says No
- A prohibited tank mix is a violation. If the label says "do not tank mix with X," mixing with X is a use inconsistent with the labelling under FIFRA, regardless of whether the jar test looks fine.
- Arkansas Class H: from April 16 through June 30, in-crop agricultural dicamba applications are prohibited from using tank mixes of glyphosate-containing products with in-crop dicamba products. This is a state rule sitting on top of whatever the federal label allows.
- Arkansas Enlist exemption: tank mixes are not permitted unless research data from a source acceptable to the Board shows the mix does not increase driftable fines below 200 microns by more than 10 percent over the product alone, and in no case may more than 10 percent of the total mix's droplets be smaller than 200 microns.
- FIFRA Section 2(ee) permits mixing a pesticide with a fertilizer when the mixture is not prohibited by the labelling - which is a permission, not a licence to ignore a prohibition.
[!WARNING] Exam Trap: A Clean Jar Test Does Not Make a Mix Legal The most common wrong answer on this topic is that a successful jar test authorizes the mix. A jar test evaluates physical compatibility only. It cannot detect alkaline hydrolysis, it cannot detect antagonism, and it has no bearing whatsoever on a label prohibition or on the Arkansas glyphosate-plus-dicamba ban.
An applicator plans to tank mix an in-crop dicamba product with glyphosate on dicamba-tolerant soybeans on June 5. A jar test shows a uniform, stable mixture with no separation or precipitate. What is the correct conclusion?
A grower reports poor glyphosate performance despite correct rate and timing. The spray water comes from a shallow well and tests high in calcium and magnesium. What is the most likely explanation and the standard remedy?
Which mixing sequence is most likely to produce a well-dispersed tank mix?