Applicators often combine two or more products in a single spray tank to control several pests at once or to add an adjuvant or fertilizer. A good tank mix can save time and fuel. A poor one can form gels, sludge or foam, plug nozzles and screens, reduce the effectiveness of one or more products, or injure the crop. Understanding the basics of compatibility and mixing order helps prevent these problems, though the product labels always have the final word.
Start with the labels
Before mixing anything, read the labels of every product involved. Labels may:
- Recommend specific tank mix partners or prohibit certain combinations.
- Specify a mixing order.
- Require or forbid particular adjuvants.
- Set restrictions that apply to the whole mixture, such as the most restrictive PPE, re-entry interval and preharvest interval among the products.
When products are mixed, the applicator must comply with every label in the tank. If one label prohibits a combination, it is not allowed, even if another label is silent.
Physical versus chemical compatibility
Physical incompatibility is visible. The mixture may separate into layers, form flakes, crystals or gels, foam excessively, or produce sediment that settles quickly. These problems can clog equipment and lead to uneven application.
Chemical incompatibility may not be visible. Ingredients can react in ways that reduce the activity of one product (sometimes called antagonism) or increase effects on the crop beyond what was intended. Water quality also plays a role: hard water and water pH can affect some active ingredients, which is why some labels address water conditioning.
A mixture can look fine in the tank and still perform poorly, so physical checks do not prove chemical compatibility. Label guidance and manufacturer information are the best sources for that.
The jar test
A jar test is a small-scale trial of physical compatibility. It uses the same water source and the same products, in the same proportions and order, as the planned tank mix. A general outline:
- Wear the PPE required by the labels of the products being tested.
- Fill a clean, clear glass jar with a measured volume of the carrier water you will actually use.
- Add each product in the intended order, in amounts scaled to match the planned rates and spray volume, capping and gently inverting the jar after each addition.
- Let the jar stand and observe it for signs of separation, clumping, gels, heat or excessive foam.
- If separation occurs but redisperses easily with gentle agitation, the mix may be usable with continuous agitation; if it does not redisperse, do not use the combination.
Dispose of the test mixture according to label directions. Some labels and extension resources include more detailed jar test procedures.
Why mixing order matters
Different formulation types need different conditions to disperse properly. Dry products need water and agitation to wet and suspend; oil-based products need to be dispersed into water that already contains the other ingredients. Adding products in the wrong sequence can trap dry particles in oily films or cause gels to form before everything is evenly distributed.
A widely taught general sequence, used when labels do not specify otherwise, follows the acronym WALES:
- W: water-soluble packets and wettable powders, plus water conditioners where needed.
- A: agitate thoroughly.
- L: liquid flowables and suspensions.
- E: emulsifiable concentrates.
- S: surfactants, solutions and remaining adjuvants.
Other sequences exist, and some products, such as certain drift-reduction agents, have their own requirements. Label instructions override any general rule.
Good mixing practice
- Begin with the tank partly filled with clean water, commonly around half full, with the agitator running.
- Add one product at a time, allowing each to disperse fully before adding the next.
- Keep agitation running throughout filling, transport and application.
- Mix only what you can apply that day; mixtures left standing may settle or degrade.
- Use a closed transfer system or induction hopper where available to reduce exposure.
- Mix and load away from wells, drains and surface water, on a surface that can contain spills.
Cleaning out the sprayer
Residues from one tank mix can injure the next crop sprayed. Follow the cleanout instructions on the labels, which may specify rinse volumes, cleaning agents and how to treat screens, nozzles and hoses. Dispose of rinsate as directed. Our article on chemical storage and organization covers related handling practices.
Key takeaways
- Every label in the tank applies; the most restrictive requirements govern the mixture.
- A jar test checks physical compatibility but not chemical compatibility.
- Follow label mixing order; WALES is a general guide when labels are silent.
- Maintain agitation and mix only what you will apply promptly.
- Clean the sprayer thoroughly between different mixes.
Informational only. Always read and follow the product label and local regulations.