There are many farm chemicals, adjuvants and water conditioners that have the potential to interact when they come into contact with each other, particularly if they have not been sufficiently diluted or fully dissolved.

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See section 5 of this module and the accompanying video for more information on the jar test procedure.

These interactions may affect their solubility, where a gel or insoluble precipitate (solid) is formed. This is known as physical incompatibility, which effectively reduces the applied rate by reducing the availability of the product in the spray solution and may result in filter screen and nozzle blockages and uneven application. Physical compatibility should always be checked before mixing new products together or adding multiple products to a tank mix by conducting a jar test. It is possible that different brands of the same product may contain different formulation ingredients. This could mean that a simple switch of brand may lead to a physical incompatibility problem occurring whereas the previous brand was able to be mixed successfully with the other products. It is impossible for product manufacturers to be able to practically test every formulation with all the possible products growers may wish to mix.   

Physical incompatibility is more likely to occur when: 

• products are passed through mixing equipment without fully flushing the system with clean water before other products are added (e.g. the concentrates come into contact); 

• products are not fully dissolved or dispersed in the main spray tank before each additional product is added; 

• multiple products are mixed;  

• low water volume is used (especially if products are added to the tank before the majority of the water is added, or where batch mixing tanks are used with low water volume); 

• or products are mixed into poor quality water. 

Even when products are physically compatible (able to be mixed together), interactions can still occur that may affect the level of efficacy that can be expected from the products as compared to when they are applied individually, particularly if they are applied in a tank mix of multiple products. This is typically referred to as biological incompatibility or antagonism. 

Interactions between the products in the tank, or at the target, can reduce the level of uptake and translocation, or they may affect how a product works at a specific site within the pest or plant.  

It is important to understand how products need to be mixed to minimise interactions and incompatibility. This includes being able to remove residues of previous applications from the spraying and mixing equipment through the correct decontamination process, which will help to prevent possible interactions with new tank mixes or damage to crops during future applications.  

Always refer to product labels and manufacturers’ guides for compatibility, mixing requirements and the recommended decontamination agents.