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By Dr Brian de Villiers
December 2025
There is a common perception that any negative effect that water quality and tank mixtures may have on the efficacy of a crop protection product (CPP) only happens in the tank. However, the chemical processes whereby CPPs are bound or degraded to reduce their control can continue beyond the tank and have a secondary effect once the droplet is on the leaf. Water should be regarded as just another chemical in the spray tank. It has a pH, it contains antagonistic cations and mud in some cases, and it has a high surface tension. It is also applied at a high volume. All these properties of the water have an effect, which could occur immediately in the spray tank, or much later once when the CPP is doing its job on the leaf surface.
Antagonistic cations and turbid water
Dissolved cations in spray water bind to certain post-emergence herbicides. These chemical reactions occur in the water phase until the spray droplet has dried out. However, the reason that these herbicides are poorly absorbed is not only because of a chemical reaction. Spray droplet residuals of herbicides which have bound to cations in water can have a viscous and sticky texture that is poorly absorbed by weeds. This is why ammonium sulphate adjuvants are used, to avoid these complexes and to produce a droplet residual that is readily absorbed. On the other hand, turbid spray water can form muddy spray droplet residuals, with a physical barrier to absorption and other modes of action. Therefore, it is crucial to use clean spray water and the correct water conditioner to ensure that spray droplet residuals are absorbed readily by the target weed, without a reduction in absorption rate.
Tank mixtures
Antagonistic tank mixtures and impurities in water often have a lot in common. There is a
perception that if everything mixes well in a jar test, the application will go well and there will be no incompatibility or reduction in efficacy. The problem is that not all incompatible spray mixtures are picked up in a jar test, because agitation is often the catalyst of incompatibility. Even though tank mixtures mix effectively with agitation, there can still be antagonism and a reduction in efficacy. This is not always due to a chemical reaction between the products but could also be a physical reduction in absorption caused by a problematic spray droplet residual. Any aspect of the droplet residual that limits absorption or other modes of action, is a recipe for poor efficacy or even phytotoxicity in
some cases.
Villa’s stance
Dissolved cations, turbid water, and tank mixtures can all influence the efficacy of CPPs. This is not only the result of chemical reactions, but also because of barriers in the spray droplet residual. Avoid turbid water and antagonistic tank mixtures. Always use ammonium sulphate when it is registered with herbicides. Contact your Villa agronomist for more information about adjuvants and water quality.
