Ammonium sulphate

By Dr Brian de Villiers

February 2025

Ammonium sulphate was first applied commercially with glyphosate as a water conditioner about 40 years ago. It’s hard to believe that after all these years, it is still used extensively and successfully with cation-sensitive herbicides. It remains the water conditioner of choice for many herbicide registration holders, including Villa. Unfortunately, because the mode of action of ammonium sulphate is misunderstood, there are still many misconceptions about this highly successful adjuvant. It is important to note that the only ammonium sulphate discussed in this tip, is that of spray-grade quality, registered with an L-number.

Antagonistic cations

Many cations that occur naturally in spray water, antagonise the efficacy of sensitive herbicides. These cations form complexes with the herbicide on the leaf during droplet drying. This limits the amount and rate of absorption and reduces weed control. The four most prevalent and antagonistic cations are magnesium, calcium, sodium, and potassium. Other cations such as iron are also highly antagonistic, but fortunately occur at low concentrations. The most common misconception is that ammonium sulphate doesn’t neutralise the antagonism of all the four major cations. Fortunately, this is only a misconception, because ammonium sulphate addresses the antagonism from all four these cations equally effectively. However, ammonium sulphate is only partially effective at overcoming iron antagonism. Fortunately, because of its low prevalence, iron poses less of a threat.

Mixing time

A modern misconception is that ammonium sulphate must be mixed with the spray water for an extended period before application, ostensibly to allow the ammonium sulphate to condition the water and to reduce its antagonistic ability. This makes it less appealing because it makes the application process time consuming. However, the truth is that ammonium sulphate does not work in the spray tank because there is enough water to keep everything in solution and no reactions occur between the water conditioner and antagonistic cations. Cation neutralisation occurs on the leaf, as the droplets dries out and complexing with cations occurs. Therefore, once the ammonium sulphate and the herbicide have been mixed into the spray solution, it is immediately ready to fulfil its function on the leaf.

Villa’s stance

Ammonium sulphate remains Villa’s first choice when it comes to a water conditioner for cation sensitive herbicides. It is an effective adjuvant that negates the effect of all four major naturally occurring cations in water. It also doesn’t need an extended time to condition the water, as its mode of action occurs on the leaf during droplet drying. Ammonium sulphate is an integral component of any water conditioner for herbicides. It also has further efficacy-enhancing properties that make it a crucial part of your spray programme when it is registered for use with your herbicide of choice. Contact your Villa agronomist for more information about adjuvants and water quality.