Ammonium Sulphate (AMS) and Glyphosate

It has been well documented that certain cations in hard water can antagonize glyphosate activity (eg Ca++). This antagonism may result from the formation of glyphosate salts of low solubility. These glyphosate salts are not taken up as readily by the plant, and therefore weed control may be reduced. U.S. growers add ammonium sulfate (AMS) to glyphosate as a cost-effective way of overcoming this antagonism, as AMS hinders the formation of low soluble glyphosate salts. The addition of AMS to glyphosate is not registered for use in Ontario.

Spray application water is not the only source of cations. Plant tissue can produce antagonistic cations that leach onto the leaf surface when dew or rainfall wets the leaves. This may explain why the addition of AMS to glyphosate has improved control of certain weed species even when glyphosate is mixed with de-ionized water.

A study by Gavin Hall and colleagues (2000) looked at the composition of cations on the leaf surface of velvetleaf, field bindweed, and johnson grass. Control of these weeds was evaluated when applying glyphosate with and without the addition of AMS. Their results found that the greatest response in control from the addition of AMS to glyphosate was seen on velvetleaf, followed by field bindweed. There was little difference in the response of johnson grass to treatments of glyphosate with and without AMS. The response to AMS mirrored the relative concentrations of calcium within the foliar tissue as velvetleaf contained the highest concentration of Ca⁺⁺ followed by field bindweed and johnson grass. Results from the Hall study suggest that calcium on the leaf surface and within the plant tissue can antagonise glyphosate activity.

The addition of AMS to glyphosate is not registered for use in Ontario. The benefits to adding AMS will depend on the weeds that you are going after, and the size of that weed. Certain weed species, show no increased response from the addition of AMS. Other weed species like velvetleaf, are more than adequately controlled by glyphosate alone provided that they are small and actively growing. However, if either the weed is under environmental stress or is nearing the upper end of its application window, the Hall Study suggests that AMS may be of benefit.

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