Spray Solution pH and Glyphosate Activity

    Published: Feb 2011

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    It was hypothesized 110 years ago that substances that dissolve in lipids pass more easily into cells than those that dissolve in water. Thus, weak acids such as glyphosate should more readily pass through the non-polar plant cuticle in the non-dissociated, more non-polar form. The objective of this study was to test this hypothesis in a series of studies in the greenhouse involving three weed species. The pH of the glyphosate spray solution was varied between 1.5 and 6. The glyphosate activity on giant foxtail, common lambsquarters, and velvetleaf (pH) was not affected by pH values between 1.5 and 6 when adjusted with hydrochloric acid (HCl). If diammonium sulfate, was also present, the results were similar; however, the glyphosate activity on velvetleaf was much greater. If the pH was lowered with sulfuric acid, H2SO4, an increase in the glyphosate activity on velvetleaf became evident as the pH was lowered from 6.0 to 1.5. This was attributed to the water conditioning effect of the SO4= in hard water. With tank-mixing glyphosate with 1 % NT2, a proprietary blend of surfactants and monocarbamide, which lowered the pH to 2.0, the water conditioning benefit of NT was decreased on velvetleaf as the pH was raised from 2.0 to 7.0. In summary, the hypothesis was disproved with respect to glyphosate.


    diammonium sulfate, Overton’s rule, sulfuric acid, weed control

    Author Information:

    Penner, Donald
    Dept. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI

    Michael, Jan
    Dept. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI

    Committee/Subcommittee: E35.22

    DOI: 10.1520/STP152720120010

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