Water Quality

Hard water containing Calcium and Magnesium salts or water rich in Iron can reduce efficacy of glyphosate, especially with sub-optimum dose rates, in high water volumes and where the ion content is high.


Most UK water contains 1-10 ppm of Iron which poses no threat to Roundup. However at high levels, usually above 20 ppm, poor performance can result as oxides of this metal can lock up glyphosate.  Hard Water coupled high Iron levels can also present a problem.  Boreholes and artesian wells may contain iron derived from surrounding sandstone; rusting metal bowsers and old cast iron supply pipes can also cause problems if water is left standing in them.

Hard Water


In hard water chelating cations of Ca ²+ or Mg ²+ can bind to the glyphosate  rendering it biologically inactive and reducing the herbicidal effect of the diluted spray solution

Hardness is a measure of Calcium, Magnesium and Manganese salts and is expressed either as degree of hardness or as parts per million of Calcium Carbonate, (Ca CO3).

  • One degree of hardness is 17.9 ppm CaCO3
  • Soft water has less than 50 ppm CaCO3
  • Moderately Soft water has 50-100 ppm CaCO3
  • Slightly Hard water has 101-150 ppm CaCO3
  • Moderately Hard water has 151-200 ppmCaCO3
  • Hard water has more than 201-300 ppm CaCO3
  • Very Hard water has over 300 ppm CaCO3

Normal mains water has a hardness in the range of 100-400 ppm. In calcareous areas, bore holes and artesian wells may on occasions contain levels up to 1,000 ppm. They may also contain Iron from sandstone or high organic matter/ acid levels from springs, bogs and moors.

Kits are available to test on farm supplies by simple titration, but local water companies are obliged to give out a detailed analysis of mains water to customers on request.

The Effects of water quality and rate on efficacy of glyphosate on volunteer cereals

In general at levels in excess of 500 PPM there will be a decrease in glyphosate activity, but it may only be noticeable at low rates.  At levels greater than 1,000 PPM overall performance of glyphosate may be variable, particularly on perennial weeds, or with sub-label dose rates.


Acidifying surfactants claim to boost efficacy and minimise hydrolysis of glyphosate, but Monsanto trials showed no impact in performance of Roundup using water between pH 4 and 8. The ideal pH for performance of glyphosate is between 4 and 5.  Note that glyphosate acid itself is a strong acid (pH of a 1% solution ≈ 2).

Neutralising with Ammonium sulphate

In all cases where a water quality issue is diagnosed, the problem can be overcome by the addition of a proprietary water conditioner.

Typically hard water and high pH occur together but the combination is not significantly greater than the individual factors, with hardness being the most significant.