Water pH and the Performance of Weak Acid Herbicides

Weak-acid herbicides must remain in their non-ionised (acid) form to move efficiently through the plant cuticle. When spray water is too alkaline, the herbicide molecule becomes ionised.

Ionised molecules struggle to penetrate the waxy leaf surface, resulting in:

• Reduced absorption
• Slower translocation
• Increased binding to dissolved minerals
• Overall loss of field efficacy

This behaviour is well established in international reviews examining the influence of water quality on herbicide performance.

Townwater pH

Most municipal water supplies across Australia fall between pH 6.7 and 7.9, including Melbourne Water’s typical range. While ideal for drinking, this pH range is not optimal for weak-acid herbicides, which perform better under mildly acidic conditions.

Preferred pH range

Industry guidance, including Indigo Specialty’s technical recommendations, confirms that weak-acid herbicides such as glyphosate perform best when the spray solution is adjusted to pH 5.5–6.5.

Within this range:

• The herbicide remains predominantly in its acid form
• Cuticle penetration is improved
• Rain fastness is enhanced
• Less product is wasted binding with dissolved minerals or hard-water cations

Impact of high pH on glyphosate

When glyphosate is mixed in water above pH 7.2–7.5, several performance losses occur:

• A larger proportion of the molecule becomes ionised, reducing leaf penetration
• The herbicide binds more readily with cations such as Ca²+, Mg²+ or Fe³+
• Efficacy decreases, especially on tolerant species or stressed vegetation
• Slower uptake may increase drift potential and off-target movement

Field studies have shown measurable efficacy losses, including ~6% reduction due to elevated pH and ~10% reduction under high hardness. These trends are consistent with global findings on water quality effects.

Corrective Action 

Adjust spray water to pH 5.5–6.5 before adding glyphosate or other weak-acid herbicides. This is achieved using an appropriate acidifying or buffering conditioner.  Always add the conditioner to the tank first to ensure water chemistry is corrected prior to introducing any herbicide.

Summary

Town water is generally too alkaline for optimum glyphosate performance. Bringing the spray solution into the 5.5–6.5 range increases absorption, improves consistency, reduces herbicide wastage, and ensures the active reaches its intended site of action within the plant.