The water quality used in spray tanks can affect herbicides efficacy.
Water is the primary carrier for herbicide applications. In fact, it usually makes up over 99% of the spray solution. Considering that, it should be no surprise that the chemistry of water added to the spray tank greatly impacts herbicide effectiveness.
Weak acid herbicides
Acids are compounds that release H+ ions when dissolved in water. Weak acids are compounds that release H+ ions, but just slightly. Postemergence herbicides used by primary producers that are weak acids include: glyphosate (Roundup), paraquat (Gramoxone), bentazon (Basagran), and 2,4-D (many products).
When mixed with water, especially alkaline water, a portion of weak acid herbicides will dissociate (break down into two charged portions) and the remainder of the herbicide molecules will not (the compound remains whole). The extent to which the herbicide dissociates is a critical factor because the herbicide that has dissociated is LESS readily absorbed by plant foliage. How much the herbicide dissociates depends primarily on the pH of water in the spray tank.
Dissociated herbicide molecules have a negative charge and some may bind with other positively charged cations. This binding action can have a bit of ‘hit or miss’ result as binding to some cations improves herbicide uptake and absorption while binding to others decreases absorption or activity in the cell.
Water pH is a measure of the H+ ion concentration in water. As water pH decreases, it becomes more acidic and the number of H+ ions increases. Water with a pH between 3 and 6 (acidic) is most suitable for mixing weak acid postemergence herbicides. When water pH exceeds 7 (alkaline) adjuvants need to be added to lower the water pH.
Weak acid herbicides dissociate less under acid conditions and are more readily absorbed across plant cell membranes. Avoiding herbicide dissociation is the primary reason water used in pesticide mixing should be acidic.
Sulfonylurea (SU) herbicides are different and it is thought that increasing pH can increase the solubility of this class of herbicides, and theoretically increase their activity.
Water Quality Problems
Hard water contains high levels of calcium (Ca), magnesium (Mg), sodium (Na), or iron (Fe). Other cations can cause hard water, but these are the usual suspects.
Ca, Mg, Na, and Fe cations (positively charged ions) attach to negatively charged herbicide molecules. Often, the binding of herbicide molecules and these cations renders the herbicide ineffective.
High pH (alkaline) and hard water act together to reduce herbicide effectiveness. High pH causes more of the herbicide to dissociate while high concentrations of cations bind with the dissociated herbicide to reduce its effectiveness. Hard water by itself is not normally problematic but when the concentration of all cations exceeds 400 ppm action may be necessary
When labels permit, additions of ammonium sulfate to the spray tank overcome many interactions with herbicides and cations
Alkalinity refers to the presence of carbonates and bicarbonates in water and this becomes a problem with some herbicides when levels exceed 300 ppm. At this level (>300 ppm) corrective action in the spray tank may be warranted.
Turbid water (water containing suspended soil and organic substances) can reduce effectiveness of postemergence herbicides. As a general rule, water should be clean and clear for all pesticide applications, however, some pesticides are not as sensitive to turbidity as others.
The herbicides glyphosate (Roundup) and paraquat (Gramoxone) have very high soil organic carbon sorbtion coefficient (Koc) values. Because of their high Koc, these herbicides will bind to soil and organic matter particles suspended in water and will not be available for absorption into weed foliage. Comparatively, dicamba (Banvel) has a low Koc and has been found to be relatively unaffected by suspended solids in spray water.
For effective action, ensure that water to be used with glyphosate and paraquat is clear and free of suspended soils or organic matter. If the water is noticeably murky or discolored, find an alternate water source.
Glyphosate is the active ingredient in Roundup and numerous other products. Different formulations of Roundup and other products utilize different surfactants and additives, but in every case glyphosate is the active ingredient. Glyphosate kills plants by binding to an enzyme called EPSP synthase. When bound to EPSP synthase, the enzyme cannot function and the plant cannot produce three critical amino acids. Plant death ensues.
Glyphosate has a high Koc value (24,000 mL/g, ppm to mg/l calculator) and therefore rapidly and tightly adsorbed to soil particles and organic matter. As described above, turbid water with soil and sediment will greatly reduce herbicidal activity.
Hard water also affects glyphosate. Ca, Mg, Fe, or Na can form a complex with the glyphosate molecule so that it is unable to bind to EPSP synthase. If glyphosate cannot bind to the enzyme, it will not provide control.
Adding ammonium sulphate to the spray tank overcomes adverse effects of hard water. The ammonium cation preferentially attaches to the glyphosate molecule and thus prevents Ca, Mg, Fe, or Na from doing so. When ammonium is attached, the molecule binds readily to EPSP synthase and the herbicide functions normally.
Some plants contain high levels of Ca in their intracellular spaces. Just like hard water in a spray tank, high Ca levels between plant cells can reduce Roundup effectiveness. Ammonium sulphate in the spray tank also alleviates physiologically-induced Ca interference.
Different types of Herbicides containing 2,4-D are available in two broad categories, ester and amine formulations. Many producers prefer the amine formulation because it is less volatile and less prone to drift. However, the amine formulations are more sensitive to poor water quality than the esters.
Amines of 2,4-D can be sensitive to hard water. While specific guidelines are currently not available, some reports indicate that hardness greater than 600 ppm or alkalinity greater than 500 mg/L CaCO3 can reduce 2,4-D effectiveness. If water analyses indicate that your water is approaching these levels, use of a more pure water source is indicated.
Active Ingredient Comments
Atrazine Decomposes slowly in alkaline solution, more rapidly if lime is present.
Diquat Stable in neutral or acid solutions, but decomposes in alkaline conditions.
Glyphosate Performs best at pH 3.5 to 4.
Paraquat Stable in neutral or acid solutions, but decomposes in alkaline conditions.
Simazine Decomposes slowly in alkaline solution, more rapidly if lime is present.
Chlorpyrifos Stable in neutral and weakly acidic conditions.
Dicofol Stable at pH 5.5 to 6.
Dimethoate Decomposes in alkaline conditions – presence of iron accelerates this
Endosulfan Undergoes some degree of alkaline hydrolysis.
Fenthion Incompatible with alkaline material.
Malathion Hydrolysed rapidly in alkaline conditions. Performs best at pH 5 to 6.
Methomyl Stable in slightly acidic solutions.
Omethoate Undergoes alkaline hydrolysis
Guazatine Unstable in alkaline conditions.
Iprodione Undergoes alkaline hydrolysis. Performs best at pH 7.
Bore water quality can negatively affect some postemergence herbicides. Water pH in the alkaline range can antagonize herbicides and render them ineffective as can the presence of Ca, Mg, and Na salts, and suspended soil and organic solids. Chemical corrective action is possible but the use of clean, purified water in the herbicide mixing tank is a preferred course of action. High capacity (Hi-Flow) reverse osmosis water purification systems are available for agricultural use.