The goal when applying any pesticide is to optimize its effects on the targeted pest by applying the proper rate at the proper time with calibrated equipment. The quality of the water carrier can be another important factor that should be considered to optimize pest control. Minerals and the effects of pH in spray water can diminish the effectiveness of many herbicides and some insecticides.

Hard Water

Hard water, when used as a carrier, may adversely affect the effectiveness of certain salt-formulated herbicides such as Roundup (glyphosate), Poast (sethoxydim), Pursuit (imazethapyr), and Liberty (glufosinate).

Natural waters usually contain ions of calcium (Ca²⁺, magnesium (Mg²⁺) and ferric (Fe³⁺). The degree of hardness is a measure of the total concentration of the above ions in water, usually expressed as parts per million (ppm) or as grains per U.S. gallon (one grain per gallon = 17.1 ppm).

Water with a hardness 50 ppm or less is considered “soft” water, 50 to 100 ppm is medium hard, and 100 to 200 ppm is considered “hard.” Water hardness exceeding 250 ppm is considered objectionable in drinking water.

Hard water ions can bind with salts of certain herbicides and with some surfactants to form an insoluble salt. These insoluble salts then “fall out” out of solution decreasing herbicide or surfactant efficiency. In the case of isopropylamine salt formulations of glyphosate, the positively charged cations of calcium (Ca²⁺) and magnesium (Mg2+) salts compete with the isopropylamine in the formulation for association with the glyphosate anion (negatively charged). This results in the herbicide having a greater difficulty absorbing into the plant leaf.

In addition, research has shown that extremely hard water, 600 ppm (35 grains/US gallon), can almost completely antagonize 2,4-D amine applied at a low rate of about 4 to 8 ounces per acre.

Reducing the Effect of Hard Water

Ammonium Sulfate (NH₄SO₄). Ammonium sulfate (AMS) has been used successfully to increase herbicide efficacy on a broad spectrum of weed species. This is particularly true for the weak-acid herbicides like Roundup (glyphosate), 2-4-D, Pursuit (imazethapyr), Poast (sethoxydim) and Basagran (bentazon). The AMS adjusts the pH so that more of the active herbicide is transported across the leaf surface and into the plant. An added benefit is that

sulfate ions (SO₄) bind up with hard water minerals. In addition, ammonium-herbicide combinations are more easily absorbed by some weed species. A general rule-of-thumb for adding AMS is the addition of 2% AMS by weight or 17 lb of dry AMS per 100 gallons of water for most applications. AMS should be added to the spray carrier solution prior to the herbicide and always, consult the pesticide label for mixing instructions. There may be limitations on the use of fertilizer-based surfactants.

Urea-Ammonium Nitrate (28% liquid N). Urea-ammonium nitrate or 28% liquid nitrogen can increase glyphosate efficacy on many weed species. The nitrate (NO₃) ion forms conjugate salts with the hard water ions and NH₄- glyphosate molecule is more readily adsorbed. However, the nitrate (NO₃) component is not as strong a "driver" of the system as sulfate (in AMS) and therefore AMS is preferred to 28% N.

Organic Acids. Another way to reduce hard water antagonism is the use of organic acids. The addition of an organic acid such as food grade citric acid will effectively remove hard water ions from solution. Organic acids are effective because the conjugate base (negative portion) of the acid binds to and removes positively charged cations from solution. A weak acid, such as citric acid, will provide a stronger conjugate base, and therefore, will be more effective than a strong acid such as nitric or hydrochloric acid. The addition of the organic acid will also lower the spray solution pH because of the addition of hydrogen (H+) ions. Organic acid is added to the water carrier prior to the addition of the herbicide. A use rate of 2.2 lbs of citric acid per 100 gallons of water should be adequate for water with 250 ppm of Ca²⁺.

Note: Acidifiers should not be used in conjunction with some organo-silicone adjuvants as increased acidity may enhance chemical breakdown of the adjuvant. In addition, sulfonyl urea herbicides can degrade in acidic environments below 7. READ THE LABEL!

Low Volume Rates. Decreasing the spray carrier volume has also been found to reduce hard water antagonism. Basically, one hard water ion can bind up two or more herbicide molecules. When the volume of carrier water is reduced the number of antagonistic cations (Ca²⁺and Mg²⁺) is also reduced. Reducing the spray carrier volume will therefore help reduce antagonism due to hard water. You can also increase your pesticide rate to provide more pesticide molecules but remember to stay within the labeled rate. Still, adding AMS or an organic acid in addition to a nonionic surfactant is the most prefer method to offset hard water problems.

Soft Water. Water treated with ion exchange water softeners contains sodium (Na+) cations in place of hard water cations. Some natural waters can also be considered “soft.” Although generally not as antagonistic as hard water cations, soft water concentrations of 500 ppm have also been found to antagonize the activity of Roundup (glyphosate) and 2,4-D. Soft water has also been shown to decrease the activity of Poast (sethoxydim) at 1000 ppm and the activity of Select (clethodim) and Achieve (tralkoxydim) at 500 ppm.

Surface Water Sources. Surface water sources such as streams or ponds generally have significant levels of dissolved solids and organic particulate matter….dirt! These soil particles decrease Roundup (glyphosate) and paraquat activity and can cause equipment wear. This type of antagonism cannot be corrected by adding AMS or an organic acid. Always choose a water source that is free of dirt, grit, and organic matter.

Adjuvants and Surfactants. Water softening additives designed for pesticide applications are available to offset hard water problems. While nonionic surfactants will generally enhance herbicide activity on most weed species, they will not overcome the antagonism between salt-based herbicides and hard water. Therefore, under hard water conditions, AMS or organic acids should be used in conjunction with nonionic surfactants to maximize herbicide absorption. Read the label of surfactants that you buy. Some AMS surfactants already have a nonionic surfactant added.

Spray Water pH

The term pH is used to measure the concentration of hydrogen ions in water and indicates the breakdown of water into a positively-charged hydrogen (H+) ion and a negatively-charged hydrogen-oxygen molecules (HO). pH is measured on a scale from 1 to 14 with 1 being highly acid, 7 is neutral and 14 is highly alkaline or basic. pH is easily measured with an electronic pH meter, a pH test kit such as those used for swimming pools, or pH test paper.

In general, the ideal pH for water used for spraying pesticides is slightly acidic (pH 4 -6). However, there are always exceptions. Make sure to read the label of the pesticide you intend to use. There may be pH restrictions. Some herbicides like the sulfonyl ureas have been known to break down in a spray tank when the pH is too acidic (pH less than 7). Higher temperatures will also cause these herbicides to inactivate at a higher rate.

Insecticides and fungicides, on the other hand, have been shown to break down in water that has a high pH (alkaline). Fungicides such as Benlate, Bravo and Captan can also inactivate quickly if left in the spray tank in an alkaline water source.

Use the following general guidelines once you have determined the pH is of your spray water.

Remember, READ THE PESTICIDE LABEL.

pH 3.5-6.0 Satisfactory for most spraying and short-term (12 to 24 hours) storage of most pesticide mixtures in the spray tank. Read the label. Not suitable for sulfonyl urea herbicides
pH 6.1-7.0 Adequate for immediate spraying of most pesticides. Do not leave the spray mixture in the tank for over 1 to 2 hours to prevent loss of effectiveness.
pH 7.0 and higher. Add buffer or acidifier.

You can offset the effects of water pH by adding certain adjuvants (additives) that can either change the pH or your spray mixture or maintain (buffer) the pH if it already at the desirable level.