Water Quality and Pesticide Performance
Please read the pesticide label prior to use. The information contained at this web site is not a substitute for a pesticide label. Trade names used herein are for convenience only. No endorsement of products is intended, nor is criticism of unnamed products implied.
Water is the most common liquid used to dilute pesticides and carry them to the pests they are intended to control. There are three (3) water quality variables that can short circuit the activity of many pesticides. These variables are:
- Acidity and Alkalinity
- Minerals Dissolved in Water
- Suspended Soil Particles or "Dirty Water"
1. Acidity and Alkalinity
A water's acidity or alkalinity is measured in terms of pH. Simply put, pH is a chemical term used to measure the concentration of hydrogen ions in water. Ions are atoms that are either positively or negatively charged, cationic and anionic respectively. pH indicates the breakdown of water (H2O), normally a stable molecule, into a positively-charged hydrogen (H+) ion and a negatively-charged hydrogen-oxygen molecule (HO) and 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 can be measured with an electronic pH meter, a pH test kit such as those used for swimming pools, or pH test paper available from chemical supply dealers.
As a general rule of thumb, the ideal pH for water used in 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 as there are always some exceptions. For example, some herbicides have been known to break down in a sprayer tank when the pH is too acidic (pH less than 7). The sulfonylureas such as Ally, Escort, Amber, Harmony Extra, Express, and Accent may inactivate if left in the sprayer tank mixed in acidic water. 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). This means that an insecticide begins to break down once mixed with high pH water in the sprayer tank. 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 what 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.
- 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 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. These buffering agents/acidifiers include:
- Buffer Xtra Strength (Helena Chemical Co.)
- Buffer PS (Helena Chemical Co.)
- Class Ballast (Cenex/Land O'Lakes)
- Combine (Cenex Industries)
- Induce pH (Helena Chemical Co.)
- LI 700 (Loveland Industries)
- Request (Helena Chemical Co.)
- Trifol (Wilbur Ellis)
- Super Spread 700 (Wilbur Ellis)
2. Mineral Dissolved In Water
In recent years, research efforts have concentrated on the effect of the mineral content of water in herbicide performance. The activity of some herbicides can be adversely affected certain minerals that are dissolved in water. This is especially true of salt-formulated herbicides such as glyphosate (Roundup), Poast, Pursuit, and Liberty. The first important property to measure where dissolved minerals may be a problem is the Total Dissolved Solids (TDS expressed in parts per million or PPM).
Many chemical elements can be dissolved in water but six major ions (charged atoms) make up the dissolved material in most
water. The dissolved chemical elements are present as ions which carry a positive or a negative charge. The major constituents are summarized as follows:
Positively charged (cations) .............Negative Charge (anions)
VX (Ca++) .....................................Sulphate (SO4-)
Magnesium (Mg++) ........................Chloride (C1 -)
Sodium (Na+) ................................Bicarbonate (HCO3-)
Small amounts of potassium (K+), iron (Fe++, Fe+++), nitrate (NO3-) or other ions may be present, but in most natural waters, the six ions listed in the above table are the only dissolved minerals usually present in significant quantities.
The TDS can be determined by evaporating a sample to dryness and weighing the minerals that remain or it can be determined by measuring the concentration of the six major ions and calculating the sum of the ions. For example, if a water analysis showed:
Calcium = 666 ppm
Sulphate = 2434 ppm
Magnesium = 234 ppm
Chloride = 32 ppm
Sodium = 130 ppm
Bicarbonate = 346 ppm
The TDS by the sum of ions method is 3842 ppm.
Waters with a TDS of >500 mg/L may have a salty taste. In addition, bicarbonate levels in >500 mg/L can affect the performance of some grass herbicides such as Achieve and Poast, and 2,4-D amine.
Another method used to measure TDS is to determine the electrical conductivity (EC) as a measure of the total dissolved mineral material. EC is generally expressed in expressed in microsiemens per cm (uS/cm) @ 25°C. The EC or other measures of TDS is a useful tool in indicating water quality. If the EC is less than 500 uS/cm, it is unlikely that pesticide performance will be affected.
The terms 'hard water' refers to presence certain minerals in the water. Hard water becomes 'hard' because of the presence of carbonates, sulfates, chlorides of calcium, magnesium, and iron. Hardness is generally expressed as milligrams per liter (mg/L or ppm) of calcium carbonate (CaCO3) plus the amount of magnesium present as calcium carbonate equivalent. Water containing calcium and magnesium can reduce the effectiveness of glyphosate and 2,4-D amine. The following table shows a standard classification systems for describing water hardness.
Hardness Range Mg/L (ppm) of CaCO3
75-150 Moderately Hard
>300 Very Hard
From "Water Quality and Treatment, American Water Works Association, 1990"
In terms of water hardness, alkalinity is defined as a measure of the water's ability to neutralize acids and bases. In other words, it's the buffering capacity of the water and is measured in ppm (when talking in terms of water hardness). In order for water to be classified as hard, its calcium carbonate concentration must be >150 ppm and its alkalinity must also be over 150 ppm. pH levels are usually relatively stable in hard water, but we can actually use acidic chlorine to achieve a normal pH balance in water. However, hard water ions could also form an insoluble compound.
Research has shown that water with a hardness of 600 ppm (35 grains/US gallon) can almost completely antagonize 2,4-D amine applied at a low rate of about ½ cup per acre. The use of a surfactant at 0.1% (1/2 quart of surfactant per 125 gallons of spray mixture) can overcome these effects.
3. Suspended Soil Particles or "Dirty Water"
Recent research has shown that the effectiveness of some herbicides can be reduced by the presence of suspended silt and organic matter, both of which can reduce the activity of the following herbicides:
- Diquat (Reglone, Reglone A)
- Paraquat (Gramoxone)
- Glyphosate (Roundup, Laredo, Wrangler, Rustler)
These products are very susceptible to inactivation by silt and organic matter so it is important to use only clear, clean water for mixing these products. It should be noted that the same kind of inactivation can occur when these products are applied to plant surfaces that are covered with a layer of dust. Dust kicked up during the spray operation may also result in reduced control, especially directly behind the sprayer.
Practical solutions if water quality is a concern:
- Test your water source and assess for suitability for spraying pesticides.
- Reduce the water volume to the minimum required for good coverage and performance. Check the label for water volume specifications.
- Use a pesticide that is least affected by water quality. For 2,4-D, use the LV ester formulation or use the maximum rate of the amine formulation. Use a non-ionic surfactant if an amine formulation of 2,4-D is used.
- Seek alternative water source.
- Spray as soon as possible after adding the pesticide to the sprayer tank.
- Where hard water is a concern with glyphosate applications, use spray grade ammonium sulphate fertilizer (21-0-0-24) at a rate of 8 ½ to 17 pounds per 100 gallons of water. Use the higher rate where the water is extremely hard. There are many commercial adjuvants on the market that act as water conditioners. These products include: Bronc and Cayuse Plus (Wilbur Ellis), Choice (Loveland Industries), Quest (Helena Chemical Co.), and Request (Helena Chemical Co).
- Where bicarbonate occurs in water, avoid using Achieve, Select or Poast where there is more than 500 ppm of bicarbonate. When there are concentrations of >500 ppm, use the maximum recommended rate of herbicide for the target weed and apply the herbicide at the optimum growth stage of the weeds. Research has also demonstrated that the use of liquid ammonium sulphate fertilizer at about 1 ½ quarts per acre will overcome the antagonistic effects of bicarbonate in the spray water. Since 2,4-D activity can be affected by bicarbonate ions, use the LV ester formulation if practical. Use MCPA amine or ester rather than 2,4-D amine if MCPA is recommended. Use a non-ionic surfactant.
- Do not use water where iron is shown to occur. When iron that is dissolved in groundwater is exposed to air, it can oxidize and produce a precipitate which can plug screens and nozzles.
It can generally be assumed that water quality should not be a potential problem with pesticides other than those mentioned previously in this article. Nevertheless, always have the source of your spray water tested.
Water Quality and Herbicides. Farm Facts. Saskatchewan Agriculture
Water Quality and Pesticide Performance. Westco Fertilizer.
A Compendium of Herbicide Adjuvants. Southern Illinois University, 1998.
Adjuvant and Surfactant Guide. Wilbur-Ellis.
Spray Adjuvant Overview. 1998 Weed Control Manual. Meister Publishing
Minerals Hurt Spray Effectiveness. Crop Protection Manager. No. 3/97.