• Print
• Share

Water Quality Affects Herbicide Efficacy

The quality of water used in spray tanks can affect herbicide efficacy. Water is the primary carrier for herbicide applications, usually making up more than 99% of the spray solution. Therefore, it is not surprising that the chemistry of the water added to the spray tank can greatly impact herbicide effectiveness.

Water used for spray solution should be analyzed for pH, Ca, Mg, Na, alkalinity and total suspended solids (turbidity).

Adjusting carrier water can be accomplished through the use of adjuvants. A pesticide adjuvant can be defined as any substance added to the spray tank, separate from the pesticide formulation that will improve its physical qualities and effectiveness. Buffers and water conditioners are adjuvants.

Water pH

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. Acidic conditions (pH 3 to 6) are most suitable for mixing postemergence herbicides classified as weak acids, such as; glyphosate (Roundup), paraquat (Gramoxone), sethoxydim (Poast) and 2,4-D (many products).

Weak acids dissociate (split into two pieces) under acid conditions where H+ ions are high. Dissociated herbicides are absorbed more slowly across plant cell membranes. Ideally, spray water pH should be between 5.0 and 8.0 for weak acid herbicides to avoid dissociation.

There are three methods for measuring pH; a probe and meter, litmus paper, and a field kit (e.g. www.Preiser.com, www.Hoskin.ca). The most accurate and reliable method is the probe and meter. This method is no less convenient than the other methods, but requires a more expensive piece of equipment (around $30). Following the manufacturers directions, you would typically immerse the probe in a container of the water you are testing for a set amount of time to accurately read the pH.

When water pH exceeds 7 it could degrade certain pesticides, so consider adding acidification agents to lower the pH. Extremely acidic water can cause corrosion in sprayer parts and may have to be managed with alkalinisation agents. Always follow label directions to avoid incompatibilities that could result in colloids forming and plugging sprayer equipment.

Hard Water

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 most common.

Ca, Mg, Na and Fe cations (positively charged ions) attach to negatively charged herbicide molecules. The association between herbicides and these cations may render the herbicide ineffective. Extremely alkaline water can also cause scale to form in the sprayer and may have to be managed with water conditioning agents. These agents attract and bind to the cations, freeing the herbicide molecules and allowing them to perform more effectively. Any scale from the tank and lines is most likely to break free early in the season. If a strainer contains any sort of deposit, clean the tank and lines thoroughly.

Total Water Hardness test kits can be purchased (i.e. www.H2OKits.com). They are packaged as individual test strips. One test strip is carefully sealed in its own individual foil packet to protect it from exposure to moisture before you are ready to use it and to ensure the integrity of the test kit. The foil packets are small enough to fit in your pocket, and are ideal when testing out in the field (around $10 for 40 tests).

High pH 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 effectiveness. If the sum of the concentration (ppm) of all cations exceeds 400 ppm, action may be necessary (follow label directions).

Alkalinity

Total alkalinity is the measure of the amount of alkaline buffers (primarily carbonates (CO3)2 and bicarbonates (HCO3-) in your water. These alkaline substances buffer the water against sudden changes in pH. Total alkalinity is considered the key to water balance. Alkalinity only becomes a problem with some herbicides when levels exceed 300 ppm. Simple test strips can be used to measure alkalinity (i.e. what you would use to test you pool or spa).

Turbidity

Turbidity is a measure of the relative clarity of water: the greater the turbidity, the murkier the water. Turbidity increases as a result of suspended solids, soil or organic matter that reduces the transmission of light. Turbid water can reduce the effectiveness of postemergence herbicides. For example, glyphosate (Roundup) and paraquat (Gramoxone) herbicides will bind to soil and organic matter particles suspended in water and therefore, will not be available for absorption into weed foliage.

Turbidity can be measured using a simple device called a Secchi disk (more appropriate for waterways), or a more precise instrument known as a turbidimeter (very expensive). Another option is to use an inexpensive turbidity test available from the LaMotte Company (www.lamotte.com). This test involves viewing a black dot at the end of a tube. As the turbidity of a sample increases, the dot becomes increasingly blurred. The turbidity of the sample is then compared with an identical amount of clear water to which a standardized turbidity reagent has been added.

Filters and strainers are effective means of capturing debris and suspended solids. Constructed from steel or fibreglass mesh they screen out debris to prevent nozzles from plugging. Most sprayers have four opportunities to filter carrier water, each with tighter mesh than the last: the tank hatch screen, the main (or suction) screen, the line screen(s) and finally the nozzle screens. The smallest screen should be less than or equal to the nozzle orifice size. They should all be cleaned regularly and/or replaced annually.

Examples of Optimum Water pH for Herbicides used in Perennial Cropping Systems

Summary

It is extremely important to know the pH and hardness of your water because more than 99% of your herbicide spray solution is water, therefore, its quality could greatly affect herbicide performance.

Disclaimer: This article is for educational purposes only. Mention of a specific product should not be interpreted as an endorsement, nor should failure to mention a product be considered a criticism. Always read the product label prior to using any herbicide and contact the registrant or point of sale for more information on compatibility.

References

• http://oregonstate.edu/dept/nursery-weeds/feature_articles/spray_tank/spray_tank.htm
  J.M. Witt. Agricultural spray adjuvants. c.2000. Oregon State University.