Why Test Water: The Importance of Spray Water Quality & Treatment
Poor water quality is often overlooked as a cause of poor pesticide performance. Water can carry impurities which tie-up spray products. Some problems can be identified by sight, such as clay and high iron. Other issues, like dissolved calcium or high pH, do not change appearance of water. In addition, water can have a combination of issues, both visible and not.
Best Management Practice is testing water quality yearly at minimum. This inexpensive step can identify factors leading to poor pesticide performance or fertilizer incompatibilities. Spray mixes can then be customized to treat issues and/or pre-planned to use less sensitive pesticides.
pH is a measure of acidity (less than 7 or lower) and alkalinity (greater than 7, caustic or high) in your water, with pH 7 being neutral.
- Some of the most widely used pesticides, such as glyphosate, as well as most nutritionals, are charged; charges react and can interfere with activity and uptake. Pesticides with a charge, like salts and amines, are affected.
- The optimum pH range for most pesticide products is 3.0-6.0. A limited number of actives have greater stability in alkaline (high) pH ranges.
- The source water pH is just one factor in what the final pH of the mix will be. A portable pH meter may be needed to test spray mix pH for on-site adjustments.
Hardness is a measure of all the dissolved mineral content (Ca, Mg, Fe, Mn, Al), and is typically calculated as just calcium or just calcium and magnesium dissolved in the water. Hardness of >180 ppm has the potential for severe spray solution impact.
- With glyphosate, hardness of > 500 ppm overwhelms standard rates of water conditioners. A high rate of water conditioner should alleviate the need for higher than standard rates of glyphosate.
- Dissolved minerals will inactivate and/or salt out a number of widely used pesticides, such as glyphosate, glufosinate, 2,4-D amine, dicamba salts and clethodim.
- High levels of hardness can also interfere with ECs and oils emulsifying.
Iron (Fe2+, Fe3+) is a measure of how much iron is dissolved in the water. Iron is NOT treated with AMS, like calcium and magnesium. Iron is a part of hardness, but it is also measured separately due to potential staining and odor issues.
- Iron will inactivate the same pesticides calcium and magnesium will and also hinder ECs and oils emulsifying. Like with hardness from calcium and magnesium, with glyphosate a hardness from iron > 500 ppm overwhelms standard rates of water conditioners.
Carbonates/Bicarbonates measure the amount of CO32- and HCO3- dissolved in water. Both will raise the pH of water.
- They occur in both hard and soft water. You can have low hardness and still have high carbonates.
- The presence of either carbonates or bicarbonates can raise pH and inactivate certain pesticides, such as clethodim, sethoxydim, glyphosate, carbamates, organophosophates and others.
Sodium can react with some anionic pesticides and reduce efficacy or compatibility. Also, in the presence of chloride, it can be detrimental to sensitive plants.
- Sodium can inactivate glyphosate, clethodim, sethxydim and others. It can also affect soil permeability and cause infiltration problems. (SAR-sodium absorption ratio)
Chloride (Cl-) is often contained in spray water sources from municipalities. Such water can have high enough levels of chloride impacting tank mix compatibility, potentially, reducing efficacy.
- Elevated levels of chloride can hurt rhizobium inoculants.
Solids (clay content) are often found in surface water.
- Silt and clay can antagonize glyphosate, paraquat and diquat. Find an alternate water source if spraying these herbicides.