Correcting Pesticide Source Water Poor Quality
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As discussed in the article, Poor Water Quality May Negatively Impact Pesticide Performance, pesticide half-life and performance can be drastically reduced due to poor water quality. Regrettably, many of the commonly used herbicides are salt-based products that are especially susceptible to poor water quality and many of the fungicides used will degrade within minutes when mixed with water of poor quality. Too, certain aspects of water quality such as hard water or water with high alkalinity may reduce pesticide absorption. Lastly, improper solutions pH can rapidly degrade pesticides or effect the ability of the pesticide to dissolve properly.
There is no single action to correct for all poor water quality. Corrective measure will vary according to the test results of the water source as well as to the chemistry of the pesticide product. There is no single solution for all products. As such, always refer to the product label.
Fortunately, pesticide applicators within North Carolina may easily test source water quality by submitting 16-20 oz samples to the North Carolina Department of Agriculture and Consumer Services Agronomic Services Solution Lab. This inexpensive test will provide analysis of water and corrective recommendations. Below are some of the more common corrective measures.
High pH – Additions of acid or acidifying adjuvants are recommended when solution pH values are too high. Generally, solution pH above 7 may need to be reduced. However, as noted earlier, this may vary by product so refer to the product label for solution pH adjustments. Rates of products used to correct the solution pH will vary from just a few ounces to a 1-2 quart per 100 gallons of water. Some products may change color to indicate the solution pH. If not, utilize pH strips or pH meters to gauge the final solution pH. Refer to the product selected for specific instructions.
Low pH – Extremely low solutions pH (3.5 or lower) may reduce the ability of a product to dissolve fully in water or may rapidly degrade some pesticides. There is no additives that will alter a solution pH that is too low. If solution pH is below 3.5, it is recommended to find another water source to be safe.
High Carbonates (Hard water) and/or Bicarbonate (Alkalinity) – Ammonium Sulfate (AMS)* can be used to increase herbicide efficacy for hard water or water with high alkalinity. This is especially true for the weak-acid herbicides (salt-based) like glyphosate, 2-4-D, imazethapyr, sethoxydim and bentazon. AMS added to solution will slightly adjust the pH and the sulfate ions will bind with the hard water minerals so that more of the active herbicide is transported across the leaf surface and into the plant. Similarly, other pesticide may mix more readily or be absorbed by pest more readily when AMS is used. Generally, a 2% AMS by weight or 17 pounds of dry AMS per 100 gallons of water is recommended. Add the AMS to the source water prior to addition of other products.
Urea Ammonium Nitrate (UAN) – may also reduce the water hardness or alkalinity but this is generally slightly less effective than AMS.
Organic Acids – Acids such as citric acids can be used to correct for source water with high alkalinity. Numerous products are available. Read and follow the directions.
Acidifying and Buffering Agents – Numerous products are available to correct for either solution pH and/or high alkalinity. Some pesticide labels may recommend a specific product while others allow multiple products. Refer to the product label for instructions.
* Do NOT add AMS to solutions when using volatile products such as 2,4-D or dicamba. This will increase the probability of volatilization and off-target movement.