by Greg LaBarge, Fulton County
A variety of factors affect the effectiveness of pesticide products. One factor to check this spring is the pH and hardness of the water you are using for spraying. These two factors, if out of range, will result in products being tied up to the point that the products are ineffective. Check product labels for specifics on spray water characteristics suited for the product you are using.
The first factor to consider is the pH of the spray water source. Most pesticides are formulated as weak acids and perform best when spray water is a pH of 4-6.5. The stability of the product in the tank is greatly influenced by the water pH. This is particularly important if the product sits in the tank for an extended period such as being rained out of a field. Herbicides and fungicides are particularly prone to short half lives in tanks with alkaline pH’s.
For example, a herbicide at pH 9 may have a half life of 10 minutes, while at pH 7 it has a half life of 17 hours and at a pH of 5 the half life is 16 days. So if your spray water pH is 9, half of the product has degraded before you are able to get it applied to the field. At pH of 7 if the spray solution sits in the tank due to be rained out of the field more than 17 hours half the product has degraded.
The second factor to know about your water is the hardness. Hardness measures the dissolved mineral in the water which includes calcium, magnesium and iron. Hardness is particularly important with glyphosate, which is a negatively charged ion that will bind with minerals in the water. Ammonium sulfate (AMS) is used with glyphosate which gives a sulfate ion that binds the minerals in the water keeping the glyphosate free in the spray water to do its job. Water hardness above 150 ppm may require conditioning depending on the product. Levels above 342 ppm are considered hard water. Refer to product labels for specific concerns by product.
Because a water source is out of the range for the product being used does not mean it can not be used. Instead conditioner products added to the tank can bring water into the range needed to increase product effectivness. Water conditioning can be the difference between success and failure if we are using low rates of product, have difficult to control weeds or using low rate use products.
Test strips which can be found for pool use are very useful to test water for hardness and pH. These test strips can be found in many stores and are an inexpensive solution to monitor water quality for your spray water sources.