Remember high school chemistry demonstrations where the teacher poured two innocent-looking liquids into a beaker, and the beaker erupted into a volcano of foam? Similar, but less dramatic, chemical reactions can happen when incompatible herbicides are not mixed correctly.
“The days of glyphosate and water made us forget everything we knew about mixing farm chemicals,” says Jim Reiss, vice president of development and technical services for Precision Laboratories. “But as we return to tank mixing products, we’re dealing with complex chemistries that can be very sensitive to chemical compatibility, mixing sequence and other subtleties that can lead to big problems if they’re ignored.”
Here are considerations that can determine whether a tank of blended herbicides provides effective weed control or becomes a weird science chemical disaster.
Mix In Sequence.
“There are situations where products are compatible and work great if you mix Product A with Product B, then add Product C,” Reiss says. “But if you mix A with C then add B, it could turn to cottage cheese. Mixing sequences are detailed in the fine print on product labels, or you can use our free ‘Mix Tank App’ at precisionlabs.com to see if products are compatible. The app also details what sequence to mix them, and other information that can help prevent problems in a spray system.”
Make Sure You Use Enough Water.
“Mixing concentrated pesticides together in a 1,000-gal. solution tank with only 100 gal. or 200 gal. of water in the bottom, or in a mixing vat or induction cone without water at all, is a mixing sin,” says Dr. Bryan Young, professor of weed science at Purdue University. “Mixing instructions on labels recommend filling any mixing container half full of water before adding any product to make sure there’s enough water to absorb all the chemicals.”
Proper Mixing Takes Time.
“People will glug one jug of chemical into an inductor, then glug another one right in behind it before the first chemical can fully mix with the water or carrier,” Young says. “We’re trying to mix things as fast as we can, and we’re forcing too much chemistry to happen all at once.”
Hard Water Can Affect Chemicals.
“Glyphosate is the poster child for problems with mineral hardness,” says Dr. Fred Whitford, director of Purdue University pesticide programs. “If you’re using Glyphosate in hard water, you’re definitely not getting full effect of the glyphosate on weeds. Hardness levels above 200 mg/L of calcium can cause problems with some chemicals.”
Be Aware of Water pH and Mineral Content.
“Some chemicals, like glyphosate, work best in slightly acidic water, around 5.0 to 5.5,” Young says. “Other chemicals are more soluble at higher pH, while some pesticides may not be stable at the higher pH.”
Understand Agitation.
All spray mixes require agitation at each step of mixing, from pre-mixing through application. “Physical agitation is best,” Reiss says. “The turbulence of just pumping chemicals into a tank isn’t enough. Sloshing down the road in a transport tank isn’t enough. If a transport tank sits on the side of a road for an hour, waiting to refill a sprayer, it should be agitated before transferring to the sprayer.”
Sitting Means Separation.
If a transport tank sits for an hour on the side of the road waiting to fill a sprayer, or overnight, it should be re-agitated before use. Even if the components don’t come out of suspension and settle to the bottom of the tank, they can stratify. Stratification means the solution at the bottom of the tank is “hotter” than the layers at the top, which can lead to mysterious in-field streaking and poor weed control as the load is sprayed off.
Don’t Trust Tank Markings or Free Measuring Tubes.
The gradations on plastic transport and sprayer tanks can be off by 100 gal. in a 1,000-gal. tank. Adding enough chemicals to treat 1,000 gal. to a tank that actually only holds 900 gal. wastes money and risks over-application.
“Misjudging volume can cost tens of thousands of dollars over a spraying season,” Whitford says. “Calibrated flow meters provide good accuracy. Or, knowing that water weighs 8.34 lb. per gallon, you can weigh a tank full of water to know exactly how many gallons it actually it holds.”
The tubes and measuring cups provided with dry chemicals aren’t especially accurate. “They warn you on the tube or cup that they can be off by 7% to 10%,” he says. “A good set of scales only costs $75 to $100 and can pay for itself by not over- or under-applying.”
When in Doubt, Add Water.
If products clump, change color or look weird during mixing, add water immediately. Water is a universal solvent, and since many coagulation and consistency problems with spray mixes start with mixing chemicals too quickly or without enough water, adding water can sometimes rescue an early-stage disaster.
“But in many cases,” Young says, “once an actual chemical reaction between two products has developed, there’s no going back. It may be best to stop and clean the system while things are still flowable, because if things actually coagulate into cottage cheese, you’ll have to clean out the tank and every valve, screen and hose on the machine.”
