Notes on testing and mixing
Tank mixing-combining two or more pesticides or fertilizers in one mix-is a common practice in the turf and ornamental industry. Because tank mixing allows you to combine multiple applications into one, it can significantly increase your efficiency. However, tank mixing also increases the potential for incompatibility. This happens when a component of the mix chemically reacts with or negatively affects the performance of one or more other components.
Many incompatibilities are easily visible. Typical signs include precipitates (solids that form within the liquid mix), surface scum, oily droplets, gels or clumps. Sometimes these can hinder the operation of the sprayer by clogging nozzles and screens. (The formation of finely divided clumps that readily disperse with mild agitation and that pass cleanly through a fine-mesh screen is usually not a serious indication of trouble.) Other tank-mix incompatibilities are not so visible and simply reduce a product's effectiveness or, even worse, alter the mix so that it becomes phytotoxic to desirable plants.
Guidelines to remember Remembering certain guidelines will help you avoid known risks and remind you when to be especially wary. Fertilizers often have substantial effects on the chemistry of a spray mix. Some slow-release forms are buffered at a pH of 9.0 to 10.5-a level that can cause some pesticides to break down-and make a spray mix difficult to acidify.
Fertilizers, especially potassium, can increase the salt load of a spray mix, which can cause a variety of incompatibilities. If you add soluble fertilizers and trace elements to your tank mix, add them individually, and do not exceed 1 ounce of dissolved solids per gallon of spray mix.
Iron sulfate is incompatible with amine formulations of certain phenoxy herbicides. Such an incompatibility can cause a cottage-cheese-like precipitate to form.
Never tank mix emulsifiable-concentrate insecticides with other chemicals (you can, however, tank mix insecticides with each other).
Do not mix more than one soluble chemical, such as an emulsifiable concentrate, with one or more insoluble products, such as wettable powders or flowables.
Avoid mixing strongly acid materials with strongly alkaline materials.
Apply sprays soon after mixing. Some pesticides start to degrade after several hours, especially when pH is alkaline.
Following these guidelines will help you avoid many problems. However, because the number of possible tank-mix combinations is virtually limitless, you still need a way to test for incompatibility when trying new combinations. Anytime you use a tank-mix combination with which you are unfamiliar, you should first take the following steps to ensure its safety.
Read the label The first thing you should do when you apply any chemical is to read the product's label. Manufacturers list known incompatibilities in addition to other important information, such as the optimal pH for the product and whether you should use an adjuvant. The chance always exists that you could encounter some incompatibility not known to the manufacturer, so reading the label is no guarantee of safety. But it is the obvious first step.
Perform a jar test The so-called jar test is a practical method of checking for visible signs of incompatibility. It consists of the following: 1. Pour 1 pint of water into a clean 1-quart glass jar. 2. Add components of the mix, in the correct proportions, to the water using the proper sequence (see "Notes on testing and mixing," below). Shake the jar for 10 seconds after adding each component. 3. Let the mix stand for at least 15 minutes. (Some authorities recommend waiting up to 24 hours.) 4. Check for precipitates, surface scum, sludge, gels, oily droplets or other unusual signs. A compatible mix will be consistent and smooth.
Test pH Many incompatibilities result from extremes in pH. This is especially important to remember when you include fertilizers-some can raise spray-mix pH to 9.0 or higher-because many pesticides degrade rapidly in alkaline conditions. Other additives can cause incompatibilities by excessively lowering pH.
In either case, it is a simple matter to check the pH of the mix. Pocket pH meters or pH test strips are inexpensive and easy to use. If the mix has a pH above 8.0 or below 6.0, you may need to adjust it with a buffer.
It is possible that your water has an undesirable pH, so don't forget to check your water source as well. After all, water is the largest component of most mixes. (Sediment in the water can inactivate some pesticides, so other aspects of water quality also are worth checking.)
Make a test application It is always a good idea to apply a product you have not previously used to a small test area. This includes tank mixes you have not tried before, even if they show no sign of incompatibility during mixing. A test application should demonstrate if the mix has become phytotoxic or lost effective-ness. Overlap your applications in some places to determine your margin of safety. Wait a few days for signs to become visible.
Mixing sequence When you perform a jar test or load a tank, you should add products in the following order: wettable powders, water-dispersible granules, dry flowables, emulsifiable concentrates, solubles and adjuvants. If you're loading a mix that you already know will require a compatibility agent, it should be the first ingredient you add. The tank's agitator should be running throughout the loading process.
When you add wettable or soluble powders, first mix each in a separate container with enough water to create a paste or slurry. Then add them to the tank. If you are using more than one emulsifiable concentrate, add them separately-concentrated forms sometimes react with one another.
What to do if the jar-test mix is incompatible If a jar-test mix shows signs of incompatibility, add six drops of a compatibility agent, shake and check again. If this causes no change, add six more drops, shake and check once more. If this doesn't work, discard the mix and try again. This time, add the compatibility agent first, and then add the other ingredients.
If the mix is still incompatible and you wish to keep trying, consider changing water sources, trying different pesticide formulations or a change in the mixing order. Only make one change at a time, but realize that some mixes are incompatible no matter what you try.
Disposing of incompatible mixes If you have an incompatible mix, you must dispose of it legally. Some states, but not all, allow you to do so by applying it to a site for which all the products in the mix are labeled. Check with your local extension office for regulations.
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