It’s always tempting to add a number of different chemicals to the spray tank and spray an area once, rather than spraying the same area or crop two or more times with different chemicals. Tank-mixing different chemicals can save you time, as well as the expense of travelling over the same area twice – unless of course, the chemicals are incompatible, in which case you’ve wasted time and money, and you’ve possibly also damaged your crop.
There are two types of incompatibility that we need to be aware of; physical and biological incompatibility.
Physical incompatibility happens when two or more chemicals react and the spray mixture changes physically; for example, the chemicals dissociate into two or more layers in the spray tank; or one or more of the chemicals precipitate and settle on the bottom of the tank; or they react to form a gel, globules or strands like spaghetti that block filters and nozzles.
Biological incompatibility happens when different chemicals appear to be physically compatible, but the results are unexpected; for example one or more of the chemicals doesn’t work, or maybe the chemical mixture damages the crop that is sprayed. The problem with this is that it may be up to several weeks before you realise that you had an incompatibility problem, and in that time the weed, pest or disease has continued to cause problems, or the crop yield or quality has been reduced.
How can we check the compatibility of different products before we mix them together?
- Firstly, check the labels of the products that you want to mix for advice on compatible/incompatible products.Be aware that label instructions may be brand specific. A farmer once told me that for years he had been spraying a tank-mix of trace elements and various glyphosate products pre-sowing, with no adverse effects. Then one year he tried a new brand of glyphosate herbicide and the spray mixture turned to a gluggy mess and had to be shovelled out of the spray tank.
- If there are no clear directions on the label(s), seek advice from a reputable agronomist or advisor, or from the manufacturers of the products.
- If you still can’t get the advice you need, you could try diluting small quantities of the chemicals to their normal application dilutions, then mixing them in a jar to see if they are compatible. Note there are some limitations to the ‘jar test':
- You need to wait long enough after mixing the chemicals in the jar to allow them time to react. This may be as long as 15-20 minutes.
- This procedure only tests for physical incompatibility, and gives no indication of whether the products are biologically compatible.
Be aware that it isn’t just the obvious situations where incompatibility can be an issue. For example, tiny amounts of Group B sulfonylurea herbicide (such as found in boom end-caps, recirculation/agitation systems, and nozzles filters) may have a ‘synergistic’ effect on Group A herbicides, causing crop damage where you wouldn’t normally expect it.
Mixing order is important. Check the correct mixing order with your advisor or a chemical manufacturer.
For many tank mixes, it is important to keep the mix agitated, so that it doesn’t settle on the bottom of the tank.