Spraying tips: Don’t mix chemicals

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When spraying different pests at the same time, many farmers try to kill two birds with one stone by making elaborate mixtures.

The whitish ‘dusting’ on this bean leaf is a layer of minerals deposited by borehole water in dry, windy weather. It can reduce chemical efficacy.
The whitish ‘dusting’ on this bean leaf is a layer of minerals deposited by borehole water in dry, windy weather. It can reduce chemical efficacy.
Photo: Bill Kerr

In a previous article, I discussed the importance of choosing the appropriate adjuvant. In this article, we’ll see that the same applies to pesticides.

Pesticides are formulated by mixing various chemicals. Add other chemicals to the tank and you run the risk of reducing the efficacy of each chemical. You could even end up with a toxic mixture that may damage the plants.

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In my early farming days, I was also guilty of this. I added various products to the tank, thinking that each would somehow remain separate. Different chemicals might well be compatible, but never assume this without first making sure.

Spray compatibility charts show which compounds can go into a tank mix, but it’s worth double-checking with someone from the chemical company. Copper-based compounds, for example, are incompatible with many chemicals. A client of mine who favoured elaborate mixtures once made a concoction that turned the water into jelly!

Many foliar feed products on the market are already a mixture of dozens of fertilisers. You might think they give the crop everything it needs. But the Agricultural Research Council has on occasion issued bulletins warning farmers to be careful about using foliar feeds, as such mixtures do not take into account the elements already in the soil that the plant takes up.

In fact, none of the trace minerals listed on the label will correct a deficiency because the quantities are far too small. I have done many controlled trials with such products over the years and have never seen any benefit, even when there was a trace element deficiency in the soil.

Supply the deficient element only
Another client of mine used to apply one such mixture to his lettuce. He stopped when I told him that I could see he had used the product again, which had caused the plants to suffer stress.

In all but the most exceptional cases, the symptoms of a deficiency will manifest unevenly on the land. The trick is to identify the deficiency in these areas and then foliar-feed the lacking element only, or another element in some cases. For example, for a molybdenum deficiency, you can apply boron at the same time for convenience.

Spraying under dry conditions
In summer rainfall areas, farmers often struggle with mineral build-up when carrying out pest control in early summer before the rain starts. This is especially so when they use overhead irrigation from a borehole, stream or dam, with a high mineral content. Evaporation on the leaf surface can cause minerals to accumulate on the leaves.

Buffering the water in the spray tank can solve the problem, but sometimes, when the product lands on the leaf, the chemicals are rapidly neutralised.When this happens, add more buffer to the water and spray (as much as you’re able to) under the leaves, where the accumulation does not occur.

In addition, use products that are less sensitive to a high pH.

Bill Kerr is a vegetable specialist and a breeder of a range of vegetables.

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