Using hard water to make up tank mixes could impair pesticide performance this spring, warns one agronomist ahead of the key spring spraying season.

“Imagine the headlines if pesticide containers were supplied only half full. That is effectively what is happening when farmers spray with hard water,” says Lincolnshire agronomist Robert Boothman from Boothmans Agriculture.

Hard spraying water, containing dissolved calcium and magnesium ions, lock on to pesticide active ingredients and form insoluble complexes. At this point, the pesticide active becomes insoluble and can’t be taken up by plants or insects.

Mr Boothman adds that many products are susceptible to this lock-up process, including graminicides (sulfonyl-ureas, dims and ALS inhibitors), insecticides, blight fungicides and some other herbicides such as glyphosate.

“Hard water is a problem, especially in the main arable areas, with water samples commonly exceeding 300ppm of calcium carbonate equivalent. This could easily deny the user 20-50% of a pesticide’s potential activity.”

“Farmers are well aware of losses from incorrect application equipment, but these could be insignificant compared with using suboptimum spraying water. With hard water lock-up reducing pesticide activity, there is an increased threat of suboptimal doses encouraging the build-up of pesticide resistance.

Water conditioners can alleviate this problem. First-generation products were acid based, which although effective, could result in extremely acidic spraying water (pH <4) when targeting very hard water samples. Certain ALS herbicides are less effective at these exceptionally low pHs, Mr Boothman says.

Newer-generation water conditioners are not acid based and therefore do not have this issue. They form chelated complexes with the calcium and magnesium ions and buffer water to an optimum pH level.

For a water conditioner to work effectively, it should be added to the spray water before the addition of pesticides. Once these ions are complexed, the active ingredient is safe from further lock-up.

Free water tests are available for the first 100 respondents at 
www.boothmans.co.uk