A John Deere Publication

An airplane spraying a large field with herbicides.

Farmers and agrologists spend a lot of time considering which herbicides to use on every field.

Agriculture, Education February 01, 2025

The Right Source

Not all water is suitable for spraying.

by Lorne McClinton

Farmers and agrologists spend a lot of time considering which herbicides to use on every field. Crop selection, weed pressure, herbicide tolerance, and re-cropping restrictions all come into play. Few though give the water source a second thought. That can cause problems because using the wrong one can have a major impact on how effective your herbicides will be. That's why spraying guru Tom Wolf, with Agrimetrix in Saskatoon, Saskatchewan, recommends testing it first.

"Rule number one is to get a water test done to learn what's in it," Wolf says. "Ask for a spray water test that will focus on the bicarbonates (alkalinity), the pH, and the hard water cations—sodium, potassium, magnesium, calcium, and also iron. These are the most problematic ones. They will inhibit glyphosate and make mixing some herbicides, particularly Group 1 chemistries, more difficult. Grass killers will be sensitive to bicarbonates."

Pat Bourns has a near limitless supply of groundwater at his farm in Cartwright, Manitoba. His well has more than enough capacity for all his farm needs, even for spraying. There's just one problem; it's so hard, and has such a high iron and other mineral content, that it's unusable for anything without filtration.

"If I pumped the water tanker full, and let it sit overnight, the water would run red when I opened the valve in the morning," Bourns says. "If you drank it straight from the well you'd wonder what in the world you were drinking. It reacted with chemicals and would leave residue in the sprayer tank. The filters and the nozzles would plug. It was just a real pain. Plus, any time I sprayed glyphosate, I had to use at least a full liter to get any sort of a decent kill. I just finally got tired of it."

Bourns decided to solve his problem by installing a large capacity reverse osmosis filter along with a 16,000 gallon storage tank, to purify his spraying water. He says it's been fantastic. He's completely eliminated the problems he was having with plugged filter and screens. Now he just fills up and goes.

Bourns says a system like his is expensive to install and operate. He estimates the increased efficiency he's getting from the chemicals he's spraying has paid for it in three years. Wolf says that most farmers wouldn't need such an elaborate system.

Above. Farmers devote lots of time deciding what herbicides to spray but few give much thought to the quality of the water they're using. That’s why Tom Wolf suggests getting a sprayer water test done to ensure it's not a problem.

Surface or well water? Many farmers spray with surface water from ponds and dugouts that are fed by rain or snow melt runoff. It's soft water so it's unlikely to require any kind of treatment, Wolf says. However, producers do need to be concerned about turbidity caused by suspended solids like clay sediments. A small number of products, including glyphosate and Reglone®, are sensitive to them. Algae and other things that can plug your filters can also cause headaches, but these can either be filtered out or dealt with using copper sulphate.

Well water, and even water from municipal sources, can be problematic and should be tested, Wolf says. Any pH level that falls between 4 and 7 is considered acceptable. However, he cautions, some herbicides especially Group 2s, do require particular pH levels to dissolve properly.

Ultimately, the numbers that you should be most concerned about on a water test is the total hardness. Some hardness isn't a problem but if it's 300 parts per million or more, you should be concerned. The easiest way to ameliorate this is by taking the hardness cations out of the solution by adding ammonium sulfate to your spray water.

"Once the sprayer tank is mostly filled with water just add between one and three percent of ammonium sulfate by water volume [in either powder or liquid form] into the sprayer tank," Wolf says. "The sulfate anions will preferentially bind to calcium, sodium, potassium and iron cations to create calcium sulfate, magnesium sulfate [and so on]. These are non-soluble and precipitate out into the bottom of the tank and you never see them again. Since all the hardness cations are tied up, they aren't free to interact with the herbicide."

Producers shouldn't need to look too hard to find a lab to conduct a spray water test, Wolf says. Several public and private labs, as well as some retail outlets, will provide them in most regions.

For more information Wolf suggests visiting his and OMAFRA Spray expert, Jason Deveau's website (sprayers101.com). Enter water quality and spray application in the search bar; all kinds of articles come up. ‡