One of the most abundant compounds on Earth, water is one of the most important chemicals to humanity and to agriculture. But all water isn’t the same and it is important to understand the influence water quality can have on pesticide efficacy.

What does it look like?

Water molecules are less than one billionth of a metre across, meaning they are great at penetrating into small places.

An example of a complex structure with water.

Water is generally expressed in the empirical formula H2O, consisting of two hydrogen atoms (H) and one oxygen atom (O). Hydrogen has a positive charge, while the oxygen has a negative charge. This polarity allows water to carry out complex interactions with materials which are soluble. Because of this polarity, water molecules are also strongly attracted to other polar compounds, including themselves.

All water isn’t created equally

Pure water is a myth. Each drop has unique qualities, many of which are dictated by underlying ground geology. This means that water quality can differ significantly depending on your source.

Water typically contains varying amounts of dissolved minerals such as calcium, magnesium, sodium, potassium, iron and many more. They typically dissolve in water as it moves through the soil profile or moves across the surface.

Theses minerals or cations give water its hardness. The effects of hard water are often noticeable in household appliances with the build-up of limescale. This occurs when the water is temporarily hard, as its minerals separate from the water and deposit on the appliances. However, water can also be permanently hard, meaning that the minerals won’t separate as easily.

Rainwater is generally soft until it lands and starts to roll over a surface. If rainwater is collected on an iron roof, or stored in an iron tank, then it may become hard. However, the most typical source of hard water on Irish farms is water extracted from boreholes.

Hard water and pesticides

The active ingredients of certain pesticides can bind to minerals (particularly iron, calcium, and magnesium) in water, causing poorer pesticide performance. After binding to the minerals, the active molecules may become too large to penetrate the plant.

Some actives are more prone to water lock-up than others, and many common brands have conditioning agents built into their products.

In the case of straight glyphosate, the active ingredient has a negative charge, so it associates with minerals in water that have a positive charge. The more minerals in the water with a positive charge (particularly calcium), the more the active may be locked up. Other actives which may be affected by hard water include many of the sulfonylureas, Dicamba, diquat, 2,4-D, aminopyralid, MCPA, trinexapac-ethyl, rimsulfuron, mecoprop-P and fluroxypyr.

Water conditioning agents can be added to the tank. These are adjuvants that provide sacrificial molecules which bond with dissolved minerals in water. When using a conditioning agent, the product should be put into the water before adding the pesticide.

Test your own water

To get your free hard water testing strips, simply email your name and contact details to the address contained in this year’s crop protection magazine. A packet of testing strips will be posted to you free of charge courtesy of Whelehan Crop Protection. Simply dip a strip into your water source (one to two seconds) and shake off excess. The more blue indicators that turn pink, the harder the water.

Hard water demonstration

To demonstrate the effects of hard water, the Irish Farmers Journal conducted a simple farm-scale demonstration earlier in the spring. A 20ac field of perennial ryegrass in the northwest was used to demonstrate the impact hard water and soft water had on the efficacy of the glyphosate. The field was grazed over winter by sheep.

The hard water for the demonstration was sourced from a local well while the soft water was sourced from a local reservoir. All four blue indicators turned pink on the hard water test, while only one turned pink on the soft water test.

Overview of the trial site.

The demonstration was conducted over six 1ha trial plots. A generic brand of glyphosate was chosen which had a concentration of 360g. Product application rates ranged between 2.5l/ha and 5.0l/ha. The water rate was a constant 250l/ha across all six trial plots. For two of the hard water plots, a water conditioner was added to the tank.

The plots were sprayed on 1 March 2019 using an Amazone UX 2400 sprayer on 24m tramlines equipped with GPS auto steer and guidance, accurate to 2cm.

Results

The trial plots were monitored from the ground and the sky on a regular basis. While the demonstration was sprayed in good conditions, the weather turned cooler in the weeks which followed. This slowed rate of kill across the entire field.

A samle of hard water testing strips.

Despite this, there were clear differences between the trial plots. The most striking difference was between the hard water (1) and soft water (3) plots at 2.5l/ha of glyphosate.

The hard water was much slower to show the effects of the glyphosate and 27 days post-spraying there were still high levels of green leaf visible in that plot. There was little difference between the hard water plots at 2.5l/ha which received a water conditioner (2) and those which hadn’t (1).

The same contrast could be seen between the hard water (4) and soft water (6) plots at 5.0l/ha of glyphosate. The hard water plots were much slower to turn, with more green leaf visible 27 days post-spraying. Again, there was little difference between the hard water plots at 5.0l/ha which received a water conditioner (5) and those that hadn’t (4).

The remainder of the field which received Round-Up Flex began to die within two weeks, with a much stronger apparent kill.

Water effects

From the demonstration, there was a clear difference in the speed of kill observed between the plots sprayed with hard water and soft water at both the 2.5l/h and 5.0l/ha rates. In each case, grass sprayed with glyphosate mixed with soft water began to respond to the active sooner.

There wasn’t a significant difference between the hard water plots treated with a conditioner and those which weren’t.

This demonstration only served to highlight the potential effect of hard water on the speed of kill and didn’t look at the longevity of the kill, largely due to time constrictions. A slow kill may not necessarily mean a bad kill and this is a question which remains to be answered.

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