It’s common advice: Test your water before using it as a spray carrier. You dutifully sample the well or dugout and await lab results. And what comes back is a whole lot of numbers. How to make sense of it all?
All three of these tests report a large number of properties and identify specific minerals and other solutes. Which ones are important in spraying? Here is the order in which I look at the numbers.
Conductivity: This property is expressed as micro Siemens per cm (µS/cm) and simply identifies how many ionic solutes are in a sample. It doesn’t differentiate between any minerals or other molecules, and therefore has limited information. But it does tell us if there is a large or small issue with water quality. If conductivity is below 500 µS/cm, the water is probably good for spraying. If the value is around 1000 to 2000, further investigation is necessary. Some water samples return conductivity of more than 10,000 µS/cm, and it’s important to identify which salts are causing that problem. Note that Total Dissolved Solids (TDS) are often listed, and these are closely related to conductivity and add very little additional information.
Bicarbonate: Bicarbonates are HCO3 and their concentration is measured in milligrams per Litre (mg/L), which is the same as parts per million (ppm). Bicarbonates can antagonize Group 1 modes of action and the common threshold is 500 ppm.
Total Hardness (calculated): This is one of the important parameters. Hardness antagonizes glyphosate and Liberty, and also ties up surfactants which can result in problems with mixing and compatibility. Hardness is caused by five main cations, in order of importance these are iron (Fe++), magnesium (Mg++), calcium (Ca++), sodium (Na+), and potassium (K+).
The Total Hardness (ppm) reported in water tests is done by taking the most common two cations, calcium and magnesium, and using this formula: 2.497*Ca + 4.118*Mg. Note that some tests report hardness in Grains per Gallon, in this case, multiply grains by 17.1 to get ppm.
While this calculation usually gives an accurate prediction of hardness, you may need to have a look at iron and sodium as well. Iron is less common, but some well waters are high in sodium and this would not be captured in the Total Hardness parameter. A water test low in Total Hardness may be high in sodium, these are typically the samples with high conductivity.
The threshold for Total Hardness depends on the herbicide, its rate, and the water volume. The most common quoted values are 350 ppm for the lower rates of glyphosate (1/2 L/acre equivalent), and 700 ppm for the higher rates. Lower water volumes increase the concentration of herbicide, and reduce the impact of water hardness.
pH: This parameter is a bit over-rated because it is later affected by the herbicide and adjuvant dissolved in it. I am usually not concerned with pH between 6 and 8, and very rarely see it outside this range. I am not a fan of changing the pH of water unless it is required on the label for mixing, because pH affects pesticide solubility in different ways and compatibility is an ever increasing concern as our tank mix complexity increases.
If your water is questionable for spraying, there are three common choices:
- Select a different well or dugout
- If the problem is barbonates or hardness, treat water with a conditioner such as Ammonium Sulphate (AMS), available in pure form as 21-0-0-24
- Use a municipal treated water source.
Using good quality water lowers the likelihood of problems with mixing and overall performance and that pays significant dividends later.