Pest Patrol: the importance of doing a “pot test” with your herbicides

Every year I get more questions about mixing the liquid fertilizer, ammonium thiosulfate (ATS), with herbicides. With sulfur deposition in the atmosphere reduced, Ontario research has shown positive yield responses in some crops when sulfur fertilizers are applied, so it makes sense that farmers are looking for ways to add sulfur to their culture effectively. However, my experience with mixing ATS with herbicides is that physical compatibility is an issue.

I have yet to mix ATS well with herbicides when used exclusively as a carrier (Figure 1 above, Figure 2 below). However, in some cases, ATS can be successfully added to a mixture when water or urea ammonium nitrate (UAN) are the primary carriers, and ATS is added last after the pesticide and vector have mixed well. Let’s walk through the compatibility determination process using a recent survey.

Q: I would like to mix Blackhawk Herbicide with water (10 gallons/acre) and ATS (five gallons/acre). Is it compatible and what mixing order should I use?

A: Step 1: Refer to product label for mixing advice. Searching for the term “fertilizer” on an electronic version of the Blackhawk label is a logical place to start. Although there is a paragraph that provides wording about mixing the herbicide with fertilizers, no real specific guidance is provided, other than a general warning that mixing the herbicide with fertilizer may decrease the level of weed control and increase the risk of crop damage. You are cautioned that if you decide to mix the herbicide with a fertilizer, you are responsible for any results associated with the mix and should contact the manufacturer for further information.

2nd step: Contact the manufacturer. The manufacturer put me in touch with a technical representative who recommended that a pot test be done and suggested that the herbicide be mixed with water first and then ATS added last. The influence of this mixture on the level of weed control and crop safety was unknown.

Step 3: Perform a jar test. I converted the rate of each component to one thousandth of an hectare (for simple conversion to pot size) and tried different operation orders to see if it mattered.

Water at 10 gallons/acre. (10 gal./ac. x 3.79 l/gal. = 37.9 l/ac. x 2.5 ac./ha = 94.8 l/ha x 1,000 ml/l x 1/1,000 ha = 8 ml d ‘water). NOTE: Full disclosure, I rounded up to 100ml of water in the jar to make measuring a bit easier.

Blackhawk application rate = 440 ml/acre. or 1.1 l/ha (1.1 l/ha x 1000 ml/l x 1/1000 ha = 1 ml).

ATS at 5 gal./ac. (5 gal./ac. x 3.79 l/gal. = 19 l/ac. x 2.47 ac./ha = 46.9 l/ha x 1000 ml/l x 1/1000 ha = 9 ml of water)…again, I rounded up to 50ml per test jar for simplicity.

Figure 2 (left): An attempted pot test of Liberty 200 SN with ATS as the carrier, resulting in unacceptable mixing; Figure 3 (right): Blackhawk Herbicide mixed with water as a carrier, according to normal best practice.


Jar Test Results: Four Jar tests were performed. In the first test, the herbicide was mixed with 15 gal./ac. of water as a vector (Figure 3 above). This becomes the standard by which other mixtures containing ATS are compared. In the second pot trial, ATS was added first, followed by water and herbicide last (Figure 4 below). In this test, the herbicide did not mix with the carrier, it separated and formed a line along the surface of the mix. The inability of the herbicide to mix with the carrier solution would most certainly cause performance issues and should not be used.

In the third pot trial, water was added first, ATS second, and herbicide last. Once again, the herbicide failed to mix properly and the results were no different from the previous test shown in Figure 4 below. In the last pot test, water was added first, herbicide was added second and allowed to mix completely, then ATS was added last (Figure 5 below) . This mixing order appeared to result in successful mixing that was similar in appearance to our standard (Figure 1 top). This pot test was invaluable in identifying mix orders that resulted in unacceptable incompatibility.

Figure 4 (left): Blackhawk herbicide added to a mixture of water and ATS as a carrier. Note the thin line of herbicide that has separated and gone to the surface; Figure 5 (right): Successful mixing of water with Blackhawk herbicide first and addition of ATS last. This mix looks good but needs to be tested on a small area first to identify any performance issues.


Step 4: Even after a successful pot test, unless the manufacturer is willing, in writing, to support the mix you’ve been talking about, it remains to be determined whether the mix will adversely affect crop safety, weed control, and may -be even drift off target. . The label always states that you are responsible for any results of applying the mixture. Therefore, testing the mixture on a small area and in an isolated environment would be a wise decision, so that you can assess its performance, as well as the risks and benefits of using it.

(For more information, search “jar test” on

Have a question you want answered? Hashtag #PestPatrol on Twitter for @cowbrough or email Mike at [email protected].

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