Q: Several weeks ago you wrote about the half-life of insecticides after application. I assume that the same breakdown occurs in herbicides. Does the degeneration occur after mixing, or only after application? I have been mixing Roundup(tm) in five gallon containers and using it a gallon or so at the time. I noticed that the last application did not seem very effective.

A: Your intuition is correct – there IS a consequence to storing the diluted glyphosate (Roundup, etc.) and using it as you need it.

One of the reasons glyphosate has such a good safety record is that the molecule is “polar”. Like a bar magnet, one end is negatively charged and the other end is positively charged. Since soil particles are also charged, the chemical, once applied, sticks to them. Bacteria and fungi are then able to make a meal of the chemical and break it down further.

Tap water, though, has a high pH: usually 8.0 to 10.0. Its alkalinity causes the glyphosate molecules to break apart within a few days. Conversely, in acid water, glyphosate is much more stable. Other pesticide molecules may act similarly.

The upshot? Mix just as much pesticide as you need immediately; don’t store it for future use.
————————-
Note:
The information above was taken from this note from a researcher, who translated it from the very technical note below that.
—————————————————————–

When you add Roundup to water, it will form a solution around pH 4.5. Roundup performance is better in slightly acid spray solutions.

There is no buffering agent added to Roundup. The glyphosate formulation itself is slightly acid so when mixed with water it will form a solution between pH 4.0 to 5.0, depending on the pH of the water.

Over time, the glyphosate can be tied up by cations in the water or it can be biodegraded by bacteria. This will vary with the quality of the water and the amount of Roundup in the solution. A large field sized sprayer with a low rate of Roundup shouldn’t set for more that a couple days before using. A small backpack sprayer with a 2% or greater solution of Roundup mixed with clean tap water could probably set for an extended period and still be used. However, it is always best to mix a fresh solution and use it immediately.
———————————————————–

Glyphosate (acid) is soluble in water at 25