This is the second edition of the Nursery Weeds List. I received
a call last week from a grower wanting to know whether or not to add ammonium
sulfate to his Roundup spray. Should you do such a thing? We
talked for a bit, and I thought it was such a good question that I wanted
to share with you some of the things we discussed.
Can you add ammonium sulfate (AS) to Roundup? Yes, the label
allows you to add up to 17 lb. AS to a 100 gallon spray tank. The
label also correctly warns that adding AS does not preclude you from using
the proper surfactant. So you are legally allowed, but should you?
Does it make any difference in the level of control?
Yes it does, under two situations. But first let's clear a few
Roundup uses the active ingredient glyphosate. Different
formulations of Roundup (and mimics from other companies) utilize different
surfactants and other additives, but in every case it is glyphosate doing
the dirty work. Glyphosate kills plants by binding to an enzyme called
EPSP synthase. This prevents the normal functioning of the enzyme,
and when the enzyme is blocked, the plant cannot form 3 critical amino acids
and soon dies.
'Hard water' is water with high levels of calcium (Ca), magnesium
(Mg), and/or sodium (Na). Other cations can cause hard water, but these
are the usual suspects.
Hard water in the spray tank will reduce Roundup
effectiveness. But adding AS alleviates problems caused by hard water.
To determine how hard your water is, have it analyzed by a laboratory.
There are many in the Willamette Valley, contact your local extension (or
me) if you need to find one. When you get your results, use the following
equation developed by North Dakota State University to determine how much
AS to add to the spray tank.
AS (lbs./100 gal) = 0.005*ppm Na + 0.002*ppm K
+ 0.009*ppm Ca + 0.014*ppm Mg
So, why does hard water interfere with Roundup? Ca, Mg, or
Na can form a complex with the glyphosate molecule. When this happens,
the molecule is rendered ineffective in the plant because the glyphosate
molecule is unable to bind to EPSP synthase. Therefore, the ability
of Roundup to kill plants is reduced when hard water is used. When
adding AS, the ammonium preferentially attaches itself to the glyphosate
molecule and thus prevents Ca, Mg, and Na from doing so. When ammonium
is attached, the molecule can function normally, thus the effects of hard
water are neutralized by adding AS.
Let's suppose your water is NOT hard. Is there still a reason
to add AS to the spray tank? Yes again, but only in some situations.
Some plants contain high levels of Ca in their intracellular spaces.
Just like hard water in a spray tank, high Ca levels between plant cells
can reduce Roundup effectiveness.
Some plants have natural defense mechanisms for reducing Roundup
effectiveness. Upon misting the leaf surface of velvetleaf (Abutilon
theophrasti), the plant will release Ca from within the leaf onto the
leaf surface! (Hall et al., 1999) Nobody knows
why this happens, but the effect is the same, Ca interferes with the ability
of the glyphosate molecule to function properly inside the cell.
Again, adding AS to the spray tank alleviates this physiologically-induced
Velvetleaf, lambsquarter, and a few other weeds have specialized thrichomes
(hairs) called chalk glands on the leaf surface. These are a source
of Ca and other cations, which will interfere with Roundup if AS is not
Adding AS (assuming water is NOT hard) only improves effectiveness
against plants that have elevated Ca levels described above. For
plants with inherently low levels of Ca, adding AS has no noticeable effect.
Velvet leaf is the plant most notably affected by adding AS. Velvetleaf
is not common in the Willamette Valley (although some have reported an increasing
population of this weed). Another plant in which control is also improved,
and one that is a HUGE problem in the Willamette Valley, is quackgrass (Agropyron
Most weeds have not been tested for their response to Roundup + AS,
so I can't provide many details on specific weeds. It will improve
control for some, not for others (again, assuming hard water is not an
Common lambsquarter (Chenopodium
album) is a common weed in Willamette Valley nurseries.
This close-up picture of a lambsquarter
seedling vividly shows the chalk glands that could interfere with Roundup
If hard water is an issue in your nursery, AS will improve
control when using Roundup.
If you're not sure, AS is pretty cheap, and might be a good insurance
Take care, and we'll talk again in about 2 weeks,
Hall, G.J., C.A. Hart, and C.A. Jones. 1999. Twenty-five
years of increasing glyphosate use: the opportunities ahead. February
Pest-Management-Science. 56: 351-358.