Tuesday, August 9, 2011

Increase the effectiveness of glyphosate herbicide by adding nitrogen

Shawn weighs out ammonium sulfate in the Mesa County CSU Lab.
Glyphosate, the herbicide ingredient most commonly known as Monsanto’s Roundup went off patent several years ago. As a result many companies now produce and sell this herbicide sometimes at a lower cost than previously.

While this herbicide is much more effective on grassy weeds than broadleaf weeds (dandelions, purslane, bindweed, etc.) you can increase the overall effectiveness of this chemical by adding nitrogen to the spray. A spray mix containing 0.5 to 1% nitrogen is recommended.

So how do you figure out the amount of a nitrogen fertilizer to add to the glyphosate/water spray?

Let’s say you are mixing up five gallons of spray for a backpack sprayer. One gallon of mixed glyphosate and water weighs about 8.35 pounds, thus 5 gallons of this spray will weigh 41.75 pounds. The steps are as follows:

1. We first need to convert 0.5% to a decimal by moving the decimal point over two places to the left. This gives us 0.005.

2. We next multiply 41.75, the weight of the five gallons of spray by 0.005. (0.005 X 41.75 pounds = 0.209) That is the weight in pounds of nitrogen we need to add to the five gallons of spray.

3. We then can convert 0.209 to ounces. There are 16 ounces in a pound and 0.209 X 16 ounces = 3.34 ounces. Now we know the amount (3.34 ounces) of Nitrogen in ounces we need to add to the each five gallon tank of spray, but not the amount of fertilizer.

4. Fertilizers contain varying amounts of Nitrogen depending on their formulation. Ammonium sulfate contains 21% N; Ammonium nitrate 33%; and Urea 48%. For this exercise I’m going to use Ammonium sulfate.

5. We next have to convert 21% (the percentage of nitrogen in ammonium sulfate) to a decimal. WE do that by moving the decimal point two places to the left. (21% = .21 – there is a decimal between the 21 and the %, you just can’t see it.) We then divide 3.34 ounces (the amount of nitrogen we need) by .21. This gives us the amount of ammonium sulfate needs to be added to every 5 gallons of glyphosate spray. (3.34 divided by 0.21 = 15.9 ounces or one pound).

If you have an eight ounce cup handy, you would need to add two of these, full to the top, to the 5 gallons of spray. This will give you the 0.5% nitrogen mix you need.

To figure out the amount of ammonium nitrate or urea to use, divide the amount of nitrogen needed by the percentage (changed to a decimal) of nitrogen in the fertilizer.


  1. Why does the N presence enhance the effectiveness?

  2. Is there a difference with NH4 vs NO3 forms of N? I.e. Ammonium sulfate vs ammonium nitrate (aside from the fact there's more N in the latter)

    1. Each molecule of NH4 and NO3 have exactly the same amount of nitrogen, that is, one atom of nitrogen. The difference is in how the nitrogen is bound to the 4 hydrogen atoms vs the 3 oxygen atoms and how these molecules bind to atoms in other molecules like water and any other molecules found in the standard fertilizers. So the ability to get more useful nitrogen out of each standard fertilizer differs. Production profits, effectiveness for the primary use, or additional uses create the different products.