Ammonium Sulfate Requirements with Glyphosate

Labels of various glyphosate formulations state that ammonium sulfate may be added to increase the herbicide’s performance under stressful conditions. One of these situations is when hard water is used as the spray carrier. Hard water is the description of water with high concentrations of minerals like calcium, magnesium, sodium, and iron. These minerals have a positive charge and are called "salts". High concentrations of these salts can antagonize or interfere with the activity of glyphosate. The positive charge of these salts is attracted to the negative charge of the glyphosate molecule and results in glyphosate-salt complexes. Unfortunately, some glyphosate-salt complexes are not absorbed as easily into leaves as others.

A glyphosate-calcium complex may be less readily absorbed than a glyphosate-ammonium or glyphosate-isopropylamine complex. The ammonium salt is the salt included in the Touchdown IQ formulation and the isopropylamine salt is the salt included in Roundup and most generic glyphosate formulations. Adding ammonium sulfate to spray water increases the ammonium salt concentration to out compete the unfavorable glyphosate-salt complexes.

It has been asked when and how much ammonium sulfate needs to be added with glyphosate to overcome hard water problems. It will depend on how hard the water is. So, the first step is having your water tested to determine the hardness of your water. Water hardness is measured either in grains or in parts per million (ppm). The hardness in grains times 17.1 will convert the value to ppm. Table 1 lists how water conditioning companies name different levels of hardness.

Table 1. Levels of water hardness described by the Water Quality Association of the U.S.

x	in grains:	in ppm:

Soft	< 1	< 17
Slightly hard	1 - 3.5	17 - 60
Moderately hard	3.5 - 7	60 - 120
Hard	7 - 10.5	120 - 180
Very hard	> 10.5	> 180

Research in North Dakota has shown that water with 300 ppm of sodium or greater than 150 ppm of calcium causes noticeable antagonism to glyphosate. This falls in the hard or very hard water range using the terms above. Fortunately, most ground water in Wisconsin has low or moderate mineral concentrations. Generally, concentrations of potassium and iron are quite low. Examples of the average and highest mineral concentrations for sampled wells are listed for nine counties from across the state (Table 2). Potassium and iron concentrations averaged less than 3 ppm across the state and were not included in the table.

Table 2. Mineral concentrations for ground water in nine selected counties. Adapted from UWEX Geological and Natural History Survey. 1981. Ground-water-quality atlas of Wisconsin. Information Circular 39.

x	Calcium		Magnesium		Sodium
County	Ave.	High	Ave.	High	Ave.	High

x	-------------------------------- ppm ---------------------------------
Chippewa	15	48	6	18	3	7
Dane	68	110	35	61	4	10
Grant	70	211	34	120	5	63
La Crosse	58	78	22	32	3	12
Manitowoc	81	368	38	101	14	107
Pierce	66	121	25	34	6	8
Rock	69	90	35	52	4	9
Waukesha	82	340	35	73	13	240
Waushara	38	76	20	41	7	19

Although these minerals are generally at concentrations below levels reported to cause antagonism, some wells have mineral concentrations that may antagonize herbicide activity. This is especially true for some wells in eastern Wisconsin as shown with Manitowoc and Wauskesha counties. Also, the antagonistic effect of minerals on herbicides is additive so water with 150 ppm of calcium and 100 ppm of sodium will cause more antagonism than water with only 150 ppm of calcium. The spray volume also affects the level of antagonism. At the same mineral concentration, minerals will cause more antagonism if glyphosate is sprayed in 20 gal/a than in 10 gal/a of water.

After all of that background information, there are three possible ways to determine the amount of ammonium sulfate (AMS) needed for glyphosate applications.

1. Use the following equation, which was developed by North Dakota State University.

AMS (lb/100 gal) = 0.005 x (sodium ppm) + 0.002 x (potassium ppm) + 0.009 x (calcium ppm) + 0.014 x (magnesium ppm)

For example, if your water test reported:

Sodium (Na)	Calcium (Ca)	Magnesium (Mg)
70 ppm	230 ppm	90 ppm

AMS = (0.005 x 70) + (0.009 x 90) + (0.014 x 90)

AMS = 0.35 + 0.81 + 1.26 = 2.42 lb/100 gal

2. Follow Monsanto recommendations for Roundup formulations and add 8.5 to 17 lb AMS per 100 gallons of water when the total of calcium, magnesium, and iron exceed 500 ppm. The Touchdown label recommends 4.25 to 17 lb AMS per 100 gallons, but I am not aware of a recommendation from Syngenta that is based on the mineral concentration.

3. Follow the North Dakota State University recommendation that AMS at 8.5 lb/100 gal is adequate for most water. They also state that AMS at 17 lb/100 gal will overcome antagonism from their highest concentrations of minerals. (They have some well water with 1600 ppm sodium.)

After reviewing data on mineral concentrations in Wisconsin well water and the equation from North Dakota, it seems that 8.5 lb/100 gal of AMS should be sufficient for most glyphosate applications in Wisconsin. However, you should have your water hardness tested to ensure that you don’t have extremely high mineral concentrations that warrant higher AMS rates.

May 2001


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