1. Impact of Water Quality on Herbicide Efficacy
Curtis Rainbolt
Everglades REC
Spray Smart 2007

2. Water Quality and Herbicides
Water is the primary carrier for herbicide applications
Spray solutions commonly contain 95% or more water
Interaction is determined by water chemistry and herbicide chemistry

3. Water Chemistry
Water pH, hardness, alkalinity, and turbidity can affect herbicide efficacy
Water pH is the concentration of H+ ions in the water
Hard water is caused by high levels of calcium, magnesium, sodium, or iron

4. Water Chemistry (cont.)
Hardness is expressed as an equivalent of calcium carbonate in water analysis tests
Turbidity is caused by suspended solids, soils, or organic matter
Alkalinity refers to carbonate and bicarbonate levels in water

5. Common Dissolved Materials
Below are the 6 most common ions found in water
Other ions can also have an effect
Cations (+ charge)
Calcium
Magnesium
Sodium
Anions (- charge)
Sulfate
Chloride
Bicarbonate

6. TSD and EC
The sum of all the minerals dissolved in a water sample is the TSD
TSD is total dissolved solids
The higher the TDS, the more electrical current water can conduct
EC is electrical conductivity
EC is used as a simple method to test for TSD

7. Turbidity The dirt and color you see in the water
They can easily deactivate herbicides with Koc values
Clean water is very important for these herbicides
Herbicides with low Koc values are not affected

8. Herbicide Chemistry
Weak acids-compounds that release some H+ ions when dissolved in water
Many herbicides are weak acids: glyphosate, paraquat, sethoxydim
Weak acid herbicides partially dissociate in water
For most herbicides, the portion that does not dissociate is more easily absorbed by plant leaves

9. Herbicide Chemistry (cont.)
The solubility of sulfonylurea herbicides (Envoke, Sandea) increases with higher pH
The amount of dissociation depends on the pH of the water
Dissociated herbicide molecules have a negative charge
Herbicides with high Koc values are sensitive to turbid water
Koc is the organic carbon sorption coefficient

10. Glyphosate
Hardness higher than 350 mg/L can affect low rates and over 700mg/L can affect high rates
Glyphosate has a high Koc and is greatly affected by turbidity
Effects can be reduced by-
Using higher rates
Adding ammonium sulfate to the spray solution
Using the spray immediately after mixing
Reducing carrier volume

11. 2,4-D Amine
Water hardness levels of above 600 mg/L will likely cause interference
Alkalinity above 500 mg/L can reduce effectiveness
Consider alternate formulations or water sources

12. Clethodim, Sethoxydim, others
These grass herbicides are weak acids
Have low Koc values so they aren’t affected by turbidity
At pH values below 6 they are not affected by hard water
Higher pH allows dissociation and cation formation

13. Koc Values of Herbicides Commonly used in the EAS
2,4-D, Ametryn, Asulam, Atrazine, Dicamba, Diuron, Halosulfuron, Hexazinone, Metribuzin, Sethoxydim, Trifloxysulfuron-low
Fluazifop- moderate
Glyphosate, Pendimethalin- high
Paraquat-very high