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How Chemicals Move Through Soil
A Review
How Water Moves Through Soil
Pesticide Characteristics
• Chemical characteristics of a pesticide will determine how it behaves in the environment.
• Four main characteristics:– Solubility– Adsorption– Half-life (aka. Persistence)– Volatility
Solubility• Amount of chemical
dissolved into a solution
• PPM• > 30 ppm = high
tendency to leach or runoff.
• Tordon = 400- 430• 2,4-D = 890• Assert = 1370• Ally/Escort
– 1750 (pH 5)– 2790 (pH 7)– 213,000 (pH 9)
• Paraquat = 7000• Roundup = 12,000
Which is the higher solubility?
50 or 500 ppm?
500 ppm
Water soluble pesticides can move easily with water
Most lower solubility pesticides will bind more readily to clay and OM than those with higher solubility.
Adsorption • The binding of a
pesticide molecule to a soil particle
• Clay & Organic matter!!
• More organic matter = more binding
Measuring Adsorptivity - Koc
• Koc
• Organic carbon partition coefficient - universal index
• How tightly a pesticide will bind to the organic matter in the soil.
Wet soils limit adsorption.
Cool & dry is bestfor highly water soluble
pesticides
Koc of Common Pesticides
• Banvel – 2
• Stinger – 6
• Pursuit – 10
• Tordon – 16
• 2,4-D – 20
• Assert – 35 to 66
• 2,4-D Ester – 100
• MCPA – 110
• Broadstrike - 700
• Methyl Parathion – 5100
• Lorsban - 6070
• Treflan – 7000
• Roundup – 24,000
• Buctril – 10,000
• Capture – 216,500
• Paraquat – 1,000,000
Runoff vs. Leaching
Solubility & AdsorptionSome things to ponder
• Read the label! – Look for restrictions based on soil type. Example –
Tordon restriction on loamy sand to sandy soils.
• Calibrate! Calibrate! Calibrate!• Do not over apply • Know the soil type in the area you are spraying.• Point vs. Non-point contamination
Soil texture has a big influence on leaching of pesticides
More adsorptive
Even a jar test will give you an idea!
Gravity has more influence on leaching in saturated soils
In dry soils, capillary
action influences movement
• Degradation– Microbes near soil surface – more O2 and
nutrients– pH – Sunlight– Rate applied
• Expressed as “half-life.”• Time required to degrade 50% of
existing concentration
Persistence (Residual)
Pesticide Drift• Physical drift
caused by small droplets– Improper
nozzles– Improper
pressure• Chemical drift
– Volatilization
Volatility• Conversion of a liquid or
solid to a gas• Lower vapor pressure =
lower volatility• 2,4-D ester = high• Banvel = high• Banvel SGF = moderate• Tordon = low• Roundup = low
Volatility is also affected by:
• Weather
– High temperatures
• 2x more 2,4-D ester volatilizes at 80o than 70o
– Low humidity = more volatilization
– Air Inversions
Protecting Water Resources
Protect Sensitive Areas!
Aerial Spraying100 feet
Vehicle Spraying 25 feet
Hand Application 10 feet
Protecting Water Resources
Practice IPM
Environmental considerations
Well Locations?
Calibrate and use only what is needed
Mix and load carefullyPrevent back-siphoning
Consider the weather
Select, store and dispose of pesticides carefully
Drift
23%
38%
26%
13%Nozzle
Applicator
Physical
Other
Applicator
Nozzle
WHAT IS DRIFT?• Movement of ..
• spray particles • vapors off-target
• Less effective control • Possible injury to susceptible
vegetation and wildlife.
DRIFT POTENTIAL
BOOM HEIGHT
HIGH PRESSURE
SMALL DROPLETS Drift Production
Movement
Comparison of Micron Sizes(approximate)
• 2000 m
• 850 m
• 420 m
• 300 m
• 150 m
• 100 m
#2 Pencil lead
paper clip
staple
toothbrush bristle
sewing thread
human hair
Driftability of Spray Driftability of Spray Droplets of Varying Sizes Droplets of Varying Sizes
10
fee
t 1
0 fe
et
Crosswind @ 3 mphCrosswind @ 3 mph
20 20
50 50
100 100
150 150
400400
8 ft 22 ft. 48 ft. 178 ft. 1065 ft. 8 ft 22 ft. 48 ft. 178 ft. 1065 ft.
\
HEIGHT OF SPRAY NOZZLES ABOVE TREATED SURFACES
Nozzle Drop Size ClassesNozzle Drop Size Classes
Very Fine to Fine < 200 um
Medium to Coarse – 200 to 450 um
Very Coarse > 450 um
Tip Spray Selection by Drop Size
Turbo TeeJet Flat-fan
Temperature & humidity Air movement (direction and velocity)Air stability/inversionsTopography
Weather and Other Factors Affecting Drift
Evaporation of Droplets
Wind
High Relative HumidityLow Temperature
Low Relative HumidityHigh Temperature
Fal
l Dis
tanc
e
Evaporation and Deceleration of Various Size Droplets*
DropletDiameter(microns)
TerminalVelocity(ft/sec)
Final Dropdiameter(microns)
Time toevaporate
(sec)
Decelerationdistance
(in)
20 .04 7 0.3 <1
50 .25 17 1.8 3
100 .91 33 7 9
150 1.7 50 16 16
200 2.4 67 29 25
*Conditions assumed: 90 F, 36% R.H., 25 psi., 3.75% pesticide solution
7 seconds for a 100 micron size droplet to evaporate at 90oF at
36% RH
Strategies to Reduce Drift
• Avoid adverse weather conditions• Buffer zones• Consider using new technologies:
– drift reduction nozzles– drift reduction additives– shields, electrostatics, air-assist
• Increase drop size
• Increase flow rates - higher application volumes• Use lower pressures
• Use lower spray (boom) heights
Shielded Sprayers