Reeves Petroff Pesticide Education Specialist Montana State University Extension.

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