SeminarON

FATE OF HERBICIDE IN SOIL

SpeakerSpeakerPravir Kumar PandeyPravir Kumar Pandey

M.Sc. (Ag.)final. M.Sc. (Ag.)final. Dept. of AgronomyDept. of Agronomy

College of Agriculture RaipurCollege of Agriculture Raipur

“For the price of a green field, we are poisoning our next generation.”

Family Circle magazine, 1991

FATE OF HERBICIDE

Herbicide Decomposition/Fate

� Adsorption to soil components

� Leaching out of plant available zone

� Volatility - escapes into air and degrades

� Photodecomposition - degraded by sunlight

� Chemical decomposition - broken down by reactions

� Microbial degradation - primary means

Pesticides are degraded into inactive Pesticides are degraded into inactive substances (e.g., COsubstances (e.g., CO2 2 ) or rendered inactive by ) or rendered inactive by several mechanismsseveral mechanisms::

Pesticide degradation

Cl

Cl

OCH2COOH

Cl

Cl

OH

2,4-D

COOH

CH2

CH2

COOH

CO2

H2O

Cl-

CO2

H2O

Pesticide degradation

Her

bic

ide

con

c. in

so

ilMinimum concentrationfor good weed control

Maximum concentration for safe recrop

Time

Critical concentrations for soil-applied or residual herbicides

Pesticide effects on earthwormsMost herbicides are harmless to earthworms

� Triazines (atrazine, simazine) appear to have moderate effects on earthworms

� Removing weeds may have indirect effects on earthworms by decreasing plant cover and food supply.

A. What are the fates of a herbicide after leaving the nozzle1. Adsorbed to soil particles

2. Photo-degradation = photodecomposition

3. Chemical degradation – hydrolysis

4. Lost through

a) Wind erosion

b) Runoff water

c) Volatilization

d) Vapor drift

e) Leaching

5. Actually hit the target (weed or soil)

a) Taken up by plants (weeds or crops)

6. Deposited on the crop

Fate of Herbicides in Soil

The vast majority of herbicides applied eventually end up in the soil and their ultimate fate is determined by soil properties or characteristics. This is true for even those herbicides which are primarily used through postemergence applications

What are the fates of a herbicide after leaving the nozzle

What factors (environmental or other) affects a herbicide’s fate (doing it’s job)

What factors (environmental or other) affects a herbicide’s fate (doing it’s job)1. Environmental

a) Wind

b) Rainfall – activation, runoff

c) Microbial population

d) Humidity

e) Soil - pH, moisture, compaction, OM, texture, fertility, slope of soil

f) Vegetation1) Affects deposition to target

What factors (environmental or other) affects a herbicide’s fate (doing it’s job)

2. Components of the spray

a) Spray volume1) High (chemigation) - runoff

2) Low (drift)

b) Application pressure1) Higher – smaller droplet, more drift

2) Low pressure – larger droplet, less drift

c) Carrier1) Water vs. fertilizer vs. oil

d) Herbicide formulation – liquid vs. dry1) Ester vs. amine formulation of 2,4-D

2) Microencapsulated vs. non-

a. Volatility, plant uptake, leaching, half life

e) Incorporation – time after application- volatility, depth, runoff

Processes that Affect Herbicide Activity

1. Degradation -- the destruction of the herbicide molecule into non-phytotoxic components

a) Biological, Chemical, Photodecomposition

2. Transfer -- the inactivation of a herbicide, but not the destruction of the herbicide

a) Adsorption, (leaching), Volatility, & Runoff

3. Both Degradation and Transfer processes will result in the loss of herbicide activity--that’s important!

Degradation vs Transfer

1. Degradation - herbicide is gone

2. Transfer - may still be around but may not be available for weed control

Degradation Process

a) Biological decomposition or degradation

1) soil microorganisms

2) plants

b) Chemical decomposition

1) hydrolysis in the soil or even spray tank

2) oxidation, etc.

c) Photodecomposition

1) breakdown by sunlight

Biological Degradation

a. Algae, fungi, actinomycetes, and bacteria

I. May use herbicide for N, C, S source

II. Fungi--smaller number than bacteria, but larger in size

III. Bacteria--large number, but small size

IV. Herbicides have generally not caused damage to fungi or bacteria--however, these organisms have caused herbicides to degrade

Factors Affecting Biodegradation

a. Just about anything that would affect the activity of soil microorganisms

I. Temperature (80 to 90 °F)

II. Water (50 to 100% field capacity)

III. Oxygen

IV. Mineral nutrient supply

V. Soil pH (6.5 to 8)

b. Cold and/or dry conditions are not favorable

I. Why is this important?

Chemical Decomposition

3) The breakdown of a herbicide by a chemical process or reaction in the absence of a living organism.

Examples of Chemical Decomposition

a. Oxidation, reduction and hydrolysis

b. These are heavily influenced by soil pH changes

c. Sometimes, hydrolysis can occur while the herbicide is in the spray tank mixed with water

Photodecomposition

1) Breakdown of herbicides by lighta. Breakage of chemical bonds

2) Incorporation of herbicides helps reduce this with soil applied herbicides

3) Can also occur with some POST applied herbicides.

4) Can also occur in the spray tank.

Transfer Processes

a) Does not mean that the herbicide is broken down, but rather it is removed or unavailable for plant use.

b) Text book has some of these processes under “chemical processes”, but do not confuse this with chemical “degradation”

Transfer Processes

1) Adsorption-desorption

2) Leaching--tied directly to Adsorption

a. Strong adsorption--no leaching

b. Weak adsorption – “may be” leached

3) Volatility

4) Vapor drift

Adsorption

a. Soil has a net “negative charge”

b. Positively charge ions – “cations” are adsorbed to the soil

c. Negatively charged ions – “anions” are repelled by the soil and will sometimes (depending on solubility) leach readily.

d. Soil texture, pH, and OM content important

Adsorption / Equilibrium

I. The adsorbed herbicide (bound herbicide) is usually in an equilibrium with desorbed (unbound herbicide); however, in some cases the adsorption is irreversible and complete.

i. Triazines--in equilibrium

ii. paraquat--complete and irreversible

Herbicides in the EnvironmentE. Residual vs. Persistent

1. Residual herbicidea) herbicide that remains active in the soil, and

continues to control weeds for a period of time after the actual application

b) A certain amount of residual activity is usually desirable--PPI & PRE herbicides

2. Persistent herbicidea) herbicide that remains active in the soil long

enough to injure rotational crops the following growing season

2. Persistent herbicicea) Phytotoxic after effects (carryover) may cause

injury to rotational crops thus restricting the planting of some potential rotational crops

b) Herbicides which persist for 3 to 12 months provide weed control for the entire growing season and have uses in perennial crops. Longer persistence--total vegetation control.

Herbicides in the EnvironmentE. Residual vs. Persistent

Soil Colloids that Adsorb Herbicides

1. Organic matter--the most adsorptivea) 4 X more adsorptive than

montmorilloniteb) 20 X more adsorptive than kaolinite

2. Clay--the second most adsorptive soil component. Clays do differ

a) Montmorillonite--3 to 7 X of kaolinite3. Silt has low adsorptive capacity4. Sand has the lowest adsorptive capacity5. CEC--cation exchange capacity

Ionic Herbicides

a) Some herbicides behave as anions, others as cations, some are acids, some are bases, and yet others are not ionic at all.

b) Adsorption of nonionic herbicides generally increases as their water solubility decreases.

Leaching1. The downward movement of a substance

with water through the soil.

a) Capillary flow can cause upward movement

2. Some leaching is desirable--herbicides are moved into the rooting zone

3. Too much leaching is undesirable

a) Herbicide dilution and poor weed control

b) Herbicide injury to crops

c) Ground water contamination

Factors and Leachinga) Adsorption (ionic bonding)

1) Soil type (affects adsorption)a. Affects CEC of soilb. Impacts permeability of soil

2) Soil organic matter (affects adsorption)a. Affects CEC of soilb. Impacts permeability of soil

b) Herbicide solubility1) Atrazine = 33 mg/L2) Pendimethalin = 0.275 mg/L

c) Rainfall amountd) Rainfall intensitye) Herbicide chemical properties

Leaching5. Adsorption is the most important factor

influencing the leachability of herbicides in soil.

6. “Adsorption bonds” are considered more important than water solubility in determining the leaching of herbicides.

7. Herbicide solubility is important, but “adsorption bonds” are the most important. MSMA, paraquat

8. Do all herbicides have the same potential to leach?

Herbicide Volatilization

1. All chemicals and compounds have a vapor pressure. Water can vaporize slowly. Gasoline can easily vaporize. Herbicides can also vaporize.

2. When a herbicide vaporizes and moves off-target, it is lost as a herbicide and is now a pollutant. It can damage non-target plants--this is not desirable.

3. Herbicides that volatilize still possess herbicide activity because the herbicide molecule is still intact--it is just in a different physical form (it is now a gas and not a liquid or solid)

4. Sunlight will attack most herbicides which are volatile, but this does NOT result in immediate decomposition of the herbicide.

Herbicide Volatilization

5. Volatility increases as temperature increases.

6. Incorporation (mechanical or water) can reduce volatility losses.

a) Command - clomazone

H. Herbicide Volatilization

Herbicide Drift (Particle Drift)

1. Drift – movement of spray droplet

2. Affected by

a) Droplet size (pressure, tip size, etc.)

b) Height of release

c) Wind velocity

d) Humidity?

Do all herbicides have the same…a) potential to volatilize?

b) potential to drift?

Do all pesticides have the same…

a) potential to volatilize?

b) potential to drift?

Drift vs. Volatilization1. Drift – movement of spray droplet

a) Particle drift--the movement of small spray droplets. This type of drift can occur with any sprayed herbicide

1) Small spray droplets can move off target by 100’s or 1000’s of feet.

2. Volatilization – change from liquid to vapor

a) Vapor drift--the movement of intact herbicide molecules that have changed to a gaseous state & are now herbicide vapors

1) Herbicide vapors can move off target for miles.

Surface Runoff

1. Herbicides can leave the site of application as

a) Dissolved herbicides from soil or foliar applications

b) Herbicides attached to soil can erode away1) Sedimentation

c) Both the dissolved herbicides and those attached to soil sediments can pollute our creeks, rivers, and lakes.