1. Seminar
ON
FATE OF HERBICIDE IN SOIL
SpeakerSpeaker
Pravir Kumar PandeyPravir Kumar Pandey
M.Sc. (Ag.)final.M.Sc. (Ag.)final.
Dept. of AgronomyDept. of Agronomy
College of Agriculture RaipurCollege of Agriculture Raipur
2. “For the price of a green field, we
are poisoning our next generation.”
Family Circle magazine, 1991
4. 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 inactivePesticides are degraded into inactive
substances (e.g., COsubstances (e.g., CO22 ) or rendered inactive by) or rendered inactive by
several mechanismsseveral mechanisms::
7. Pesticide effects on earthworms
Most 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.
8. A. What are the fates of a
herbicide after leaving the nozzle
1. 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
9. 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
10. What are the fates of a herbicide
after leaving the nozzle
12. 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) Vegetation
1) Affects deposition to target
13. What factors (environmental or other) affects
a herbicide’s fate (doing it’s job)
2. Components of the spray
a) Spray volume
1) High (chemigation) - runoff
2) Low (drift)
b) Application pressure
1) Higher – smaller droplet, more drift
2) Low pressure – larger droplet, less drift
c) Carrier
1) Water vs. fertilizer vs. oil
d) Herbicide formulation – liquid vs. dry
1) 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
14. 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!
15. Degradation vs Transfer
1. Degradation - herbicide is gone
2. Transfer - may still be around but may
not be available for weed control
16. 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
17. 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
18. 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?
19. Chemical Decomposition
3) The breakdown of a herbicide by
a chemical process or reaction in
the absence of a living organism.
20. 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
21. Photodecomposition
1) Breakdown of herbicides by light
a. 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.
22. 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”
23. 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
24. 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
25. 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
26. Herbicides in the Environment
E. Residual vs. Persistent
1. Residual herbicide
a) 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 herbicide
a) herbicide that remains active in the soil long
enough to injure rotational crops the following
growing season
27. 2. Persistent herbicice
a) 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 Environment
E. Residual vs. Persistent
28. Soil Colloids that Adsorb Herbicides
1. Organic matter--the most adsorptive
a) 4 X more adsorptive than
montmorillonite
b) 20 X more adsorptive than kaolinite
2. Clay--the second most adsorptive soil
component. Clays do differ
a) Montmorillonite--3 to 7 X of kaolinite
3. Silt has low adsorptive capacity
4. Sand has the lowest adsorptive capacity
5. CEC--cation exchange capacity
29. 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.
30. Leaching
1. 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
31. Factors and Leaching
a) Adsorption (ionic bonding)
1) Soil type (affects adsorption)
a. Affects CEC of soil
b. Impacts permeability of soil
2) Soil organic matter (affects adsorption)
a. Affects CEC of soil
b. Impacts permeability of soil
b) Herbicide solubility
1) Atrazine = 33 mg/L
2) Pendimethalin = 0.275 mg/L
c) Rainfall amount
d) Rainfall intensity
e) Herbicide chemical properties
32. Leaching
5. 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?
33. 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.
34. 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
35. 5. Volatility increases as
temperature increases.
6. Incorporation (mechanical or
water) can reduce volatility losses.
a) Command - clomazone
H. Herbicide Volatilization
36. 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?
37. 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?
38. Drift vs. Volatilization
1. 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.
39. 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 away
1) Sedimentation
c) Both the dissolved herbicides and those
attached to soil sediments can pollute our
creeks, rivers, and lakes.
