Physical removal and decomposition of applied herbicides in soil.

1. DEGRADATION
OF
HERBICIDES IN
SOIL
By SUSHMITA GARG
Submitted to: DR. N.K.CHOUBEY
11/27/2018

2. BEFORE WE START
Whatis degradation?
Degradation means devaluing through any process that
make the substancelose its ability to perform the function
for which it was meant for.
Is decompositionsame as degradation?
Often these two words are used synonymously but they
are not same. Decomposition means change in original
composition of the chemical substanceand this structural
change results in functional change that is detoxification
of herbicide whereas degradation is a broader term that
includes decomposition within it along with all other
processes that results in loss of herbicide and ultimately
herbicide is detoxified for the environment. Therefore,
process for decomposition and degradation is different but
their effect or result is same.
 We are applying herbicides for the management of weeds,
then why are we talkingabout it’s degradation?
We are talking about degradation because degradation of
herbicide occurs. Herbicide is harmful to the environment
(weed is a componentof environment that is harmed by
the herbicide). And anything we will release into the
environment that can harm the ecosystem, the
environment in response, is going to tackle with that

3. harmful substance to detoxify it and that is what we call
degradation.
Also knowledge of persistence of herbicide is provided by
its ability to undergo degradation and this knowledge is
helps us to design a non-hazardouschemical weed
management strategy.
Besides the above, being a technical person of agriculture,
we should know what is happeningto the chemical we are
releasing to our field.
 To what extent, degradationis desirable?
Degradation is desirable based on site and purposeof
application of herbicide.
 Under cropping situation, a herbicide should have a
reasonabledegradability i.e. within the crop period.
In this case, a readily degradableherbicide may not
be able to control the weeds emerging later and a
slowly degradable herbicidemay injure the next crop
to be grown in rotation.
 Under non-croppingsituation where vegetation-free
are is desired such as railway lines, roadside, etc, a
non-degradableor very slowly degradable herbicide
will be preferred..

4. HOW DO HERBICIDE REACHES THE SOIL?
 Directly through :
 Pre-emergence application of herbicides.
 Pre-plant Incorporation of herbicides.
 Herbicides injected into the soil.
 Indirectly through :
 Foliage-applied spray droplets reaching the
ground
 Residues of previous crop treated with
herbicide.

5. FATE OF THE APPLIED HERBICIDE
Herbicides are recommended at rates far beyond the
actual amount utilised for herbicidal activity . This
is because there are several process that this agro-
chemical undergoes pertaining to it’s loss.
Amount of
herbicide lost in the
environment
Amount used for
HERBICIDAL
ACTIVITY

6. CATEGORISATION OF WHAT HAPPENS
TO THE APPLIED HERBICIDE
HERBICIDAL
ACTIVITY
PHYSICAL
REMOVAL
DECOMPOSITION

7. PHYSICAL PROCESSES OF REMOVAL OF
HERBICIDES
 Physical processes take the herbicides away
from the root zone of plants resulting in the direct
loss of herbicides in original form.
 Physical loss is attained through:
 Adsorption
 Vaporization
 Leaching
 Run-off/Wash-off
 Codistillation
 But the activity of the above processes and the
fact that which process will dominate the physical
removal is highly dependent on several soil &
climatic factors.

8. 1) ADSORPTION
Adsorption is the adhesion or simply holdingof herbicides
molecules or ions at the specific soil colloid sites.
Soil colloids- both clays(inorganic colloids)and organic
matter/humic substances(organic colloids)act as adsorbent
whereas herbicide moleculeis the adsorbatehere.
Under normal edaphicconditions, herbicide molecules
adsorbed with soil colloidsremain under equilibriumwith free
herbicide molecules in soil solution.
Adsorbed herbicide is not readily available for uptake by
plants unless the soil condition changes to an extent that
favours its desorption.
A strongly adsorbed herbicide decomposes very slowly while
free herbicides in the soil solution decomposes easily.
Thus adsorption affects herbicide’s availability to plants and
it’s decomposition and persistence.

9. Kinds of adsorption
Based on the type of interaction between the herbicidemolecule
and the soil colloid:
Chemical/IonicAdsorption : It involves completetransfer of
electronsbetween the herbicide moleculeand soil colloids. It
is the most dominating form of herbicide adsorption.
e.g. – Triazines, Triazinones, Bipyrilidiums.
Hydrogen Bonding : This interaction uses hydrogen bond
formation for adsorption. Hydrogen bond is a bond formed
between two highly electronegative atoms (Oxygen &
Nitrogen) through bridging by the highly electropositive atom
i.e. Hydrogen.
The carboxylic group (-COOH) in the herbicide molecule
binds with the hydroxyl group (-OH) or amino group (-NH2)
of organic soil colloidsby hydrogen bonding.
e.g. – Aliphatics(aldehydes), Chloro-aliphaticacids, etc
Physical/Vander waals adsorption : An electricallystable
moleculemay form a dipole(one side negatively charged and
otherside positively charged) which gives rise to van der waal
forces between this moleculeand other charge present in its
vicinity. Due to such forces, weak bond forms between them
which is distance-dependentand is called Van der waals
adsorption.

10. Bigger herbicide molecule acts as a dipole and gets
adsorbed on the surface of negatively charged soil
colloids.
e.g. – Sulfonylureas.
HydrophobicAttraction : It is the adsorption of water
insoluble,non-ionicherbicide molecules on the surface of
hydrophobicmoleculespresent in the soil such as lipids,
waxes, fats, resins, etc. All the non-ionicherbicides adsorb
through this mechanism only and none other.
e.g-Dinitroanilines,Anilides,Phenylureas,Benzonitriles,etc
Ligand Exchange: Ligands are the heavier molecules which
take part coordinate bond formation with heavy metals.
Coordinatebond is a bond in which both the electronsin the
bond is derived from the same element i.e. heavy metal.
In ligand exchange mechanism of adsorption, herbicide
moleculereplaces the original ligand and itself forms
coordinatebond with the heavy metal. It is of very minor
importance in herbicide adsorption.
Order of increasing binding strength (heat of adsorption)of
adsorption:

11. FACTORS AFFECTING ADSORPTION
SOIL pH : Soil pH affects ionization of herbicides.
SOIL COLLOIDS – Inorganic colloids (aluminosilicate
minerals) follow the following order for their adsorption
capacity :
MONTMORILLONITE(expanding type 2:1) >
VERMICULITE(limited expanding type 2:1) > ILLITE(non-
expanding type 2:1) > KAOLINITE(1:1 type)
All the inorganic clays have lower sorption capacity than organic
matter since organic colloidsdominates over clays in surface area.
Higher the organic matter content, greater is the adsorption of
herbicide and less herbicide will be available for herbicidal
activity.
SOIL MOISTURE CONTENT – This is inversely
proportional to herbicide adsorption.Drier the soil, greater is
the adsorption. If soil is moist, some sites on the adsorptive
colloid will be occupied by water molecules and fewer sites
will be available for herbicide adsorption.
SOIL TEXTURE – Smaller the soil particles greater is the
surface area for adsorption. Hence, clayey soil will have more
adsorption than sandy soils.
SOIL TEMPERATURE – It affects only physical adsorption
and generally weakens the adsorption force in high
temperatures.

12. 2) VAPOURIZATION
 Vapourization refers to the physical process of transformation
of a matter from solid or liquid to vapour state in response to
changes in temperatureand/orpressure.
 In case of herbicide dissipation, the original compound does
not change chemically but simply change in the state of matter
occurs.
 Therefore, instead of the term volatilization(thatrenders
chemical change), the term “vaporization”may be appropriate
to indicate the loss of herbicide.
Examples – Trifluralin, Fluchloralin,EPTC, etc.
Factors affecting vaporization
1)Herbicidal factors
a. Vapour Pressure
b.Herbicide chemical composition
c. Herbicide formulation
2)Environmentalfactors
a. Soil Moisture
b.Temperature
c. Soil colloid content
d.Wind speed
e. Depth of incorporation

13. Factors affecting vaporization
1)HERBICIDAL FACTORS
a.Vapour Pressure : Vapour pressure is the inherent
propertyof a substance and is the pressure of vapour
which is in equilibriumwith solid or liquid phase of
that chemical containedin a closed vessel at given
temperature.
A herbicide with higher vapour pressure will be
proneto have more losses due to vaporization
S.No. Vapour Pressure (mm Hg) Herbicide class
1 < 10-6
Non-volatile
2 10-6
to 10-4
ModeratelyVolatile
3 >10-4
Highly Volatile
b.Herbicidechemical composition : A water soluble
herbicide is less proneto loss due to vaporization
because it will get dissolved in soil water whereas a
non-ionicherbicide which is water insolublewill
escape away from soil water through vaporization.
e.g. Prophamis more volatile than chlorpropham
by virtue of its low solubilityin water.
c. Herbicideformulation : Ester formulation (2,4-D
ester) is more volatile than salt formulation(2,4-D
sodium salt).

14. 2)ENVIRONMENTAL FACTORS
a.Soil Moisture : Higher the soil moisture lesser will
be the vaporization of herbicide because moisture
may trap the herbicide in it.
b.Temperature : Temperature has a direct relationship
with the rate of vaporization of herbicide as it
provides heat for change of state of matter.
c. Soil colloidcontent : Presence of high amount of soil
colloidsbind the herbicidemolecule on its surface
through adsorptionand thus reduce vaporization
significantly.
d.Windspeed : Higher wind speed encourages more
vaporization loss of herbicide.
e. Depth of incorporation: A volatile herbicide if
incorporated into soil will volatilize less than when
sprayed on the soil surface.

15. 3) LEACHING
Leaching of herbicides is the vertical downward movement of
herbicides with water through soil profiles to deeper layers.
Leaching can make herbicides available or unavailable to
plantsdependingon how much leaching has occurred.
Leaching may affect selectivity of herbicides. Excessive
leaching to the deeperlayers may render the herbicideless
effective on shallow-rootedweed species but could make it
effective on deep-rooted species.
Factors affecting Leaching
ADSORPTION – Leaching has inverse relationshipwith
adsorption becausestrong adsorptionof any herbicide will
make it unavailable to flow down with water.
ORGANIC MATTER - Leaching is inversely related with soil
organic matter content becauseof increased adsorptionof
herbicides due to the presence of high amount of organic
colloids.
SOIL TEXTURE : Leaching is more in sandy soil than in
clayey soil because in latter, adsorptive forces of the soil
colloidsare not strong enough to hold the herbicidemolecules
tightly.

16. CHEMICAL NATURE OF HERBICIDES - The degree of
leaching of a herbicide is determined by –
1) Adsorptive characteristics of herbicide
For example, in Triazines, leaching occurs in the order :
Prometryne>Simazine>Atrazine>Propazine(this is in the
reverse order of its adsorptive characteristics
2) Water solubility of a herbicide
Herbicides which are more water solublereadily leach out of
surface soil.
However, apolarherbicides with low water solubility have reduced
leaching.
QUANTITY OF WATER INFLUX PASSING
DOWNWARD THROUGH A SOIL COLUMN : When the
water influx is more i.e. during flooding method of irrigation
or heavy rainfall, preferential flow of herbicide is most
operative resulting in greater leaching.

17. 4) RUN-OFF AND WASH-OFF
Herbicides from the soil surface or treated foliage are
subjected to lateral movement with run-off water along
gradient of the field if it rains immediately or even within few
days after application of herbicides.
Wash-off is the carrying away of adsorbed herbicide
molecules or ions along with the adsorptive surface or
adsorbent which remain suspended all through the run-off
water.
Factors governing run-off & wash-off losses:
Intensity, Quantity & duration of rainfall
Water solubilityof Herbicide
Adsorption behaviour of herbicide
Soil characteristics
This run-off may serve as a direct source of contamination to
the water bodies where it ultimately reaches.

18. 5) CO-DISTILLATION
Codistillationinvolves loss of herbicides with water
evaporating from the soil surface.
The herbicide binds with water physically or chemically, with
the product being more volatile than the parent material.
It is not a dominant mode of herbicide loss and therefore not
immensely studied.

19. DECOMPOSITION
Decomposition is chemical alteration of herbicide molecules
yielding metabolites toxic, less toxic or non-toxiccompared to
original herbicides.
It is predominantlya deactivation process & producesnon-
toxic metabolites in majority cases.
Decomposition can further be of two types:
• BIOTIC DECOMPOSITION (Plant and
microorganisms)
• ABIOTIC DECOMPOSITION (Photodecomposition
and Chemical degradation)
Decomposition of any herbicide may involve one or more of
the following processes listed below:
Oxidation Reduction Hydrolysis
Hydroxylation Demethylation Decarboxylation
N-dealkylation Dethioation Deamination
Dehalogenation Conjugation Ring cleavage

20. MICROBIAL DECOMPOSITION
Microbial decomposition includesdecomposition by soil
microflora viz. bacteria, fungi, actinomycetes, moulds, algae
& protozoa,etc.
Microbial decomposition is one of the major processes of
decomposition of herbicides in soil.
Herbicide degradation by soil microflora is brought about due
to one of the following interactionsbetween microbes and the
herbicide:
Herbicides do not affect the soil microflora and are
degraded only after a lag period or rapidly degraded
by them because it may serve as a source of food &
energy for the microbes.
Herbicides are inhibitory to some microbes reducing
their population.Thus, only resistant ones survive
and thereafter those resistant strains build up new
microbial populationto degrade the toxic herbicide.

21. Generalised STEPS of MICROBIAL degradation
The adaptive mechanism enables the microbes to develop the
capacity to degrade the chemicals in the soil.
Soil microbes develop this capacity through the formation of
adaptive enzymes or through mutation to form new strains.
During this adaptive phase, also called lag phase, the
populationof microbes is built up to the levels required for
detoxification.
Soil microbe degrade herbicides both intracellular(inside their
bodies) and extracellular(outsidetheir bodies) by utilizing
enzymes excreted by other organisms respectively.
The degradation proceedsby use of carbon atoms of herbicide
particles for their energy requirements resulting in the release
of CO2 in all the degradation mechanism.

22. FACTORS AFFECTING MICROBIAL
DECOMPOSITION
TEMPERATURE : Temperatures between 24°C and 32°C is
most favorable to microorganisms for their growth & activity.
Extremely cold weather and frosting in the atmosphere as well
as in the soil reduce the activities of microorganisms. Activity
is drastically reduced at or below 5°C.
MOISTURE : Excess moisture(waterlogging) or deficit
moisture(dry soil) reducetheir growth & subsequently
decomposition. Microbial decomposition is optimum when
soil moisture is at or near field capacity. It takes place between
50% to 100% of field capacity.
SOIL pH : Bacteria and actinomycetes are active at moderate
to higher pH & their activities decrease at acidic or lower pH
(below 4.5). Fungi, on the contrary, are less sensitive to soil
pH and work well in all pH ranges. They, therefore,
predominatein lower pH.
ORGANIC MATTER & ADSORPTION : If huge amount of
organic matter present in soil, it may result in increased
herbicide adsorption.Once herbicides are adsorbed, it is likely
that they are less prone to decomposition by microorganisms.
Also, if organic matter is abundantlyavailable, the microbes
may use that as a source for food & energy and herbicide
decomposition will have a second preference.

23. HERBICIDE’S CHEMICAL COMPOSITION : Microbes
may have specificity towards herbicides. For example -
Extra-large herbicide moleculesare difficult to be
embodied by microbial cells.
Chlorinemoiety on herbicides is less proneto be attacked
by microorganisms than methyl moieties
OXYGEN AVAILABILITY : Aerobic microbes under
anaerobicconditionswill highly reduce their activity towards
decomposition of organic residues including herbicides.
NUTRIENT AVAILABILITY : If there is readily available
mineral nutrientsin soil or othereasily decomposableorganic
sources for food and energy of microbes, herbicides may be
less preferred by microbes and there would be lower
decomposition.
EXTENDERS : Extenders is used to increase the span of weed
control.They are inhibitors to microorganisms and thereby
extend persistence of a herbicide or pesticide in soil. Example:
R-33865is an extender of EPTC.
Boron for 2.4-D.
Vorlex(fungicide) for linuron.

24. MICROBIAL DECOMPOSITION OF 2,4-D
DECOMPOSITION BY Pseudomonas and Arthrobacterium
DECOMPOSITION BY Pseudomonas
DECOMPOSITION BY Flavobacterium

25. ABIOTIC DECOMPOSITION
1) PHOTODECOMPOSITION
Photodecompositionmeans detoxification or decomposition of
herbicides in soil by sunlight, predominantin fields where
surface applicationsare made without subsequent
incorporation or receival of rainfall or irrigation.
Important herbicide groups undergoing photodecomposition:
Aliphatic Acids, Amides, PhenylaceticAcids, Bipyridiliums,
Carbamates, Dinitroanilines, Triazines, Phenylureasand Uracils.
Most of the herbicides exhibit their principal absorption
maxima in the UV band of light because UV band provides
sufficient energy to cause photodecompositionviz.
Energy required by one mole of herbicide to decompose
=50-120 kcal
Energy provided by UV band of light to each mole of herbicide=
60-143 kcal
Few Examples of photodecomposition
Dalapon reduction Propionic acid
Paraquat dichloride demethylation& ring cleavage
4-carboxy-1-
methylpyridiniumion + Methylamine

26. General mechanism of photodecomposition is
illustrated below:
Herbicides which do not undergo photodecomposition at all are
Arsenicals, Benzamides, Benzoics, Benzothiadiazoles, Pyridate
and glyphosate. This is because either they are highly mobile in
soil or their half-life is very long.
Absorption of sunlight by the herbicide
Herbicide molecule gets energized
Excitation of electrons of the constituting
atoms
Excited electrons takes part in either of:
Alteration of innate biological activity of
the herbicide

27. 2) CHEMICAL DECOMPOSITION
It is the deactivation of herbicides by chemical reactions
causing alteration in its structuredue to interaction of
herbicide molecules with water, oxygen, resins etc present in
the soil layers for long duration.
In soil, it occurs in absence of living organisms & is
essentially a non-enzymaticprocess.
The rate of chemical degradation of a herbicide is proportional
to its concentration.
For chemical degradation to occur, the herbicideshould have
reasonablemobility and reactivity.
Chemical degradation does not have any role at all for
dissipation of aliphatics, certain acetamides (acetochlor&
metolachlor), benzothiadiazoles(bentazon), benzoicsand
bipyrilidiums,etc.
Whereas it constitutesa major pathway for decomposition of
Dinitroanilines, Diphenyl ethers, Sulphonylureas, triazines,
etc.
Few Examples are :
Sulfonylureas ring cleavage Sulfonamide + heterocyclicamide
Atrazine hydrolysis Hydroxyatrazine
Trifluralin reduction & n-dealkylation Detoxified product

28. WHY IS DECOMPOSITION IMPORTANT ?
Presence of herbicide in the soil will give rise to following
problems :
Herbicidal residue may prove hazardousfor soil flora & fauna.
Contributeto soil pollution.
Loss of soil fertility.
Check and reduce soil cation & anion exchange capacity.
Affect soil pH
Therefore, decomposition of herbicide is very essential for
controllingthe above menace.

29. REFERENCE :
 Weed Science : Basics and Applications
- T.K. DAS
 Principles of Weed Science
- V.S. RAO
 en//Wikipedia.com
 researchgate.com/uttrakhand+agriculture+university
+weedscience