Factors Influencing the Activity
of Foliar and Soil Applied
Herbicides
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Factors Influencing the
Effectiveness of a Herbicide
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Affect of soil characteristics on herbicide
activity
Type of herbicide, plant species, climatic
variation influence effectiveness of herbicide
treatment
Type of herbicide treatment
Surface vs. incorporation
Depth of weed species germination
Uniformity of germination of weed seeds
Persistence of the herbicide in the rooting zone
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Factors Influencing the Effectiveness
of Foliar Applied Herbicides
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Reach the plant
Be retained on the leaf
Penetrate the leaf
Move to the site of action
Remain toxic long enough to exert its
action
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Reach the plant
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1. Spray drift
2. Volatilization
3. "Canopy" effect
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Be retained on the leaf
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1. Morphology of the plant
2. Waxiness of the leaf
3. Characteristics of the spray solution
4. Volatility from the leaf
5. Spray droplet size
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Penetrate the leaf
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Four factors affecting leaf penetration
1. Remain on the surface as a crystal or liquid
 2. Enter the cuticle and remain dissolved in the
nonpolar portion
 3. Enter and move in the aqueous phase along
cell walls to the vascular system
 4. Enter and move directly into living cells and
through them to the vascular system
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The site of action
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Movement is affected because of: Type of
herbicide used
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Contact herbicides
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Kill only the tissues they come into contact with
Systemic herbicides
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Move from the point of application to other
parts of the plant
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Herbicide movement in plant tissues:
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Symplast
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Comprises the sum total of living protoplasm of
a plant (phloem)
Apoplast
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Comprises the total nonliving cell wall
continuum of the plant (xylem)
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Plants manufacture or store
carbohydrates (sugars)
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"Source to Sink" concept
Source - site within the plant where sugars are
manufactured
 Sink - site within the plant where sugars are
being used
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A herbicide must remain toxic long
enough to exert its action
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May be subject to deactivation through
plant metabolism
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Factors Influencing the Effectiveness
of Soil Applied Herbicides
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1. Decomposition by microorganisms
2. Chemical decomposition
3. Adsorption on soil colloids
4. Leaching
5. Volatility
6. Photodecomposition
7. Removal by plants by harvesting
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1. Decomposition by microorganisms
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Algae
Fungi
Bacteria
Actinomyces
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Factors affecting microorganisms
include:
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organic compounds
organic matter
 organic herbicides
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may increase decomposition of herbicide
microbial populations may increase and cause rapid
herbicide breakdown
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Other factors affecting microorganisms
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Food supply
Temperature
Water
Oxygen
Nutrients
Soil pH
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Other factors (con't)
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Herbicides may remain toxic for extended
periods of time if the soil is cold, dry,
poorly aerated or other conditions are
unfavorable for microorganism growth.
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2. Chemical decomposition
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May destroy some herbicides and activate
others
Hydrolysis (addition of water), oxidation
(loss of electrons), isomerization
(rearrangement of atoms), ionization
(charge), salt formation (Mg, Ca, Na, K)
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Hydrolysis and oxidation are the most
important
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3. Adsorption on soil colloids
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Soil colloids - microscopic organic and
inorganic particles that have large
adsorptive capacities
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One cubic inch of colloidal clay has 200 to 500
square feet of adsorptive surface
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Adsorption of a herbicide on soil colloids
reduces the concentration of herbicide
freely available in the soil solution.
Organic matter and clay have a larger
surface area than sand.
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4. Leaching
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Is the downward movement of a herbicide
in solution through the soil.
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Herbicide leaching is determined by:
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1. Adsorptive relationships between the
herbicide and the soil,
2. solubility of the herbicide in water,
3. and amount of water passing downward
through the soil.
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5. Volatility
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Herbicidal loss from evaporation.
Herbicides can evaporate from the soil and
be lost to the atmosphere as volatile gases.
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Example: Soil fumigants such as methyl
bromide or vapam.
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6. Photodecomposition
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Degradation of herbicides by light.
Herbicide molecules absorb light energy,
causing the molecule to breakdown and
become deactivated.
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7. Removal by plants by harvesting
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Plants absorb herbicides from the soil.
The herbicide is then removed when the
crop is harvested.
Example: Corn may be used to remove
unwanted simazine and atrazine in areas
where ornamental are to be grown.
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Herbicide Concentration in the Soil
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Affected by two factors:
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1. Weight of the soil.
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Average 3.5 million lb/acre foot.
3.5 lb of herbicide in the top foot of soil = 1 ppm
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2. Water holding capacity of the soil.
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The concentration of herbicide in the soil
solution depends on the amount of water
available in the soil.
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Water holding capacity of the soil
(example).
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Soil one = 300,000 lb of water/acre foot
Soil two = 1,500,000 lb water/acre foot
A water soluble herbicide would have a
concentration (in solution) 5 times as
great in soil one than in soil two.
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