Factors Influencing the Activity of Foliar and Soil Applied Herbicides

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Factors Influencing the Activity of Foliar and
Soil Applied Herbicides
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Factors Influencing the Effectiveness of a
Herbicide

• 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

• 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
• 1. Spray drift
• 2. Volatilization
• 3. "Canopy" effect

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• Be retained on the leaf
• 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
• 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
• Movement is affected because of Type of
  herbicide used
• Contact herbicides
• Kill only the tissues they come into contact with
• Systemic herbicides
• Move from the point of application to other parts
  of the plant

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• Herbicide movement in plant tissues
• Symplast
• Comprises the sum total of living protoplasm of a
  plant (phloem)
• Apoplast
• Comprises the total nonliving cell wall continuum
  of the plant (xylem)

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• Plants manufacture or store carbohydrates
  (sugars)
• "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
• May be subject to deactivation through plant
  metabolism

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Factors Influencing the Effectiveness of Soil
Applied Herbicides

• 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
• Algae
• Fungi
• Bacteria
• Actinomyces

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• Factors affecting microorganisms include
• organic compounds
• organic matter
• organic herbicides
• may increase decomposition of herbicide
• microbial populations may increase and cause
  rapid herbicide breakdown

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• Other factors affecting microorganisms
• Food supply
• Temperature
• Water
• Oxygen
• Nutrients
• Soil pH

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• Other factors (con't)
• 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
• 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)
• Hydrolysis and oxidation are the most important

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• 3. Adsorption on soil colloids
• Soil colloids - microscopic organic and inorganic
  particles that have large adsorptive capacities
• 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
• Is the downward movement of a herbicide in
  solution through the soil.

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• Herbicide leaching is determined by
• 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
• Herbicidal loss from evaporation.
• Herbicides can evaporate from the soil and be
  lost to the atmosphere as volatile gases.
• Example Soil fumigants such as methyl bromide or
  vapam.

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• 6. Photodecomposition
• 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
• 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
• Affected by two factors
• 1. Weight of the soil.
• 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.
• 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).
• 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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