Pesticide Properties

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Pesticide Properties
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Runoff movement of material away from
application location over soil surface
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Leaching the downward movement of material into
the soil profile with soil water
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Factors Affecting Pesticide Runoff

• Soil infiltration rate
• precipitation or irrigation rate is greater than
  what the soil or media can absorb
• Pesticide solubility and/or adsorption
  characteristics
• material may be carried off-site in runoff water
  or, if bound tightly to soil particles, it can be
  carried with sediment in runoff water

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Pesticide Properties

• Solubility (ppm)
• Adsorption (Koc)
• Persistence or half-life (T1/2)
• affected by biotic and abiotic factors
• microbes
• photodecomposition
• pH
• others

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Solubility

• Amount of material that can be dissolved
• Reported in ppm or mg/l (same thing)
• The higher the number, the more soluble the
  herbicide
• glyphosate (isopropyl amine salt) (Roundup)
  900,000ppm
• atrazine (Aatrex) 33ppm
• oryzalin (Surflan) 3ppm

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Adsorption

• The process by which a material associates with a
  surface (stickiness)
• Reported as a Koc value
• If pesticide is not adsorbed at all Koc0
• Pesticides with a low Koc are more likely to
  leach all other things being equal

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General Classification for Koc Values

• Kocgt1,000 pesticide attaches strongly to soil
  unlikely to move unless soil erosion occurs.
• Koclt 500 tend to move with water high potential
  to leach or move off-site with surface water
  runoff.
• 500ltKoclt1000 potential to move depends on other
  cultural or environmental factors.

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Adsorption

• Soil organic matter and clay content will
  increase adsorption
• Examples average Koc
• glyphosate (isopropyl amine salt) (Roundup)
  24,000
• atrazine (Attrex) 100
• oryzalin (Surflan) 600

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Pesticide Persistence
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Soil Properties

• Texture
• proportions of sand, silt, and clay
• Permeability
• how well water is absorbed
• Organic Matter Content
• affects adsorption

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Organic Matter
Lower Adsorption
Higher Adsorption
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Scheduling

• Most herbicide that is lost from runoff or
  leaching is at the first watering event
• Apply only enough water to activate herbicide
  for 1st irrigation
• Better to pulse water or subirrigate

from Wilson et al. 1994
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Reducing Pesticide Leaching

• USE IPM
• Follow the label
• Select pesticides which have a low solubility and
  high adsorption rate
• Check irrigation systems for output and
  uniformity
• Use drip or micro-sprinklers
• Herbicides For first irrigation after
  application apply only enough water to move
  herbicide to active zone

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Reducing Surface Runoff

• Dont overirrigate
• Use vegetative buffer strips
• Use as close a pot spacing as possible
• Place containers on gravel or fabric

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Greatest Chance of Runoff

• Pesticide
• High adsorption (high Koc)
• Persistent (long half-life)
• Soil
• Fine or easily eroded
• High soil moisture
• Management
• Over-irrigation
• Lack of filter strips or buffers
• Nursery bed surface (gravelltfabricltplastic)

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Aquatic Toxicity

• Select product with lowest toxicity to aquatic
  organisms
• High LC50 (concentration that is lethal to 50 of
  tested population)
• Usually in ppm
• Acephate LC50 730, Diazinon LC50 0.09
• (based on rainbow trout exposure)

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