Pesticide Properties

Runoff: movement of material away from application location over soil surface soil surface offsite

Leaching: the downward movement of material into the soil profile with soil water

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

Pesticide Properties Solubility (ppm) Adsorption (Koc) Persistence or half-life (T 1/2 ) –affected by biotic and abiotic factors microbes photodecomposition pH others

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

Adsorption The process by which a material associates with a surface (“stickiness”) Reported as a Koc value If pesticide is not adsorbed at all Koc=0 Pesticides with a low Koc are more likely to leach all other things being equal

Koc>1,000= pesticide attaches strongly to soil; unlikely to move unless soil erosion occurs. Koc< 500 tend to move with water; high potential to leach or move off-site with surface water runoff. 500<Koc<1000 potential to move depends on other cultural or environmental factors. General Classification for Koc Values

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

Pesticide Persistence

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

Organic component Organic Matter Lower Adsorption Higher Adsorption

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

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

Reducing Surface Runoff Don’t overirrigate Use vegetative buffer strips Use as close a pot spacing as possible Place containers on gravel or fabric

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 (gravel<fabric<plastic)

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

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