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Crop Management - Dr. Robert G. Hartzler, Iowa State University - Pesticide Application Rate Based on Soil Type
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Crop Management - Dr. Robert G. Hartzler, Iowa State University - Pesticide Application Rate Based on Soil Type

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Attendees will gain a basic understanding of the processes influencing herbicide availability which is essential to diagnose performance issues in the field.

Attendees will gain a basic understanding of the processes influencing herbicide availability which is essential to diagnose performance issues in the field.

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  • The majority of herbicide within the soil at any given moment is bound to soil colloids, only a small fraction is in solution where it is available to control weeds
  • The positive charge on triazines increase adsorption, so triazines are more active in high pH soils. Plus, binding to colloids increases degradation rates, thus persistence increases in alkaline soils
  • Too much rain can dilute herbicide within profile or cause it to reach crop seed (dicamba)
  • As soils dry, more herbicide moves onto colloids, therefore reducing the performance of herbicides. In this illustration, the moist soil has 3X the ‘killing power’ as the dry soil
  • Transcript

    • 1. Herbicide-SoilInteractionsBob HartzlerExtension Weed ScienceDepartment of Agronomy
    • 2. Herbicide Application Timing.% Acres Treated - 1985 Corn SoybeanEarly Preplant 10 5PP Incorporated 50 85Preemergence 36 20TOTAL Residual 96 100Postemergence 35 16 Wintersteen and Hartzler. 1987. Pm-1288.
    • 3. Fate of Soil-Applied Herbicides
    • 4. Pools of Herbicide in Soil Desorption Bound to In soil soil solution Adsorption
    • 5. Pools of Herbicide in Soil Desorption Bound to soil Adsorption In soil solution
    • 6. Soil Properties InfluencingHerbicide Adsorption• Mineral components – Sand – Silt – Clay• Organic matter (humus) – 4 to 20 times more adsorptive than clay
    • 7. Organic matter vs crop residue
    • 8. Retention of atrazine on crop residue Soil Residue 100 80% of applied herbicide 60 40 20 0 0 0.2" 0.5" 1.0" Cumulative rainfall Ghadiri et al. 1984. Univ. Nebraska
    • 9. Sorption Coefficient KD = Amount on soil / Amount in water H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H KD= 7 H2O Soil Air Soil KD= 24 2O H Air 21 molecules bound / 3 free 24 molecules bound / 1 free
    • 10. Herbicide Sorption Herbicide Koc paraquat 100,000 glyphosate 24,000 trifluralin 7000 atrazine 100 dicamba 6
    • 11. Herbicide rates by soil type Metribuzin rate recommendations (lbs) < 2% OM 2 to 4% OM > 4% OM Coarse Do not use ½ 2/3 Medium ½ to 2/3 2/3 to 5/6 5/6 to 1 Fine 2/3 to 5/6 5/6 to 1 1 to 1 1/6 Corvus rate recommendations (fl oz) Coarse soils, 2.0 % OM or less All other soils 3.33 5.6
    • 12. Influence of soil pH on soilrelations of herbicides• Triazines – High pH: less adsorption, greater persistence• Sulfonylureas – High pH: greater persistence• Imidazolinones, isoxaflutole – Low pH: greater persistence
    • 13. Influence of Rain onPRE Herbicide Performance• Leaching – Pro: Move to depth where weed seeds germinate – Con: Can dilute herbicide and reach tile lines/groundwater• Amount of herbicide in solution
    • 14. Herbicide Activation Herbicide Soil profile (upper 2”)
    • 15. Movement of two herbicidesthrough soil1 (six inches rain) Depth (in) Alachlor Metribuzin Koc = 124 Koc = 60 0-3 95% 88% 3-6 5% 10% 6-12 0% 2%1Sandy Amount 0.6% OM loam, remaining 30% 37% 20 DAA Jones et al. 1990. Weed Sci. 38:589-597.
    • 16. Soil Moisture Affect onHerbicide Availability KD = Amount in soil / Amount in water Example : KD = 7 H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H HH H H H H H H H H H H H Soil H2O Soil H2O Air Moist soil Dry soil
    • 17. Herbicide degradation100806040 LD95%20 0 0 1 2 3 4 5 6 7 8 Weeks after planting
    • 18. Factors influencingdegradation• Herbicide properties• Environment – Rainfall – Temperature• Soil characteristics – pH (certain herbicides)
    • 19. Enhanced herbicidedegradation100 1970’s - Eradicane, Sutan+ 2000’s - atrazine80 Non-history soil: ½ life = 17 days History soil: ½ life = 9 days6040 LD95%20 History soil 0 0 1 2 3 4 5 6 7 8 Weeks after planting
    • 20. Summary• Only a small fraction of PRE herbicides is absorbed by plants• Only herbicide present in soil water available for absorption by weeds• Sorption characteristics of a herbicide critical in determining performance