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Herbicide Behavior in Soil Pesticide Applicator Recertification Training Prepared by Alan C. York N. C. State University and Timothy L. Grey University.

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Presentation on theme: "Herbicide Behavior in Soil Pesticide Applicator Recertification Training Prepared by Alan C. York N. C. State University and Timothy L. Grey University."— Presentation transcript:

1 Herbicide Behavior in Soil Pesticide Applicator Recertification Training Prepared by Alan C. York N. C. State University and Timothy L. Grey University of Georgia 1

2 2

3 Why is it important to understand herbicide behavior in soil? That behavior can affect: success or failure of weed control presence or absence of crop injury persistence of the herbicide (length of control; potential for carryover) environmental impact 3

4 Environmental Fate of Herbicides Herbicide Adsorbed by clay and organic matter Leaching Runoff and erosion Chemical decomposition Microbial decomposition Plant uptake Photochemical decomposition Volatilization 4

5 Environmental Fate of Herbicides Herbicide Adsorbed by clay and organic matter Leaching Runoff and erosion Chemical decomposition Microbial decomposition Plant uptake Photochemical decomposition Volatilization 5

6 What is Herbicide Adsorption? Binding of herbicide to soil colloids (clay and organic matter fraction of soil) Determines herbicide availability to plants Also impacts leaching, volatilization, and microbial degradation 6

7 Factors Affecting Herbicide Adsorption 1. Organic matter content 2. Clay content 3. Soil moisture 4. Chemical properties of herbicide 5. Soil pH (affects some herbicides, not others) 7

8 Factors Affecting Herbicide Adsorption 1. Organic matter content High capacity to adsorb herbicides Higher organic matter = greater herbicide adsorption 8

9 Humic Matter vs Organic Matter Herbicide labels base application rates on organic matter content Humic matter binds herbicides Humic matter is the highly degraded organic fraction of soil 9

10 Factors Affecting Herbicide Adsorption 1. Organic matter content High capacity to adsorb herbicides Negative surfaces (ionic binding) and organophillic surfaces (non-ionic binding) 10

11 O C O-O- Organophillic (non-polar) Hydrophillic (polar) Figure from Stevenson, J. Environ. Qual. 1:333-344. 11

12 Factors Affecting Herbicide Adsorption 1. Organic matter content High capacity to adsorb herbicides Negative surfaces (ionic binding) and organophillic surfaces (binds non-ionic herbicides) Greater organic matter content = greater herbicide adsorption 12

13 Humic Matter vs Organic Matter Herbicide labels base application rates on organic matter content Humic matter binds herbicides Humic matter is the highly degraded organic fraction of soil NCDA soil analyses include humic matter content Humic matter and organic matter contents highly correlated; absolute values differ 13

14 Soil seriesHumic matterOrganic matter ------------------ % --------------------- Rion 0.11.0 Norfolk 0.31.2 Rains 0.61.9 Roanoke 2.83.2 Cape Fear 3.13.4 Portsmouth 3.23.8 From: Blumhorst et al., Weed Technol. 4:279-283. 14

15 Factors Affecting Herbicide Adsorption 2.Clay content Clay is negatively charged; ionic binding with positively charged herbicides More clay = more adsorption 15

16 Herbicides and Soil Adsorption A.Clay and organic matter adsorb herbicides B. Clay and organic matter content of soil affects application rate of soil-applied herbicides 16

17 Herbicide labels break soil texture into three major categories: Categories Textural classes Coarsesand, loamy sand, sandy loam Mediumloam, silt loam, silt, sandy clay loam, sandy clay Finesilty clay loam, silty clay, clay loam, clay 17

18 Application rate recommendations for Axiom herbicide applied PRE to corn. Organic matter content Soil texture 3% Coarse4-6 oz8-11 oz 11 oz Medium8-11 oz11-15 oz 15 oz Fine15 oz 15 oz15-19 oz Note the rate increase 18

19 Herbicides and Soil Adsorption A.Clay and organic matter adsorb herbicides B. Clay and organic matter content of soil affects application rate of soil-applied herbicides C. Clay and organic matter content also put limitations on use of some herbicides 19

20 Application rate recommendations for Sencor DF herbicide applied PRE to soybeans. Organic matter content Soil texture 4% lb product/acre CoarseDo not use 0.50 0.67 Medium0.50 - 0.670.67 - 0.830.83 - 1.00 Fine0.67 - 0.830.83 - 1.001.00 - 1.06 Note the rate increase 20

21 Lexar Preemergence Rate Recommendations For Corn 1 Lexar use rate Soil OM content (qt/A) Less than 3% 3.0 qt 3% or greater3.5 qt Greater than 10% Not recommended 1 Taken from Lexar label. 21

22 Factors Affecting Herbicide Adsorption 3.Soil moisture Herbicides are more tightly bound to drier soil. Due to less competition with water for binding sites under dry conditions. 22

23 HB Soil waterSoil colloid HB H20H20 H20H20 H20H20 H20H20 H20H20 23

24 Factors Affecting Herbicide Adsorption 1. Organic matter content 2. Clay content 3. Soil moisture 4. Chemical properties of herbicide 5. Soil pH (affects some herbicides, not others) 24

25 Sorption Coefficient, K OC Measures the tendency for pesticide adsorption by soil. 25

26 K OC Values Low K OC Not tightly bound to soil. Most of the herbicide in soil solution and available for uptake, leaching, volatilization, microbial degradation. High K OC Tightly bound to soil. Most of the herbicide not available for plant uptake, leaching, volatilization, microbial degradation. 26

27 Herbicide K OC Values Paraquat (Gramoxone)1,000,000 mL/g Glyphosate (Roundup) 24,000 mL/g Trifluralin (Treflan) 7,000 mL/g Alachlor (Micro-Tech) 124 mL/g Imazaquin (Scepter) 20 mL/g 27

28 HB -HB HB Low K OC High K OC 28

29 Soil pH and Herbicide Adsorption 29

30 Soil pH and Herbicide Adsorption A.Non-ionizable herbicides - Have no charge regardless of soil pH - No effect of soil pH on adsorption trifluralin 30

31 Soil pH and Herbicide Adsorption B.Cationic herbicides - Always positively charged - Very tightly bound to colloids - Soil pH has no effect paraquat 31

32 Soil pH and Herbicide Adsorption C.Basic herbicides - Charge on molecule is pH dependent - Neutral or positively charged, depending upon pH - Positively charged at lower pH; greater adsorption under low soil pH 32

33 N NN NHCH 2 CH 3 HNCHCH 3 Cl CH 3 H+H+ H+H+ N NN NHCH 2 CH 3 HNCHCH 3 Cl CH 3 Atrazine Higher pH Lower pH 33

34 Soil pH and Herbicide Adsorption C.Basic herbicides - Charge on molecule is pH dependent - Neutral or positively charged, depending upon pH - Positively charged at lower pH; greater adsorption under low soil pH - At low pH, less available for plant uptake; also less available for microbial degradation and leaching 34

35 Soil pH and Herbicide Adsorption D.Acidic herbicides - Charge on molecule is pH dependent - Neutral at low pH, negatively charged at high pH - Soil pH has no effect on adsorption Cl Cl OCH 2 C O OH = Cl Cl OCH 2 C O O - = Low pH High pH 35

36 Environmental Fate of Herbicides Herbicide Adsorbed by clay and organic matter Leaching Runoff and erosion Chemical decomposition Microbial decomposition Plant uptake Photochemical decomposition Volatilization 36

37 Herbicide Leaching Downward movement of herbicide through soil profile by water 37

38 Importance of Leaching 1.Provides for activation of PRE herbicides 38

39 Weed seed Germination zone Preemergence herbicide Rain Weed seed Germination zone 39

40 Importance of Leaching 1.Provides for activation of PRE herbicides 2. Excessive leaching may reduce weed control 40

41 Importance of Leaching 1.Provides for activation of PRE herbicides 2. Excessive leaching may reduce weed control 3.Can explain crop selectivity, or lack of selectivity 41

42 = Herbicide 42

43 Importance of Leaching 1.Provides for activation of PRE herbicides 2. Excessive leaching may reduce weed control 3.Can explain crop selectivity, or lack of 4.Can contribute to ground water contamination 43

44 Factors Affecting Herbicide Leaching 1.Herbicide chemical properties a.Water solubility b.Degree of adsorption c.Determines PLP (pesticide leaching potential) 44

45 Pesticide Leaching Potential (PLP) of Commonly Used Herbicides Herbicide PLP Pendimethalin (Prowl)very low Imazethapyr (Pursuit)low Atrazinemedium Prometone (Pramitol)very high From: North Carolina Agricultural Chemicals Manual, chapter 1. 45

46 Factors Affecting Herbicide Leaching 1.Herbicide chemical properties a.Water solubility b.Degree of adsorption c.Determines PLP (pesticide leaching potential) 2.Soil characteristics a.Texture and organic matter i. Affect adsorption ii. Affect water infiltration b.Determines SLP (soil leaching potential) 46

47 Soil Leaching Potential (SLP) of Selected NC Soils Soil Series SLP Cape Fearvery low Alamancemedium Tarborovery high From: North Carolina Agricultural Chemicals Manual, chapter 1. 47

48 SLP PLPV. LowLowModHighV. High V. Low Low Mod LowV. LowLow Mod Low Mod High LowMod High V. HighMod High V. High From: N. C. Agricultural Chemicals Manual, Chapter 1. Ground Water Contamination Potential (GWCP) for Selected Herbicide-Soil Combinations 48

49 SLP PLPV. LowLowModHighV. High V. Low Low Mod LowV. LowLow Mod Low Mod High LowMod High V. HighMod High V. High From: N. C. Agricultural Chemicals Manual, Chapter 1. Ground Water Contamination Potential (GWCP) for Selected Herbicide-Soil Combinations 49

50 SLP PLPV. LowLowModHighV. High V. Low Low Mod LowV. LowLow Mod Low Mod High LowMod High V. HighMod High V. High From: N. C. Agricultural Chemicals Manual, Chapter 1. Ground Water Contamination Potential (GWCP) for Selected Herbicide-Soil Combinations 50

51 SLP PLPV. LowLowModHighV. High V. Low Low Mod LowV. LowLow Mod Low Mod High LowMod High V. HighMod High V. High From: N. C. Agricultural Chemicals Manual, Chapter 1. Ground Water Contamination Potential (GWCP) for Selected Herbicide-Soil Combinations 51

52 Factors Affecting Herbicide Leaching 1.Herbicide chemical properties a.Water solubility b.Degree of adsorption c.Determines PLP (pesticide leaching potential) 2.Soil characteristics a.Texture and organic matter i. Affect adsorption ii. Affect water infiltration b.Determines SLP (soil leaching potential) 3.Volume of water flow 52

53 Environmental Fate of Herbicides Herbicide Adsorbed by clay and organic matter Leaching Runoff and erosion Chemical decomposition Microbial decomposition Plant uptake Photochemical decomposition Volatilization 53

54 Herbicide Runoff a.Movement in surface water leaving site; herbicide dissolved or suspended in water b.Herbicide attached to soil carried by runoff water c.Herbicide can enter rivers and reservoirs 54

55 Factors Affecting Amount of Herbicide Transported by Runoff 1.Herbicide application rate 2.Time of first rainfall, intensity, amount 3.Soil texture (infiltration rate) and slope 4.Chemical properties of herbicide a.Water solubility b.Extent of adsorption 55

56 Reducing Herbicide Runoff Best Management Practices (BMP’s) a.No-till (reduced soil erosion) b.Soil tilth, residue cover (water infiltration) c.Vegetative buffer strips (trap runoff soil) d.Containment ponds around nurseries 56

57 Environmental Fate of Herbicides Herbicide Adsorbed by clay and organic matter Leaching Runoff and erosion Chemical decomposition Microbial decomposition Plant uptake Photochemical decomposition Volatilization 57

58 Herbicide Volatilization Change from a solid or liquid phase to a gaseous phase, with subsequent dissipation into the atmosphere 58

59 Factors Affecting Extent of Loss by Volatilization 1. Vapor pressure of herbicide VP Herbicide (mm Hg @ 25C x 10 -7 ) butylate (Sutan) 130,000 Trifluralin (Treflan) 1,100 Pendimethalin (Prowl) 94 Atrazine (AAtrex) 2.9 59

60 Factors Affecting Extent of Loss by Volatilization 1.Vapor pressure of herbicide 2.Application method PrePlant Incorporated (PPI) vs Pre-Emergent (PRE) 60

61 Factors Affecting Extent of Loss by Volatilization 1.Vapor pressure of herbicide 2.Application method PPI vs PRE 3.Adsorption Greater adsorption = less volatilization 61

62 Factors Affecting Extent of Loss by Volatilization 1.Vapor pressure of herbicide 2.Application method PPI vs PRE 3.Adsorption Greater adsorption = less volatilization 4.Soil temperature Greater volatilization on hotter soils 62

63 Factors Affecting Extent of Loss by Volatilization 1.Vapor pressure of herbicide 2.Application method PPI vs PRE 3.Adsorption Greater adsorption = less volatilization 4.Soil temperature Greater volatilization on hotter soils 5.Soil moisture content Greater volatilization on wet vs. dry soil 63

64 Environmental Fate of Herbicides Herbicide Adsorbed by clay and organic matter Leaching Runoff and erosion Chemical decomposition Microbial decomposition Plant uptake Photochemical decomposition Volatilization 64

65 65 Cl C l OCH 2 COOH Cl C l OH Cl C l OH OH C ClCOOH CH 2,4-D Herbicidally active  -chloromuconic acid No herbicide activity Microbial Degradation of 2,4-D

66 Herbicide Fate -- Degradation Processes 1.Microbial degradation a.Aerobic microorganisms (require oxygen) i.Rate of herbicide degradation related to population of microorganisms in soil ii.Populations and activity affected by: - Soil temp: 80 to 90F best; cool soil retards microbial degradation - Soil moisture: 50 to 100% field capacity is best; dry soil retards degradation - Soil aeration: poor aeration (and flooding) retards aerobic degradation 66

67 Herbicide Fate -- Degradation Processes 1.Microbial degradation b.Anaerobic microorganisms i.Do not require oxygen ii.Survive in flooded conditions iii.Temperature affects activity 67

68 Environmental Fate of Herbicides Herbicide Adsorbed by clay and organic matter Leaching Runoff and erosion Chemical decomposition Microbial decomposition Plant uptake Photochemical decomposition Volatilization 68

69 Herbicide Fate -- Degradation Processes 2.Chemical degradation a.Non-biological, chemical reactions 69

70 N N Cl RR N N OH RR Example of Chemical Degradation Herbicidally active 70 N N O RR Herbicidally inactive Less active

71 Chemical Degradation of Herbicides a. Non-biological, chemical reactions b. Affected by temperature; more rapid at higher temperatures 71

72 Chemical Degradation of Herbicides a. Non-biological, chemical reactions b. Affected by temperature; more rapid at higher temperatures c. May be affected by soil pH 72

73 Environmental Fate of Herbicides Herbicide Adsorbed by clay and organic matter Leaching Runoff and erosion Chemical decomposition Microbial decomposition Plant uptake Photochemical decomposition Volatilization 73

74 Environmental Fate of Herbicides Herbicide Adsorbed by clay and organic matter Leaching Runoff and erosion Chemical decomposition Microbial decomposition Plant uptake Photochemical decomposition Volatilization 74

75 Herbicide Persistence Length of time a herbicide remains phytotoxic in soil 75

76 Importance of Persistence Length of weed control Toxicity to following crop (carryover) 76

77 Herbicide Dissipation Over Time Hypothetical Case Time after application (weeks) Herbicide conc. (% of applied) Minimum concentration necessary for weed control Maximum concentration for safe recropping 77

78 Herbicide Persistence Typically expressed as half-life (t 1/2 ), or the time it takes for 50% of the herbicide to breakdown to an inactive form 78

79 Herbicide Dissipation Over Time Hypothetical Example Time after application (weeks) Herbicide conc. (% of applied) t 1/2 = 6 weeks 79

80 Factors Affecting Herbicide Persistence A.Soil properties B.Climatic conditions C.Herbicide properties 80

81 Herbicide Persistence Typically expressed as half-life (t 1/2 ), or the time it takes for 50% of the herbicide to breakdown to an inactive form Half-lives can vary considerably, depending upon soil and environmental conditions 81

82 Herbicide Dissipation Over Time Hypothetical Example Time after application (weeks) Herbicide conc. (% of applied) Favorable Environmental conditions t 1/2 = 2 weeks Less favorable environmental conditions t 1/2 = 6 weeks 82

83 Importance of Persistence Length of weed control Toxicity to following crop (carryover) Possibly illegal residues in next crop Impacts environmental fate 83

84 Environmental Fate of Herbicides Herbicide Adsorbed by clay and organic matter Leaching Runoff and erosion Chemical decomposition Microbial decomposition Plant uptake Photochemical decomposition Volatilization 84

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