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

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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

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

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

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

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

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

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

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

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

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

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

Soil seriesHumic matterOrganic matter % Rion Norfolk Rains Roanoke Cape Fear Portsmouth From: Blumhorst et al., Weed Technol. 4:

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

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

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

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

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

Application rate recommendations for Sencor DF herbicide applied PRE to soybeans. Organic matter content Soil texture 4% lb product/acre CoarseDo not use Medium Fine Note the rate increase 20

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

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

HB Soil waterSoil colloid HB H20H20 H20H20 H20H20 H20H20 H20H20 23

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

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

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

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

HB -HB HB Low K OC High K OC 28

Soil pH and Herbicide Adsorption 29

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

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

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

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

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

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

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

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

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

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

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

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

= Herbicide 42

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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