Herbicide-Soil Interactions

Bob HartzlerExtension Weed ScienceDepartment of Agronomy

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.

Fate of Soil-Applied Herbicides

Desorption

Adsorption

Pools of Herbicide in Soil

Bound to soil

In soilsolution

Desorption

AdsorptionBound to

soil

In soil solution

Pools of Herbicide in Soil

Soil Properties InfluencingHerbicide Adsorption• Mineral components

– Sand– Silt– Clay

• Organic matter (humus)– 4 to 20 times more adsorptive than clay

Organic matter vs crop residue

Retention of atrazine on crop residue

0 0.2" 0.5" 1.0"0

20

40

60

80

100

Soil Residue

% o

f app

lied

herb

icid

e

Cumulative rainfall

Ghadiri et al. 1984. Univ. Nebraska

AirH2O

KD = Amount on soil / Amount in water

KD= 721 molecules bound / 3

free

Soil AirH2OSoil KD= 2424 molecules bound / 1

free

Sorption Coefficient

H H

H

H

H

HH

H

H

H

H H

H

HH

HH

H

H

H

H

H

H

HH H

H

H

HH

H

H H

H

HHH

H

H

H

H

H

HH

H

H

H

H

H

Herbicide SorptionHerbicide Koc

paraquat 100,000

glyphosate 24,000

trifluralin 7000

atrazine 100

dicamba 6

Herbicide rates by soil type

Corvus rate recommendations (fl oz)Coarse soils, 2.0 % OM or less All other soils

3.33 5.6

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

Influence of soil pH on soil relations of herbicides • Triazines

– High pH: less adsorption, greater persistence• Sulfonylureas

– High pH: greater persistence• Imidazolinones, isoxaflutole

– Low pH: greater persistence

Influence of Rain on PRE 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

Herbicide Activation

Herbicide

Soil profile(upper 2”)

Movement of two herbicides through soil1 (six inches rain)

Depth (in) AlachlorKoc = 124

MetribuzinKoc = 60

0-3 95% 88%

3-6 5% 10%

6-12 0% 2%

Amount remaining 20 DAA 30% 37%

Jones et al. 1990. Weed Sci. 38:589-597.

1Sandy loam, 0.6% OM

Soil Moisture Affect on Herbicide Availability

Moist soilH2O

KD = Amount in soil / Amount in water

HH

H

H

H

H

HH

H

H

H

H H

H

HH

HH

H

H

H

H

Example : KD = 7

H

H

H

Soil AirH2O

HH

H

H

H

H

HH

H

H

H

H H

H

HH

HH

H

H

H

H

H H

H

Soil

H

H HH

H

H

HH

HH

HH

H

H

Dry soil

Herbicide degradation

0 1 2 3 4 5 6 7 8Weeks after planting

100

80

60

40

20

0

LD95%

Extr

a ki

lling

pow

er

Factors influencing degradation• Herbicide properties• Environment

– Rainfall– Temperature

• Soil characteristics– pH (certain herbicides)

Enhanced herbicide degradation

0 1 2 3 4 5 6 7 8Weeks after planting

100

80

60

40

20

0

LD95%

History soil

1970’s - Eradicane, Sutan+

2000’s - atrazine Non-history soil: ½ life = 17 days History soil: ½ life = 9 days

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