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Questions on Heavy Rain and Herbicide Persistence

Posted: April 19, 2011

This past Saturday, nearly 5 inches of rain fell around the Gettysburg area in a short period of time. One grower contacted me and was wondering about recently applied herbicides and the potential for washing the herbicide too deep. I thought this would be a good time to review some of our residual herbicides and the mobility or potential to be rendered ineffective due to rains. The table at the end of this article lists the common tree fruit residual herbicides and the tendency of them to leach or move across the soil. Leaching is the physical process of movement of herbicides in soil water flow and is influenced by several factors, including solubility and soil adsorption. The amount of movement is influenced by the amount of herbicide in the soil, soil texture, and the extent of water movement. The amount of herbicide in the soil solution is a function of the solubility of the herbicide and strength of soil binding (adsorption). Soil adsorption is a measure of the affinity of an herbicide to soil organic matter. Herbicides that are low in solubility and have a high affinity to soil particles will be less likely to leach. Since organic matter is the most influential soil factor governing adsorption, the Koc (ml/g) of a herbicide is a very useful measure of its tendency to move with water in soil. Herbicides that are soluble with low soil adsorption are prone to leaching, including diuron, napropamide, and norflurazon. Herbicides that are low in solubility and not prone to leaching, include, trifluralin, and pendimethalin.
Lateral movement of a herbicide due to runoff.

Lateral movement of a herbicide due to runoff.

Dr. Rob Crassweller, Penn State Professor of Tree Fruit

Volatilization of a herbicide is determined by vapor pressure (the pressure of the gas phase of the chemical). Herbicide degradation is not a fixed property, but is influenced by soil and environmental conditions. Degradation or the breakdown of herbicides can occur by microbial decomposition, chemical decomposition, photodecomposition, soil adsorption, volatilization, leaching, surface runoff, and plant metabolism. The half-life of a herbicide is often used to express the length of time required to reduce the amount of active ingredient by one-half. Herbicides that tend to degrade rapidly are generally not prone to leaching, unless they move in runoff water or enter deeper soil layers through cracks.

The water solubility of a herbicide is a measure of how readily the chemical will dissolve in water and is typically expressed as the maximum amount of the pesticide that will dissolve in one liter of water. Typical concentration units are mg per liter (mg/L) which is approximately equal to parts per million (ppm) or micrograms per liter (ug/L), which is approximately equal to parts per billion (ppb). The larger this number is, the more water soluble the pesticide, and the more readily the herbicide will be transported away from the application site by stormwater or irrigation water runoff. In most cases, but not all, as solubility declines, adsorption increases and soil mobility declines. Herbicides low in solubility are excluded from the soil water and become associated with soil colloids and organic matter. While certain herbicides may have low solubility, under certain conditions (like sandy soils or clay soils with large cracks) they may tend to leach in free-flowing water rather than adsorb to soil particles.

The adsorption coefficient, (Koc), is a measure of how strongly a chemical adheres to soil in preference to remaining dissolved in water. Koc is formally defined as the ratio of the mass of pesticide adsorbed per unit mass of soil to the mass of the pesticide remaining in solution at equilibrium. Because it is a ratio of masses, the number is unitless. The value is dependent on the type of soil and the soil pH, so it is not uncommon to see a range of values reported in the literature. Herbicides with high Koc values are typically not very water soluble and will preferentially adhere to soils rather than be dissolved in water. This means that herbicides in this class are unlikely to be carried off-site in runoff as dissolved substances; instead, they are transported on sediment particles.

The vapor pressure (Vp) of a herbicide is a measure of how readily it will evaporate. In the Pesticide Information database, vapor pressures are measured between 20-25 degrees C, an important distinction since the vapor pressure of a chemical increases with increasing temperature. The vapor pressure is a good predictor of the volatility of the chemical and allows determination of which herbicides might be prone to evaporate from leaf and soil surfaces off-site after application.

Soil half-life is the amount of time required for half of the pesticide to degrade in soil. This half-life is governed by the types of soil organisms that are present that can break down the herbicide, the soil type (e.g., sand, loam, clay), pH, and temperature.

As you can see in the table below the herbicides with the lowest potential for leaching and/or soil runoff include Chateau, Gallery, Devrinol, Surflan, Goal, Prowl, and Matrix. Solicam is the herbicide that has the most potential for lateral movement due to runoff. (See image)

Table 1. Physical properties of selected soil-residual herbicides used in trees and vines

 

Herbicide

Trade Name

Volatility

Soil Sorption Koc (ml/g))

Solubility (mg/L)

Soil Mobility

Half life (days)

diuron

Karmex

Low

42

480

Medium

90

flumioxazin

Chateau

Medium

1.5

2

Low

varies by soil pH

isoxaben

Gallery

Low

1

190-570

Low

50 - 120

napropamide

Devrinol

Low

73

700

Low

70

norflurazon

Solicam

High

28

700

High

70-180

oryzalin

Surflan

Low

2.6

600

Low

20

oxyfluorfen

Goal

Low

0.1

100,000

Low

35

pendimethalin

Prowl

Medium

0.275

17,200

Low

45

pronamide

Kerb

Medium

15

800

Medium

60

rimsulfuron

Matrix

Low

150

150

Low

7

saflufenacil

Treevix

???

500

2.5 - 210

Medium

7 - 35

simazine

Caliber 90

Low

6.2

130

Medium

91

terbacil

Sinbar

Low

1.89

710

Medium

50 - 100

Where trade names appear, no discrimination is intended, and no endorsement by Penn State Cooperative Extension is implied.