How Soil pH Affects the Activity and Persistence of Herbicides

How Soil pH Affects the Activity and Persistence of Herbicides

FS189E
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Alan Raeder, Ph.D. Candidate, WSU Department of Crop and Soil Sciences, Dr. Drew Lyon, Professor, WSU Department of Crop and Soil Sciences, Dr. Jim Harsh, Professor, WSU Department of Crop and Soil Sciences, Dr. Ian Burke, Associate Professor, WSU Department of Crop and Soil Sciences
This publication discusses how decreasing soil pH contributes to increased instances of herbicide persistence and categorizes commonly used herbicides as either a weak acid, weak base, cationic, nonionic polar, or nonpolar. It also briefly explains how the soil/herbicide interactions within each category can be affected by pH. The Soil Acidification series begins with An Introduction, covering the fundamentals of soil pH and acidification, and continues with other fact sheets on more specific topics such as the influence of pH on pathogens and microbes, recommended varieties of specific crops, crop nutrition, and liming.
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Abstract

Decreasing soil pH, also called soil acidification, is a growing concern in eastern Washington and northern Idaho. Researchers and farmers have measured soil pH values below 5.0 throughout the Palouse region, in particular. Decreasing soil pH has serious implications for the cropping systems of the Palouse.

This publication, How Soil pH Affects the Activity and Persistence of Herbicides, discusses how decreasing soil pH contributes to increased instances of herbicide persistence. The publication categorizes commonly used herbicides as either a weak acid, weak base, cationic, nonionic polar, or nonpolar and briefly explains how the soil/herbicide interactions within each category can be affected by pH.

The Soil Acidification series begins with An Introduction, covering the fundamentals of soil pH and acidification, and continues with other fact sheets on more specific topics such as the influence of pH on pathogens and microbes, recommended varieties of specific crops, crop nutrition, and liming.

Introduction

Instances of herbicide persistence contributing to crop injury in the dryland regions (DLR) of the inland Pacific Northwest (PNW; eastern Washington, northern Idaho, and northeastern Oregon) are common. A major concern related to cases of herbicide persistence is the soil pH in the region.

Soil pH can have substantial effects on the activity and persistence of herbicides in soil, particularly at pH extremes of 4.5 or below and 7.5 or above (Monaco et al. 2002). Therefore, there is a need to understand how the pH of soils in the inland PNW-DLR will influence the persistence of herbicides commonly applied. A knowledge and understanding of the chemical and physical properties associated with herbicides is needed in order to understand how soil pH influences persistence.

The most common way herbicides are grouped is by their site of action. Both the Weed Science Society of America (WSSA) and the Herbicide Resistance Action Committee (HRAC) have systems for classifying herbicides according to their site of action. For example, the active ingredient of PowerFlex HL is pyroxsulam, an inhibitor of acetolactate synthase (also known as acetohydroxy acid synthase) in plants, and is classified by the WSSA as a Group 2 (HRAC classification: B) herbicide.

Important Soil/Herbicide Interaction Terms:

adsorption. The association or binding of molecules with the surfaces of solids.

anion. An ion or molecule that has a net negative charge.

cation. An ion or molecule that has a net positive charge.

half-life. The time required for half of applied molecules to be degraded.

ion. An atom or molecule that has acquired a net electric charge by gaining or losing one or more electrons.

ionization. The process by which a molecule acquires a negative or positive charge by gaining or losing electrons to form ions.

pH. A measure of the concentration of hydrogen ions in a system, known as the master variable.

weak acid. An acid that dissociates (loses a hydrogen ion) only slightly in water to produce an acid solution.

weak base. A molecule that associates with hydrogen ions only slightly in water to produce an alkaline solution.

Classifying herbicides by mechanism of action is convenient for recalling trends in weed control and crop selectivity, as well as for designing herbicide rotation programs to avoid selection of herbicide resistant weeds. However, when considering herbicide properties and how soil pH could affect herbicide persistence in soil, it is important to classify herbicides based on charge (ionic) and polarity properties.

In chemistry, “ion” refers to an atom or molecule that bears a net positive or negative charge—that is, the number of electrons is not equal to the number of protons. Thus, properties of herbicides depend on the change of the herbicide’s overall charge from neutral to a net positive or negative charge.

The significance of understanding the charge of a given herbicide in a soil is important because soil colloids (clay and organic matter) are negatively charged (anions, or anionic particles).

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Copyright 2015 Washington State University

Published October, 2015

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