Soil Quality Information Sheet

Soil Quality Concerns: Pesticides

USDA Natural Resources Conservation Service January 1998

What are pesticides?

Pesticides are synthetic organic chemicals used to control

weeds in fields and lawns, and unwanted or harmful pests,

such as insects and mites that feed on crops. Pesticides

are divided into categories according to the target organisms they are designed to control (e.g., insecticides

control insects).

Herbicides are by far the most commonly used pesticides

in the United States. They range from non selective to

highly selective for control of specific weeds in specific

crops, with different products having postemergence,

preplant, and preemergence uses. Insecticides are second

in usage, and fungicides are third.

Effects of Pesticides on Soil

Quality

The capacity of the soil to filter, buffer, degrade, immobilize, and detoxify pesticides is a function or quality of the

soil. Soil quality also encompasses the impacts that soil

use and management can have on water and air quality,

and on human and animal health. The presence and

bio-availability of pesticides in soil can adversely impact

human and animal health, and beneficial plants and soil

organisms. Pesticides can move off-site contaminating

surface and groundwater and possibly causing adverse

impacts on aquatic ecosystems.

What are pesticide

formulations?

The formulation is the chemical and physical form in

which the pesticide is sold for use. The active ingredient

(a.i.) is the chemical in the formulation that has the

specific effect on the target organism. The formulation

improves the properties of the pesticides for storage,

handling, application, effectiveness, or safety. Examples

of formulated products are wettable powders and waterdispersible granules. A single pesticide is often sold in

several different formulations, depending on use requirements and application needs.

Pesticide mode of action

Mode of action refers to the mechanism by which the

pesticide kills or interacts with the target organism.

• Contact pesticides kill the target organism by weakening or disrupting the cellular membranes; death can be

very rapid.

• Systemic pesticides must be absorbed or ingested by

the target organism to disrupt its physiological or

metabolic processes; generally they are slow acting.

How effective the pesticides are at killing the target

organisms (efficacy) depends on the properties of the

pesticide and the soil, formulation, application technique,

agricultural management, characteristics of the crop,

environmental or weather conditions, and the nature and

behavior of the target organism.
(Prepared by the National Soil Survey Center in cooperation with the

Soil Quality Institute, NRCS, USDA, and the National Soil Tilth Laboratory,

Agricultural Research Service, USDA).

The U. S. Department of Agriculture (USDA) prohibits discrimination in its programs on the basis of race, color, national origin, gender, religion, age,

disability, political beliefs, sexual orientation, and marital or family status. (Not all prohibited bases apply to all programs.) Persons with disabilities who

require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET Center at (202)

720-2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326W, Whitten Building, 14th and Independence Avenue, SW,

Washington, DC 20250-9410 or call (202) 720-5964 (voice or TDD). USDA is an equal opportunityprovider and employer.

Fate of pesticides in the

environment

Ideally, a pesticide stays in the treated area long enough to

produce the desired effect and then degrades into harmless materials. Three primary modes of degradation occur

in soils:

• biological - breakdown by micro-organisms

• chemical - breakdown by chemical reactions, such as

hydrolysis and redox reactions

• photochemical - breakdown by ultraviolet or visible

light

The rate at which a chemical degrades is expressed as the

half-life. The half-life is the amount of time it takes for

half of the pesticide to be converted into something else,

or its concentration is half of its initial level. The half-life

of a pesticide depends on soil type, its formulation, and

environmental conditions (e.g., temperature, moisture).

Other processes that influence the fate of the chemical

include plant uptake, soil sorption, leaching, and volatilization. If pesticides move off-site (e.g., wind drift, runoff,

leaching), they are considered to be pollutants. The

potential for pesticides to move off-site depends on the

chemical properties and formulation of the pesticide, soil

properties, rate and method of application, pesticide

persistence, frequency and timing of rainfall or irrigation,

and depth to ground water.

Retention of pesticides in the soil

Retention refers to the ability of the soil to hold a pesticide in place and not allow it to be transported. Adsorption is the primary process of how the soil retains a

pesticide and is defined as the accumulation of a pesticide

on the soil particle surfaces. Pesticide adsorption to soil

depends on both the chemical properties of the pesticide

(i.e., water solubility, polarity) and properties of the soil

(i.e., organic matter and clay contents, pH, surface charge

characteristics, permeability). For most pesticides,

organic matter is the most important soil property controlling the degree of adsorption.

For most pesticides, the degree of adsorption is described

by an adsorption distribution coefficient (Kd

), which is

mathematically defined as the amount of pesticide in soil

solution divided by the amount adsorbed to the soil.

Pesticide toxicity

The toxicity level of a pesticide depends on the deadliness

of the chemical, the dose, the length of exposure, and the

route of entry or absorption by the body. Pesticide

degradation in soil generally results in a reduction in

toxicity; however, some pesticides have breakdown

products (metabolites) that are more toxic than the parent

compound.

Pesticides are classified according to their potential

toxicity to humans and other animals and organisms, as

restricted-use (can only be purchased and applied by

certified persons who have had training in pesticide

application), and general use (may be purchased and

applied by any person).

Use and application considerations

• Apply pesticides at the lowest effective level.

• Avoid unnecessary pesticide treatments.

• Use Integrated Pest Management.

• Follow all label instructions.

• Apply proper rates and times as label indicates.

• Calibrate application equipment.

• Apply formulations that minimize drift.

• Use safety equipment when handling.

• Store and dispose of pesticide containers properly.

• Use biological controls when appropriate.

• Alter farming or cropping systems to control pests.

• Use disease and insect resistant crop varieties.

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