The Damage to Soil by Weed Killer


Many gardeners favor the use of weed killers to eliminate unwanted plants. Weed killers are also used on a widespread basis in commercial agriculture. Many products exist, containing chemicals such as glyphosate, atrazine, endothall and alachlor. The manufacturers claim that their products are safe for humans and animals and that they do not remain in the soil for long periods of time. However, organizations such as the Pesticide Action Network have reported different findings.

Glyphosate Product Information

Glyphosate is the active chemical in several commercial brands of herbicides, including Roundup. Monsanto Company began manufacturing this product in 1974---it and other products that contain glyphosate are legally used in more than 130 countries. Product literature from Monsanto states that glyphosate disrupts plant enzymes that control growth, but plants growing in close proximity are not affected after their neighbor is sprayed because the chemical "binds tightly" to soil. This prevents the roots of nearby plants from taking the chemical up into their foliage.

Surfactants Cause Soil Toxicity

Britain's Pesticide Action Network has reported that some of the lesser ingredients of herbicides are "serious irritants," and has recommended that humans remain away from areas that have been treated with weed killers for 12 hours. Substances known as surfactants are commonly added to weed killers to help them stick to plants. One surfactant, polyoxy-ethyleneamine, has been implicated in causing soil toxicity and in killing fish when it runs off into nearby bodies of water.

Soil Function Is Impaired

The Pesticide Action Network has reported that glyphosate degrades to carbon dioxide after it is sprayed on the soil around unwanted plants. The chemical itself is inactivated because soil particles absorb it, but when it becomes unbounded from the soil, it remains for years. This presence prevents soil function by limiting the amount of "anaerobic nitrogen fixation" that the soil can achieve. When soil erodes, it often travels to nearby sources of water, where it breaks down very slowly.

Metabolites Remain Toxic in Soil

During the process of breaking down in soil to which it has been applied, glyphosate retains a metabolite known as aminomethylphosphonic acid (AMPA). Its half-life is 958 days, which leaves toxins in affected soil for nearly three years. The Web site reports that the metabolite causes liver and bladder damage in laboratory animals. Crops such as barley and lettuce have been found to contain AMPA even one year after the soil in which they grow has received a treatment of glyphosate. The effects on human health are not known.

Treated Soil Hurts Worms

The Extension Toxicity Network at Oregon State University has found that glyphosate harms earthworms and other creatures that live in soils exposed to this chemical. Worms fail to grow to their full size when they dwell in treated soils, according to Unlike the claims of herbicide manufacturers, this watchdog group has found that glyphosate can remain in soil for 141 days and that even sandy soil and soil low in humus "strongly adsorb" the chemical.

Keywords: herbicides chemicals, weed killer, soil damage worms

About this Author

Barbara Fahs lives on Hawaii island, where she has created Hi'iaka's Healing Herb Garden. Fahs wrote "Super Simple Guide to Creating Hawaiian Gardens," and has been a professional writer since 1984. She contributes to Big Island Weekly, Ke Ola magazine, GardenGuides and eHow. She earned her B.A. at UCSB and her M.A. from San Jose State University.