Herbicide and Pesticide Effects on Soil Bacteria

Written by misty witenberg
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Herbicide and Pesticide Effects on Soil Bacteria
Pesticides and herbicides have been used for centuries, although their use has increased dramatically in the last 70 years. (crops in rows image by david Hughes from Fotolia.com)

Pesticides and herbicides can be effective tools in ridding a crop of detrimental insects and weeds, but they can also have harmful consequences to nontarget organisms such as beneficial bacteria. A single spoonful of healthy soil holds millions of microorganisms, including bacteria.

History of Pesticides and Herbicides

The Chinese were using arsenic to counter garden pests as far back as 900 C.E. Other pesticides such as pyrethrum, lime sulphur and mercuric chloride started to become more widely used in the late 19th century, expanding even further after World War II. Today more than 1,600 pesticides are available, with more than 4.4 million tons used each year. The United States uses more than a quarter of this amount.

Benefits of Bacteria

Bacteria in the soil help to store nutrients and water and to filter pollutants, while also helping plants use the nutrients in the soil to grow. They help transform atmospheric nitrogen into nitrates, which plants need to thrive. They are also vital for the fertility of the soil. According to the University of Florida EDIS, bacteria help decompose dead animal and plant material into organic matter, such as compost, that is useful to plants.

Effects of Pesticides

Heavy use of pesticides in soil can cause beneficial bacteria in soil to deteriorate. According to SpringerLink, pesticides that are used on soil at planting still remain when plant roots are developing, interacting with bacteria. Chemical pesticides can include insecticides, nematicides and molluscicides. Dichlopropene, methyl isocyanate, chloropicrin and methyl bromide are examples of broad-spectrum soil fumigants that can kill a range of beneficial bacteria, along with other plants and animals within the soil, according to Pollution Issues.

Effects of Herbicides

Herbicides, or weed killers, can harm bacteria as well, most notably if used in high concentrations. Commonly used herbicides can upset the process of bacteria turning nitrogen into plant-friendly nitrates. Specifically, triclopyr hinders soil bacteria that change ammonia into nitrite, and glyphosate diminishes the activity and growth of nitrogen-fixing bacteria in soil and those that live in nodules on plant roots. On bean plants, they reduce nitrogen fixation of bacteria that live on the roots. According to the Journal of Basic Microbiology, glufosinate gives off a toxicity that may cause a loss of functional diversity within a microbial community.


Loss of beneficial bacteria lowers soil quality in several ways. According to Street Directory, bacteria help soil store nutrients and water, regulate water flow throughout the soil and assist in filtering pollutants. Overuse of pesticides and herbicides may work in the short term but in the long term may have an outweighing negative effect, similar to overuse of antibiotics to humans. After a prolonged period of indiscriminate use, soil that lacks helpful bacteria can't hold onto the nutrients essential to plant health. This is because the nitrogen in the atmosphere can't be converted into the nitrates necessary for plants to thrive, whether in a commercial or a home garden.

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