Pesticide Impact on Soil Microbes

Pesticides (Herbicides, Insecticides, Miticides, Fungicides etc.) may affect soil microbes in many ways, often leading to reduced diversity and even lower populations of various soil microbes.

Soil microbes are bacteria, fungi, archaea, protists, and viruses and account for an estimated 3%–4% of the total biomass on earth. Pesticides often degrade soil health and can lead to lower agricultural productivity or lower crop yields. Some pesticides change the soil structure so that soils become dense and compacted. Minimizing pesticide use is beneficial for improving soil health and microbial diversity and increasing soil microbial populations.

Broad-spectrum herbicides are those that kill or control a wide variety of pests including weeds, fungi, and insects. Since they are so non-selective, they can kill or control beneficial microbes as well as both targeted and non-targeted organisms. Often broad-spectrum pesticides may also harm pollinators and beneficial predators. When this happens, species diversity decreases along with changes to the overall population of soil microbes but also beneficial pollinators and other predators.

For example, the carabidae beetle (ground beetle) population is negatively affected by neonicotinoids (Cruiser, Poncho, Goucho) and this beetle along with fireflies, spiders, and solder beetle consume a lot of slugs and slug eggs. Other broad-spectrum insecticides include Malathion, and Sevin (carbaryl). Common herbicides include glyphosate (Roundup) and paraquat. Roundup is known to suppress anaerobic (no oxygen) bacteria for up to 6 weeks while promoting aerobic (oxygen loving) bacteria. Many anaerobic bacteria make nutrients plant available but can also cause disease if not controlled and kept in balance. Methyl bromide is a fungicide used for sterilization in agriculture. For good soil health, there needs to be a wide diversity of soil microbes with large thriving populations.

Many microbes and insects have different soil functions. Some shred and break down crop residue. Others transform the nutrients into a form the plants can use. Come consume microbes and in the process release many plant nutrients but also enzymes that speed up biological processes. Some release plant hormones that increase crop growth while others increase crop yield. Pesticides can sometimes inhibit nitrogen fixers or bacteria that make phosphorus plant available. Without a wide microbial diversity, many nutrient deficiencies occur in the soil, and this affects plant growth. Healthy plants can resist plant diseases and insect infestations. They also grow faster and can shade out weeds.

Some Pesticides inhibit certain enzymes like ureases and phosphatase. Urease (a nickel containing enzyme) takes the microbial poop called urea and breaks it down into ammonia (NH3) and carbon dioxide. Ammonia maybe quickly transformed by soil microbes into nitrate (NO3-). The nitrate form of N promotes crop growth and vegetation. The ammonia form promotes and improves crop yields. A delicate balance needs to occur in the soil to get optimal yields. Too much soil nitrate causes a lot of vegetation but lower crop yields associated with seed and grain formation. Phosphatase is an enzyme make soil phosphorus plant available. Inhibiting these enzymes slows down soil organic and crop residue break down and also reduces nutrient release.

Some soil processes are affected more long-term. For example, years of using broad-spectrum pesticides can reduces soil quality and soil health. The soil physically may get harder and more compacted. Biologically, some microbes are reduced or eliminated. Some chemicals reactions are affected like pH (more acid) balance, and new soil formation are adversely affected. The soil becomes like an old tractor; the wheels lose air, the oil needs change, parts get rusted and worn out; and while it may still do some work, it is not as good or as effective. Plant growth slows down along with crop yields.

Some microbes can become resistant to certain pesticides, however; this comes at a price. Usually it is the pathogens, the disease-causing microbes that thrive at the expense of other beneficial microbes. This microbial shift and loss of predators can change the balance so that disease organisms can infect the plant and cause the plant harm.

There are several ways to combat this soil fatigue. First, use integrated pest management (IPM strategies to minimize the over use of pesticides. Some of these pesticides degrade quite slowly and can last for years in the soil. Often not a whole pesticides but as metabolites (broken down pieces of a pesticide) may still cause harm. Sometimes rather than a pesticide, other methods can be used like crop rotation, earlier planting, thicker stands etc. to minimize pesticide usage. Bioremediation is the supplementation of beneficial microbes to restore soil health. Manure, compost teas, spring applied gypsum, and microbial biological products can help restore the soil. Cover crops and no-till all are sustainable practices that improve soil health.
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