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Soil Microbial Activity


Why soil microbes are critical to a quality crop.

Did you know that there is more life below the earth’s surface than there is above it? Even though you cannot see it, there is an extremely active community of microbes living and thriving in the fields where you plant your crops. In fact, there are more microbes in a teaspoon of soil than there are people on the earth.

Importance of microbes

Among other organisms, microbes include bacteria, fungi and nematodes. The benefits of having a healthy microbial community in your soil are immeasurable. First of all, active microbes and soil animals, such as earthworms, promote soil aeration and drainage as they move through the soil and create channels. This activity helps to promote the plants’ root penetration as well. Also, when microbes break down organic matter, they release essential nutrients and carbon dioxide back into the soil. Soil microbes also help plants defend themselves against pathogens, cement soil aggregates and degrade pest control chemicals. Finally, microbes are responsible for nitrogen fixation, the capture of inert N2 gas from the air and making it available to plants.

Influencing activities

Because of its importance, it’s useful for farmers to understand the many factors that can affect microbial activity in soil:

  • Temperature: As temperature increases, so does microbial activity, especially between temperatures from around 60 degrees Fahrenheit to around 95 degrees Fahrenheit.

  • Moisture: The ideal water holding capacity of the soil for microbial activity is between 40 and 60 percent. This is typically a bit drier than field capacity. Activity decreases as the soil becomes drier.

  • Soil pH: The soil pH affects microbes differently, depending on the type of organism. For example, fungi are more active in acidic soils, while bacteria respond better in neutral to basic soils.

  • Organic matter: One way to stimulate microbial activity in the soil is to add organic carbon. How quickly the activity increases depends on how decomposable the organic matter is. Manure and leguminous cover crops are examples of very decomposable materials that rapidly speed up activity. However, other organic material that decomposes less quickly can have a longer-lasting effect on microorganisms; sawdust and compost are examples.

  • Salinity: As salinity increases, microbes in soil must exhaust more energy to absorb water. Therefore, as salinity increases, activity can decline. How much of a decline depends on the type of microbe.

  • Nitrogen application: Generally, the application of nitrogen, and the resulting improved soil fertility, advances microbial activity in soil. Application can impact types of microbes differently, however, improving some situations while limiting others.

  • Tillage: In most cases, tillage helps to increase soil microbial activity, and compacted soil will hinder activity. Note that the rapid growth caused by tilling will probably only be temporary, but the long-term benefits of tilling as little as possible could be considerable.

Ultimately, healthy soil leads to healthy crops. Being aware of the complex properties that make up your soil is the first step in keeping your soil microbes active and healthy. For more information on microbes, please consult your local Southern States Agronomy Professional or County Extension office.

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