Our Living Soil

Our Living Soil
by Pat Vance

The United Nations declared 2015 the International Year of Soils in an effort to draw attention to the extremely important role soil plays in the health and welfare of humans around the world.

Most of us are aware that the soil in our gardens is teeming with living creatures, some visible to the eye, like our old friend the earthworm, and many, many more microscopic critters. This piece is about some of those tiny critters.

The densest population of microorganisms in the soil is near the roots of living plants, an area called the rhizosphere. Here, plants exude excess carbohydrates that soil microbes consume.

Here are some of the most active parts of this ecosystem:

Bacteria

 

Bacteria make up the largest group of denizens of the dirt. With as many as a billion bacteria in one teaspoon, there could very well be more bacteria than humans in the entire world in a mere half cup of fertile soil. Bacteria are simple single-celled organisms that are more primitive than animals and plants. Most of the bacteria in soil are decomposers that consume the carbohydrates and other organic materials in decaying plants. The by-products of this consumption include simple molecules that the living plants can then consume as nutrients.

In addition, there are bacteria that alter nitrogen, an essential elements for all living organisms. Many gardeners are familiar with rhizobia, bacteria that fix nitrogen in a partnership with legumes. Rhizobia form nodules on the roots of the legumes in a mutually beneficial collaboration. The legume exudes carbohydrates at the roots and rhizobia consume the carbohydrates. As a sort of payment, the rhizobia convert atmospheric nitrogen in the soil into ammonium, which the legume can utilize.  Additionally, if the roots are left in the ground, the next generation of plants to occupy the site will benefit from the excess ammonium that remains.

In turn, bacteria themselves are food sources for other organisms in the soil. Protozoa, for example, consume bacteria but need much less nitrogen. This results in more nitrogen available for plants in the immediate area.

Bacteria also contribute to soil structure by producing gummy substances that help hold soil particles together and help retain moisture.

Actinomycetes

Dig up a shovelful of fertile garden soil and in breathe in. That characteristic smell is produced by actinomycetes, a special group of bacteria that grow filaments much like fungi. Actinomycetes make up the second largest group of organisms in soil. They are found in soils on the alkaline, or higher, side of the pH scale and they break down more complex, harder to digest materials like lignin and cellulose. Actinomycetes produce several dark colored pigments, which add to the brown color of fertile soil And they produce many substances that are toxic to pathogenic microbes. The antibiotic streptomycin is a product of one group of actinomycetes.

Fungi

 

Fungi are found in acidic, or lower pH, soil and, like actinomycetes, they break down more complex materials like lignin and cellulose. Fungi are sometimes single-celled organisms, but they can also be enormous and complex. Mushrooms are the so-called fruiting bodies of fungi. The visible mushroom is only a small part of a very large fungus that can continue underground for great lengths. Some people consider a honey fungus in Oregon to be the world’s largest organism, extending over two miles underground.

Mycorrhizae are a special group of fungi that form associations with the roots of plants. Like rhizobia, the metabolic activity of these fungi provides nutrients for plants. In some cases, the plants will not grow properly without the presence of the correct mycorrhizae.

Filaments produced by fungi as well as actinomycetes provide structural support for soil.

Other Microbes in Soil

 

There are other groups of microbes present in the soil in smaller numbers.

Nematodes are microscopic worms that may feed on living plants, bacteria, fungi or other nematodes. Some are pathogenic, but others simply contribute to the release of nutrients in much the same way as bacteria and fungi.

We discussed protozoa, however, the amount of protozoa present in most typical garden soil is quite low.

There are undoubtedly a number of viruses in the soil. Viruses are not always classified as living things because they must be inside another living cell to reproduce. Despite this limitation viruses can lie dormant in soil for a long time, in some case many years, waiting for the opportunity to infect a living cell. Soil virology is a largely unexplored arena. Perhaps we will learn more in future years.

How much microbiology is going on in your garden soil?

 

There are many factors that influence the growth of microbes including moisture content, organic matter, and acidity or alkalinity. Also, the population of macrofauna (worms, insects, and other larger groups) will make a difference. Higher organic content in general usually means higher microbial activity.

To keep them happy, most beneficial microbes need air, so keeping soil from becoming compacted, and keeping soil moist but not waterlogged is vital. Unless you are courting some special mycorrhizal relationships, soil that is close to neutral pH (around 7.0) will also promote microbial health.

Are they all beneficial?

 

Not all dirt denizens are welcome in the garden. Some cause diseases in plants. For example, Crown gall is caused by bacteria. And a few, such as Clostridiun botulinum, can cause disease in humans. And we mentioned nematode pathogens. But that’s a topic for another time.

Sources

I consulted several reports, but most of the information in this piece comes from the USDA’s Natural Resources Conservation Service. Take a look at their newsletter PDF “Soil Microorganisms and their Functions” online.

Graphics used with kind permission from “Life from Soil: Soil Microorganisms-a picture story”

By Thomas Fester from the Helmholtz Centre for Environmental Research. You can find a Power Point presentation here: http://www.ufz.de/index.php?en=20665