Microorganisms

Microorganisms are tiny life forms. 


Until very recently, commercial agriculture viewed all soil microorganisms as disease-carrying pests that needed to be eradicated. We now know that many of those microorganisms provide nutrients for plants and help suppress disease.

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What is living in your soil?

Besides plants and worms, there are more than 30,000 different varieties of bacteria, fungi, algae, protozoa, and other microscopic critters (microbes) living in your soil. The jury is still out on whether viruses are alive or not - but they are considered microbes. According to Principles and Applications of Soil Microbiology (1998), you will find this many microorganisms in one gram of soil [one-fifth the weight of an American nickel]:
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    •    100,000,000 - 1,000,000,000 bacteria
    •    100,000 - 1,000,000 fungi
    •    1,000 - 1,000,000 algae
    •    1,000 - 100,000 protozoa
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Microscopic mite (Public Domain)

Soil microbes on petri dish (Pixabay)

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With those numbers, each handful of soil can contain more microorganisms than there are human beings on Earth. Try wrapping your brain around that, the next time you’re outside pulling weeds!


Most of these microorganisms have not yet been identified, or even named, but scientists at the Earth Microbiome Project, and elsewhere, are working on that. What we do know, in the world of soil microorganisms, is that there are beneficial microbes, and there are microbes that cause us grief.

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The Bad Guys

Some microorganisms are disease pathogens that can damage or kill plants. There are bacteria in the soil that can cause fireblight, cankers, and soft rot. Fungi may bring powdery mildew, eutypa dieback or rust to your garden. Viruses can cause spotted tomato wilt, cucumber mosaic, and many other diseases.


The Good Guys

Legumes, such as peas and beans, have evolved a symbiotic relationship with Rhizobium soil bacteria. These bacteria live in and around the roots, converting atmospheric nitrogen into a form usable by plants. This is just one part of the Nitrogen Cycle.


In similar fashion, many other soil microorganisms make nutrients available to plants. Plants trade the carbon they create through photosynthesis for mineral nutrients mined from the soil by microbes. These networks of beneficial microorganisms can extend a surprising distance from the plant. In the case of giant redwoods, the network of microorganisms that feed the tree can be over seven miles long, in all directions!


Scientists have learned that some soil microorganisms trigger defensive plant behavior, while others produce toxins that kill disease pathogens. Large populations of beneficial microbes also use up resources, making life more difficult for disease pathogens.


Networks of soil fungi allow plants to pick up on alarm signals emitted by neighboring plants, giving them a head start on producing disease-fighting enzymes. These soil fungi provide an information and material network that few could have imagined hundred years ago.


Researchers have found that plants cultivate relationships with beneficial microorganisms, depending on current needs, and that those alliances can change as conditions change. There are soil microorganisms that promote photosynthesis, improve drought tolerance and respiration efficiency, and sensitivity to high salt levels, just to name a few. Biodiversity of soil microbes is one of the best ways to maintain plant health. And those benefits are carried into future generations. Even if a beneficial microbe’s population declines, the pathways that were forged during its active phase are retained, creating the plant equivalent of a vaccine. This information is passed on to future generations, creating natural immunities. Unfortunately, as far as researchers can tell at this time, those benefits only last through the second generation. The third generation has to start the process from scratch.


Therapeutic microorganisms

One soil microorganism in particular, Mycobacterium vaccae, has recently been found capable of uplifting your mood! This particular microorganism is absorbed through tiny cuts and is inhaled on dust particles, as we garden. Once inside, these microorganisms cause a chemical reaction similar to the effects of prozac. Most gardeners claim that gardening is their therapy. Ends up, they were right!


Microorganisms, like other living things, can be poisoned with herbicides, pesticides, and fungicides. The truth is, we don't really understand all of the interactions between these tiny life forms. Throwing a chemical monkey wrench into what looks like a delicate balance is probably not in our best interest. A lab-based soil test every few years can tell you a lot about what is going on in your soil.


The best way to keep a healthy balance of microorganisms in your soil is to keep the soil healthy. Very often, chemicals cause too much change too quickly. Mulching with coarse wood chips and regularly adding aged compost to your soil are the best ways to keep your soil healthy.