In one tablespoon of healthy soil there are as many organisms as there are people on the Earth.

Soil microbes perform many functions in maintaining fertile plant grow medias including; storage of nutrients, filtering and buffering contaminants, regulating water usage, protection from pathogens, recycling plant wastes, formation and stabilization of soil aggregates and providing a habitat for beneficial micro flora and fauna. This microbial community has often been called the Microbial Food Web and, when in balance, it protects plants from diseases and pests and stimulates plant growth functions.

What makes up this microbial community?
Grow medias contain a large variety of microorganisms which include; bacteria and fungi, protozoa, nematodes and other microbes. However, the largest group of these microorganisms are various bacteria and fungi. They have been classified into several functional groups depending upon their main activity. Microbial functional groups include; Aerobic and Anaerobic Bacteria, Fungi, Actinomycetes, Pseudomonads and Mycorrhizae Fungi. There are many other specific types of microbes that are specialists such as cellulose degraders, nitrogen fixers, denitrifiers, pathogen inhibitors and hundreds of thousands more.

Examples of microbial functions in the soil:
Plants cannot function properly without help from some of these microbes.

1. Heterotrophic bacteria make up a majority of the microbes in growing systems. They are responsible for many functions including; storage of nutrients and water, the breakdown of simple carbon compounds into plant available forms, formation of soil structure, release and stimulation of plant growth compounds to name a few. They are a major indicator of Grow Media health.

2. Mycorrhizae fungi attach to plant root systems.They are essential for helping the plant uptake certain minerals such as Phosphates and Calcium as well as protecting the plant from diseases. These fungi are also responsible for breaking down more resistant organic compounds like cellulose, lignin, lipids and pectin materials. Fungi, along with earthworms, are some of the main aggregate builders in soil systems. Mycorrhizae fungi are very important for root development and growth. They usually colonize the root systems of plants and help store nutrients for the plant. There are thousands of different types of Mycorrhizae fungi and they are generally specific to each type of plant. In addition to collecting and storing nutrients, they also protect the plant from many pathogens and predators such as nematodes and root feeding bacteria and fungi. A plant will often send more that 50% of its photosynthetic sugars to the roots to attract these important fungi.

3. Actinomycetes produce plant antibiotics. They are agriculture’s main plant protectors. They produce and release antibiotics and other compounds that inhibit the growth of plant pathogens. Actinomycetes give soils their “earthy” smell. If you grab a handful of soil and it has a good earth smell, this is an excellent indicator of a strong, healthy microbial system.

4. Nitrogen-fixing bacteria Like Azotobacter, Azospirillum and Rhizobium are examples of bacteria that convert atmospheric nitrogen into nitrogen compounds used by plants to synthesize proteins. They are often associated with plant roots although there are many genera of nitrogen fixers in good grow media. They are an essential part of the nitrogen cycle, which keeps plants growing.

5. Pseudomonads
These organisms are nutritionally versatile and capable of degrading many natural and synthetic organic compounds. Pseudomonads are typically aerobic and contribute to the decomposition and nutrient release process by attacking a wide variety of organic substrates including humic acids and synthetic pesticides. Certain pseudomonads have been linked to the biological control of plant pathogens.

Checking your Grow Media’s microbial health.
You can check the diversity and strength of your grow media’s microbial balance by having a microbial bioassay done. Bioassay laboratories such as MBS (Mineral Bio-Science), A&L Labs or Midwest Laboratories will check your medium for microbial species strength and classification by functional grouping. It is as important as nutrient balance to know your grow media’s microbial content to understand what is needed to generate bio-active mediums for maximum plant growth and health.

How do you increase microbial diversity and strength?
Microbial organisms are what produce an environment that will make your grow media perform to highest level. Beneficial microbes are essential to any plant growth environment. It is important to continually stimulate the growth of beneficial microbes. Some of the foods microorganisms need are simplesugars, proteins and carbohydrates, which can come from molasses, fish emulsions, seaweed compounds, organic acids, composts and compost teas and other organic materials. Beneficial microbe populations can be best inoculated by adding to your grow media the above mentioned materials, which stimulate the indigenous populations rather than add foreign microbial inoculants. External microbial inoculations sometimes do not work as well as just building up the indigenous microbial populations. Inoculants often merely become food for the local microbial populationswho have acclimatized to the existing grow media.

Increasing microbial diversity.
Recently, Parson/Pavich Consulting conducted a research project on vegetable fields in Salinas, CA. The researchers selected two large fields and set up a control field and a test field. A bioassay microbial count in colony forming units (cfu) by functional groups was done prior to planting and after harvest. The control field was treated with a conventional fertilization program, which included high levels of nitrogen and phosphorus applications. In the test field the chemical NPK fertilizers were reduced by 50% and the researchers added a microbial based humic acid and seaweed application. Once the harvest was completed the researchers found some dramatic results. The microbial counts in the test area showed increases of 10 to 100 times higher than the standard fertilizer program (see graphic).

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In addition to increased microbial counts they found no disease in the test plots versus high levels in the conventional NPK fertilized field. The test area produced larger plants at a much lower cost due to the reduction in synthetic chemical NPK fertilizer and pesticide application.

As growers, we should be aware that by enhancing the diversity and strength of the microbial communities in our grow media, we will have healthier plants, less disease and increased utilization of nutrients applied to our crops.

– Perry O. Gooch, PhD

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