Gardening Microbes 101

As you walk up and down the aisles of your local hydroponics store, you’ve probably noticed products containing mycorrhizae, microbes and beneficial bacteria. On the back of the label, there is a long list of Latin names for each microbe species and how many spores are included. If you aren’t familiar with the many species of fungi and bacteria, this list can be overwhelming. You might be wondering what each of these critters actually does for your plants.

First things first, you need to understand what microbes are. The word microbe generally refers to fungi and bacteria. Beneficial fungi and bacteria are essential components of the soil food web. They are the reason forests are lush and green despite gardeners not being there to water and feed the trees.

Mycorrhizal Fungi

Mycorrhizal fungi form a symbiotic relationship with their host plant. In exchange for the root exudates the plant provides, mycorrhizal fungi spread their hyphae throughout the soil and bring nutrients and minerals back to the host plant. The beneficial bacteria are an indispensable part of this circle. They consume, digest and excrete nutrients and minerals back into the soil. If it wasn’t for these bacteria holding onto them, the nutrients and minerals would drain into the water table when it rains. The host plant and microbes recycle nutrients to feed one another.

Mycorrhizal fungi can be classified into three groups: ectomycorrhizae, ericoid mycorrhizae and endomycorrhizae (also known as arbuscular mycorrhizae). Ectomycorrhizal fungi colonize the roots of pine, fir, oak, eucalyptus, hazelnut and birch trees. Ericoid mycorrhizal fungi associate with blueberry, cranberry, rhododendron, azalea and Pieris plants.

Arbuscular mycorrhizal fungi associate with most crop plants and are what most gardeners need to use. This is important for you to know because you don’t want to buy the wrong type of mycorrhizal fungi for the wrong plant. There are also mycorrhizae products that contain a lot of strains you don’t need in your garden. For example, if you are growing crop plants, you don’t need ectomycorrhizal fungi from the genera Rhizopogon or Scleroderma.

Glomus is the largest genus of AM fungi. In 1995, the American Society for Microbiology published a study that investigated how lettuce plants react in stressful conditions when the roots are colonized by seven different Glomus species. The study revealed which Glomus species were the most beneficial in adverse conditions. G. deserticola stood out as the best species for drought resistance, and was also associated with the least growth reduction (9%) under stressful conditions. The others were most effective for drought tolerance in the following order: G. fasciculatum, G. mosseae, G. etunicatum, G. intraradices, G. caledonium and G. occultum.

In the study, all seven Glomus species used in the experiment increased plant growth under both well-watered and drought conditions, although their efficiency varied. Drought conditions did not significantly affect nutrient uptake on the plants colonized with G. deserticola and G. etunicatum. However, the conditions impacted the nutrient uptake of plants associated with G. occultum more significantly. Phosphorus and potassium levels stayed relatively constant, even in adverse conditions, with G. deserticola, but were reduced with G. occultum.

Glomus intraradices is effective for aiding nutrient uptake. In the study, G. intraradices significantly increased the calcium and magnesium contents of plants that were both well-watered and in drought conditions. It also improves phosphorus uptake. In fact, it is the only AM fungi currently known to control nutrient uptake amounts by individual hyphae depending on the different phosphorus levels in the surrounding soil. Its hyphae will also improve the soil tilth.

Additional Microbes

There are many types of beneficial bacteria out there that provide a plenty of benefits to your garden. The most common benefits include greater root mass, improved nutrient uptake, increased vegetation and yield, improved transplant success, improved soil tilth, better pest and disease prevention, improved drought tolerance and improved tolerance to soil toxicities.

Of the many microbes listed on labels, there are a few species that stand out. Bacillus subtilis, Bacillus pumulis and Bacillus licheniformis are three species of bacteria so closely related that in the past it has been difficult to distinguish them from one another. They are referred to as the subtilis spectrum and are some of the most widely studied bacteria. They have applications in healthcare, cosmetics, laundry detergent, explosives, safe disposal of radionuclide wastes and genetic engineering. In agricultural applications, they are used to prevent and control fungal infections. More specifically:

•     B. subtilis bacteria produce a class of lipopeptide antibiotics called iturins. The bacteria use the iturins to help them out-compete other micro-organisms by either reducing their growth rate or killing them.

•     B. pumulis produces compounds that compete with fungal diseases for amino sugars needed to build cell walls, which makes it impossible for fungal cells to build and grow. It has been shown to specifically prevent Rhizoctonia and Fusarium spores from germinating.

•     B. licheniformis has been approved to treat ornamental plants to protect against fungal pathogens. It is especially effective against fungi that cause leafspot and blight diseases. Its ability to produce important extracellular enzymes and break down complex polysaccharides also makes it useful in nutrient cycling.

Another member of the Bacillus family, B. thuringiensis (Bt), is a well-known biological pesticide. B. thuringiensis forms crystals of proteinaceous insecticidal endotoxins called cry proteins. These toxins are harmful for many insects, including caterpillars, moths, butterflies, flies, mosquitos, beetles, wasps, bees, ants, sawflies and nematodes. Bt has been used, quite controversially, in genetically engineered corn. You can usually find Bt in the pesticide section of your grow shop, and it is included in most mycorrhizal products.

The Paenibacillus genus has gained a lot of attention in the agricultural world. Once classified as Bacillus, it was reclassified as a separate, although very similar, genus in 1993. Several Paenibacillus species are plant growth-promoting rhizobacteria (PGPR). PGPR are competitive colonizers of the root zone, acting as both biofertilizers and biopesticides. They make phosphates available to plants, fix atmospheric nitrogen into a form plants can use, degrade environmental pollutants and produce important hormones. They also control phytopathogens by competing for resources such as iron, amino acids and sugars. They even produce helpful antibiotics.

You will often find Paenibacillus polymyxa, Paenibacillus azotofixans and Paenibacillus durum in store-bought mycorrhizae products. They are all nitrogen-fixing bacteria, important in the vegetative growth stage. In 1994, scientists recommended that P. durum be reclassified as a species of P. azotofixans because they are so similar, although you will still see both names on product labels. Other nitrogen fixers you will find in products from your local hydroponics store include Azospirillum brasilense and species from the genus Azotobacter.

Beneficial microbes are essential to the soil food web and should be a mainstay in our gardens. They protect plants growing in stressful conditions, and help them to thrive in ideal ones. They will even attract the gardener to the garden by way of a friendly bacterium that lives in the soil called Mycobacterium vaccae. When humans come into contact with it while gardening, it causes serotonin to be released in our brains. No wonder we love gardening so much!

Source: Maximum Yield, April 17, 2020 "Getting To Know Your Microbes" https://www.maximumyield.com/getting-to-know-your-microbes/2/1404