Microbial Inoculants

Organic growers have known for years that healthy soil grows healthy plants. But what makes some organic soils better than others? The answer lies in the rhizosphere, the area closest to the root zone where root cells and microbes interact. Plants can’t take up large organic molecules. Organic fertilizers must first be broken down by microorganisms before they can be efficiently taken up by the plant. In the process, some beneficial microbes also produce biologically-active molecules that stimulate plant growth and improve the plant’s natural resistance to pests and disease. That’s where microbial inoculants come into play. By selecting specific strains of microorganisms from the most fertile soils and applying them to the plant as a root drench, it is possible to enhance the performance of any soil.

All microbial inoculants are not alike. Some microorganisms in the rhizosphere complement one another, working together as a very efficient team, while other microorganisms compete with one another. The key is to assemble a team of microbial superstars that work well together and have the greatest impact on the plant. For example, some beneficial bacteria are great at snatching nitrogen out of the air and changing it into fertilizer. Other bacteria are better at unlocking phosphorus from the soil. And other microbes are experts at making natural growth hormones and plant protection agents. Put them together in just the right way, and you have a dream team that provides a synergistic effect on the plant. In fact, the right combination of microorganisms will feed each other while feeding the plant!

Mycorrhizae and Beneficial Bacteria

Many potting soils advertise the fact that they contain mycorrhizae, but mycorrhizal fungi alone provide only limited benefits to the plant. For example, if there is a phosphorus deficiency in non-fertile soil, plants will exude signal molecules to attract mycorrhizal fungi. The fungi penetrate the root cells, feed on sugars manufactured by the plant, and extend the root zone by sending out threadlike hypha into the surrounding soil. In exchange for the plant’s sugars, the fungi unlock unavailable phosphorus from the soil and feed it to the plant. But even the best mycorrhizal fungi work better when combined with phosphorus solubilizing bacteria. The bacteria hitch a ride on the fungi as it penetrates the soil, and swim to places in the soil solution that the fungi can’t reach. As the bacteria reproduce, they exude organic acids and enzymes that unlock phosphorus from soil particles and organic matter and feed it to the fungi. The phosphorus solubilizing bacteria also colonize the developing root hairs to directly feed the plant. So when choosing inoculants to use as a “starter blend”, make sure that it is dominated by beneficial bacteria.

Beneficial Bacteria For Rooting

In addition to mineralization, some plant-growth-promoting rhizobacteria actually make rooting hormones directly on the surface of the roots. For example, as plants grow they exude amino acids from root cells. One of the amino acids is called tryptophan. Beneficial microorganisms such as Bacillus subtilis and Bacillus firmus feed on the tryptophan and change it into a powerful auxin called IAA (indole acetic acid). The auxin stimulates new root growth. Other microorganisms on the dream team are especially adept at producing growth hormones called cytokinins. Cytokinins stimulate cell division. So the combination of auxins and cytokinins work together to stimulate more lateral root growth and more root mass. The results? Better root strike, more efficient uptake of water and minerals, and a healthier and more stress resistant plant.

Microbes For Stress Tolerance

Speaking of stress, did you know that some beneficial microorganisms can actually rescue plants from stress? During times of heat stress, drought stress or UV stress plants produce ethylene gas. The ethylene signals the plants to stop producing new root and shoot growth and the plant temporarily shuts down. But some microorganisms produce an enzyme called ACC deaminase that blocks the production of ethylene and rescues normal plant growth. The plant recovers more quickly from stress and continues to grow and reproduce instead of going into shock. Therefore, any good “maintenance blend” of microbial inoculants should contain a healthy dose of ACC+ bacteria.

Some microorganisms have been isolated from disease suppressive soils and provide an extra level of protection from pathogens. For example, Bacillus subtilis GB03 is registered as a bio-pesticide and helps protect plants against root rot. Other beneficial microorganisms such as actinomycetes and Pseudomonas fluorescens provide protection against a wide range of pathogens, including infectious bacteria, fungi and even viruses. But one of the best defensive players on the microbial dream team is trichoderma sp. Trichoderma is actually a beneficial fungus, but it parasitizes and feeds on other fungi! It also helps feed the plant, and activates the plant’s natural immune system.

Teaming With Bacteria, Teaming With Fungi

Generally speaking, it’s much better to inoculate plants with multiple forms of plant-growth-promoting rhizobacteria than it is to use a single strain. For example, P. fluorescens has been isolated from disease-suppressive soils, but when used alone can decrease yields. It’s also been found that some yeasts (Saccharomyces cerevisiae) don’t play well with our friendly trichodermas. So choose carefully when selecting a polymicrobial blend, and know what you’re looking for. If you’re using water soluble nutrients and choose a blend that’s loaded with endo- and ecto-mycorrhizal fungi, you are wasting your money! By using the perfect blend of growth promoting and disease suppressing microbes, you can have the best of both worlds!

It’s also important to choose a blend that’s formulated specifically for the correct stage of plant growth. A good “starter blend” should be loaded with phosphorus solubilizers combined with a healthy complement of growth hormone producers. On the other hand, a good “bloom” or “maintenance blend” should be better at stress protection and growth promotion. Some microbes even produce a bouquet of volatile organic compounds that help the plant produce more blooms and bigger blooms. Volatile organic compounds produced by specific bacillus sp. can activate up to 600 different growth regulating genes in the plant! By switching on the genes that tell the plant to keep manufacturing and storing sugars, it is possible to keep your favorite plants biologically active all the way to the day of harvest.

In short, microorganisms are the biostimulant factories, with different microorganisms performing different roles at different times. The right team of microbes can help your plants reach their true genetic potential. So get to know the key players in the root zone. The more you know, the better you’ll grow!

By Harley Smith