Soil Solutions

Nursery Management asked Josiah Shelton Hunt, the founder of Hawaii Biochar Products, to explain the role of beneficial microbes in plant health. These biological wonders can help fight off pathogens and improve a plant’s ability to access water.

What are beneficial microbes?

A. In the context of plant growth, the term “beneficial microbes” refers to those microbes that benefit plant growth and health, opposed to the well-known pathogens such as powdery mildew, Phytophthora and the gang.

What role do they play in plant health?

A. Beneficial microbes can aid in assimilation of raw nutrients, protection from pathogenic microbes, extending of plants’ ability to access water and nutrients, and other interesting phenomena.

Where can you find beneficial microbes (i.e. in the soil, on roots, etc.)

A. Beneficial microbes can be found in the soil, plant roots, plant surfaces and even within the plant’s body (root nodules and mycorrhizal relationships). They are literally everywhere, but then again, so are the pathogens, so how might you select for beneficial rather than detrimental microbes? This is an art. Here in Hawaii, a series of techniques, recipes and philosophies known as Korean natural farming has become popular. One particular recipe includes using rice wash (the water after rinsing it) and milk to make a concentrated group of beneficial bacteria. So why the rice wash? I looked into it a bit more and discovered that it was bringing a beneficial strain of Bacillus subtilis to the mix.

Are they naturally occurring in growing media or field soil?

A. In natural systems, plant surfaces both above and below the ground, are literally covered with microorganisms. It is different in man-made systems where growing media can often begin virtually devoid of microorganisms important in plant growth and health. While sterilization assures that there will be no pathogens, it also leaves the media without beneficial microbes. As it will eventually become home to a population of microorganisms, it is wise to actively choose your species at this time.

Beneficial microbes can be found in field soil. The diversity and density depends on environmental factors and cultural practice in the field. While some fields with cultural practices that embrace healthy soil biology may have sufficient populations, most will show a benefit by actively incorporating and cultivating beneficial microbes.

Is it possible to increase the amount of beneficial microbes in container or field-grown ornamentals?

A. Yes. First step is to consider soil biology when choosing potting media or preparing field soil. Second is to actively incorporate beneficial microorganisms suited to your habitat and crop. Another important way to increase the beneficial microbes is to use inputs that will stimulate them such as organic material based fertilizers and foliar sprays.

At any given moment there is a dynamic balance between several dozen to several thousand species of microorganisms surrounding your plants. There is an art in tilting that balance to your favor and it is proving to be a highly advantageous skill to have.

Is it possible to kill off beneficial microbes? How?

A. Beneficial soil microbes can be killed off by lack of drainage/flooding, high salt content and pesticides. In field conditions, perhaps the most damaging to beneficial soil microbes is direct sunlight. Groundcovers and/or mulch will dramatically increase the diversity and abundance of microorganisms found in the topsoil.

How do they react with insecticides, fungicides or herbicides that some nurseries may use on crops?

A. Many herbicides and insecticides can be used without killing all the beneficial microbes if done with care. Fungicides will inherently reduce the amount of beneficial fungi present, though these can be re-introduced through active management. In pesticide-intensive systems, beneficial microorganisms are highly subdued if not completely absent, but regularly reintroducing them with the use of chosen inoculants can be effective.

Are they most active during a certain season? Do they go dormant?

A. The population dynamics change throughout the seasons. Some go dormant while others become more active, a sort of changing of the guards. There are definitely seasons in which they are most needed. For instance, in advance of the onset of a known pathogen, say powdery mildew, this would be a highly advantageous time to apply a foliar application of one of the strains of Bacillus subtilis, which is known to suppress the pathogen. While in a healthy system there will be a natural ebb and flow, predators following prey, pathogens can be suppressed by actively supporting their predators and competitors.

Are there products on the market that allow growers to apply beneficial microbes?

A. There are many products available containing strains or combinations of beneficial microorganisms. These can be found in a variety of forms and for a variety of purposes. There are some that target specific problems, some that provide a broader range of benefits, and many that are being developed as we speak.

Perhaps the longest standing and most recognized blend of selected microorganisms used today is known as EM. EM stands for effective microorganisms, and as its name implies, has proven to be effective in many uses. Popular now is the use of mycorrhizal fungi as an inoculant in potting media or as a stand-alone product. Mycorrhizal fungi/plant relationships were once thought to be an occasional occurrence but are now recognized as almost ubiquitous in nature. Regular old compost can serve as great source of beneficial microbes -- a well maintained compost pile can have a highly diverse and active population of beneficial microorganisms. If properly aerated and allowed to reach temperatures above 140°F for a time, there should be virtually no pathogens and plenty of beneficial microbes in the compost as the mentioned conditions favor the latter.

What is the shelf life of such products?

A. The shelf life of such products can vary, in general though, the sooner used the better.

When is the best time to apply them?

A. For foliar applications of beneficial microbes, it is generally best to apply in the evening or on a cool, overcast day. For soil applications, it is best to apply when the soil is moist.

But the best time to apply beneficial microbes is before there is a problem. You can guarantee that microbes will find your plant attractive, so you can either choose your team or you can leave it up to chance.

Josiah Shelton Hunt is the founder and owner of Hawaii Biochar Products LLC in Pahoa, HI; www.hawaiibiocharproducts.com. He received a Bachelor of Science degree in agroecology and environmental quality from the University of Hawaii at Hilo.