All Hydro 101

Establish and Maintain a Healthy Root Microbiome

Plants consist of several basic organ systems, and roots are an important one. In both soil culture and hydroponics, roots generally keep to themselves, out of sight, doing their critical work of locating and transporting water and nutrients up to the shoots. However, they do not perform their duties alone.

In soils, roots are surrounded by a complex mixture of solids, liquids, gases, a vast array of organic and inorganic molecules, and a highly variable and dynamic biological community.

This community may consist of anything (and everything) from macroscopic earthworms and other invertebrate animals, all the way down to the smallest microorganisms.

Since plants and microorganisms have co-existed for a long time, they have also evolved together to form both antagonistic as well as beneficial relationships. As microorganisms live and die in the root zone, they directly or indirectly provide many substances that can eventually be broken down and used to support plant growth. And, through the release of phytohormones and other root exudates, plants have some ability to actively encourage the presence of some microbes while discouraging the proliferation of others. It’s a complex world down there in the rhizosphere and plants clearly have preferences as to which other species they can tolerate or even prefer to share their environment with.

In nature, it is the norm rather than the exception for plants to succeed only with the help of many different bacteria, and fungi, in association with their roots either directly or indirectly. Although conditions may be quite a bit different for plants grown hydroponically, interactions between plants and microorganisms in the root zone are just as important. Growers should encourage the establishment of healthy, beneficial organisms, while inhibiting antagonistic or disease-causing species.

It is normal for a thriving community of microbiota to exist in the liquid phase of a properly maintained hydroponic system. Not only is it undesirable to have a root zone and nutrient supply system free of microbes for biological and horticultural reasons, it is an essentially impossible condition to achieve. Bacteria and fungi just can’t be kept out except in the most rigorous, difficult to implement, and expensive laboratory operations. Growers have a choice in the matter: either let their systems be colonized naturally by whatever adaptable organisms exist in the growing environment, or inoculate with species known to offer benefits to the plants.

People generally do not prepare their own microbes for inoculation. Commercial preparations are the norm, and reputable products consist of a well defined and quantified mixture of beneficial species. Introducing a well-defined community of organisms to the root zone by direct application to the substrate in flow-through systems or by addition to the nutrient reservoir for recirculating systems allows a healthy and beneficial root microbiome to develop before a random and uncontrolled selection of environmental microbes can take hold. In most cases a naturally occurring microbiome will not cause problems, but there is always the chance that an unwanted species may find a home. And, although a system may seem to perform well, it might perform even better if better control of the root microbiome was established and maintained.

Inoculating has clear advantages over letting nature take its course:

(1) A beneficial root microbiome confers resistance to colonization of the roots by pathogenic microbes including Pythium, Fusarium, and many others. It is more difficult for an invader to get a foothold if there is a large number of microbes already occupying the desired territory.

(2) A large population of prokaryotes (bacteria and actinomycetes) at the rhizoplane (the root surface and surrounding region in close contact with the root) makes for a hot bed of chemical activity that improves nutrient availability and results in more efficient transfer of materials between the root and its environment. Which species are present at the rhizoplane has a significant impact on root growth and function.

(3) Inoculation of plant roots with mycorrhizal fungi in solid substrates (coco fiber, coir, some ceramic materials) allows for a healthy fungal community that provides the same advantages in terms of nutrient uptake as it does under natural conditions. Even in liquid culture, mycorrhizal roots are denser with greater branching compared to non-mycorrhizal roots. Roots colonized by mycorrhizal fungi are more resistant to disease-causing organisms than non-mycorrhizal roots. The vast majority of plants on earth have naturally-occurring relationships with mycorrhizal fungi, so why should hydroponically grown plants be any different?

In the end, it’s the plants that tell the story. The research literature is rich with examples of increased plant productivity resulting from mutualistic relationships between plants, bacteria, and fungi— plants are just better off in the presence of a large population of beneficial microorganisms. Thousands of species of bacteria, fungi, and protists, are commonly found in association with plant roots. Many of these species are little known and poorly characterized. Some are good, a few are bad, and for others, we just don’t know. Growers who take steps to proactively develop a healthy community of microorganisms in their plant roots reduce the probability of catastrophic failure from disease and have a better chance of reliably producing strong healthy plants over the long term.

by Philip McIntosh