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Plants Are in Control

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The rhizosphere is an area of interaction between the surface of a plant root and the area surrounding it. Bacteria and other microorganisms, as well as soil debris, fill the area. 20,0005. Photograph by Sandra Silvers, USDA-ARS
The rhizosphere is an area of interaction between the surface of a plant root and the area surrounding it. Bacteria and other microorganisms, as well as soil debris, fill the area. 20,0005. Photograph by Sandra Silvers, USDA-ARS

[sidebar]Healthy soil is teaming with life—not just earthworms and insects, but a staggering multitude of bacteria, fungi, and other microorganisms. The term “soil food web” refers to the interactions of these organisms, which feed on one another and, in doing so, provide an environment in which plants can grow and prosper.

Jeff Lowenfels and Wayne Lewis
Teaming with Microbes: A Gardener’s Guide to the Soil Food Web[/sidebar]

Most gardeners think of plants as only *taking up nutrients through root systems and feeding the leaves. Few realize that a great deal of the energy that results from photosynthesis in the leaves is actually used by plants to produce chemicals they secrete through their roots. These secretions are known as exudates. A good analogy is perspiration, a human’s exudate.

Root exudates are in the form of carbohydrates (including sugars) and proteins. Amazingly, their presence wakes up, attracts, and grows specific beneficial bacteria and fungi living in the soil that subsist on these exudates and the cellular material sloughed off as the plant’s root tips grow. All this secretion of exudates and sloughing-off of cells takes place in the rhizosphere, a zone immediately around the roots, extending out about a tenth of an inch, or a couple of millimeters. The rhizosphere, which can look like a jelly or jam under the electron microscope, contains a constantly changing mix of soil organisms, including bacteria, fungi, nematodes, protozoa, and even larger organisms. All this “life” competes for the exudates in the rhizosphere, or its water or mineral content.

At the bottom of the soil food web are bacteria and fungi, which are attracted to and consume plant root exudates. In turn, they attract and are eaten by bigger microbes, specifically nematodes and protozoa (remember the amoebae, paramecia, flagellates, and ciliates you should have studied in biology?) that eat bacteria and fungi (primarily for carbon) to fuel their metabolic functions. Anything they don’t need is excreted as wastes, which plant roots are readily able to absorb as nutrients. How convenient that this production of plant nutrients takes place right in the rhizosphere, the site of root-nutrient absorption.

At the center of any viable soil food web are plants. Plants control the food web for their own benefit, an amazing fact that is too little understood and surely not appreciated by gardeners who are constantly interfering with Nature’s system. Studies indicate that individual plants can control the numbers and the different kinds of fungi and bacteria attracted to the rhizosphere by the exudates they produce. During different times of the growing season, populations of rhizosphere bacteria and fungi wax and wane, depending on the nutrient needs of the plant and the exudates it produces.

Soil bacteria and fungi are like small bags of fertilizer, retaining in their bodies nitrogen and other nutrients they gain from root exudates and other organic matter (such as those sloughed-off root-tip cells). Carrying on the analogy, soil protozoa and nematodes act as “fertilizer spreaders” by releasing the nutrients locked up in the bacteria and fungi “fertilizer bags.” The nematodes and protozoa in the soil come along and eat the bacteria and fungi in the rhizosphere. They digest what they need to survive and excrete excess carbon and other nutrients as waste.

Left to their own devices, then, plants produce exudates that attract fungi and bacteria (and, ultimately, nematodes and protozoa); their survival depends on the interplay between these microbes. It is a completely natural system, the very same one that has fueled plants since they evolved. Soil life provides the nutrients needed for plant life, and plants initiate and fuel the cycle by producing exudates.

Soil Life Creates Soil Structure

The protozoa and nematodes that feasted on the fungi and bacteria attracted by plant exudates are in turn eaten by arthropods (animals with segmented bodies, jointed appendages, and a hard outer covering called an exoskeleton). Insects, spiders, even shrimp and lobsters are arthropods. Soil arthropods eat each other and, in turn, are the food of snakes, birds, moles, and other animals. Simply put, the soil is one big fast-food restaurant. In the course of all this eating, members of a soil food web move about in search of prey or protection, and while they do, they have an impact on the soil.

Bacteria are so small they need to stick to things, or they will wash away; to attach themselves, they produce a slime, the secondary result of which is that individual soil particles are bound together (if the concept is hard to grasp, think of the plaque produced overnight in your mouth, which enables mouth bacteria to stick to your teeth). Fungal hyphae, too, travel through soil particles, sticking to them and binding them together, thread-like, into aggregates.

Worms, together with insect larvae, moles, and other burrowing animals, move through the soil in search of food and protection, creating pathways that allow air and water to enter and leave the soil. Even microscopic fungi can help in this regard. The soil food web, then, in addition to providing nutrients to roots in the rhizosphere, also helps create soil structure: the activities of its members bind soil particles together even as they provide for the passage of air and water through the soil.

Soil Life Produces Soil Nutrients

When any member of a soil food web dies, it becomes fodder for other members of the community. The nutrients in these bodies are passed on to other members of the community. A larger predator may eat them alive, or they may decay after they die. One way or the other, fungi and bacteria get involved, be it decaying the organism directly or working on the dung of the successful eater. It makes no difference. Nutrients are preserved and eventually are retained in the bodies of even the smallest fungi and bacteria. When these are in the rhizosphere, they release nutrients in plant-available form when they, in turn, are consumed or die.

Without this system, most important nutrients would drain from soil. Instead, they are retained in the bodies of soil life. Here is the gardener’s truth: when you apply a chemical fertilizer, a tiny bit hits the rhizosphere, where it is absorbed, but most of it continues to drain through soil until it hits the water table. Not so with the nutrients locked up inside soil organisms, a state known as immobilization; these nutrients are eventually released as wastes, or mineralized. And when the plants themselves die and are allowed to decay, the nutrients they retained are again immobilized in the fungi and bacteria that consume them.

The nutrient supply in the soil is influenced by soil life in other ways. For example, worms pull organic matter into the soil, where it is shredded by beetles and the larvae of other insects, opening it up for fungal and bacterial decay. This worm activity provides yet more nutrients for the soil community.

Healthy Soil Food Webs Control Disease

A healthy food web is one that is not being destroyed by pathogenic and disease-causing organisms. Not all soil organisms are beneficial, after all. As gardeners, you know that pathogenic soil bacteria and fungi cause many plant diseases. Healthy soil food webs not only have tremendous numbers of individual organisms but a great diversity of organisms. Remember that teaspoon of good garden soil? Perhaps 20,000 to 30,000 different species make up its billion bacteria—a healthy population in numbers and diversity.

A large and diverse community controls troublemakers. A good analogy is a thief in a crowded market: if there are enough people around, they will catch or even stop the thief (and it is in their self interest to do so). If the market is deserted, however, the thief will be successful, just as he will be if he is stronger, faster, or in some other way better adapted than those that would be in pursuit.

In the soil food web world, the good guys don’t usually catch thieves, though it happens with some forms of nematodes; rather, they compete with them for exudates and other nutrients, air, water, and even space. If the soil food web is a healthy one, this competition keeps the pathogens in check; they may even be out-competed to their death.

Just as important, every member of the soil food web has its place in the soil community. Each, be it on the surface or subsurface, plays a specific role. Elimination of even one group can drastically alter a soil community. Birds participate by spreading protozoa carried on their feet or dropping a worm taken from one area into another. Too many cats, and things will change. Dung from mammals provides nutrients for beetles in the soil. Kill the mammals, or eliminate their habitat or food source (which amounts to the same thing), and you won’t have as many beetles. It works in the reverse as well. A healthy soil food web won’t allow one set of members to get so strong as to destroy the web. If there are too many nematodes and protozoa, the bacteria and fungi on which they prey are in trouble and, ultimately, so are the plants in the area.

Excerpted from Teaming with Microbes: A Gardener’s Guide to the Soil Food Web, an important new book due from Timber Press (www.timberpress.com) in August 2006. Watch for future excerpts in the pages of Pacific Horticulture.




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