Organic gardeners are faced with a unique challenge; by putting health and environment first, we set ourselves up for a neverending struggle against pests, disease, and lackluster growth. In the absence of chemicals, the organic grower has to get creative and work twice as hard to achieve the desired results.
This, at any rate, is the popular narrative, one that undoubtedly prevents many hobbyists from going the organic route. Sure, we'd all love to do the environmentally friendly thing, but we also want a flourishing garden without too many hassles, and we're conditioned to believe this is only possible through chemical applications. But what many people don't realize is that by relying on chemical fertilizers and pesticides, they actually create more work, rather than less, for themselves.
Most gardeners have probably heard that synthetic chemicals destroy beneficial organisms in the soil, but for many this concept is too abstract to grasp. Who are these organisms anyway? What are they up to? How can I know they're helping if I can't even see them? In their book Teaming With Microbes, Jeff Lowenfels and Wayne Lewis demystify the world of the soil food web, identifying key players and their roles and offering detailed advice on how to protect and manage this ecosystem for maximum results.
Lowenfels and Lewis make the compelling argument that many of problems plaguing our lawns and gardens are the direct result of the home horticulturalist's reliance on chemical products which destroy microbial life. The result is a garden that depends on application after application of fertilizer and pesticides in order to survive. Conversely, a healthy soil ecosystem controls pathogen populations, provide plants with essential nutrients, and form relationships that encourage vigorous growth and healthy immune systems.
The book is a real eye-opener; considering that I've been using organic growing practices for several years now, I was amazed by how little I really knew. Teaming With Microbes is a must-read for anyone interested in truly understanding the microscopic forces at work in the garden. But the growing season is already upon us, so for hasty, reactive growers like myself, I've distilled some key concepts that will change the way you garden forever.
Fungi and Bacteria: Your New Best Friends
At the center of a healthy soil food web is a robust population of bacteria and fungi. These microscopic organisms are the soil food web all-stars, the foundation upon which the ecosystem rests, and their presence (or absence) affects everything from pH to nutrient content to soil structure to pathogen control.
Bacteria and fungi are the world's most prolific decomposers. Without these organisms, the nutrients in the materials used in organic fertilizers would be inaccessible to plants. In the course of their feeding activities, bacteria and fungi convert nutrients in these materials into forms plants are capable of absorbing, immobilizing those nutrients in the process and preventing them from being leached away by runoff. When these organisms die -- often from being eaten by larger organisms -- those nutrients become available to other members of the food web -- most notably, your plants. Better yet, these microbes are actually attracted to plants, drawn by chemicals plant roots secrete, known as exudates. To that end, fungi and bacteria like to set up camp in the area immediately surrounding the roots, known as the rhizosphere, where their activities will have the maximum impact on your garden.
Contrary to our experiences with mushrooms and molds, soil fungi are largely invisible, but their contributions are truly astonishing.
Fungi, being more complex than bacteria, perform several additional functions. Contrary to our experiences with mushrooms and molds, soil fungi are largely invisible, but their contributions are truly astonishing. Certain species of fungi are able to decay tougher materials -- things like wood chips, insect shells, and bone -- that elude the abilities of bacteria, which can only tackle softer matter composed of simple sugars. Further, while bacteria are tiny, unicellular organisms who travel very little during the course of their existence, soil fungi grow in long, multicellular threads known as hyphae. These hyphae are capable of traveling long distances to find nutrients and transporting those nutrients back to the rhizosphere.
Perhaps the most remarkable contribution fungi can make to your garden is the symbiotic relationships -- known as mycorrhizae -- certain fungi form with plants. Mycorrhizal fungi attach themselves to plant roots and feed off their exudates. The hyphae of these fungi in turn act as extensions of the host plant's roots, effectively extending their reach; in the case of trees, mychorrhizal fungi can increase root surface area by up to 1,000 times! Moreover, mychorrhizal fungi play in essential role in allowing plants to access phosphorus, a nutrient that's often difficult for plants to absorb without the help of their fungal friends. It's estimated that up to 95 percent of plants form this relationship.
Once a foundation of fungi and bacteria is in place, the rest of the soil food web -- the protozoa, nematodes, right on up to the mites, beetles, earthworms, spiders, and all the other creepy-crawlies we typically observe in our gardens -- will follow. These creatures play a variety of roles in the soil community; they shred up organic matter, making it easier for bacteria and fungi to decompose; they manage one another's populations, both by preying on one another and by forming symbiotic relationships; they and limit pathogenic populations by eating them and competing with them for resources; they transport bacteria and fungi to other parts of the garden; they mineralize nutrients, converting them into a form that's accessible to plants; and their tunneling creates passageways through which air and water can circulate.
Contrary to what many of us might think, few of our gardens' inhabitants are objectively “bad.” Creatures commonly considered pests --slugs, for example -- actually carry out many beneficial functions in the garden. The problems arise when the soil community is somehow damaged and certain populations are left unchecked. This deficit can almost always be traced back to -- and remedied by -- the bacterial and fungal foundation.
Assess Your Microbes
Healthy soil is brimming with activity, and if the microbial foundation is in place you should be able to spot a diverse array of critters going about their day. Spiders, centipedes, ants, rollie pollies, and even a few slugs or snails are all good news for the gardener. If, however, you get up close, scoop up a few handfuls of dirt and still have a hard time spotting anything moving about, you'll probably need to take action to replenish your soil's microbial populations.
But there's more to assessing microbes than just determining that they're there. While a thriving soil community is a wonderful thing, it's also important to determine which microbes dominate the soil. Ultimately, it's a choice between fungi and bacteria. While both organisms are essential to the soil food web, certain plants prefer bacterial domination to fungal domination, while others thrive under the opposite conditions. According to Lowenfels and Lewis, fungally dominated soils are better suited to support perennials such as trees and shrubs, while annuals such as vegetables, lawns, and annual flowers prefer soil that is bacterially dominated.
A simple pH test can help determine which class of organism reigns supreme. A low pH (acidic soil) indicates fungal domination while a high pH (alkaline) suggests bacterial domination. The presence of moss, too, can be an indicator, as mosses prefer acidic conditions, and obviously any mushrooms you might spot are indicative of a thriving fungal population.
More precise methods of assessing a soil's microbial makeup become increasingly complicated and costly. Lowenfels and Lewis suggest tools such as homemade insect traps and sending soil samples to a lab for testing. If you have the means and the motivation to devote to these endeavors, you can learn a tremendous amount about the life in your garden, but such tools needn't be considered a necessity. Simply knowing what to look for and how to care for your garden microbes is a major step in the right direction.
Boost Your Microbes
Whether you're trying to replace a depleted microbe population, alter the fungi to bacteria ratio, or you simply want to keep your soil inhabitants healthy, there are several things you can do to encourage microorganisms to call your garden home.
One of the most important things a gardener can do to support his garden's microbes is to add lots of organic matter, most commonly in the form of either compost or mulch. These are familiar tools anyone who already uses organic gardening methods, though the logic behind this practice is not always fully understood. We may think we're feeding our plants with delicious leaf mulch or well-rotted compost, but we're actually feeding the microorganisms who support our plants. This is why organic fertilizers often appear to be less effective than chemical ones; chemical fertilizers deliver a blast of plant-available nutrients (usually nitrogen) directly to the roots, resulting in a noticeable boost in growth and driving away our microbe friends in the process. Organic matter, on the other hand, though rich with nutrients, requires the help of microbes to make these nutrients available to plants -- and the microbes are happy to oblige. The process takes a little longer than the application of steroid-like chemicals, but ultimately fosters a strong soil community that will support healthy plant growth season after season.
So what do we feed our microbes? The answer depends on the kind of organisms you most want to support. The choice ultimately comes down to whether you want your soil to be fungally or bacterially dominated. Simply put, bacteria like fresh, nitrogen-rich green matter like table scraps and lawn clippings, while fungi are more attracted to older, carbon-rich brown materials such as dead leaves and twigs.
The concept of browns versus greens is a familiar one to anyone who composts. Off-site composting (as opposed to in-ground composting, which takes place right in the garden bed) is a terrific way to add microbes to your garden. The compost heap is a favorite home to all kinds of soil food web champions attracted to the abundant supply of food, and their prowess for decomposition is essential to making a successful batch of the stuff. Adding a topdressing of compost to the garden not only imparts valuable nutrients, it also dumps these beneficial organisms back into the soil where they can do the most good. As such, it's important to know ahead of time what kind of organisms you most want to attract to the heap. Many composters have been led to believe that browns and greens must be present in equal measures for the compost to break down properly. In truth, many factors go into making a successful batch of compost, but altering the ratio of bacteria to fungi in a compost heap can be as simple as changing the ratio of browns to greens to better feed the microbes you want.
Mulches are another favorite tool of the organic gardener. In addition to their nutritional value, mulches are a natural way to snuff out weeds, improve soil quality, and insulate roots against sudden temperature changes. While mulches might not necessarily add microbial life to the garden the way compost does, they certainly help support existing populations. Here the same rule applies: if you want to increase your garden's fungal population, mulch composed of dead leaves, wood chips, and/or sawdust is a good option. To boost bacteria populations, a green mulch of grass clippings is the way to go.
Manure is another popular soil additive among organic gardeners. Lowenfels and Lewis don't have much to say on the subject of manure, except pointing out that adding it to compost or using it to brew tea increases the risk that E. coli bacteria will make an appearance. Other sources, however, suggest that the use of animal manures as a fertilizer can have positive effects on the soil community. A study released by Michigan State University Extension found that adding manures to the soil increases the numbers and diversity of microbial life, improves soil structure, and plays a role in suppressing certain diseases. Bacteria appear to be more responsive to manure than fungi, making it a useful tool for gardens that prefer bacterial domination. The slow-release nature of the nutrients in manure can be both a blessing and a curse, supplying the food web with nutrients year after year but possibly prompting the inexperienced grower to to apply more manure than the community can absorb, resulting runoff and groundwater contamination as well as air pollution. Such problems, however, can be avoided by digging manure into the soil and choosing a manure that matches the crop's needs. A fact sheet by Colorado State University Extension offers further guidance as to the nutrient content -- particularly the carbon to nitrogen ratio (C:N) -- of various forms of manure.
A fourth option for gardeners looking to increase microbe populations is an application of actively aerated compost tea, or AACT. Lowenfels and Lewis recommend this tool as a means of injecting microbiology into the rhizosphere in a way that gets it there much faster than an application of compost or mulch. AACT is also an effective way to treat leaves that may be suffering from pest or fungal infections. Important to note that this means of concocting compost tea might differ from teas you're familiar with which involve steeping compost or manure for a few days and then watering with the resulting brew. As its name implies, brewing AACT is an active process, one that requires the use of an air pump to keep the mixture oxygenated and to separate microbes from the organic matter. AACT brewers are available commercially, but the industrious gardener can build his own brewer at a relatively low cost (check out this article by Mother Earth News for detailed instructions and recipes). An application of bacterial compost tea can serve as a quick fix for a wide range of pathogenic fungi diseases, including dime spot, damping off, and the ubiquitous powdery mildew, and is also an effective means of pest control against insects such as weevils and grubs.
Finally, in much the same way earthworms and ladybugs are available for purchase, gardeners looking to capitalize on that magical, root-extending relationship certain fungi form with plants (remember mycorrhizae?) can order spores online. Mycorrhizal fungi fall into two general categories, ectomycorrhizal and endomicorrhizal. The name refers to the manner in which the fungus bounds with the host's roots, but for our purposes the important thing to remember is that ectomycorrhizal fungi are best suited for use with hardwoods and conifers while endomicorrhizal fungi partner with most other annuals, perennials, grasses, and softwood trees. A few plants can benefit from both types of mycorrhizal fungi. Luckily, mycorrhizal fungi products are often labeled according to the kind of plants they're best suited for, keeping the confusion to a minimum. Mycorrhizal fungi can be applied during seeding or transplanting and can also be injected into the roots of established plants.
Protect Your Microbes
At this point it should be apparent that the first rule of protecting your microbes is to banish all non-organic chemical products. This includes chemical fertilizers, herbicides, fungicides, and insecticides, all of which can disrupt the soil ecosystem and drive away essential organisms.
Organic products are a little trickier. Awhile ago I wrote an article exploring the use of chemicals in organic gardening. Organic pesticides contain chemicals that generally decay very quickly, won't leach into groundwater, and pose minimal toxicity or long-term health risks to humans; some of these chemicals, however, can still disrupt microbe populations. Organic fungicides are particularly dicey, as their very purpose is to kill off fungi, often with no discrimination between beneficial organisms and pathogens. If you plan to inoculate plants with myccorhizal fungi, this list can help you determine which fungicides might be used in case of emergency. GrowOrganic.com also offers a useful overview on how different kinds of organic pesticides affect the soil food web.
When it comes to fertilization, Lowenfels and Lewis advise gardeners to avoid fertilizers with nitrogen/phosphorus/potassium (NPK) numbers greater than 10/10/10. Luckily, most organic fertilizers already meet this criteria, so this is generally not an issue. Colorado State University Extension further advises that fertilizer with a C:N ratio of 20:1 or higher can upset the nutrient balance and lead to nitrogen deficiency.
Perhaps the most surprising way gardeners inadvertently damage their soil ecosystems is by turning the soil, either through the use of hand tools or rototillers. Many of us have been taught that the practice of turning the soil helps increase air circulation. In reality, however, this practice actually destroys the tiny tunnels and fissures dug by various microorganisms. Worse, tilling is an excellent way to tear apart the delicate fungal hyphae that prove so useful in distributing nutrients, and larger organisms -- earthworms, for example -- can get caught in the fray as well. Of course, gardening nearly always requires some digging, whether it's seeding, transplanting, or digging in organic material, but in general it's best to disrupt the soil in your garden as little as possible.
Tilling the soil contributes to another major threat to the soil community: compaction. This might seem counter-intuitive, considering that tilling initially loosens the soil, but it also breaks the soil into tinier particles that are more prone to compaction. Compaction kills fungi and restricts air and water circulation. Whenever possible, avoid driving vehicles or heavy machinery on your lawn, and avoid walking on garden beds. Adding organic matter to the soil also helps prevent compaction by improving soil structure.
No one ever said gardening wasn't work -- for many of us, getting our hands dirty is half the fun -- but when we raise a garden that depends on chemical intervention to survive, an enjoyable hobby can quickly become an expensive, stressful chore. Unlocking the power of the soil food web gets everything right: it's inexpensive, environmentally friendly, doesn't expose anyone to potentially dangerous chemicals, and ultimately makes gardening easier. With a harmonious soil ecosystem in place, many -- if not all -- of the problems that had you reaching for the pesticides will resolve themselves naturally. Plus, if you're a big garden nerd like me, discovering what those critters in the dirt are up to is actually pretty fun to boot.
For a more in-depth look at the soil food web and its inhabitants, I highly recommend picking up a copy of Teaming With Microbes and checking out the sites in the Further Reading section below to stay current on the latest advancements in microbiology and how they apply to your garden.
Further Reading:
Soil Food Web Inc
Gardening With Microbes
THE SOIL FOOD WEB: TAKING ORGANIC GARDENING TO THE NEXT LEVEL by UrbanSculpt Staff Writer Leslie McIntyre is licensed under a Creative Commons Attribution 4.0 International License.
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