Overstory #81 - The Soil Foodweb: It's Role in Ecosystem Health
The Soil Foodweb
What is the soil foodweb? It's the set of organisms that perform the functions that allow plants to grow normally, without the need for toxic chemical inputs. A set of healthy organisms, a healthy soil foodweb if you will, gives plants the following necessary functions:
A healthy foodweb includes thousands of species of bacteria and fungi, protozoa and nematodes all of whom combat diseases causing organisms. Unfortunately, they are among the first to be killed when using pesticides and high levels of inorganic fertilizers.
Ever wonder why fertilizers have to be added every year in chemically intensive systems? Because it is the organisms in the healthy foodweb that retain those nutrients, and once those organisms are killed, soil cannot hold on to those nutrients. N, P, S, and calcium, as examples, will leach from the soil without the nutrient-retention abilities of the millions if not billions or more living bacteria and fungi that should be present in your soil.
If bacteria and fungi hold onto the soil's nutrients, then how do those nutrients become available back into leachable forms that the plant requires? And this process has to happen to the greatest extent around the root system of the plant. Of course, the organisms that will do this are present in soil, and were present in soil long before higher plants came into existence. The important critters in this function are protozoa, who eat bacteria and release nutrients in plant available forms, beneficial nematodes, who eat bacteria, or fungi or each other, (99% of the nematodes in soil are beneficial to plant growth, not root-feeding or foliar-feeding nematodes as pesticides salesmen would have you believe), and microarthropods and earthworms, the power-cyclers of the soil, that eat bacteria, fungi, protozoa, nematodes, and anything else they can chow down on. Most of this happens in the root-zone, because plants make exudates (cake and cookies for soil bacteria and fungi!) which they release right around the roots, in order to grow the disease-suppressive bacteria and fungi right around their roots. These bacteria and fungi growing on plant-made goodies immobilize nutrients right there, around the roots. Then the protozoa, beneficial nematodes and microarthropods are attracted to the root zone, and release those nutrients right where the plant can grab them! Plants are "smarter" than we thought!
Decomposition of plant residues and plant-toxic compounds.
Many microbiologists have the attitude that any organic compound that can be made will be decomposable by an already existing bacterium or fungus. There's no need to make genetically-engineered species - plants and microbes already exist which perform whatever function humans might need. But can you sell it to someone? That's a different story, but we need a healthy soil foodweb present in our soils to get rid of any toxins that might be there. Plant litter ought to decompose and be long gone by spring-time. If it isn't, there's something wrong with your foodweb, and you need to fix it!
You are stuck with the sand, silt and clay that you were given in your soil, but you can improve the life in your soil immensely. And by improving that life, the right life, you improve water-infiltration into the soil (does water puddle on the surface of your soil? You have an unhealthy foodweb, fix it!), you'll improve water-holding capacity so you don't have to water so much in the summer, and you'll improve root growth into your soil, so you don't need the pesticides, fertilizers, etc. Imagine less work to do all the time on your lawn! Let the critters in the soil do that work for you!
How Many of Each Functional Group?
Per gram of healthy soil, which is about a teaspoon of soil plus organic matter, the following organisms - most of whose names are not known to scientists - are found:
- 1 million (in arid soils) to 100,000 million bacteria (in forest soils) [1 million=10^6]. Bacteria break down easy to-use organic material (sugars, proteins, carbohydrates), retain nutrients, like N, P, and S in the soil and combat disease causing organisms. About 60% of the carbon in those organic materials is respired as carbon dioxide, but 40% of that carbon is retained as bacterial biomass and organic matter. The waste products bacteria produce become soil organic matter. This "waste" material is more recalcitrant than the original plant material, but can be used by a large number of other soil organisms, exemplifying the classic statement that "One man's garbage is another's treasure." Productive garden soil should contain more bacteria than any other kind of organism, although care must be taken to make sure beneficial bacteria, instead of disease-causing bacteria, are most prevalent. How do you do that? By feeding the foods the suppressive bacteria like. That also means, no nitrate fertilizers (which selects for the disease-causing bacteria and fungi) and no compaction (that means poorly structured soils, and reduced oxygen levels which help the root-rot fungi more than anything else).
- 150 to 500 micrograms of fungal hyphae. Fungi break down the more recalcitrant, or difficult-to-decompose, organic matter (like newspaper, cardboard, bark, sawdust, corn stalks) and retain those nutrients in the soil as fungal biomass. Just like bacteria, fungal waste products become soil organic matter, which are used by other organisms. Gardens require some fungal biomass for greatest productivity, but in order for best CROP growth, there should be an equal biomass of bacteria as compared to fungi. Most grasslands or pastures have less fungi than bacterial, while all conifer forests have much more fungal, as compared to bacterial, biomass. As with bacteria, we need to feed the "good-guy" fungi, not the "bad-guys." Feed the soil complex mixtures of humic acids and algae, and avoid nitrate fertilizers and prevent soil compaction.
- 10,000 to 100,000 protozoa. These organisms are one-celled, highly mobile organisms that feed on bacteria and on each other. Because protozoa require 5 to 10-fold less nitrogen than bacteria, N is released when a protozoan eats a bacterium. That released N is then available for plants to take up. Between 40 and 80% of the N in plants can come from the predator-prey interaction of protozoa with bacteria.
- 15 to 500 beneficial nematodes. Beneficial nematodes eat bacteria, fungi, and other nematodes. Nematodes need even less nitrogen than protozoa, between 10 and 100 times less than a bacterium contains, or between 5 and 50 times less than fungal hyphae contains. Thus when bacterial- or fungal-feeding nematodes eat bacteria or fungi, nitrogen is released, making that N available for plant growth. However, plant-feeding nematode are pests because they eat plant roots. These "bad" nematodes can be controlled biologically, as they are in natural systems, by fungi that trap nematodes, by having fungi that colonize root systems and prevent nematode attack of roots, or by predation of nematodes by arthropods. In cases of extreme outbreaks, however, the only answer may be the use chemicals to control these plant-feeding nematodes. However, once a chemical is used which kills the beneficial nematodes as well as the plant-feeding ones, the beneficial nematodes need to be replaced through inoculation. What inoculants are there for these beneficial fungi? Compost, and compost tea are the only commercially available sources of the whole community of these beneficial nematodes, or protozoa, for that matter.
- A few to several hundred thousand microarthropods. These organisms chew the plant leaf material, roots, stems and boles of trees into smaller pieces, making it easier for bacteria and fungi to find the food they like on the newly revealed surfaces. The "comminuting" arthropods can increase decomposition rates by 2 - 100 times, although if the bacteria or fungi are lacking, increased decomposition will not occur because it takes ALL the organisms working together to make nutrient cycling work. In many cases, however, the arthropods carry around an inoculum of bacteria and fungi, making certain the food they want (bacteria and fungi) are inoculated onto the newly exposed surfaces! Because the C:N ratio of arthropods is 100 times greater than the bacteria and fungi, they release nitrogen which then is available for plant growth. Some arthropods eat pest insects, while others eat roots. Again, it's important to encourage the beneficial ones and discourage the ones that eat plants!
The Web of Life Can Be Lost
The interactions between these organisms form a web of life, just like the web that biologists study above ground. What most people don't realize is that the above ground world wouldn't exist without below ground systems in place and functioning. Soil biology is understudied, compared to life above ground, yet it is important for the health of gardens, pastures, lawns, shrub lands, and forests. If garden soil is healthy, there will be high numbers of bacteria and bacterial-feeding organisms, which means the beneficial, disease-suppressive organisms will be present. If the soil has received heavy treatments of pesticides, chemical fertilizers, soil fungicides or fumigants that kill these organisms, the tiny critters die, or the balance between the pathogens and beneficial organisms is upset, allowing the opportunist, disease-causing organisms to become problems.
Overuse of chemical fertilizers and pesticides have effects on soil organisms that are similar to over-using antibiotics. When we consider human use of antibiotics, these chemicals seemed a panacea at first, because they could control disease. But with continued use, resistant organisms developed, and the organisms that compete with the disease-causing organisms were lost. We found that antibiotics couldn't be used willy-nilly, that they must be used only when necessary, and that some effort must be made to replace the normal human-digestive system bacteria killed by the antibiotics.
Soils are similar, in that plants grown in soil where competing organisms have been knocked back with chemicals are more susceptible to disease-causing organisms. If the numbers of bacteria, fungi, protozoa, nematodes and arthropods are lower than they should be for a particular soil type, the soil's "digestive system" doesn't work properly. Decomposition will be low, nutrients will not be retained in the soil, and will not be cycled properly. Ultimately, nutrients will be lost through the groundwater or through erosion because organisms aren't present to hold the soil together.
The best way manage for a healthy microbial ecosystem is to routinely apply organic material. To keep garden soil healthy, the amount of organic matter added must be equal to what the bacteria and fungi use each year.
Indiscriminate use of chemical fertilizers and pesticides should be avoided. If the soil is healthy for the type of vegetation desired, there should be no reason to use pesticides, or fertilizers. If a decision is made to change from grass to garden, or forest to lawn, a massive change in the soil foodweb structure is required but all that is needed is addition of the right kind of compost with the right kinds of organisms to do that conversion. Once the correct soil foodweb structure is in place, there should be no reason to apply chemicals.
If both bacteria and fungi are lost, then the soil degrades. If bacteria are killed through pesticide or chemical applications, and especially if certain extremely important bacteria like nitrogen-fixing bacteria or nitrifying bacteria are killed, the wrong kinds of bacteria, or too much fungal biomass can result (often seen in tropical soils!) can take over and crop production can be harmed. For example, current research indicates that the reason moss takes over in lawn ecosystems is because the soil is converted from a bacterial dominated system to one dominated by fungi. Or the soil may become saturated with water because of poor soil structure during heavy downpours, and the iron in the soil is reduced into plant unavailable forms. Without the right biology present, reduced iron cannot be taken up by grass, and weeds flourish.
Without the right biology present, nutrients are lost, erosion increases and plant yield is reduced. If inorganic fertilizers are used to replace the lost nitrogen, the immediate effect may be to improve plant growth. However, as time goes on, it is clear that inorganic fertilizers can't replace the other kinds of food that bacteria and fungi need and may damage soil through accumulation of salts. After a while, fertilizer additions are a waste of money, because there aren't enough soil organisms to hold on to the nutrients added. Surface and groundwater will become contaminated with the lost nutrients, causing problems.
This article was updated by the author in April 2001 from an article that appeared online at Soil Foodweb.
About the Author
Elaine R. Ingham, B.A., M.S., Ph.D., is an adjunct Associate Research Professor in the Department of Forest Science at Oregon State University. She is also President and Director of Research at Soil Foodweb Inc., a small business that grew out of her University research program. Her research focuses on what organisms are present in the soil and on the foliage of plants, which organisms benefit which types of plants, which organisms harm plants, how can these organisms be managed to grow plants with the least expensive inputs into the system while maintaining soil fertility. The research and practical understanding and application of soil organisms continues at Soil Foodweb Inc. while much of the academic work remains at the University. Elaine can be reached through: Soil Foodweb, Inc., 1128 NE 2nd St., Suite 120, Corvallis, OR 97330, USA.