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EAP Publication - 39
BY DR. STUART B. HILL
In most parts of the world , there is more life beneath the soil surface than above it. Of 611 the groups of animals represented there my favorite is the mites. In a few shovelfuls of forest soil there may be more than 1,000 different types of organisms, including 100 types of mites.
A square meter of soil 4 inches deep may be home to more than 10 million individual nematodes and protozoa, 1 million mites and springtails, and thousands of other invertebrates, including several hundred earthworms. In even a teaspoon of soil, as many as 10 million bacteria and more than a mile of fungal filaments can thrive.
The soil is the decomposer part of the natural cycle that flows from plant and animal production to consumption and use, to waste decomposition and recycling, and then back to plant production. If there were no organisms in the soil, we would soon be up to our ears in dead animals and other forms of organic matter.
In recent decades, however, these soil organisms are having a harder and harder time getting their work done. Nearly everything that humans do to the soil kills these fragile creatures--directly or indirectly. Pesticides and some fertilizers poison them. Cultivation and baresoil tillage cause physical injury or expose them to the damaging rays of the sun or to predators.
What's more, we're starving soil life of a balanced diet by our failure to properly return plant and animal wastes to the soil, and by our tendency to grow the same crop on the same land year after year. The resulting monotonous diet makes for an unhealthy soil population. In addition, dessication, flooding, fire, compaction from heavy machinery and contamination with a vast range of pollutants add to the hazards of life in the soil.
Yet it is upon these organisms and their free regeneration service that the productivity of our farms and forests, and ultimately, we ourselves rely.
To learn how to farm in cooperation with our willing soilborne allies, imagine exploring as a mite the topsoil beneath your feet.
About half the volume of average soil is solid material, mostly mineral particles. The other half consists of spaces between the particles. Water that occurs as a film around the particles fills about half the spaces.
This situation gave rise to the evolution of three primary strategies for living in soil:
· Protozoa, nematodes and some other small organisms swim or creep around in the water film, feeding primarily on bacteria and one another.
· Mites, springtails and other small arthropods wander around in the air spaces (up to their knees in water) browsing on fungi, nematodes and one another.
· Earthworms, slugs, snails and some insects and other larger arthropods burrow through the soil, regardless of the size of the air and water spaces, feeding primarily on the dead organic matter and on the microorganisms that colonize it.
The key soil management question: What can we do to provide these organisms with optimal food and living conditions so they can get on with their jobs?
Clearly, the answer is to encourage the return of a suitable mix of uncontaminated organic wastes to the soil and to avoid stressing the system physically, chemically and biologically.
As well as the obvious benefits of earthworm tunnels and fertile castings, most soil organisms have many subtle, positive effects. For example, the breakdown of leaves requires the action of a succession of different species of fungi. But because fungi have limited abilities to move, they soon run out of suitable food. They also flood their micro-environment with antibiotics, making it difficult for other fungi to grow.
Soil animals, especially mites and springtails, partially solve this problem.
Most fungal-feeding mites prefer certain species of fungi. The mites generally digest only the filaments of the fungal bodies they consume, leaving the spores , to pass through their guts undamaged. They deposit these seeding parts in little packages of their manure throughout the soil. Because this waste is fertile, like potting soil, these spores will germinate and form new fungal colonies in new locations. This is a not-so-primitive form of farming by mites. They deposit their waste in a piece of cast-off gut lining that contains substances that prevent other, less desirable, fungi from growing there. This amounts to a fairly sophisticated method of "weed" control. What's more, hairs on their bodies may disperse the spores of the fungal species on which they prefer to feed. It is humbling to think that this sort of farming has been going on in the soil for more than 400 million years.
These delicate, regulating relationships between soil organisms are important for maintaining soil fertility and health. Regular use of chemical fertilizers has partially masked loss of productivity from diminished soil life. But as is the case with many similar processes in the human body, using an artificial form of a naturally produced substance will inhibit its natural production and create dependence on repeated applications.
There are many functions performed by the vast array of creatures in the soil, many more than we are fully aware of. Whenever we eliminate one of these organisms, we inherit its job--a job at which it is an expert and we are, at best, novices.
The message for farmers and everyone is clear: We must care for our biological allies in the soil. We must help them to support all of humanity and the other organisms that share this planet with us.
Editors Note: Dr. Stuart B. Hill is an associate professor of entomology at McGill University, Montreal, Quebec. He currently is examining relationships between psychological health and the design and management of sustainable agro-ecosystems. This essay first appeared in Seasons, a publication of the Federation of Ontario Naturalists.
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