Overstory #28 - Microlife
Microlife: The unseen community we depend on
Microorganisms permeate the soil, water, and air of our planet. They existed for billions of years before any plants or animals existed, and continue to be the most abundant form of life on Earth. Microorganisms created the atmosphere, turned bare rock and lava into soil, made possible the eventual evolution of larger life-forms, and continue to dominate the lives and functions of all living plants and animals. They are of key importance to every ecological process happening on the planet.
The general public normally only hears about microorganisms in terms of viruses or harmful bacteria, to be battled with soap, disinfectants, and other weapons in our modern arsenal. However, in general the functions of most microorganisms are benign or very much to our benefit, and to destroy them would be to destroy ourselves. The human body, for example, is teeming with microlife--about 10% of the average human's body weight is made up of microorganisms! Each square centimeter human skin hosts an average of 100,000 bacteria, maintaining the health of the skin, and countless millions occupy our intestines. Our vital abilities to breathe and digest food are all intricately linked to the microorganisms that reside within us and make our life possible. As the biologist Lynn Margulis said, "Beneath our superficial differences we are all of us walking communities of bacteria." The web of life depends on this vast network of microorganisms.
For agriculturalists, the greatest interest in microlife is in the complex communities of microorganisms that are part of the soil. One gram (the same weight as a small paperclip) of healthy soil can contain between one and ten billion microorganisms. If the microbes underground were spread over the land surface of Earth, they would make a layer 5 feet thick. Next time you see cows or sheep raised in a pasture, remember that the invisible soil microorganisms on that same piece of land can outweigh the livestock per hectare (2.4 acres) by factors of 10 or 100 times!
For the past century, the trend in university research and in modern farmer's practice has been to focus on the physical and mechanical properties of soil, and only recently has the living component gained recognition for its central role in land productivity and plant health. Communities of bacteria, fungi, algae, protozoa, and other microorganisms aerate the soil, make nutrients available to plants, create water and air channels, maintain soil structure, and recycle nutrients and organic matter that allow vegetation to grow. Every chemical transformation that happens in soil involves microorganisms. Some microorganisms excrete enzymes and other growth substances that stimulate plant feeding. Microorganisms provide a living reserve of nutrients like nitrogen and sulfur that would otherwise be easily leached. A healthy population of soil microorganisms can also maintain ecological balance, preventing the onset of major problems from the viruses or other pathogens that live in the soil.
Introduction to a few soil microorganisms
Thousands of species of microorganisms have been recognized and named, but the number of unknown species is estimated to be in the millions. Almost every time microbiologists search in a soil sample, they discover a previously unknown species. A few that are known to be of benefit to agriculture and forestry are listed below, as examples.
Mycorrhizae- a fungi that associates with the roots of many kinds of plants, acting as a conduit for soil nutrients. Many scientists believe it was tree's association with this fungi that enabled them to colonize large tracts of land, giving rise to the forests. Widely used in forestry (see Overstory # 8).
Rhizobia-- bacteria that live in nodules on the roots of nitrogen-fixing plants, gathering the atmospheric nitrogen for plants like legumes (see Overstory # 4).
actinobacteria--a kind of bacteria whose members break down the difficult-to-digest cellulose of trees on forest floors. Some kinds of actinobacteria, like Frankia, can also fix atmospheric nitrogen and can form nitrogen-fixing nodules with non-leguminous plants like alder trees.
cyanobacteria (blue-green algae)--These tiny entities can photosynthesize, and some 100 strains of cyanobacteria are also able to fix nitrogen from the air. This enables them to colonize and thrive on barren areas, such as immediately following a lava flow--or even a nuclear blast! They are important in the early stages of pioneering harsh conditions. Some forms of cyanobacteria have also been used traditionally in rice agriculture, to fix nitrogen for rice crops
slime molds-- colonies of microorganisms that help decompose and recycle organic matter in forest soils
lichens--symbiotic beings that are combinations of fungi and algae, lichens can break down solid rock, releasing phosphates and nitrates, which generates food for plant's roots.
Other fungi, bacteria, yeasts, algae, protozoans, and many, many more play crucial roles in maintaining the health and integrity of the soil, and the productivity of the land.
Improving productivity of land with microorganisms
Often in dealing with land that has been cleared of natural vegetation and depleted of microlife, modern farmers have tried to improve productivity by adding soluble fertilizers. However, given the increasing recognition of the role of microorganisms, the way the the future may focus on fostering and, when necessary, reintroducing healthy populations of microlife in the soil to enhance productivity. The use of mycorrhizae and rhizobium inoculants has become almost standard in forestry and agriculture. Other ways to increase productivity with microlife are being researched and practiced worldwide. These techniques range from simply bringing in small amounts of healthy soil to inoculate new plantings in degraded areas, to the manufacture and application of special biostimulants to feed and encourage microlife, or to introducing blends of selected beneficial microorganisms to be used in nursery or field production.
Ways to foster microlife: The daily activities of farmers and foresters can help in maintaining and improving healthy populations of soil microorganisms, and therefore improving the productivity of the land. The use of mulch and organic matter is vital for healthy soil life, providing the moist, fertile conditions that allow microlife to thrive.
Other activities that are highly beneficial include:
- Using no-till practices
- Using green manure or cover crops
- Maintaining species diversity of vegetation (encourages diversity of soil organisms) * Reduce or eliminate the use of soluble fertilizers
- Reduce use of fungicides, disinfectants, and other chemicals that kill microlife unnecessarily
- Control erosion
Intimately connected to the subvisible world are the tiny meso and macrofauna like earthworms, centipedes, beetles, termites, snails, and others. Many sustainable growers are enthusiastic about these small animals when they find them in their soils, not only for the services that these larger animals provide, but also because their presence indicates a healthy and thriving population of microlife on a level that we cannot see, but that is essential to the health of our crops.
Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors by L Margulis, D Sagan, L Thomas. UC Press.
Mycorrhizae in Sustainable Agriculture (ASA Special Publication Number 54) by R G Linderman, G J Bethlenfalvay. 1992. American Society of Agronomy.
The Soul of Soil: A Guide to Ecological Soil Management by G Gershuny and J Smillie. July 1996. Chelsea Green Pub.
Secrets of the Soil: New Age Solutions for Restoring Our Planet by P Tompkins and C Bird. Out of Print.
Soil Organic Matter Dynamics and Sustainability of Tropical Agriculture: Proceedings of an International Symposium by K. Mulongoy, R. Merckx (Editor) Although this book is currently out-of-print, you may be able to find it in your library.
Soil Microorganisms and Plant Growth by N.S. Subba, Prof. Rao Hardcover 3rd edition (December 1995) Science Pub.
Related Editions to The Overstory
- The Overstory #70--Rhizosphere
- The Overstory #61--Effects of Trees on Soils
- The Overstory #33--Mushrooms in Agroforestry
- The Overstory #8--Mycorrhizae