Meet your hard working helpers in the rizosphere -- the world beneath your feet
By Darrell Smith
Healthy soil often has more life below the surface than above it, says rhizosphere ecologist Jill Clapperton of Agriculture and Agri-Food Canada - a governmental agency similar to USDA. But until now. except for "macro-fauna" -- earth worms and insects -- most soil organisms have been invisible to the naked eye.
Clapperton and her colleagues at the Lethbridge Research Centre, Lethbridge, Alberta, are putting faces and job descriptions on the creatures that convert soil from an anchor for roots into a living system and a partner in producing food. She thinks, someday farmers will manage the animals in their soil as intensively as their above-ground livestock, producing crops with less risk to the environment.
Clapperton and her staff are identifying and photographing the denizens of the "rhizosphere" -- the portion of the soil inhabited by plant roots and soil fauna -- that break down crop residue, convert it to organic matter, release nutrients and help (or occasionally harm) plant roots in various ways. A tool called a scanning electron microscope and a new chemical and genetic techniques for handling samples of roots and soil make the photographs possible.
On this page is some of the micro-organisms whose portraits have only recently been captured on film. The heretofore invisible, and often unknown, organisms fall into three categories. The first consists of bacteria, fungi, protozoa and amoebae. The second includes micro-fauna -- tiny animals such as mites, collembola and nematodes. The third category is macro-fauna, or earth worms and insects.
Fungi and Bacteria
The organisms that work most intimately with plants are vesicular-arbuscular mycorrhizal (VAM) fungi. They form a mutually beneficial relationship with plants, penetrating the root cells without harming them.
"Their greatest role is to increase early establishment of plants, especially under difficult conditions," says Clapperton. "VAM appear to be essential to the establishment, growth and survival of most cereals -- particularly warm season grasses such as corn and sorghum. They also aid establishment and growth in some non-cereal crops, such as sunflower, flax and potatoes."
From the root, the VAM fungi extend a network of hyphae, silk-like threads, through the soil. "VAM fungi are like an effective pipeline moving water and nutrients to the plant," says Clapperton. In return, VAM gain access to carbon-rich compounds that the plant produces by photosynthesis.
You might say VAM enjoy releasing phosphorus so much that if there's no work to do they go away. "VAM are very sensitive to soil levels of available phosphorus," says Clapperton. "Too much phosphorus inhibits colonization.
VAM benefits don't end with minerals. "When roots are colonized by VAM, the plants' rate of photosynthesis increases, water use becomes efficient and they move more and different kinds of carbon compounds to the roots," says Clapperton. "All the organisms associated with the roots and rhizosphere change. There are fewer pathogens and more nitrifiers that convert ammonia to nitrate, which is easy for plants to absorb."
"VAM fungi also increase resistance to root diseases. The hypae and their secretions also tie or glue soil particles into erosion-resistant aggregates."
Besides VAM, there are two other groups of fungi. A second group is pathogenic and includes crop diseases such as seedling blights and root rots. "But bad fungi usually are not competitive with the good ones.," says Clapperton. "So higher populations of good fungi prevent disease."
The final group, saprophytic fungi, help decompose plant material, recycle nutrients by making them available to plants and help improve soil structure.
Fungi and bacteria feed on dead cells sloughed from plant roots, as well as carbon compounds that leak from roots, such as sugars, amino acids and organic acids. They also destroy pollutants such as nitrous oxides and methane (greenhouse gases), and some pesticides.
Bacteria produce antibiotic compounds and hormones that keep roots healthy and help them grow. They cycle nutrients more rapidly than fungi, but over a shorter period of time. They also build aggregates of soil and aid in improving structure. Sometimes bacteria and fungi work together. In legumes, VAM fungi supply the phosphorus required by Rhizobium bacteria to fix nitrogen efficiently.
Cepheidae (top right), Anachipteria (bottom right) and Liochthonius (top left) mites are found in Great Plains soil. They eat bacteria, fungi, nematodes and other soil animals, chew litter into small pieces and distribute bacteria and fungi throughout the soil. Galumna (bottom left), a mite found in pastures, eats plants, litter and other organic matter. The images have been enhanced by computer colorization.
Mycorrhizal fungi (left) live within the roots of most crop plants. Soil fungi (right) help decompose organic matter and form soil aggregates. Most soil fungi are beneficial, rather then pathogenic.
Next in the food chain is micro-fauna -- mites, collembola and nematodes. "They feed on organic matter, fungi, bacteria, and sometimes each other," says Clapperton.
These organisms recycle nutrients caught in bacteria, fungi and protozoa. They break down organic matter by chomping on it, creating more surface area for fungi and bacteria to colonize. They leave nutrients in a more stable form and, in dung pellets, make them more available to plants. They also create tiny channels as they move in the soil," she explains.
Soybean cyst nematodes attack plants, but most nematodes are beneficial. "In greenhouse studies, plants grown in soil with added bacterial- and fungal-feeding nematodes had more shoot growth and a higher yield than in plants grown in soil without the nematodes, " says Clapperton.
Earthworms consume dead plant material, fungi, protozoa and bacteria. Worms' castings are rich in nitrogen, and they bind and stabilize smaller soil particles into larger aggregates, which improves soil fertility and structure. Worm burrows improve air and water infiltration, nitrogen availability and microbial activity, and provide channels for roots to follow.
The role of soil organisms will become more important as farmers seek to reduce inputs for economic and environmental reasons., Clapperton predicts. "If you have a healthy soil, you recycle nutrients such as nitrogen and carbon, rather than losing them to groundwater or the atmosphere," she says.
Before we become efficient soil managers, we still have much to learn about soil organisms and how they interact with various crops. There are some things you can do now to improve soil health.
**No-till. "By improving habitat, no-till increases the population and diversity of soil animals, especially soil mites that feed on fungi," says Clapperton. "With no-till, residue is decomposed by fungi that accumulate nitrogen in their hyphae. In response, the population of fungal feeding mites increases rapidly. They use some of the nitrogen from the fungi and release the rest into the soil. Nitrogen is recycled into the system more regularly over the growing season than with conventional tillage."
** Rotate. "Above-ground diversity increases below-ground diversity," says Clapperton. "Even with corn and soybeans --although its hard to call two crops a rotation-- the corn promotes organisms that are good for the soybeans and vice versa. Include a perennial crop, if possible. Grow a cover crop or intercrop-- anything that adds diversity. Fall rye, mustard, oats, George black medic, hairy vetch, sunflowers, oilseed hemp and sweet clover all increase populations of VAM fungi, as well as inhibiting weed growth."
**Add manure or compost. "Organic matter improves soil structure and provides food," says Clapperton. "Many of the organisms that rapidly colonize organic matter are antagonistic to disease-causing organisms." If you want to increase VAM fungi, she adds, be careful not to build soil phosphorus levels too high.
No-till, rotations and organic matter could turn your soil into a regular zoo -- and that's just what you want.
The Heartland Microbes have over 18 microbial families, which create more than 125 different species.
Call to order your products today and see the difference for yourself.