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College of Food, Agricultural, and Environmental Sciences


A Peek into the World Beneath Our Feet

May 23, 2007

WOOSTER, Ohio -- There's more to soil than just dirt.

Within the top six to eight inches of the Earth's crust lies a world of chemical processes and very small forms of life whose function and purpose are so closely intertwined that they form the support for plant establishment, growth and stability. Any imbalance to this community impacts soil health and, consequently, plant health.

Brian McSpadden Gardener, an Ohio State University plant pathologist with the Ohio Agricultural Research and Development Center, said that gaining a better understanding of how the "living" soil works will aid in improving plant health and productivity.

"Most growers are aware that soil microbes decompose crop residue," said Gardener. "But it's also important to remember that, as a whole, soil organisms are the living catalysts that change and move all of the nutrients in the soil."

Gardener said that maintaining a healthy soil community can be achieved largely by managing the amount and types of carbon inputs. Soil carbon comes from plant residue and inputs such as manures or composts.

"Plants fix atmospheric carbon and put it into solid forms that can be ingested by other life forms. Providing a food source for soil organisms can encourage a greater abundance and diversity of soil life which typically increases the availability of other nutrients and translates into more vigorous crops," said Gardener. "It's impressive to think that by simply managing soil carbon, you can have more soil life and healthier plants."

Soil carbon is the catalyst for a healthy soil community. From there, bacteria and fungi consume the soil carbon, recycling it and other nutrients directly from plant residues. Secondary consumers then feed on bacteria, fungi, and, to some extent, on each other, releasing waste products that are the chemical forms most readily assimilated by plants.

The following is a who's who of organisms that rely on that carbon and are an important part of that soil "circle of life:"

• Bacteria -- Bacteria are the tiniest and most diverse of all soil organisms. A single teaspoon of topsoil typically contains more than 100 million bacteria that belong to over 1,000 different species. Bacteria help to decompose residue in the soil and increase nutrient availability for plants by dissolving phosphorus and fixing atmospheric nitrogen. Some bacteria that live on plant surfaces can also prevent plant diseases, either by antagonizing pathogens or stimulating the plant's own immune systems.

• Fungi -- Fungi are important in the break down of plant residue, and they can transport nutrients through the soil profile. Because they are tougher than bacteria, they tend to release nutrients more slowly so that plants can have access to them throughout the growing season. Some fungi, called mycorrhizae, develop beneficial relationships with plants that allow for nutrient and water absorption by plant roots. Fungal cells also help to stabilize soil structure by secreting a sticky gel that glues mineral and organic particles together into aggregates. These aggregates allow for natural breaks to occur in soil, allowing for greater aeration and water infiltration.

• Protozoa -- Protozoa are single-celled animals that act as secondary consumers of organic matter. They feed on bacteria, fungi, other protozoa, and organic molecules. Protozoa are believed to be responsible for mineralizing a small fraction of the nitrogen in soils.

• Nematodes -- Nematodes are simple worms that are less than one-tenth of an inch long. They help breakdown organic residues and feed on bacteria, fungi and protozoa. They also convert some nitrogen into usable forms for plants.

• Free-living mites -- Soil mites are arthropods that graze on decomposing organic matter, fungi, algae and nematodes. Mite populations are slow to develop, so their appearance indicates a highly stable soil environment.

• Springtails -- Springtails are arthropods found in decaying material. They are one of several biological agents responsible for the creation of soil, and are considered to be the most abundant of all macroscopic animals living in the topsoil.

• Earthworms -- Earthworms round out the list as important keepers and restorers of soil fertility. While less numerous than nematodes, they account for up to 10 times the biomass of the other secondary consumers. Earthworms feed on bacteria-laden plant residues and organic matter mixed with mineral particles. The resulting material is given off as worm casts, which are generally higher in available nitrogen, calcium, magnesium and phosphorus than the soil itself. Earthworms also constantly bring soil up to the surface, providing anywhere from one to 100 tons per acre of organic-laden soil for plant use each year.

"There are countless other organisms that contribute to the soil community. The variety of soil organisms is so high that even today we don't have the tools to say what an average community really looks like from a small scale to a large scale," said Gardener. "But it can be said that a healthy soil is often marked by multiple trophic levels of diverse microflora, microfauna and mesofauna."

So what are the characteristics of a healthy soil? Good tith, good aeration, high organic matter content, and balanced soil fertility all qualify as identifying markers -- all of which are made possible through the contributions of the creatures both great and small that call the soil their home.

Candace Pollock
Brian McSpadden Gardener