The plow is one of the most ancient and most valuable of man’s inventions; but long before he existed the land was in fact regularly ploughed, and continues to be thus ploughed by earthworms.

—CHARLES DARWIN, 1881

When soil organisms and roots go about their normal functions of getting energy for growth from organic molecules, they “respire”—using oxygen and releasing carbon dioxide to the atmosphere. (Of course, as we take our essential breaths of air, we do the same.) An entire field can be viewed as breathing as if it is one large organism. The soil is like a living being in another way, too—it may get “sick” in the sense that it becomes incapable of supporting healthy plants.

The organisms living in the soil, both large and small, play a significant role in maintaining a healthy soil system and healthy plants. One of the main reasons we are interested in these organisms is because of their role in breaking down organic residues and incorporating them into the soil. Soil organisms influence every aspect of decomposition and nutrient availability. As organic materials are decomposed, nutrients become available to plants, humus is produced, soil aggregates are formed, channels are created for water infiltration and better aeration, and those residues originally on the surface are brought deeper into the soil.

soil organisms and their role in decomposing residues. Modified from D.I. Dindal (1972).

We classify soil organisms in several different ways. Each can be discussed separately or all organisms that do the same types of things can be discussed as a group. We also can look at soil organisms according to their role in the decomposition of organic materials. For example, organisms that use fresh residues as their source of food are called primary (1°), or first level, consumers of organic materials (see figure 4.1). Many of these primary consumers break down large pieces of residues into smaller fragments. Secondary (2°) consumers are organisms that feed on the primary consumers themselves or their waste products. Tertiary (3°) consumers then feed on the secondary consumers. Another way to treat organisms is by general size, such as very small, small, medium, large, and very large. This is how we will discuss soil organisms in this chapter.

There is constant interaction among the organisms living in the soil. Some organisms help others, as when bacteria that live inside the earthworm’s digestive system help decompose organic matter. Although there are many examples of such mutually beneficial, or symbiotic, relationships, an intense competition occurs among most of the diverse organisms in healthy soils. Organisms may directly compete with each other for the same food. Some organisms naturally feed on others— nematodes may feed on fungi, bacteria, or other nematodes, and some fungi trap and kill nematodes. There are also fungi and bacteria that parasitize nematodes and completely digest their content.

Some soil organisms can harm plants, either by causing disease or by being parasites. In other words, there are “good” as well as “bad” bacteria, fungi, nematodes, and insects. One of the goals of agricultural production systems should be to create conditions that enhance the growth of beneficial organisms, which are the vast majority, while decreasing populations of those few that are potentially harmful.