4.1: Introduction

… long before [humans] existed the land was in fact regularly ploughed, and continues to be thus ploughed by earthworms.

—Charles Darwin, 1881

Soils are alive and the organisms living in them, both large and small, play a critical role in maintaining a healthy soil system and healthy plants. A handful of soil contains billions of bacteria and fungi, plus other organisms, and soils are a major reservoir of life on Earth. Living organisms in the top 6 inches of an acre of soil with 3% organic matter will weigh about 1.5 tons, the equivalent weight of two Holstein milk cows.

When soil organisms go about their normal functions of getting energy for growth from organic molecules, they “respire,” just as plant roots do, by 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, with oxygen diffusing into the soil and carbon dioxide diffusing out into the atmosphere. The soil is like a living being in another way, too; it may get “sick” in the sense that it has difficulty supporting healthy plants.

Although soil organisms are involved in many different types of activities with a variety of outcomes, one of the reasons for our interest in these organisms is their role in breaking down organic residues and incorporating them into the soil. Soil organisms influence every aspect of decomposition and nutrient availability, and they have profound effects on promoting good structure. As organic materials decompose, 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. And while we are interested in maintaining good amounts of organic matter in soil, we also want to maintain active populations of diverse organisms.

We can discuss soil organisms in several different ways. Each can be considered separately or all organisms that do the same types of things can be discussed as a group. We can also look at soil organisms according to their role in the decomposition of organic materials. For example, organisms that use residues as their source of food are called primary (1°), or first-level, consumers of organic materials (see Figure 4.1). Many of these 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. The many types of soil organisms participate in a complex multi-path food system (Figure 4.1), usually called a food web (compared to a food chain, which involves only one direction).

Figure 4.1. The soil food web. Modified from D.L. Dindal (1972). Illustration by Vic Kulihin.

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.