10.1: 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!

Organic farmers want many beneficial soil micro-organisms to thrive in the habitat. These micro-organisms are 'soil livestock'. They may be the most important livestock on the farm. Farmers should do as much as they can to help them multiply and increase. They, in turn, grow and recycle soil nutrients that feed plant crops. Legumes, plants that have a beneficial relationship with soil bacteria, are another important type of livestock. Legumes form nodules, a habitat for the bacteria, that pluck free nitrogen from the air and then fix it into their bacterial and plant bodies – free nitrogen! It doesn’t get any better than that.

Figure \(\PageIndex{1}\): Nitrogen-fixing bacteria nodules on a legume plant. USDA-NRCS.

Figure \(\PageIndex{2}\): Fungal hyphae in soil. USDA-NRCS.

Soil worms, fungi, bacteria, and other microbes, much like the soil structure itself, need air space water space, and open space. Soil that is alive with good microbes must not be compacted nor sterilized by salt-based fertilizers, herbicides, pesticides, and heavy tractor traffic. Did you know that 90% of vascular plants typically have a mycchorhizal fungal association that works with roots to reach further and deliver nutrients? – these fungi are killed by plowing, compaction, and herbicides. Organic farmers want a lot of beneficial soil life so that the food we grow uses nutrients produced by natural, biological cycles rather than by synthetic means. We want good organisms to overwhelm and out-compete the pathogens so that we don’t need to spray and kill all the soil life. We are not afraid of microbes, we embrace them as mini-colleagues that do their part in the system. We trust the way biology has worked for millennia to grow the gifts of good food from rich soil of which we are stewards at this time.

Some of the farming practices used to enhance soil health include; managed animal grazing, composting, crop rotation, planting perennial legumes, and cover crops, and plowing a field only every fifth year. It is very important (if at all possible) to have animals on any crop farm because of the manure they produce. The livestock adds manures and beneficial bacteria back into the farm crop system. Composting adds organic matter, that jungle gym habitat, back into the soil. Rotating crops keeps non-beneficials from gaining a foothold in the system. Using legumes adds nitrogen to the soil and perennials keep living roots in the soil year-round. Roots exude what Soil Microbiologist Dr. Elaine Ingham describes as “cakes and cookies” into the soil for microbes to feed on. The perennials and cover crops leaf out very early in spring and have leaves later into the fall and this keeps the soil covered with “many little tents” which prevents raindrop impacts from dislodging soil particles which may then flush them away with the rain.

Figure \(\PageIndex{3}\): Water droplet.

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 into the atmosphere. They do not have lungs of course, we are describing cellular respiration. 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 the 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 \(\PageIndex{4}\)). 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, such 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, 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 \(\PageIndex{4}\)), usually called a food web (compared to a food chain, which involves only one direction).

Figure \(\PageIndex{4}\): 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.