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 an organism in another way too — a field also 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.
We classify soil organisms in several different ways. Each organism 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. 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 them14 selves or their waste products. Tertiary (3°) consumer 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 other organisms, as when bacteria that live inside the earthworm’s digestive system help decompose organic matter. Although there are
many examples of such mutually beneficial 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.
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.
SOIL MICROORGANISMS
Microorganisms are very small forms of life that can sometimes live as single cells, although many also form colonies of cells. A microscope is usually needed to see individual cells of these organisms. Many more microorganisms exist in topsoil, where food sources are plentiful, than in subsoil. They are especially abundant immediately next to plant roots, where sloughed off cells and chemicals released by roots provide ready food sources. These organisms are important primary decomposers of organic matter, but they do other things, such as providing nitrogen through fixation to help growing plants. Soil microorganisms have had another direct importance for humans — they are the origin of most of the antibiotic medicines we use to fight various diseases.
Bacteria
Bacteria live in almost any habitat. They are found inside the digestive system of animals, in the ocean and fresh water, in compost piles (even at temperatures over 130°F), and in soils. They are very plentiful in soils; a single teaspoon of topsoil may contain more than 50 million bacteria.
Although some kinds of bacteria live in flooded soils without oxygen, most require wellaerated soils. In general, bacteria tend to do better in neutral soils than in acid soils. In addition to being among the first organisms to begin decomposing residues in the soil,
bacteria benefit plants by increasing nutrient availability. For example, many bacteria dissolve phosphorus, making it more available for plants to use. Bacteria are also very helpful in providing nitrogen to plants. Although nitrogen is needed in large amounts by plants, it is often deficient in agricultural soils. You may wonder how soils can be deficient in nitrogen when we are surrounded
by it — 78 percent of the air we breathe is composed of nitrogen gas. Yet plants as well as animals face the dilemma of the Ancient Mariner, who was adrift at sea without fresh water: “Water, water, everywhere nor any drop to drink.” Unfortunately, neither animals nor plants can use nitrogen gas (N2) for their nutrition. However, some types of bacteria are able to take nitrogen gas from the atmosphere and convert it into a form that plants can use to make amino acids and proteins. This conversion process is known as nitrogen fixation.
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