The Living Soil

The Living Soil

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 incorporat-ing 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 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 materi-als. 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 organ-isms living in the soil. Some organisms help others, as when bacteria that live inside the earthworm’s digestive

Soil organisms and their role in decomposing residues. Modified from D.L. Dindal (1972).

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 nema-todes, 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 caus-ing 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 microor-ganisms exist in topsoil, where food sources are plenti-ful, than in subsoil. They are especially abundant in the area immediately next to plant roots (called the rhizo-sphere), where sloughed-off cells and chemicals released

by roots provide ready food sources. These organisms are primary decomposers of organic matter, but they do other things, such as provide nitrogen through fixation to help growing plants, detoxify harmful chemicals (tox-ins), suppress disease organisms, and produce products that might stimulate plant growth. Soil microorganisms have had another direct importance for humans—they are the source of most of the antibiotic medicines we use to fight 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. Although some kinds of bacteria live in flooded soils without oxygen, most require well-aerated soils. In general, bacteria tend to do better in neutral pH 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, which they need in large amounts but is often deficient in agricultural soils. You may wonder how soils

can be deficient in nitrogen when we are surrounded by it—78% of the air we breathe is composed of nitro-gen gas. Yet plants as well as animals face a dilemma similar to that 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 nitro-gen 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. Some nitrogen-fixing bacteria form mutually benefi-

cial associations with plants. One such symbiotic rela-tionship that is very important to agriculture involves the nitrogen-fixing rhizobia group of bacteria that live inside nodules formed on the roots of legumes. These bacteria provide nitrogen in a form that leguminous plants can use, while the legume provides the bacteria with sugars for energy.

People eat some legumes or their products, such as peas, dry beans, and tofu made from soybeans. Soybeans, alfalfa, and clover are used for animal feed. Clovers and hairy vetch are grown as cover crops to enrich the soil with organic matter, as well as nitrogen, for the following crop. In an alfalfa field, the bacteria may fix hundreds of pounds of nitrogen per acre each

year. With peas, the amount of nitrogen fixed is much lower, around 30 to 50 pounds per acre.

The actinomycetes, another group of bacteria, break large lignin molecules into smaller sizes. Lignin is a large and complex molecule found in plant tissue, espe-cially stems, that is difficult for most organisms to break down. Lignin also frequently protects other molecules like cellulose from decomposition. Actinomycetes have some characteristics similar to those of fungi, but they are sometimes grouped by themselves and given equal billing with bacteria and fungi.

Root heavily infected with mycorrhizal fungi (note round spores at the end of some hyphae). Photo by Sara Wright.

Fungi

Fungi are another type of soil microorganism. Yeast is a fungus used in baking and in the production of alcohol. Other fungi produce a number of antibiotics. We have all probably let a loaf of bread sit around too long only to find fungus growing on it. We have seen or eaten mush-rooms, the fruiting structures of some fungi. Farmers know that fungi cause many plant diseases, such as downy mildew, damping-off, various types of root rot,

and apple scab. Fungi also initiate the decomposition of fresh organic residues. They help get things going by softening organic debris and making it easier for other organisms to join in the decomposition process. Fungi are also the main decomposers of lignin and are less sensitive to acid soil conditions than bacteria. None are able to function without oxygen. Low soil disturbance resulting from reduced tillage systems tends to promote organic residue accumulation at and near the surface. This tends to promote fungal growth, as happens in many natural undisturbed ecosystems.

Many plants develop a beneficial relationship with fungi that increases the contact of roots with the soil. Fungi infect the roots and send out rootlike structures called hyphae (see figure 4.2). The hyphae of these mycorrhizal fungi take up water and nutrients that can then feed the plant. The hyphae are very thin, about 1/60 the diameter of a plant root, and are able to exploit the water and nutrients in small spaces in the soil that might be inaccessible to roots. This is espe-cially important for phosphorus nutrition of plants in low-phosphorus soils. The hyphae help the plant absorb water and nutrients, and in return the fungi receive energy in the form of sugars, which the plant produces in its leaves and sends down to the roots. This symbiotic

interdependency between fungi and roots is called a mycorrhizal relationship. All things considered, it’s a pretty good deal for both the plant and the fungus. The hyphae of these fungi help develop and stabilize larger soil aggregates by secreting a sticky gel that glues min-eral and organic particles together.

Algae

Algae, like crop plants, convert sunlight into complex molecules like sugars, which they can use for energy and to help build other molecules they need. Algae are found in abundance in the flooded soils of swamps and rice paddies, and they can be found on the surface of poorly drained soils and in wet depressions. Algae may also occur in relatively dry soils, and they form mutually ben-eficial relationships with other organisms. Lichens found on rocks are an association between a fungus and an alga.

on the root, sometimes cause greater disease severity and more damage than the nematode itself. A number of plant-parasitic nematodes vector important and damaging plant viruses of various crops. However, there are many beneficial nematodes that help in the break-down of organic residues and feed on fungi, bacteria, and protozoa as secondary consumers. In fact, as with the protozoa, nematodes feeding on fungi and bacteria help convert nitrogen into forms for plants to use. As much as 50% or more of mineralized nitrogen comes from nematode feeding. A number of nematodes alone or with special bacteria parasitize and kill insects such as the larvae of the cabbage looper and the grubs of the Japanese beetle. Finally, several nematodes infect ani-mals and humans, causing serious diseases such as river blindness and heartworm.

Protozoa

Protozoa are single-celled animals that use a variety of means to move about in the soil. Like bacteria and many fungi, they can be seen only with the help of a microscope. They are mainly secondary consumers of organic materials, feeding on bacteria, fungi, other protozoa, and organic molecules dissolved in the soil water. Protozoa—through their grazing on nitrogen-rich organisms and excreting wastes—are believed to be responsible for mineralizing (releasing nutrients from organic molecules) much of the nitrogen in agricultural soils.

SMALL AND MEDIUM-SIZE SOIL ANIMALS Nematodes

Nematodes are simple multicellular soil animals that resemble tiny worms but are nonsegmented. They tend to live in the water films around soil aggregates. Some types of nematodes feed on plant roots and are well-known plant pests. Fungi such as Pythium and Fusarium, which may enter nematode-feeding wounds

Earthworms

Earthworms are every bit as important as Charles Darwin believed they were more than a century ago. They are keepers and restorers of soil fertility. Different types of earthworms, including the night crawler, field (garden) worm, and manure (red) worm, have different feeding habits. Some feed on plant residues that remain on the soil surface, while other types tend to feed on organic matter that is already mixed with the soil.

The surface-feeding night crawlers fragment and mix fresh residues with soil mineral particles, bacteria, and enzymes in their digestive system. The resulting mate-rial is given off as worm casts. Worm casts are generally higher in available plant nutrients, such as nitrogen, calcium, magnesium, and phosphorus, than the sur-rounding soil and, therefore, contribute to the nutrient needs of plants. They also bring food down into their burrows, thereby mixing organic matter deep into the soil. Earthworms feeding on debris that is already below the surface continue to decompose organic materials and mix them with the soil minerals.

A number of types of earthworms, including the

surface-feeding night crawler, make burrows that allow rainfall to easily infiltrate the soil. These worms usually burrow to 3 feet or more, unless the soil is saturated or very hard. Even those types of worms that don’t normally produce channels to the surface help loosen the soil, creating channels and cracks below the surface that help aeration and root growth. The number of earthworms in the soil ranges from close to zero to over a million per acre. Just imagine, if you create the proper conditions for earthworms, you could have 800,000 small channels per acre that conduct water into your soil during downpours.

Earthworms do some unbelievable work. They move a lot of soil from below up to the surface—from about 1 to 100 tons per acre each year. One acre of soil 6 inches deep weighs about 2 million pounds, or 1,000 tons. So 1 to 100 tons is the equivalent of about .006 of an inch to about half an inch of soil. A healthy earthworm popula-tion may function as nature’s plow and help replace the need for tillage by making channels and bringing up subsoil and mixing it with organic residues.

Earthworms do best in well-aerated soils that are supplied with plentiful amounts of organic matter. A study in Georgia showed that soils with higher amounts of organic matter contained higher numbers of earth-worms. Surface feeders, a type we would especially like to encourage, need residues left on the surface. They are harmed by plowing or disking, which disturbs their burrows and buries their food supplies. Worms are usu-ally more plentiful under no-till practices than under conventional tillage systems. Although many pesticides have little effect on worms, some insecticides are very harmful to earthworms.

Diseases or insects that overwinter on leaves of crops can sometimes be partially controlled by high earthworm populations. The apple scab fungus—a major pest of apples in humid regions—and some leaf miner insects can be partly controlled when worms eat the leaves and incorporate the residues deeper into the soil.

Although the night crawler is certainly beneficial in farm fields, this European introduction has caused problems in some northern forests. As fishermen have discarded unused worms near forest lakes, night crawlers have become adapted to the forests. They have in some cases reduced the forest litter layer almost completely, accelerating nutrient cycling and changing species com-position of the understory vegetation. So some forest managers view this organism, considered so positively by farmers, as a pest!

Insects and Other Small to Large Soil Animals

Insects are another group of animals that inhabit soils. Common types of soil insects include termites, spring-tails, ants, fly larvae, and beetles. Many insects are secondary and tertiary consumers. Springtails feed on fungi and animal remains, and in turn they themselves are food for predacious mites. Many beetles, in particu-lar, eat other types of soil animals. Some beetles feed on weed seeds in the soil. Termites, well-known feeders of woody material, also consume decomposed organic residues in the soil.

Other medium-size to large soil animals include millipedes, centipedes, mites, slugs, snails, and spiders. Millipedes are primary consumers of plant residues, whereas centipedes tend to feed on other organisms. Mites may feed on food sources like fungi, other mites, and insect eggs, although some feed directly on residues. Spiders feed mainly on insects and keep insect pests from developing into large populations.

VERY LARGE SOIL ANIMALS

Very large soil animals, such as moles, rabbits, wood-chucks, snakes, prairie dogs, and badgers, burrow in the soil and spend at least some of their lives below ground. Moles are secondary consumers, their diet consisting mainly of earthworms. Most of the other animals exist on vegetation. In many cases, their presence is consid-ered a nuisance for agricultural production or lawns and

gardens. Nevertheless, their burrows may help conduct water away from the surface during downpours and thus decrease erosion. In the southern U.S., the burrowing action of crawfish, abundant in many somewhat poorly drained soils, can have a large effect on soil structure. (In Texas and Louisiana, some rice fields are “rotated” with crawfish production

PLANT ROOTS

Healthy plant roots are essential for good crop yields. Roots are clearly influenced by the soil in which they live and are good indicators of soil quality. If the soil is compact, is low in nutrients or water, includes high populations of root pathogens, or has other problems, plants will not grow well. On the other hand, plants also influence the soil in which they grow. The physical pres-sure of roots growing through soil helps form aggregates by bringing particles closer together. Small roots also help bind particles together. In addition, many organic compounds are given off, or exuded, by plant roots and provide nourishment for soil organisms living on or near the roots. The zone surrounding roots is one of espe-cially great numbers and activity of organisms that live off root exudates and sloughed-off cells. This increased activity by microorganisms, plus the slight disruption caused as roots grow through the soil, enhances the use of active (“dead”) organic matter by organisms—also enhancing nutrient availability to the plant. A sticky layer surrounding roots, called the mucigel, provides close contact between microorganisms, soil minerals, and the plant (figure 4.3). Plant roots also contribute

The soil population must be considered from the point of view of a biological complex; it is not sufficient to separate it into different constituent groups.

greatly to organic matter accumulation. They are usu-ally well distributed in the soil and may be slower to decompose than surface residues, even if incorporated by plowing or harrowing.

For plants with extensive root systems, such as grasses, the amount of living tissue below ground may actually weigh more than the amount of leaves and stems we see above ground.

BIOLOGICAL DIVERSITY, ABUNDANCE, AND BALANCE

A diverse biological community in soils is essential to maintaining a healthy environment for plant roots. There may be over 100,000 different types of organisms living in soils. Most are providing numerous functions that assist plants, such as making nutrients more available, producing growth-stimulating chemicals, and helping form soil aggregates. In a teaspoon of agricultural soils it

is estimated that there are from 100 million to 1 billion bacteria, several yards of fungi, and several thousand pro-tozoa. It may hold 10 to 20 bacterial-feeding nematodes and a few fungal-feeding and plant parasitic nematodes. Arthropods can number up to 100 per square foot, and earthworms from 5 to 30 per square foot.

Of all the organisms in soils, only a small number of bacteria, fungi, insects, and nematodes might harm plants in any given year. Diverse populations of soil organisms maintain a system of checks and balances that can keep disease organisms or parasites from becoming major plant problems. Some fungi kill nematodes, and others kill insects. Still others produce antibiotics that kill bacte-ria. Protozoa feed on bacteria and may attack fungi. Some bacteria kill harmful insects. Many protozoa, springtails, and mites feed on disease-causing fungi and bacteria.

Beneficial organisms, such as the fungus Trichoderma and the bacteria Pseudomonas fluorescens, colonize plant roots and protect them from attack by harmful organ-isms. Some of these organisms, isolated from soils, are now sold commercially as biological control agents. The

effects of bacteria and fungi that suppress plant disease organisms are thought to arise from competition for nutrients, production of antagonistic substances, and/ or direct parasitism. In addition, a number of beneficial soil organisms induce the immune systems of plants to defend the plants (systemic acquired resistance; see discussion in chapter 8). Also, roots of agronomic crops usually have their own characteristic microbial communi-ties with numerous interactions.

Soil management can have dramatic effects on soil biological composition (see figure 4.4 for manage-

ment effects on organisms). For example, the less a soil is disturbed by tillage, the greater the importance of fungi relative to bacteria. Thus, promotion of cropping practices that encourage abundance and diversity of soil organisms encourages a healthy soil. Crop rotations of plants from different families are recommended to keep microbial diversity at its maximum and to break up any potential damaging pest cycles. Additional practices that promote the diversity and activity of soil organisms include low amounts of soil disturbance, use of cover crops, maintaining pH close to neutral, and routine use of organic sources of slow-release fertility.

SUMMARY

Soils are alive with a fantastic number of many types of organisms, most of which help to grow healthy plants and protect them from pests. The food for all the soil’s organisms originates with crop residues and organic materials added from off the field. These provide the fuel that powers the underground life that has such a positive effect on the soil’s chemical and physical properties, as well as, of course, maintaining a system of equilibrium that helps regulate the populations of organisms. Soil