Biological Properties of Soil
Soil Microorganisms
Microorganisms are invisible to the naked eye. However, their effect on numerous soil properties is far-ranging. Microorganisms represent the largest and most diverse biotic group in soil, with an estimated one million to one billion microorganisms per one gram of agricultural top soil. Although soil organisms are involved in many different types of activities, their main role is breaking down fresh animal, crop and pasture residues, using the carbon and nutrients for food and growth. In the process they produce new compounds, which can be used by a large variety of organisms, or they incorporate some of the carbon and nutrients that were in the organic matter into their own microbial biomass. By breaking down carbon structures, soil microbes play a significant role in nutrient cycling processes. Soil organisms have profound effects on promoting good soil structure. As organic materials decompose, nutrients become available to plants, humus is produced, soil aggregates are formed. Most microorganisms exist in topsoil, where food sources are more plentiful, than in subsoil. They are especially abundant in the area immediately next to plant roots (called the rhizosphere), where sloughed-off cells and chemicals released by living roots provide ready food sources and other organic matter deposits.
Bacteria
Bacteria are simple, single-celled organisms that lack a nucleus; they are the most abundant inhabitants of the soil and the most numerous of the microflora (Figure 6.2). Scientists believe that there are hundreds of millions of species, and most of them are not yet discovered. After fungi, they are the second-greatest decomposer in the soil community. Bacteria do not move very far in the soil, so most movement is associated with water, growing roots, or hitching a ride with other soil fauna like earthworms, ants, spiders, etc. Bacteria usually grow as small colonies on the surface of soil particles and in smaller pores.
Soil Benefits from Bacteria
Bacteria perform many important ecosystem services in the soil including improved soil structure and soil aggregation, and recycling of soil nutrients. Many bacteria produce a layer of polysaccharides or glycoproteins that coats the surface of soil particles. These substances play an important role in cementing sand, silt, and clay soil particles into stable microaggregates that improve soil structure. These microaggregates are further grouped and glued together by the glomalin produced by mycorrhizal fungi.
Nitrogen-Fixing Bacteria
Of the mutualistic bacteria, there are four bacteria types that convert atmospheric nitrogen (N2) into nitrogen for plants. The most well-known of these are the nitrogen-fixing Rhizobium bacteria (i.e., mutualists) that form symbiotic associations with the roots of legumes (alfalfa, soybeans) or clovers (red, sweet, white, crimson). Visible nodules are created where bacteria infect a growing root hair (Figure 6.3). The plant supplies the carbon to the Rhizobium in the form of simple sugars and the bacteria convert atmospheric nitrogen (N2) into a nitrogen form the plant can use.
Fungi
Fungi are another group of soil organisms that are multi-celled and more complex than bacteria. Due to their ability to produce a wide variety of extracellular enzymes, they are able to break down all kinds of organic matter, decomposing soil components and thereby regulating the balance of carbon and nutrients. Fungi are not as numerous as bacteria, but because of their larger size they may account for the greatest total mass of the soil microbes.
Mycorrhizae
Mycorrhizae (my-cor-ry´-zee) are fungi that live either on or in plant roots and act to extend the reach of root hairs into the soil (Figure 6.4). Mycorrhiza is a symbiotic association between a fungus and a root of a vascular plant. It is a mutualistic association which provides the fungus with carbohydrates such as glucose and sucrose synthesized by the plant, and the plant, in return, benefits of the huge mycelial network that adsorbs water and nutrients from a larger volume of soil. Mycorrhizal mycelia are much finer in diameter than the smallest root and can explore a large volume of soil for absorption of water and nutrients. This extension of the plant root system increases its efficiency, providing perhaps ten times as much absorptive surface as the root system of an uninfected plant.
Actinomycetes
Actinomycetes (ac-tin-o-my´-cetes) resemble fungi in that they form a network of threads. They have been classified as fungi and bacteria at various times, but are now classified in a category of their own. The actinomycetes resemble bacteria in that they have a very simple cell structure and are about the same size in cross section. They resemble filamentous fungi in that they produce a branched filamentous network. The network compared to fungi, however, is usually less extensive. Like the bacteria, they help decompose organic matter into humus, releasing nutrients.
Protozoa
Protozoa are free-living microorganisms that crawl or swim in the water between soil particles. Most are considerably larger than bacteria. Most soil-inhabiting protozoa are mainly secondary consumers of organic materials, feeding on bacteria, fungi, other protozoa and organic molecules dissolved in the soil water.
Algae
Many different species of algae live in the upper half-inch of the soil. Unlike most other soil organisms, algae produce their own food through photosynthesis. Most grow best under moist to wet conditions, but some are also very important in hot or cold desert environments.
Conditions Affecting Soil Microorganisms
Soil microorganisms are controlled by many factors with soil organic matter, primarily nitrogen content, having a major influence on soil organism activity.
Organic Matter Content
Organic matter decomposition serves two functions for microorganisms, providing energy for growth and suppling carbon for the formation of new cells. Microbes need regular supplies of active soil organic matter in the soil to survive in the soil. Soil organic matter is basically all the organic substances (anything with carbon) in the soil, both living and dead.
Soil Temperature and Moisture
Microbial activity is generally greatest when temperatures are 68 to 104 degrees F (20–40°C). The warmer end of this range tends to favor actinomycetes. Soil temperature extremes seldom kill bacteria and commonly only temporarily suppress their activity. Changes related to soil water are particularly important. Soil microorganisms live in water films surrounding soil particles. Different types of microorganisms prefer different moisture conditions for growth.
Soil pH
As mentioned earlier, soil microbes are influenced by pH. This is especially true of bacteria. Under acid conditions (below pH 6), bacterial activity is greatly reduced. Soil fungi responsible for breakdown of soil organic matter are generally less affected by low pH.
Management Practices
Tillage enhances the growth of soil microorganisms in the short-term by aerating the soil and by breaking apart soil aggregates to expose organic matter that had been protected from microbial decay. Their activity increases the loss of carbon respired as carbon dioxide, and triggers population explosions of bacterial predators such as protozoa. However, over the long term with repeated tillage, these populations are likely to decline because of the lack of surface residue rather than because of the mechanical action of tillage.
Click on the following topics for more information on biological properties of soils.
