SOIL MICROORGANISMS

The soil is considered as the land surface of the earth which provides the substratum for plant and animal life. The soil represents a favourable habitat for microorganisms and is inhabited by a wide range of microorganisms, including bacteria, fungi, algae, viruses and protozoa. The physical structure, aeration, water holding capacity and availability of nutrients are determined by the mineral constituents of soil, which are formed by the weathering of rock and the degradative metabolic activities of the soil microorganisms. Cultivated soil has relatively more population of microorganisms than the fallow land, and the soils rich in organic matter contain much more population than sandy and eroded soils. Microbes in the soil are important to us in maintaining soil fertility, cycling of nutrient elements in the biosphere and sources of industrial products such as enzymes, antibiotics, vitamins, hormones, organic acids etc. But certain microbes in the soil are the causal agents of various human and plant diseases.  The plant and animal remains deposited in the soil contribute organic substances. Soil microorganism’s breakdown a variety of organic materials and use a portion of these breakdown products to generate or synthesize a series of compounds that make up humus, a dark coloured amorphous substance composed of residual organic matter not readily decomposed by microorganisms. The three major fractions of humus are humic substances, poly-saccharides and other non-humic substances, and humin. These materials impact on the physical, chemical and bio-chemical properties of soil in many ways. Humus improves the texture and structure of the soil, contributes to its buffering capacity and increase the water holding capacity of the soil.

Soil organisms are creatures that spend all or part of their lives in the soil. There are several criteria that can be used to classify soil organisms. For example, soil organisms are classified according to their size into:  

(1)   macro-organisms ( > 2 mm in width),
(2)   meso-organisms (0.2 to 2 mm in width), and
(3)   micro-organisms (< 0.2 mm in width).  

Based on the ecological functions that soil organisms perform they can be classified as:

(1)   herbivores that subsist on living plants,
(2)   detritivores that subsist on dead plant debris,
(3)   predators that consume animals,
(4)   fungivores that eat fungi,
(5)   bacterivores that eat bacteria,
(6)   parasites that live off, but do not consume, other organisms.  

Another classification of soil organisms groups them into:
(1)   heterotrophs that rely on organic compounds for their C and energy needs, and
(2)   autotrophs that obtain their C mainly from CO₂ and their energy from photosynthesis or oxidation of various elements

Microbial flora of soil

The vast differences in the composition of soils, together with differences in their physical characteristics and the agricultural practices by which they are cultivated, result in corresponding large differences in the microbial population both in total numbers and in kinds. The great diversity of the microbial flora makes it extremely difficult to determine accurately the total number of microorganisms present.

MICROORGANISMS IN SOIL

Microorganisms in soil are important because they affect the structure and fertility of different soils. Soil microorganisms can be classified as bacteria, actinomycetes, fungi, algae, and protozoa. Each of these groups has different characteristics that define the organisms and different functions in the soil.

Bacteria

Bacteria and Archaea are the smallest organisms in soil apart from viruses. Bacteria and Archaea are prokaryotic. All of the other microorganisms are eukaryotic, which means they have a more advanced cell structure with internal organelles and the advanced ability to reproduce sexually. A prokaryote has a very simple cell structure with no internal organelles. Bacteria and archaea are the most abundant microorganisms in the soil, and serve many important purposes, one of those being nitrogen fixation among other biochemical processes.

 Biochemical processes

One of the most distinguished features of bacteria as a whole is their biochemical versatility. A bacterial genera called Pseudomonas can metabolize a wide range of chemicals and fertilizers. In contrast, other genera known as Nitrobacter can only derive its energy by turning nitrite into nitrate, which results in a gain of oxygen and is known also as oxidation. Furthermore, the genera Clostridium is also an example of bacteria’s versatility because it, unlike most species, can actually grow in the absence of oxygen, respiring anaerobically. Several species of Pseudomonas, such as Pseudomonas aeruginosa are able to respire both aerobically and anaerobically, using nitrate, as the terminal electron acceptor.

 

Nitrogen fixation

Bacteria are responsible for the process of nitrogen fixation, which is the conversion of atmospheric nitrogen into nitrogen-containing compounds (like ammonia) which can be used by plants to uptake. Autotrophic bacteria, or bacteria that derives its energy making its own food by oxidation, like the Nitrobacters species, rather than feeding on plants or other organisms. The bacteria that are autotrophic are responsible for nitrogen fixation, and the amount of autotrophic bacteria is small compared to heterotrophic bacteria (the opposite of autotrophic bacteria, heterotrophic bacteria acquires energy by consuming plants or other microorganisms), but are very important because almost every plant and organism require nitrogen in some way, and would have no way of obtaining it if not for nitrogen-fixing bacteria.

Actinomycetes

Actinomycetes are soil microorganisms. They are a type of bacteria. They are similar to both bacteria and fungi, and have characteristics linking them to both groups. Actinomycetes are often believed to be the missing evolutionary link between bacteria and fungi, but they have many more characteristics in common with bacteria than they do fungi. Belong to soil microflora (heterotrophs, aerobic).  They are single-celled, prokaryotic, filamentous and often profoundly branched organisms. They are of great importance in the decomposition of soil organic matter and the mineralization of nutrients, especially in alkaline soils. Many actinomycetes produce antibiotic compounds that kill other microorganisms (e.g. streptomycin is produced by growing soil actinomycetes in pure culture).  

Ability to produce antibiotics

One of the most notable characteristics of the actinomycetes is their ability to produce antibiotics. Streptomycin, neomycin, erythromycin and tetracycline are only a few examples of the antibiotics derived from actinomycetes. Streptomycin is used to treat tuberculosis and infections caused by certain bacteria and neomycin is used to reduce the risk of bacterial infection during surgery. Erythromycin is a very important antibiotic that is used to treat certain infections caused by bacteria, such as bronchitis; pertussis (whooping cough); pneumonia; and ear, intestine, lung, urinary tract, and skin infections. This ability to produce these useful antibiotics is the basis of our entire pharmaceutical industry and has saved human lives.