Introduction to Soil Microbiology

What is soil and how it formed?

The word soil refers to the loose outer material of Earth’s surface, a layer distinct from the bedrock that lies underneath. Soil develops over long periods of time through complex interactions among the parent materials (rock, sand, glacial drift materials, and so on), the topography, climate, and living organisms. Soils can be divided into two broad groups: mineral soils are derived from the weathering of rock and other inorganic materials, and organic soils are derived from sedimentation in bogs and marshes. Most soils are a mixture of these two basic types. Although mineral soils, which are the primary focus, predominate in most terrestrial environments, there is increasing interest in the role that organic soils play in carbon storage.

Soils are composed of at least four components. These include

Inorganic mineral matter, typically 40% or so of the soil volume
Organic matter, usually about 5%;
Air and water, roughly 50%; and
Microorganisms and macro organisms, about 5%.

Particles of various sizes are present in soil. Soil scientists classify soil particles on the basis of size: Those in the range of 0.1–2 mm in diameter are called sand, those between 0.002 and 0.1 mm silt, and those less than 0.002 mm clay. Different textural classes of soil are then given names such as “sandy clay” or “silty clay” based on the percentages of sand, silt, and clay they contain.

A soil in which no one particle size dominates is called a loam. Physical, chemical and biological processes all contribute to the formation of soil. An examination of almost any exposed rock reveals the presence of algae, lichens, or mosses. These organisms are phototrophic and produce organic matter, which supports the growth of chemoorganotrophic bacteria and fungi. More complex chemoorganotrophic communities composed of Bacteria, Archaea, and eukaryotes then develop as the extent of the earlier colonizing organisms increases.

Carbon dioxide produced during respiration becomes dissolved in water to form carbonic acid (H₂CO₃), which slowly dissolves the rock, especially rocks containing limestone (CaCO₃). In addition, many chemoorganotrophs excrete organic acids, which also promote the dissolution of rock into smaller particles. Freezing, thawing, and other physical processes assist in soil formation by forming cracks in the rocks. As the particles generated combine with organic matter, a crude soil forms in these crevices, providing sites needed for pioneering plants to become established. The plant roots penetrate farther into the crevices, further fragmenting the rock; the excretions of the roots promote development in the rhizosphere (the soil that surrounds plant roots and receives plant secretions) of high microbial cell abundance. When the plants die, their remains are added to the soil and become nutrients for more extensive microbial development. Minerals are rendered soluble, and as water percolates, it carries some of these substances deeper into the soil. As weathering proceeds, the soil increases in depth and becomes able to support the development of larger plants and small trees. Soil animals such as earthworms colonize the soil and play an important role in keeping the upper layers of the soil mixed and aerated. Eventually, the movement of materials downward results in the formation of soil layers, called a soil profile. The rate of development of a typical soil profile depends on climatic and other factors, but it can take hundreds to thousands of years.

Soil Microbiology

Soil represents a medium or substrate in which numerous microorganisms live and bring about a great variety of processes which are responsible for continuation of the cycle of life in nature. The numerous living forms which spend all or part of their life in soil ranging from sub microscopic forms to the lower animal forms. With the growing recognition of the numerous processes carried out by the microorganisms in the soil there gradually emerged a branch of microbiology, which came to known as soil microbiology. It is a branch of soil science concerned with soil inhabiting microorganisms and their functions and activities.

Since soil microbiology concerns with soil microorganisms and their processes, it is closely associated with soil biochemistry. Medical bacteriologists were interested in the soil as a medium for the growth and survival of disease producing organisms. Agricultural chemists are also interested in the soil processes that result from the activities of microorganisms. General bacteriologist, zoologist, botanist were interested in certain special group of organisms found in soil. Recently, soil microbiology has expanded to include the study of the role of soil microorganisms in genetic engineering, in the biological control of pests and diseases, the degradation of pollutants, production and destruction of radioactive gases and its transfer. Thus microbial participation in several important processes emphasizes that soil microbiology has become a global science.

Soil microbiology: Deals with the microscopic organisms of the soil; Their population and activities; Role of soil microbes in various transformations taking place in the soil and Their importance in plant nutrition and crop production

Distinct phases of soil microbiology

Ecological phase: It concerns with the study of the quantitative and qualitative composition of the microscopic and ultramicroscopic soil population.

Experimental or physiological phase: Study of the physiology and biochemistry of the organisms, their role in the cycle of life in nature and their utilization for the formation of valuable metabolic products.

Agronomical phase: Application of microbiological activities to soil fertility and crop production.

Pedological phase: Studies the Importance of microorganisms in soil formation and soil structure.

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