• Functional soil is a soil embedded with organic matter and soil microbes that work together to hold onto nutrients in the soil and convert nutrients locked in the soil.
  • Soil is an excellent culture media for the growth and development of various microorganisms.
  • Living organisms present in the soil are grouped into two categories as Soil and Soil fauna.
  • Soil is dynamic habitats or living system that provides shelters for many animal types, such as worms and insects (invertebrates) mammals like rabbits, moles, fox and badgers. It also provides habitats colonised by a staggering variety of microorganisms. All these forms of life interact with one another along with soil to create continually changing conditions. This allows an on-going evolution of soil habitats.
  • A normal soil is made up of solid, liquid, and gaseous constituents. These can be broadly divided into five groups: 1. Mineral Particles. 2. Plant and Animal Residues. 3. Living Systems. 4. Water. 5. Gases.

Distribution of Microorganisms in soils

  • Soil microorganisms can be classified as bacteria, actinomycetes, fungi, algae and protozoa.
  • Rhizosphere is the zone of plat roots. It is a very dynamic environment where plants, soil, micro-organisms, nutrients and water meet and interact.
  • Microorganisms constitute < 0.5% (w/w) of the soil mass, yet they have a major impact on soil properties and processes.
  • About 60-80 % of the total soil metabolism is due to the microflora. These are the smallest organisms (<0.1 mm in dm.) and are extremely abundant and diverse.
  • They include algae, bacteria, cyanobacteria, fungi, yeasts, mycomycetes and actinomycetes. Most of them are able to decompose almost any existing natural material.
  • Soil organisms represent a large fraction of global terrestrial biodiversity. They can be grouped on the basis of size, species and function.
  • One gram of topsoil may contain: as many as one million bacteria, up to 100 million actinomycetes, one million fungi and 100 nematodes.

Importance of Soil Organisms

  • Micro-organisms transform organic matter into plant nutrients that are assimilated by plants.
  • Responsible for cycling of C, N and other nutrients.
  • Enhance soil structure.
  • Relocate and decompose organic materials.
  • Maintain soil quality and health.
  • Increase soil aeration and penetrability.
  • Soil microbes break down organic matter.
  • Soil microbes create humus.
  • Soil microbes fix nitrogen.
  • Soil organisms promote plant growth.
  • Soil microbes control pests and diseases.

Soil environment & Soil life

Soil microorganisms are influenced by biotic and abiotic factors, such as plant cover or edaphic Parameters. pH strongly influences soil bacterial communities.

Soil life is classified into 3 groups:

  1. Macrofauna

These are visible to the naked eye (>2mm in diameter). Macrofauna  includes  vertebrates (snakes, lizards, mice, rabbits, moles, etc) that  primarily  dig  within  the  soil  for  food  or  shelter, and invertebrates (snails, earthworms, ants, termites, millipedes, centipedes, caterpillars, beetle larvae and adults, fly and wasp larvae, spiders, scorpions, crickets and cockroaches) that live in and feed in or upon the soil, the surface litter and their components.

In fact, soil macrofauna are important regulators of decomposition, nutrient cycling, soil organic matter dynamics, and pathways of water movement through their feeding and burrowing activities.

  1. Mesofauna

Mesofauna (0.1-2 mm in diameter) includes nematodes, micro-arthropods, like pseudo-scorpions, springtails, mites, and the worm-like enchytraeids. These have limited burrowing ability and generally live within soil pores, feeding on organic materials, microflora, microfauna and other invertebrates.

  1. Microfauna

The microfauna (<0.1 mm in diameter) includes mainly protozoa. These generally live in the soil water films and feed on microflora, plant roots, and other microfauna. They are important to release nutrients immobilized by soil microorganisms.

Soil Flora

It includes Macroflora (Vascular plants, Mosses, etc.) and Microflora (Bacteria, Actinomycetes, Fungi, Alge).

Soil Bacteria

  • These are very small, one-celled organisms that can be seen with electron microscope.
  • They constitute the highest biomass of soil organisms. They more abundant near roots, one of their food resources.
  • For agriculture point of view, Rhizobium and actinomycetes are important.
  • Bacteria are important in agricultural soils because they contribute to the carbon cycle by fixation (photosynthesis) and decomposition.
  • Some bacteria are important decomposers; some like actinomycetes are effective to break down cellulose (which makes up the cell walls of plants) and chitin (which makes up the cell walls of fungi).

Soil Actinomycetes

  • Transitional group between bacteria and fungi is called Actinomycetes.
  • Active in degrading more resistant organic compounds.
  • Optimal growth at alkaline pH.
  • Produce antibodies and geosmin.
  • Negative impact- potato scab.

Soil fungi

  • Responsible for decomposition in terrestrial ecosystems as they degrade and assimilate cellulose, the component of plant cell walls.
  • Dominate the soil biomass.
  • Obligate aerobes.
  • Can survive desiccation.
  • Dominate in acid soils.
  • Beneficial- penicillium
  • Negative impact- Apple replant disease (Rhizoctonia, pythium, fusarium and phytophthora), powdery mildew.

Nutrient cycling & Role of microbes

  • The reactions involved in elemental cycling are often chemical in nature, but biochemical reactions also play an important part in the cycling of elements.
  • Carbon cycle- Photosynthetic plants and microbes are the primary producers of organic carbon compounds and these provide nutrients for other organisms. These organisms consume organic carbon and break down organic material in the processes of fermentation and respiration. Chemo organotrophic microbes break down organic carbon compounds to release carbon dioxide. Chemo lithotrophic bacteria can assimilate inorganic carbon into organic matter in the dark.
  • Nitrogen cycle Inorganic nitrogen compounds such as nitrates, nitrites and ammonia are converted into organic nitrogen compounds such as proteins and nucleic acids in the process of nitrogen assimilation. Many bacteria reduce nitrates to nitrites and further reduce nitrites to ammonia. Nitrifying bacteria are responsible for the biological oxidation of ammonia.

Frequently Asked Question (FAQ)

What is Soil microbiology?

It is the study of organisms in soil, their functions, and their effects on soil properties. So, Soil microbiology is concerned with the study of all biological aspects of the life that exist in the soil environment.

What is Biogenic structure?

It is created biologically by a living organism. Three main groups of biogenic structures are commonly found in agricultural systems: 

  • Earthworm casts and burrows
  • Termite mounds and
  • Ant heaps.

Optimal growth of Actinomycetes is found?

Alkaline pH (>8.5).

Soil fungi dominate in which soil?

Acid soil.

Among Edaphic parameter which one greatly affects soil bacterium communities?


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