Agriculture such as earthworms. Soil microbes (or microorganisms) are

Agriculture
or cultivation of land is done to support the human needs of food, clothing and
shelter. Soil is the important component of agriculture which needs to be taken
care of. With the ever-increasing population, the limited available arable land
is exploited for increased productivity and fulfillment of our requirements; leading
to deterioration lands. It is threatened because of deforestation, desertification,
salinity, paving, contaminants, loss of biodiversity, and climate change. We should practice/emphasize sustainable agriculture to maintain the
soil health. The conservation
of soil and water can be achieved through conservation tillage, efficient water
management and organic waste management using mechanical, cultural and
biological approaches.

Soil harbors
numerous diverse organisms such as bacteria,
algae, fungi, and protozoa, to the larger organisms such as earthworms. Soil microbes (or microorganisms) are too small
(i.e., smaller than 0. 1 mm) to be seen with naked eye. Bacteria are the most
abundant microorganisms in soil, with a population of 1010–1011 individuals and 6,000–50,000 species per gram of soil
and a biomass of 40-500 grams per m2. These microbes can be
utilized for establishing favorable soil microbial population. Soil microorganisms
are involved in various activities such as nutrient cycling, regulating the
dynamics of soil organic matter, soil carbon sequestration, modifying soil
physical structure and water regimes, increase the efficiency of nutrient
acquisition by the plants and improve plant health.

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Beneficial rhizobacteria are strains from genera of Pseudomonas, Azospirillum,
Azotobacter, Bacillus, Burkholdaria, Enterobacter, Rhizobium, Erwinia and
Flavobacterium.
Among actinomycetes, Streptomyces
is the predominant genus followed by Actinomadura,
Microbispora, Micromonospora, Nocardia, Nonomurea, Mycobacterium, Frankia,
Actinoplanes, Saccharopolyspora, and
Verrucosispora. The important fungal genus includes Trichoderma and Glomus.

            Micro-organisms have both direct and
indirect mechanisms to influence the plant growth and protection. The direct
mechanisms involve the production of vital factors for crop growth such as
growth hormones and the assistive actions on nitrogen fixation, phosphate solubilization,
and iron acquisition. They indirectly influence the plant growth by controlling
and minimizing the deleterious effects of external stresses of either biotic or
abiotic sources through the following modes: competition for nutrients,
cell-wall degrading enzymes, and antibiotics.

Two kinds of
nitrogen-fixing bacteria are
recognized. The first kind, the free-living (non-symbiotic) bacteria, includes
the cyanobacteria (or
blue-green algae) Anabaena and Nostoc and genera such as Azotobacter, Beijerinckia, and Clostridium. The second kind comprises the mutualistic
(symbiotic) bacteria; examples include Rhizobium, associated with leguminous plants (e.g., various members of
the pea family); actinobacteria Frankia, associated with certain dicotyledonous species
(actinorhizal plants); and certain Azospirillum species, associated with cereal grasses. All biological nitrogen fixation is done by nitrogenases enzymes
encoded by nif genes. Phosphate Solubilzing Microorganisms (PSM) are able to convert inorganic
and organic soil P respectively into the bioavailable form facilitating uptake
by plant roots through various mechanisms of solubilization and mineralization
(release of organic
acids, phosphatases and phytases). Predominant PSM include Pseudomonas and Bacillus bacteria and Aspergillus and Penicillium fungi. 
Iron
in soil is known for its un-availability to both plants and microbes due to its
normal presence as insoluble hydroxides and oxyhydroxides. This is made
available by the synthesis of siderophores. Besides the context of plant
nutrition, siderophore also offers for plant protection through the control of
phytopathogens. They acquire iron thereby create a competitive environment for
other pathogenic microbes in the root vicinity.

Various
micro-organisms such as Bacillus, Pseudomonas, Trichoderma and
Streptomyces synthesize different enzymes like ligninases, lipases, peroxidases,
phenyl ammonia lyase, chitinases, etc. These enzymes have biocontrol potential
as it degrades the cell wall of pathogenic fungi and bacteria. Actinobacteria account for about two-third of
antibiotics production. These antibiotics also are involved in plant disease
control. The production of these molecules contributing to plant resistance to
diseases is termed as induced systemic resistance (ISR).

 Microbes produce phytohormones
such as indole acetic acid and giberrellic acid which help in plant growth
promotion. The root elongation in plants helps in absorption of nutrients from
deeper strata of soil, which in turn improves plant yield. Increased root
biomass can help in efficient water uptake thereby supporting survival of
plants under water stressed conditions. Also, inoculants have been known to
support crop tolerance to saline conditions. Ethylene is produced in plants
under abiotic stress conditions which affects plant survival. Association of beneficial
microbes mitigates abiotic stress through ACC deaminase by lowering ethylene
production. The exopolysaccharide production, IAA production, biofilm
formation, production of volatiles, glycine betaine and such molecules supports
plant survival under abiotic stress; which is termed as induced systemic
tolerance. A biofilm helps in aggregation of soil particles and increased water
retention capacity of soil. This improves crop productivity and physiochemical
properties of soil.      

Micro-organisms are important in agricultural soils because
they contribute to the carbon cycle by fixation  (photosynthesis) and
decomposition. Organic residues added to soil are first attacked by
bacteria and fungi and later by actinomycetes, because they are slow in
activity and growth than bacteria and fungi. Some bacteria are particularly effective at breaking down
tough substances like cellulose, lignin and the chitin to produce
dark black to brown pigments which contribute to the dark colour of
soil humus. The breakdown of these materials makes nutrients available to
plants. During the process of composting mainly thermophilic (adapted
to high temperatures) and thermo tolerant actinomycetes are responsible for
decomposition of the organic matter at elevated temperatures.

Soil and water ecosystems are
getting heavily polluted because of modernization, urbanization, excessive use
of chemical fertilizers, pesticides, and industrial discharge of heavy metals
and dyes. Biodegradation is a less expensive alternative to physical and
chemical means of pollutant detoxification. Pathways of biodegradation have
been characterized in heterotrophic microorganisms, mostly isolated from soil,
some of which have been used for remediation of soil, water and other polluted
sites. As cyanobacteria are photoautotrophic and some also fix atmospheric
nitrogen, their use for decontamination of polluted soil systems can be a very
effective tool for sustainable and green agriculture.

Recent molecular studies using different – ‘omics’ approaches such as metagenomics, transcriptomics,
proteomics and metabolomics also support the involvement of microbes for
improved soil health leading to increased crop productivity. So, we can
emphasize the use of microbes for rhizosphere engineering contributing to sustainable
agriculture.