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Global food production needs to be increased in order to feed the world’s growing population and at the same time, the reliance on inorganic fertilizers and pesticides should be minimized. To accomplish this goal, the various beneficial associations between plants and soil microorganisms should be explored. The soil microbes are bacteria, actinomycetes, viruses, fungi, nematode, and protozoa. They have an important soil function that has fulfilled several useful tasks in the soil system. Microbes support biological nitrogen fixation of different biological transformations that support the accumulation and utilization of key nutrients, support root and shoot growth processes, disease control, and improve soil quality in crop cultivation. Soil microbes offer nutrient-dense nourishment improved crop production and recycle soil solutions. They play an essential role in decomposing organic matter, cycling nutrients, and fertilizing the soil. Besides, they improve plant growth on various physiological parameters of plants by a number of mechanisms. The mechanism involved in growth promotion includes plant growth regulators, production of different metabolites, and conversion of atmospheric nitrogen into ammonia in direct and indirect ways. In addition, soil microbes offer resistance against diseases. This review outlines the significant impact of soil microbes on sustainable agricultural growth, the benefits of microbes in maintaining soil health, and their interactions
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Introduction
Soil contains millions of microbes involved in soil fertility
betterment and crop production (Gougoulias et al., 2014). Soil
microorganisms are important for the conservation of core soil
processes linked to literal decomposition, nutrient availability,
and crop yields. Soil physicochemical properties rely on the
volume and quality of soil organic content, pH, biomass density,
and constraints of redox potential (Pandey et al., 2020). These
all have important effects on the composition, dynamics, and
soil structure of the microscopic culture (Lombard et al., 2011).
Soil assessments give valuable insight into nutrient
requirements and assistance on fertilizer for crops (Shrestha et
al., 2020a). Healthy and fertile soil functions as a robust living
system offering various ecological functions that preserve
the quality of water and enhance soil productivity (Shrestha,
et al., 2020b) regulating the decomposition and recycling
of soil nutrients, and the disposal of greenhouse gasses.
Soil performance is directly related to the accumulation
of water content for the greater productivity of crops and
sustainable agriculture as the variety and involvement of soil
microorganisms which are core elements of soil health (Glick,
2018). Doran and Zeiss (2000) have described soil health as “a
soil’s ability to act as an extremely important living system within
the environment and within the borders of land use, to support
living organisms’ development, preserve or improve water and
air quality and support animals and plants welfare”. Soils are
critical for the stability of the terrestrial ecosystems to remain
intact or recover from disruption, including drought, climate
change, insect infestations, deforestation, and exploitation
of human capital, including agriculture (Ellert et al., 1997).
Interactions between plants and microbes (mutual association)
are essential areas of interest within the agricultural ecosystem
and form the basis for all ecosystems (Be´langer and Avis,
AGRICULTURAL SCIENCE AND TECHNOLOGY, VOL. 13, No 2, pp 109-118, 2021
Published by Faculty of Agriculture, Trakia University, Bulgaria
ISSN 1313-8820 (print)
ISSN 1314-412X (online)
http://www.agriscitech.eu
DOI: 10.15547/ast.2021.02.019
Review
Role of soil microbes in sustainable crop production and soil health: A review
K.K. Shah1*, S. Tripathi2, I. Tiwari2, J. Shrestha3, B. Modi4, N. Paudel5,6, B.D. Das7
1Institute of Agriculture and Animal Science, Gokuleshwor College, Tribhuvan University, Nepal
2Himalayan College of Agricultural Science and Technology, Purbanchal University, Kathmandu, Nepal
3Nepal Agricultural Research Council, National Plant Breeding and Genetics Research Centre, Khumaltar, Lalitpur, Nepal
4Central Department of Chemistry, Tribhuvan University, Kathmandu, Nepal
5National Institute of Horticultural and Herbal Science, Rural Development Administration, 55365 Wanju, Republic of Korea
6Department of Applied Plant Science, Kangwon National University, 24341 Chuncheon, Republic of Korea
7Department of Botany, Mahendra Morang Adarsh Multiple Campus, Biratnagar, Tribhuvan University, Nepal
(Manuscript received 12 August 2020; accepted for publication 29 March 2021)
Abstract. Global food production needs to be increased in order to feed the world’s growing population and at the same time, the reliance
on inorganic fertilizers and pesticides should be minimized. To accomplish this goal, the various benecial associations between plants and
soil microorganisms should be explored. The soil microbes are bacteria, actinomycetes, viruses, fungi, nematode, and protozoa. They have
an important soil function that has fullled several useful tasks in the soil system. Microbes support biological nitrogen xation of different
biological transformations that support the accumulation and utilization of key nutrients, support root and shoot growth processes, disease
control, and improve soil quality in crop cultivation. Soil microbes offer nutrient-dense nourishment improved crop production and recycle soil
solutions. They play an essential role in decomposing organic matter, cycling nutrients, and fertilizing the soil. Besides, they improve plant
growth on various physiological parameters of plants by a number of mechanisms. The mechanism involved in growth promotion includes plant
growth regulators, production of different metabolites, and conversion of atmospheric nitrogen into ammonia in direct and indirect ways. In
addition, soil microbes offer resistance against diseases. This review outlines the signicant impact of soil microbes on sustainable agricultural
growth, the benets of microbes in maintaining soil health, and their interactions.
Keywords: crop production, microbes, nutrients, soil health
*e-mail: agri.kabita35@gmail.com
110
2002). Microorganisms play a major part in the absorption and
decomposition of organic compounds by nitrogen, sulfur, and
phosphorus. This impacts the global processing of nutrients
and carbons (Pankhurst et al., 1997). In nitrogen, sulfur, and
phosphorus oxidation, microorganisms are key participants in
the decomposition of organic residues. This impacts nitrogen
and carbon cycles worldwide (Pankhurst et al., 1997). The
production and deterioration of raw products and synthetic
organic compounds were also correlated with microorganisms
(Torstensson et al., 1998). Microorganisms adapt rapidly to
changes, so they react quickly to environmental conditions.
The microorganisms that are best adapted will be the ones that
ourish (Kennedy and Papendick, 1995). Soil microbiomes are
implicated in the decomposion of soil organic matter, and both
the existence of soil microorganisms and the quality of organic
matter supplies depend on the rate of decomposition. Soil
microorganisms are a core factor in the reaction through the
different nutrient cycles as well as the soil carbon sequestration
of the evolving environment in agriculture. Soil microorganisms
are involved in many biogeochemical processes. We depict
a concise summary of the role of various bacteria of the
microorganisms (nitrogen xation bacteria, azobacteria,
actinomycetes, and rhizobacteria), fungi, algae, protozoa,
and viruses in this scenario. Worldwide focus is required to
enhance or rebuild the health of soils. Evaluation of indices of
soil quality can expand our awareness of the factors that lead to
sustainable agriculture. This review highlights research results
on various categories of items of microorganisms found in soil
and soil health management practices. It aims to enhance the
awareness of soil microbe’s benets, interaction between soil
and microbes, and ecological balance among plant, soil and
microbes.
Types of soil microorganisms
Bacteria
Bacteria are so basic in structure that they are sometimes
referred to as bags of enzymes and/or fertilizer soluble bags
(Dick, 2009). In crop processing, soil bacteria have a key role
as they take part in the processes that provide soil nutrients
(Davison, 1988), boosting plant growth, e.g. plant hormone
development, regulating or inhibiting plant pathogens,
improving the composition of the soil as well as bioaccumulation
and inorganic microbial leaching. Nitrogen content (10-30%
N, 3 to 10 C:N ratio) in bacteria is higher than most microbes
(Hoorman and Islam, 2010). Ingham (2009) denes the four
main functional groupings of soil bacteria as decomposers,
mutualists, pathogens, and lithotrophs. Each functional bacteria
category plays a role in recycling soil nutrients.
Nitrogen xing bacteria: Microorganisms employed to
enhance the availability of nutrients, viz., nitrogen (by xing
atmosphere N). Nitrogen plays an important role in the
production of food and promotes plant growth (Pandey et al.,
2020), it is also essential for the synthesis of cellular enzymes,
chlorophyll, proteins, RNA and DNA. In agriculture currently,
65% of the nitrogen is utilized through the process of biological
nitrogen xation and will remain to be vital in upcoming
sustainable systems of crop production.
Bacteria commonly referred to as “Rhizobia” are believed
to trigger the nodules at the roots of leguminous plants (and,
rarely, stalks). The distinguished “bacteroid” types help in the
xation of nitrogen from the atmosphere within those nodules
and subsequent ammonia is used as a xed nitrogen supply.
This symbiotic relationship is an exceptional niche for a
bacterium and a specialized supply of nitrogen is obtained by
the plants (Andrew et al., 2007). Bacteria such as Azotobacter,
Azospirillum, Rhizobium, MesoRhizobium, SinoRhizobium are
recognized for enhancing plant growth (Gonzalez et al., 2005).
Azospirillum: Nitrogen fixing Azaspirillum species
correspond to the optional endophytic diazotrophic classes
colonizing the surface and inside of roots, which is globally
recognized as the bio-nitric nitrogen xation. Azospirillum
specically promotes plants, which boost root and root growth
and increase the rate of water and mineral intake per root, and
maintain soil quality (Gonzalez et al., 2005).
Azotobacter: Azotobacter is a mandatory aerobe, even
though at low oxygen concentration it can develop. The
ecological spread of this bacterium is dynamic and is linked
to different matters that decide that this bacterium occurs or is
absent in a given soil (Sumbul et al., 2020).
Actinomycetes: Actinomycetes are gram-positive aerobic
bacteria that belong to the order of Actinomycetes known by its
substrate and aerial mycelium production. They form associations
with some non-leguminous plants and x N, which is then
available to both the host and other plants in the near vicinity.
They have a crucial role in the cycling of organic matter; inhibit
the growth of several plant pathogens in the rhizosphere. They
decompose complex mixtures of polymer in dead plant, animal
and fungal material resulting in production of many extracellular
enzymes which are conductive to crop production (Bhatti et al.,
2017). Actinomycetes inhabit the rhizosphere of agricultural
crops, where they increase soil fertility through recycling of
organic matter and solubilizing phosphate (Hozzein et al., 2019).
Cyanobacteria: Cyanobacteria are photoautotrophic gram-
negative bacteria that are plentiful in several soils and take
an active part in soil build-up and sustain fertility (Prasanna
et al., 2009). Their impact on soil quality and plant tolerance
to abiotic and biotic stresses, including plant diseases, is
important. Cyanobacteria are sustainable sources of the
biomass of the solubilized organic matter, that are mineralized
by soil microorganisms (non-ribosomic peptides, isoprenoids,
ribosomal peptides, alkaloids, and polyketides) which
then in turn help farm crop growth (Kultschar et al., 2018).
Cyanobacteria may improve N levels and the population of
microorganisms in poor semiarid soils (Acea et al., 2003).
Rhizobacteria: The rhizosphere is the narrowed area
of the soil that is specically inuenced by root secretions
and related soil microbiomes (Antoun and Prevost, 2005).
The term ‚rhizosphere‘ was rst used by Lorenz Hiltner to
describe the effect of plant root secretion on edaphon. Plant-
111
microbe interactions have attracted strong interest since the
last century due to their crucial role in promoting plant growth
(Hütsch et al., 2002). Rhizobacteria also play a signicant role
in enhancing soil structuring and the growth and stabilization
of mineral phosphates, other nutrients, and soil aggregates,
a-gluconase, antibiotics, and dehydrogenase, soil aggregates
(Miller and Jastrow, 2000). The physical, chemical, and
biological characteristics of the rhizosphere differ signicantly
from those of the surrounding soil. The rhizosphere is a quite
special and narrow area of soil that is inuenced by the plant
root system (Giri et al., 2005). The number of microorganisms
and invertebrates around the rhizosphere is higher than that
in bulk soil. Rhizosphere microbes are considered to be
the second set of genomes in plants and play a vital role in
promoting plant growth and development, nutrient acquisition,
inhibition of fungal plant pathogens. It should be noted that the
structure of bacterial microbiota is a hereditary trait that has
been veried in the rhizosphere of Arabidopsis and wheat,
suggesting that its population could stabilize or undermine
plant growth (Donaldson et al., 2016). Soil and crop health are
the basis of the rhizobacterial population in the soil and vary
from species to species (Tilak et al., 2005).
Microora available in the soil (especially in the rhizosphere)
has shown its potential to control the soil-borne disease by
the method of biological control, and microbes used in this
technique are referred to as bioagents or biocontrol agents
(Cook et al., 1978). Rhizosphere composition of organic acids
from root exudates is about 30% and its composition consists of
simple carbohydrates, amino acids, and complex nutrients that
support plant growth (Sandnes et al., 2005). The key elements
of biological control is the colonization by microbes, especially
bacteria, of the soil of rhizosphere or the external/internal root
zone (Bahme and Schroth, 1987).
Fungi
Fungi are very productive inhabitants of soil, due to their
high plasticity and ability to adopt different forms in response
to adverse or unfavorable conditions (Sun et al., 2005).
Fungi also perform an important part in the stabilization and
decomposition of soil organic matter (Shah et al., 2021). Fungi
transform dead organic matter into carbon dioxide, organic
acids, and biomass. Many fungal species can function as an
important biosorbent of toxic metals such as cadmium, copper,
mercury, lead, and zinc, by collecting them in their fruiting
bodies. The fungi are well-equipped for the preservation
and restoration of the soil that consequently allows plants to
thrive and ourish (Baldrian, 2003). Among all the microbes,
fungi are more resistant than bacteria and actinomycetes,
while others have quite the resistance for the uctuating soil
disruptions, or no soil disturbs (Hoorman and Islam, 2010;
Meliani et al., 2012). In fact, they perform a signicant part
in the stabilization and decomposition of soil organic matter.
In almost every environment, fungi can be observed and can
live in a wide range of pH and temperature (Frac et al., 2015).
Various biotic (plants and other organisms) and abiotic (soil pH,
moisture, salinity, structure, and temperature) factors regulate
the diversity and activity of fungi (Rouphael et al., 2015). Soil
fungi can be classied into three functional groups, including
(1) biological inspections, (2) ecosystem regulator, and (3)
species involved in the decomposition of organic matter and
transformation of compounds. For example, mycorrhizal fungi
enhance plant growth through improved nutrient intake and
defense from pathogens (Bagyaraj and Ashwin, 2017). The
fungal population is highly affected by the plant community‘s
diversity and composition, and in return trigger plant growth
through mutualism, pathogenicity, and their impact on nutrient
availability and cycling. Because of their capacity to generate a
wide range of extracellular enzymes, they can break down all
forms of organic matter, decompose soil components and thus
control the carbon and nutrient balance.
Algae
Algae are a very wide and varied group of simple, usually
autotrophic organisms which can perform photosynthesis and
capture energy from sunlight; they also play a signicant role in
the soil where they are used as biofertilizer and soil stabilizers
(Neilands, 1995). Like other species, algae contained in various
types of soil can help the soil improve its characteristics such
as carbon content, texture, and aeration (Ibraheem, 2007).
Soil algae emit substances that promote growth, such as
hormones, vitamins, amino acids, and organic acids, which in
many ways inuence other species (Wilson, 2006). The key
roles and functions of algae in the soil are the maintenance of
soil fertility, particularly in the tropical soils; they increase the
organic carbon amount and other organic matter which helps
to maintain soil health (Johns, 2017). By joining up soil particles
in a manner that eliminates and avoids soil erosion, the algae
help improve the potential for preservation of water in soils over
longer periods of time; retain massive quantities of oxygen in
the soil through the photosynthesis cycle and thereby help to
control the depletion of nitrates; through leaching and drainage,
especially in up-cropped soils; and helping in the weathering of
rocks and building up of soil structure (Johns, 2017).
Protozoa
These are colorless, single-celled animal-like organisms.
They are larger than bacteria, varying from a few microns to
a few millimeters. Their population in arable soil ranges from
10,000 to 100,000 per gram of soil and they are abundant in
surface soil (Bhatti et al., 2017). They can withstand adverse soil
conditions, as they are characterized by a protected, dormant
stage in their life cycle. Most protozoans derive their nutrition from
feeding or ingesting soil bacteria and, thus, they play an important
role in maintaining microbial/bacterial equilibrium in the soil. Some
protozoa have been recently used as biological control agents
against organisms that cause harmful diseases in plants.
Viruses
Soil viruses are very signicant as they can have an effect
on soil biology by moving genes from host to host and as a
112
possible source for microbial mortality (Bhatti et al., 2017).
Consequently, viruses are major players in global cycles,
inuencing the turnover and concentration of nutrients and
gases (Mitchell, 1973); the eld of soil virology is understudied
in light of this signicance. Studies are carried out on virus
diversity and abundance in various geographic zones in
order to investigate the role of viruses in plant health and soil
quality. Viruses have been shown to be extremely common
in all environments observed to date, including conditions
under which bacterial species of the same ecosystem vary
signicantly.
Benets of soil microbes
Disease control
Soil microbes play a signicant role in the tolerance to
plant diseases. They can prevent pathogen infection by
inducing plant systemic disease resistance and by coating root
surfaces to physically shield the plant from getting infected
by pathogens. Soil microbes can make plants more resistant
to an aggressive disease, thus opening new possibility for
sustainable food production (University of York, 2019).
The industrial cultivation of the bacterium Bacillus
thuringiensis (Bt) in the soil for caterpillars, pests is a well
prominent framework for the usage of soil microbes to
combat pests. Many Bt strains are often used for monitoring
beetles and ies. A variety of strains have been produced
as biocontrol agents of the fungal genus Trichoderma spp.,
primarily root disorders, for fungal plant diseases. In the
management of insect pests, numerous genera of fungi are
included. Efcacy of Bacillus spp. has been identied in various
crop plants such as tomato, chili, brinjal, etc. to control different
pathogens like R. solani, Colletotrichum acutatum, C. capsici,
Pythium aphanidermatum and C. gloeosporioides (Abdul et al.,
2007). Pseudomonas spp. exhibit antifungal activity against, R.
solani, Pyricularia oryzae, and Xanthomonas oryzae pv. oryzae
F. oxysporum, f. sp. udum both in vitro and eld environments
(Vidhyasekaran et al., 2001). Several soil-borne antagonists
such as Trichoderma spp. are reported to control fungal wilt of
tomato caused by F. sp. lycopersici and F. oxysporum (Singh
et al., 2013).
Availability of soil nutrients
Soil microbes may contribute greatly to enrich the soil with
nutrients which helps in maintaining soil productivity. It has
been proved that biological fertilization is an efcient method to
supply plants with their necessary nutrient. Three mechanisms
are usually suggested to understand how microbial activity can
promote plant growth:
(1) Manipulating the hormonal signaling of plants (Verbon
and Liberman, 2016);
(2) Repelling or outcompeting pathogenic microbial strains
(Mendes et al., 2013), and
(3) Enhancing plant mineral bioavailability (Bender et al.,
2016).
Numerous experimental results have shown that the
recycling of organic compounds and nutrients by the soil
microorganisms is essential (Rai et al., 2016). The need for
nutrients differs greatly according to soil productivity, climatic
factors, crop characteristics, and production (Pandey et
al., 2020). Soil microorganisms help plants by converting
atmospheric elemental dinitrogen (N2) into ammonia and are
capable of nitrogen-xing (bacteria possessing nitrogenase
enzyme). Nitrogen-xing and nutrient mineralization processes
performed by soil, the microorganism is important in plant
nutrition in natural ecosystems because these reactions
metabolize recalcitrant forms of N, P, and S to liberate their
elements providing plant nutrition (Modi et al., 2020). It must
be noted briey that in recent years, this proven paradigm
has been somewhat challenged, as several studies have
shown invasive species have a detrimental effect on nutrients
availability and in direct plant uptake of various organic-N forms
(Nasholm et al., 1998; Paungfoo-Lonhienne et al., 2008). Soil
biota plays a crucial role in the modulation of primary production
through control of decomposition and availability of nutrients as
well as affects the root grazing and plant nutrient absorption,
where invasive plants may disrupt soil associations and plant
native plants and thereby encourage the success of invasions
(Shah et al., 2020). Because of the low diffusivity of organic-N
molecules in soil, microbes are better competitors for these
nutrients and the ndings of isotope labeling studies generally
support the hypothesis that most organic-N is rst assimilated
by microbial taxa, and then assimilated by plants at microbial
turnover (Richardson et al., 2009). Wilson et al. (2006) stated
that iron contained in soil and microorganisms can cause growth
inhibition, decrease in RNA and DNA synthesis, inhibition of
sporulation, and can even alter cell morphology.
Interaction between plants and soil microbes
Soil microorganisms including bacteria, algae, fungi, and
actinomycetes are important components which interact with
the plants (Andreote et al., 2014). These soil microorganisms
serve several essential roles and and sometimes cause
negative effects. In the soil prole, the inuence of soil biota
is dynamic and nuanced, based on their location, the effect
can be negative or benecial (Parkin et al., 1996). In order to
better control the interplay between crop microbes, the role
environmental conditions must be taken into account. The
dynamic relationships between bacteria and plants, and the
soil system, as a factor for the soil formation and soil fertility,
is evidence that soil has evolved over hundreds of years
(Harrison, 2008)
Soil microbes have ability to produce auxins, gibberellins,
SA, ABA, and cytokinins in plant tissues. Phytohormones
produced by root-associated microbes may prove to be
important metabolic engineering targets for inducing host
tolerance to abiotic stresses (Egamberdieva et al., 2017).
Soil productivity and mineral rock formation involve a
wide range of chemical, physical and biological interactions.
The soil fertility rate is influenced by parameters such as
113
topography, climatic conditions, period, rock form, plants,
and microbes, which is why the nature and identity of
the microbes in the soil determine the nutrient levels
(Lombard et al., 2011). Soil microbes attach soil to roots,
recycle nuts, decompose organic matter, and quickly
respond to any changes in soil ecology, by providing an
ideal indicator of which definite soil-environment functions
(Jacoby et al., 2017). Crop productivity and soil fertility can
be improved by various microbes, as well as by organic
material bacteria. Climate change is also anticipated as
a direct result of rising surface temperatures and indirect
effects such as plant species or surface resources to
influence soil microbial community structures (Deslippe
et al., 2012). In non-disturbed grassland environments,
decomposer Actinobacteria were more prevalent than in
agricultural soils, whereas, for Bacteroidetes, the reverse
trend has been identified (Acosta-Martínez et al., 2008).
The conventional view is that soil microbes exist anywhere
and the environment determines which species prevail, i.e.
no dispersal limitations. The composition of a soil microbial
culture, including its soil form, its mineral, and texture,
the availability of nutrients (C, N, and P), its moisture, the
oxygen status as well as the related plant populations, has
been influenced by this view (Bonkowsi et al., 2009).
Different types of soil microbes have different specic roles
on plants (Table 1).
Table 1. Roles of soil microbes on plant
Bacteria Experimental conditions Effects on plants References
Bacillus
amyloliquesfaciens Laboratory conditions Increase root and shoot growth of rice He et al., 2013
Bacilus subtilis Field conditions Increase macro and micro nutrient absorption,
growth and plant production Altuhaish et al., 2014
Field conditions Increase fresh and dry shoot and root weight Turan et al., 2014
Pseudomonas spp.
Laboratory, greenhouse and
eld conditions
Increased germination, shoot and root length,
grain yield of maize Kie and Laing, 2016
Field conditions Increased grain yield and straw weight of barley Fröhlich et al., 2012
Pot experiment and eld
conditions
Improves germination, growth parameters and
yield of maize Gholami et al., 2009
Laboratory conditions Growth stimulation of tomato plants Gravel et al., 2007
Pseudomonas jessenii
Greenhouse and eld
conditions Increase yield and shoot dry weight of chickpea Valverde et al., 2006
Greenhouse conditions Increase growth of tomato
Paenibacillus
mucilaginosus Pot experiment Improve growth of trifoliate orange seedlings Wang et al., 2016
Rhizophagus intraradices Greenhouse conditions
Increase the plant growth, number of leaves,
plant height, shoot and root length and weight
of tea
Sharma and Kayang,
2017
Field conditions Increase growth of tomato Mohamed et al., 2016
Fungi
Trichodema spp. Laboratory conditions Improve growth and seed production of
soybean Paradiso et al., 2017
Laboratory conditions Growth promoter of cowpea Chagas et al., 2016
Penicilium bilaii
Rhizobox experiment Increase root length of maize Gomez-Munoz et al.,
2017
Field conditions Increase grain yield of wheat Ram et al., 2015
Field conditions Increase root length and P-content in root of
pea
Vessey and Heisinger,
2001
Trichoderma harzianum
Pot experiment Improve germination and seedling growth of
wheat El-Gremi et al., 2017
Greenhouse and laboratory
conditions Increase potato yield Buysens et al., 2016
Pot experiment Increase shoot and root length, dry mass and
grain yield of Pigeon pea Gupta et al., 2016
Pot experiment Increased root length, growth and shoot dry
weight in Brassica nigra and melon Galletti et al., 2015
Pot experiment Increase growth of Brassica juncea Galletti et al., 2015
Source: Holeckova et al., 2017
114
Plants exhibit a diverse array of interactions with the soil
microbes. Interactions between plants and soil microbes play
an important role in enhancing plant growth and development.
The interactions of soil, plant and microbes along with their
benets are given in Figure 1.
Figure 1. A conceptual theme demonstrating the important soil microbes with benets and interactions of soil, plant and
microbes (Tahat et al., 2021)
Ecological balance among plants, soil and microbes
Bio-fertilizer is also used in organic farming systems,
but at present, the option of plant cultivars and microbial
inoculants have little mechanistic understanding (Bender
et al., 2016). Balanced utilization of organic fertilizers
enhances fertilizer efciency and physical, chemical, or
biological soil environment, contributing to an increase in crop
production (Tiwari et al., 2021). Plants interact with these
soil-inhabiting species in several ways that cover the entire
spectrum of ecological possibilities (competitive, exploitative,
cooperative, commensal, and mutual). Many of the plant-
based interactions have been focused in modern sciences
on alleviating pathogens such as herbivory and infection,
or reducing abiotic stress conditions. Yaish et al. (2016)
established that microbial metabolism speeds up organic
matter decomposition, facilitates nutrient mineralization, and
enables plant absorbed nutrients. Efux of CO2 from soil stems
from two distinct factors such as root and microbial respiration
rhizosphere and soil organic microbial decomposition.
Importantly, nitrogen-xing bacteria in the rhizosphere may be
used to x nitrogen in order to provide organic and inorganic
nitrogen to plants (Dominati et al., 2010).
Protection of soil is therefore of high priority and a thorough
understanding of ecosystem processes is an important factor
in maintaining that soil remains healthy (Balasubramanian,
2017). This modication can likely allow for differential
microbial studies in the soil health study and thus can serve as
an excellent signal for improvements to soil health in microbial
populations and activities (Kennedy et al., 1995; Pankhurst et
al., 1997).
There are more microbes in a teaspoon of soil than there
are people on the earth. Soils contain about 8 to 15 tons of
bacteria, fungi, protozoa, nematodes, earthworms, and
arthropods (Hoorman, 2020). The number and biomass of
microbial species is given in Table 2. Changes in populations
of microbes can in some cases precede signicant changes
in physical and chemical soil material, which can be a sign
of early soil improvements or a warning of soil deterioration
(Pankhurst et al., 1995). One example is the microbial biomass
turnover rate.
115
Table 2. Relative number and biomass of microbial species at
0-15 cm depth of soil
Microorganisms Number/g of soil Biomass (g/m2)
Fungi 105–106100-1500
Bacteria 108-10940-500
Algae 104–1051-50
Nematodes 102–103Varies
Protozoa 103–104Varies
Actinomycetes 107-10840-500
Source: Hoorman, 2020
Conclusion
The soil microbes are bacteria, actinomycetes, viruses,
fungi, nematode, and protozoa. They produce plant growth
regulators and metabolites that affect the plant growth and
development. Soil microbes are critical to decomposing organic
residues and recycling soil nutrients. They provide nutrients to
crops, enhance soil health and crop outputs. Soil microbes play
a signicant part in the tolerance to plant diseases. Considering
the environmental damage associated with use of chemical
fertilization, a research priority on optimizing plant-soil microbe
nutritional interactions is essential for more sustainable
agricultural systems.
Conict of interest
The authors have no conict of interest.
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