Agricultural microbiology is a branch of microbiology dealing with plant-associated microbes and plant and animal diseases. It also deals with the microbiology of soil fertility, such as microbial degradation of organic matter and soil nutrient transformations.

Soil Microorganisms

Importance of soil microorganisms

* Involved in nutrient transformation process * Decomposition of resistant components of plant and animal tissue * Role in microbial antagonism

Microorganisms as biofertilizers

Biofertilizer A biofertilizer is a substance which contains living micro-organisms which, when applied to seeds, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of prim ...

s are seen as promising, sustainable alternatives to harmful

chemical fertilizers A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...

due to their ability to increase yield and soil fertility through enhancing crop immunity and development. When applied to the soil, plant, or seed these biofertilizers colonize the

rhizosphere The rhizosphere is the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Soil pores in the rhizosphere can contain many bacteria and other microo ...

or interior of the plant root. Once the microbial community is established, these microorganisms can help to solubilize and break down essential nutrients in the environment which would otherwise be unavailable or difficult for the crop to incorporate into biomass.

Nitrogen

Nitrogen is an essential element needed for the creation of biomass and is usually seen as a limiting nutrient in agricultural systems. Though abundant in the atmosphere, the atmospheric form of nitrogen cannot be utilized by plants and must be transformed into a form that can be taken up directly by the plants; this problem is solved by biological nitrogen fixers.

Nitrogen fixing Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmo ...

bacteria, also known as

diazotroph Diazotrophs are bacteria and archaea that fix gaseous nitrogen in the atmosphere into a more usable form such as ammonia. A diazotroph is a microorganism that is able to grow without external sources of fixed nitrogen. Examples of organisms that ...

s, can be broken down into three groups: free-living (ex. ''

Azotobacter ''Azotobacter'' is a genus of usually motile, oval or spherical bacteria that form thick-walled cysts (and also has hard crust) and may produce large quantities of capsular slime. They are aerobic, free-living soil microbes that play an impo ...

'', ''

Anabaena ''Anabaena'' is a genus of filamentous cyanobacteria that exist as plankton. They are known for nitrogen-fixing abilities, and they form symbiotic relationships with certain plants, such as the mosquito fern. They are one of four genera of cyanob ...

,'' and '' Clostridium)'' , symbiotic (ex. ''

Rhizobium ''Rhizobium'' is a genus of Gram-negative soil bacteria that fix nitrogen. ''Rhizobium'' species form an endosymbiotic nitrogen-fixing association with roots of (primarily) legumes and other flowering plants. The bacteria colonize plant cells ...

'' and ''

Trichodesmium ''Trichodesmium'', also called sea sawdust, is a genus of Filamentation, filamentous cyanobacteria. They are found in nutrient poor tropical and subtropical ocean waters (particularly around Australia and in the Red Sea, where they were first des ...

)'' and associative symbiotic (ex. ''

Azospirillum ''Azospirillum'' is a Gram-negative, microaerophilic, non- fermentative and nitrogen-fixing bacterial genus from the family of Rhodospirillaceae. ''Azospirillum'' bacteria can promote plant growth. Characteristics The genus ''Azospirillum'' ...

''). These organisms have the ability to fix atmospheric nitrogen to bioavailable forms that can be taken up by plants and incorporated into biomass. An important nitrogen fixing symbiosis is that between ''

'' and leguminous plants. ''

'' have been shown to contribute upwards of 300 kg N/ha/year in different leguminous plants, and their application to agricultural crops has been shown to increase crop height, seed germination, and nitrogen content within the plant. The utilization of nitrogen fixing bacteria in agriculture could help reduce the reliance on man-made nitrogen fertilizers that are synthesized via the

Haber-Bosch The Haber process, also called the Haber–Bosch process, is an artificial nitrogen fixation process and is the main industrial procedure for the production of ammonia today. It is named after its inventors, the German chemists Fritz Haber and C ...

process.

Phosphorus

Phosphorus can be made available to plants via solubilization or mobilization by bacteria or fungi. Under most soil conditions, phosphorus is the least mobile nutrient in the environment and therefore must be converted to solubilized forms in order to be available for plant uptake. Phosphate solubilization is the process by which organic acids are secreted into the environment, this lowers the pH and dissolves phosphate bonds therefore leaving the phosphate solubilized. Phosphate-solubilizing bacteria (PBS) (ex. ''

Bacillus subtilis ''Bacillus subtilis'', known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium, found in soil and the gastrointestinal tract of ruminants, humans and marine sponges. As a member of the genus ''Bacillus ...

'' and ''

Bacillus circulans ''Bacillus circulans'' is a soil-dwelling human pathogen which has been associated with "septicemia, mixed abscess infections, and wound infections", as well as with meningitis. This species has been recently transferred into the genus '' Nialli ...

)'' are responsible for upwards of 50% of microbial phosphate solubilization. In addition to the solubilized phosphate,

PBS The Public Broadcasting Service (PBS) is an American public broadcaster and non-commercial, free-to-air television network based in Arlington, Virginia. PBS is a publicly funded nonprofit organization and the most prominent provider of educat ...

can also provide trace elements such as iron and zinc which further enhance plant growth. Fungi (ex. ''

Aspergillus awamori ''Aspergillus awamori'' is a species of aspergillus that is used to make awamori and shōchū. It can produce citric acid and convert starch to sugar. ''Aspergillus awamori'' is often confused with ''Aspergillus niger'' as they have very similar ...

'' and ''

Penicillium ''Penicillium'' () is a genus of ascomycetous fungi that is part of the mycobiome of many species and is of major importance in the natural environment, in food spoilage, and in food and drug production. Some members of the genus produce pe ...

'' spp.) also perform this process, however their contribution is less than 1% of all activity. A 2019 study showed that when crops were inoculated with ''

Aspergillus niger ''Aspergillus niger'' is a mold classified within the ''Nigri'' section of the ''Aspergillus'' genus. The ''Aspergillus'' genus consists of common molds found throughout the environment within soil and water, on vegetation, in fecal matter, on de ...

,'' there was a significant increase fruit size and yield compared with non-inoculated crops; when the crop was co-inoculated with '' A. niger'' and the nitrogen fixing bacteria '' Azobacter'', the crop performance was better than with inoculation using only one of the biofertilizer and the crops that were not inoculated at all. Phosphorus mobilization is the process of transferring phosphorus to the root from the soil; this process is carried out via mycorrhiza (ex. ''

Arbuscular mycorrhiza An arbuscular mycorrhiza (AM) (plural ''mycorrhizae'', a.k.a. ''endomycorrhiza'') is a type of mycorrhiza in which the symbiont fungus (''AM fungi'', or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. ( ...

)'' . ''

'' mobilize phosphate by penetrating and increasing the surface area of the roots which helps to mobilize phosphorus into the plant. Phosphate solubilizing and mobilizing microorganisms can contribute upwards of 30–50 kg P2O5/ha which, in turn, has the potential to increase crop yield by 10–20%.

Example

* DAP * UREA * SUPER PHOSPHATE

Soil Microorganisms

Importance of soil microorganisms

Microorganisms as biofertilizers

Nitrogen

Phosphorus

Example

See also

References

Further reading