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Essay on Biofertilizers
Essay Contents:
- Essay on the Meaning of Biofertilizers
- Essay on the Need for Biofertilizers
- Essay on the Types of Biofertilizers
- Essay on PGPR (Plant Growth Promoting Rhizobacteria) as Biofertilizers
- Essay on Phosphate Solubilizing Biofertilizers
- Essay on Mycorrhizae
- Essay on the Role of Biofertilizers
- Essay on the Economic and Environmental Benefits of Biofertlizer
- Essay on the Development of Biofertilizer Industry
Essay # 1. Meaning of Biofertilizer:
Biofertilizer is a large population of a specific or a group of beneficial microorganisms for enhancing the productivity of soil either by fixing atmospheric nitrogen or by solubilising soil phosphorus or by stimulating plant growth through synthesis of growth promoting substances.
Bio-fertilizers based on renewable energy sources are cost effective supplement to chemical fertilizers, eco-friendly and can help to economize on the high investment needed for chemical fertilizer use as far as nitrogen and phosphorus are concerned.
Biofertilizer is a 100% natural and organic fertilizer that helps to provide and keep in the soil all the nutrients and microorganisms required for the benefit of the plants.
Biofertilizers do not come under the purview of the definition of the term fertilizers, as they do not contain substantial quantity of plant nutrients as other fertilizers like urea, diammonium phosphate, muriate of potash etc. contain. The term biofertilizer is a misnomer but is in wider use. They should be termed as inoculants, after the name of the microorganisms they contain, viz., Rhizobium inoculant, Azospirillum inoculant or blue green algae inoculant.
Bioinoculants or biofertilizers are different from chemical fertilizers. Biofertilizers on application remain in soils, multiply and keep benefiting the growing crops. They do not get depleted as in the case of fertilizers and, therefore, if the optimum soil conditions prevail, population of added microorganisms builds up and frequent application of biofertilizers can be avoided.
There are millions of microscopic organisms near the plants that not only provide nutrients to the plants but also help to keep the water and retain the nutrients in the soil, for its easy availability to the plants.
Biofertilizer can be defined as a substance which contains living microorganisms which, when applied to seed, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of primary nutrients to the host plant. This definition is based on the logic that the term biofertilizer is a contraction of the term biological fertilizer.
As biology is the study of living organisms, biofertilizer should contain living organisms which increase the nutrient status of host plant through their on-going existence in association with the plant. This definition separates biofertilizers from organic fertilizer (fertilizer containing organic compounds which directly, or by their decay, increase soil fertility).
Likewise, the term biofertilizer should not be used interchangeably with the terms, green manure, intercrop, or organic supplemented chemical fertilizer. Oken and Landera Gonzalez (1994) argue that rhizosphere organisms which improve utilization of soil nutrient but do not replace soil nutrients (like chemical fertilizers) should not be called biofertilizers.
Some plant growth promoting rhizobacteria (PGPR), that represent a wide variety of soil bacteria when grown in association with a host plant, result in stimulation of growth of their host. However, not all PGPR can be considered as biofertilizers. Bacteria that promote plant growth by control of deleterious organisms are biopesticides, but not biofertilizers.
Interestingly some PGPR appear to promote growth by acting as both biofertilizer and biopesticide. For example, strains of Burkholderia cepacia have been shown to have biocontrol characteristics to Fusarium spp., but also can stimulate growth of maize under iron- poor conditions via siderophore production.
The biofertilizer has a relative high nutrient concentration, and even so, it can be used directly over soil before planting. Once diluted, it constitutes a high quality foliar fertilizer, and in this form, it is known as diluted biofertilizer. Diluted biofertilizer also has all the needed conditions to be used as a complete nutrient solution in organic hydroponics. The advantages of using the biofertilizer are enormous. Not only it is very economical, but also for the high agricultural yields which it produces.
Most fertilizers add nitrogen to the soil. This can be done via chemical fertilizers, or through a process called biological nitrogen fixation (BNF). On a worldwide basis it is estimated that about 175 million tons of nitrogen per year is added to soil through biological nitrogen fixation (BNF).
The term bio means living; so bio-fertilizers refer to living, microbial inoculants that are added to the soil. These biofertilizers are products consisting of selected and beneficial microorganisms, which are known to improve plant growth through supply of plant nutrients.
The soil microorganisms commonly used in biofertilizers are- Phosphate Solubilizing microbes, Mycorrhizae, Azospirillum, Azotobacter, Rhizobium, blue green algae, and Azolla.
Biofertilizers are ready to use live formulates of such beneficial microorganisms which on application to seed, root or soil, mobilize the availability of nutrients by their biological activity in particular, and help build up the micro-flora and in turn the soil health in general.
Essay # 2. Need for Biofertilizers:
Biofertilizers have definite advantage over chemical fertilizers. Chemical fertilizers supply nitrogen whereas biofertilizers provide, in addition to nitrogen certain growth promoting substances like hormones, vitamins, amino acids, etc. Crops have to be provided with chemical fertilizers repeatedly to replenish the loss of nitrogen utilized for crop growth.
On the other hand biofertilizers supply the nitrogen continuously throughout the entire period of crop growth in the field under favourable conditions. Continuous use of chemical fertilizers adversely affects the soil structure whereas biofertilizers when applied to soil improve the soil structure. The deleterious effects of chemical fertilizers are that they are toxic at higher doses. Biofertilizers, however, have no toxic effects.
It may be borne in mind that biofertilizers are no substitute for chemical fertilizers. At present, the use of chemical fertilizers is far below the recommended level. Therefore, the aim and object of spread of biofertilizer technology as an Industry, is to build up efficiency in use of chemical fertilizers, supplemented by low cost inoculants to the extent possible. Biofertilizers are used for integrated farming-systems mainly in the agricultural sector or areas with little or no access to chemical fertilizer.
Essay # 3. Types of Biofertilizers:
Organisms used for biological fertility management, are either free living or having symbiotic association with plants. They directly or indirectly contribute nutrition to crop plants.
Based on the type of microorganisms, the biofertilizers can also be classified as follows:
1. Bacterial biofertilizers e.g., Rhizobium, Azospirillum, Azotobacter, Acetobacter Phosphobacteria.
2. Fungal bio fertilizers: e.g., Mycorrhiza.
3. Algal bio fertilizers: e.g., blue green algae (BGA) and Azolla- Anabaena association.
4. Actinomycetes biofertilizers: e.g., Frankia.
Biofertilizers are mostly cultured and multiplied in the laboratory. However, blue green algae and Azolla can be mass-multiplied in the field.
Essay # 4. PGPR (Plant Growth Promoting Rhizobacteria) as Biofertilizers
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Numerous species of soil bacteria which flourish in the rhizosphere of plants, but which may be grown in or around plant tissues, stimulate growth by a plethora of mechanisms. These bacteria are collectively known as PGPR. Research on PGPR has been increasing at an ever increasing rate since the term was first used by Kloepper and Coworkers. Not all PGPR are biofertilizers. Many PGPR stimulate the growth of plants by helping to control pathogenic organisms.
For PGPR to have a beneficial effect on plant growth via an enhancement of the nutrient status of their host, their needs to be an intimate relationship between the two, that can be categorized into two levels- (1) Rhizospheric, (2) Endophytic.
In rhizospheric relationships, PGPR many colonize the rhizosphere, the surface of the root, or even superficial intercellular spaces (although this later situation may often involve dead cell layers). In many rhizospheric relationships, the PGPR will actually attach to the surface of plants. How, ever, the means of attachment is much less known.
Azospirillum spp. is one group of PGPR where mechanism involved in attachment, have been well characterized. Colonization of root surfaces by PGPR is not uniform. For example, Kluyvera ascorbata colonizes the upper two-thirds of the surface of canola roots, but no bacteria were detected around tips.
In endophytic relationship, PGPR actually reside within apoplastic spaces inside the host plant. The best characterized symbioses involving colonization of hosts by endophytes are the legume-rhizobia symbiosis. Likewise the infection process and development of N2-fixing specialized structures in the non-legume symbiosis of Parasporia rhizobia, Alnus- Frankia, Azolla-Anabaena, Gunnera-Nostoc and Cycads-Cyanobacteria have also been well characterized.
Some PGPR and endophytic species are known to have cellulose and pectinase activities. Some endophytic PGPR may utilize other organisms as, vectors to gain access to apoplastic spaces in their host. For example both the pink sugarcane mealybug Saccharicoccus sacchari and arbuscular mycorrhizae have been implicated in the infection of host plants by the endohytic diazotroph, Gluconoacetobacter diazotrophicus (formerly Acetobacter diazotrophicus).
The means by which PGPR enhance the nutrient status of host plants can be categorized into five areas:
1. Biological N2 fixation
2. Increasing the availability of nutrients in the rhizosphere
3. Inducing increases in the root surface area.
4. Enhancing other beneficial symbiosis of the host.
5. Combination of modes of action.
A list of PGPR, for which evidence exists that their promotion of plant growth is based on their ability to fix N2 in situ are provided in Table 11.2.
Many PGPR increases the availability of nutrients for the plant in rhizosphere. It involves solubilization of unavailable forms of nutrients and/or siderophore production which helps facilitate the transport of certain nutrients (notably ferric iron). Examples of recently studied associations include Azotobacter chroococcum and wheat, Bacillus circulans and Cladosporium herbarum and wheat, Enterobacter agglomerans and tomato, Pseudomonas chlororaphis and P. putida and soybean, Rhizobium leguminosarum bv. Phaseoli and maize.
Despite wide ranges in the solubility’s and availabilities of soil nutrient species, PGPR that affect root morphology, and more specifically, increases root surface area, can have huge influence on nutrient uptake potentials. Bacterial mediated increases in root weight are commonly reported responses to PGPR inoculations. Most importantly increases in root length and root surface area are sometimes reported.
Inoculation of maize with Azospirillum brasitense resulted in a proliferation of root hours which could have dramatic effect on increasing root surface area. Due to which there is an increase in the volume of soil explored. For e.g., treatment of clipped soybean roots with A. brasilense Sp7 caused a 63% increase in root dry weight, but more than or 6-fold increase in specific root length (root length per unit root dry weight), and more than a 10-fold increase in total root length.
Many actual and putative biofertilizing PGPR produce phyto hormones that are believed to be related to their ability to stimulate plant growth. Indole 3-acetic acid is a phyto- hormone which known to be involved in root initiation, cell division, and cell enlargement. This hormone is very commonly produced by PGPR. Table 11.3 lists recent papers where the production of this hormone has been implicated in the growth promotion by biofertilizing PGPR.
Evidence of GA (Gibberellic acid) production, that are phytohormones associated with modifying plant morphology by the extension of plant tissues, by PGPR, is rare. However Guiterrez-Manero et al, (2001) provide evidence that four different forms of GA are produced by Bacillus pumilus and Bacillus licheniformis.
Inoculation of alder (Alnus glutinosa) with these PGPR, could effectively reverse a chemically induced inhibition of stem growth. The recent discoveries of the involvement of cytokinins, ACC deaminase and possibly GA producing PGPR, opens the possibility that even more plant growth regulating substances may be involved in the promotion of plant growth by some PGPR.
There are evidences in some systems that PGPR may be directly affecting root respiration which in turn leads to increase in the root growth.
Some biofertilizing PGPR sometimes enhance plant growth indirectly by stimulating the relationship between the host plant and beneficial rhizospheric fungi such as arbuscular mycorrhizae (AM) although AM by themselves are well known to enhance the uptake of various soil nutrients (especially phosphorus).
Ratti et al (2001) found that the combination of arbuscular mycorrhizal fungus Glomus aggregatum and the PGPR Bacillus polymyxa and Azospirillum brasilense maximized biomass and P content of the aromatic grass palmarosa (Cymbopogon martini) when grown with an insoluble source of inorganic phosphate. Toro et al (1997) found that both Enterobacter spp. and Bacillus subtilis promoted the establishment of the AM, Glomus itraradices, and increased plant biomass and tissue N and P contents.
Thus there is a huge potential for the use of PGPR as biofertilizing agents for a wide variety of crop plants in a wide range of climatic and edaphic conditions. There has been little extensive commercialization of biofertilizing PGPR. An exception to this is Azospirillum inoculant which is available for a variety of crops in Europe and Africa.
No doubt, the lack of consistent responses in different host cultivars and different field sites, are reasons limiting to the more widespread commercialization of biofertilizing PGPR. A better understanding of rhizosphere ecology, the influence of rhizosphere organisms on each other, must increase before we can be assured that biofertilizing PGPR in inoculants will successfully colonize host rhizosphere and consistently promote the growth of the host plant.
Essay # 5. Phosphate Solubilizing Biofertilizers
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Phosphorus is an important nutrient for plants. There are several microorganisms which can solubilize the cheaper sources of phosphorus, such as rock phosphate. Bacteria like Pseudomonas striata, and Bacillus megaterium are also important phosphorus solubilizing soil microorganisms. Many fungi like Aspergillus and Penicillium are potential solubilizers of bound phosphates. They solubilise the bound phosphorus and make it available to the plant, resulting in improved growth and yield of crops.
Soil phosphates are rendered available to plants by soil microorganisms through secretion of organic acids. Therefore, phosphate dissolving soil microorganisms play some part in correcting phosphorus deficiency in plantation soils. They may also release soluble inorganic phosphate into soil through decomposition of phosphate rich organic compounds. These microbial inoculants can substitute almost 20 to 25% of the phosphorus requirement of plants.
Phosphate solubilizing microbes can also be inoculated to coffee husk along with rock phosphate while preparing compost, to enrich the compost with available phosphorus.
Phosphorus is second most important plant nutrient responsible for various essential metabolic processes in plant growth and development. In nature phosphorus is found in soil, manure, plants and microorganisms in organic and inorganic form in various combinations, but the availability of phosphorus to the plants is critical. The plant absorbs phosphorus from the soil solutions mainly in the form of H2PO4– (orthophosphate) ionic form and smaller amount in the form of secondary orthophosphate ions (HPO4-2).
The concentration of soluble phosphorus in soil solution is 0.1 ppm out of which only an infinitesimal part is available to plants at any time. This is because of the fact that soluble orthophosphate rapidly reacts in soil to form insoluble phosphorus through precipitation and adsorption, this process called “Phosphate fixation”.
A large number of microorganisms belonging to diverse families and genera and of heterotrophic to autotrophic in nature are known to have the capacity of phosphate solubilzation. Out of which Pseudomonas striata, Bacillus polymyxa, Aspergillus awamorii, A. niger and Penicillium digitatum have been found to be most promising and are used as mother culture in the production of phosphate solubilizing biofertilizers. These formulations are popularly known as PSM, Phosphotika, Microphos etc.
When carrier based phosphetic biofertilizers are applied to the crops as seed, seedling root, or soil treatment, the microorganism present in the inoculant multiply along the developing roots and develop a thick population in the soil adjacent to roots.
Here these microorganisms develop a sort of temporary associative symbiotic type of relationship, in which they derive their food from roots in the form of root exudates and provide mineral nutrients which normally could not be solubilzed by the roots including phosphorus.
Essay # 6. Mycorrhizae:
Mycorrhizae are a group of fungi that include a number of types based on the different structures formed inside or outside the root. These are specific fungi that match with a number of favourable parameters of the host plant on which it grows. This includes soil type, the presence of particular chemicals in the soil types, and other conditions.
These fungi grow on the roots of these plants. In fact, seedlings that have mycorrhizal fungi growing on their roots survive better after transplantation and grow faster. The fungal symbiont gets shelter and food from the plant which, in turn, acquires an array of benefits such as better uptake of phosphorus, salinity and drought tolerance, maintenance of water balance, and overall increase in plant growth and development.
While selecting fungi, the right fungi have to be matched with the plant. There are specific fungi for vegetables, fodder crops, flowers, trees, etc.
Mycorrhizal fungi can increase the yield of a plot of land by 30%- 40%. It can absorb phosphorus from the soil and pass it on to the plant.
Mycorrhizal plants show higher tolerance to high soil temperatures, various soil- and root-borne pathogens, and heavy metal toxicity.
Essay # 7. Role of Biofertilizers
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Biofertilizers are inputs containing microorganisms, which are capable of mobilizing nutritive elements from non-usable form to usable form through biological processes. They are less expensive, eco-friendly and sustainable. The beneficial microbes in the soil, which are of greater significance to horticultural crops are, biological nitrogen fixers, phosphate solubilizers and mycorrhizal fungi which are the phosphate scavengers.
Nitrogen is one of the chief and important constituents of protein and nucleic acid molecules that play the basic role in cell metabolism, growth, reproduction and transmission of heritable characters. Therefore, without a constant supply of this unavoidable element, life cannot go on.
Biological nitrogen fixation (BNF) is the reduction of atmospheric nitrogen to ammonia by microorganisms in the soil. It involves highly specialized and intricately evolved interactions between soil microorganisms and higher plants for harnessing the atmospheric nitrogen. It is a fascinating biological phenomenon studied extensively to provide low-cost nitrogen and improve crop productivity.
The nitrogen fixing organisms associated with horticultural crops are the Rhizobium species, living in symbiotic relationship with the leguminous plants and free living fixers belonging to the Azotobacter family and the Azospirilla living in association with the root system of crop plants.
Some soil microorganisms play an important role in improving soil fertility and crop productivity due to their capability to fix atmospheric nitrogen, solubilize insoluble phosphate and decompose farm wastes resulting in the release of plant nutrient. The extent of benefit from these microorganisms depends upon their number and efficiency, which however, is governed by a large number of soil and environmental factors.
When the number and activity of specific microorganism called microbial inoculant or biofertilizer is used to hasten biological activity to improve availability of plant nutrient. One of the essential plant nutrients on which successful agriculture depends to a great extent is nitrogen. High crop requirements, susceptibility to gaseous and leaching losses have made nitrogen the most in demand fertilizer material followed by phosphorus.
Although air has about 80% of nitrogen, plants cannot make use of atmospheric N. The discovery of bacteria which had the ability to reduce non-usable (atmospheric nitrogen) into usable form (ammoniacal) had been a major breakthrough in the field of agricultural research.
Of late, increasing attention is being paid to harmness the potential benefits from these renewable sources of plant nutrients because of the following reasons:
1. Depleting soil fertility due to widening gap between nutrient removal and supplies.
2. Depleting feedstock/fossil fuels and increasing cost of fertilizers.
3. Growing concern about environmental hazards.
4. Increasing threat to sustainable agriculture.
A number of products are now available that are generally referred to as soil and plant additives, of non-traditional nature.
These products include:
1. Microbial fertilizers and soil inoculants contain unique and beneficial strains of soil microorganisms.
2. Microbial activators that supposedly contain special chemical formulations for increasing the numbers and activity of beneficial microorganisms in soil.
3. Soil conditions that claim to create favourable soil physical and chemical conditions which result in increased growth and yield of crops.
4. Vermicompost helps in improving soil health and fertility.
Nitrogen fixing organisms can be provided to the farmers in the name of microbial inoculants otherwise termed as biofertilizers.
The biofertilizers containing biological nitrogen fixing organisms are of utmost important in agriculture in view of the following advantage:
1. They help in the establishment and growth of crop plants and trees.
2. They enhance biomass production and grain yields by 10-20%.
3. They are useful in sustainable agriculture.
4. They are suitable in organic farming.
5. They play an important role in agroforestry/silvipastoral systems.
After the advent of the acetylene test for assessing nitrogen fixing potential, rapid progress has been made in identifying nitrogen fixing organisms. Out of a large number of microorganisms possessing the property of nitrogen fixation, only a few such as Rhizobium, Azotobacter, Azospirillum, BGA, Azolla, etc. have been commercially exploited. The reaction in biological nitrogen fixation is essential the same as in production of chemical fertilizers (Haber-Bosch process) i.e., the catalytic reduction of dinitrogen (N2) to ammonia (NH3).
In India, Rhizobium biofertilizers specific for different legumes and blue green algae are the most popular among farmers, Azospirillum and Azotobacter are at an intermediate stage of acceptance while phosphate solubilizing microorganisms (PSM), VAM, Azolla, cellulose decomposing organisms etc. are at preliminary stage.
Essay # 8. Economic and Environmental Benefits of Biofertlizer:
The production of fertilizers depends on nonrenewable energy sources and is energy intensive. The energy requirements for production of one kg fertilizer nitrogen are 11.2, phosphorus 1.1 and potash 1.0 KWH respectively. The requirement of petroleum based energy source in the production of biofertilizer is almost negligible. Organisms like Rhizobium, Azospirillum etc. assimilate 20-40 mg nitrogen g-1 of carbon.
The biofertilizer production cost is very low so is the selling price. It has been estimated that one kg of fertilizer nitrogen costs more than rupees six while biofertilizer nitrogen costs only twenty paise through Rhizobium and fifty paise through Blue green algae (BGA).
On nutrient basis, one tonne of Rhizobium biofertilizer is equivalent to 100 tonnes of fertilizer nitrogen (considering 50 kg N fixed ha-1 by application of 500 g Rhizobium biofertilizer) and one tonne of soil base BGA is equivalent to two tonnes of fertilizer nitrogen (considering 20 kg of N fixed ha-1) by application of 10 kg BGA.
Biofertilizers are cost effective as well as environment friendly. They have a favourable influence on soil health. Biofertilizers are compatible with chemical fertilizers and agrochemicals like pesticides and herbicides without demanding extra or special care in their handling. They are safe to crops and users both.
The use of biofertilizer in various cropping systems may save 20-40 kg ha-1 of fertilizer nitrogen and 10-20 kg ha-1 phosphoric acid per cropping season. The success in the transfer of nif operon from nitrogen fixing bacterium Klebsiella pneumonia into non-nitrogen fixing bacterium Escherichia coli has shown promising possibilities like achieving nodule formation in non-modulating plants, industrial preparation of nitrogenase enzymes and their use in nitrogen fixation by-passing the use of non-renewable energy sources.
Biofertilizers are available for almost all crops and for three nutrients – nitrogen, phosphorus and zinc and, therefore, the biofertilizer consumption is expected to increase many folds in coming years.
Essay # 9. Development of Biofertilizer Industry
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The prominent role of legumes in soil fertility was first noted by J. B. Boussingault (1834), a French farmer, substantiated by Hellreigel (1886) in Germany by identifying root nodule as advantageous growth and Biejerinck (1888), a Dutch Scientist, who discovered that nodule is formed by bacteria, now called Rhizobia. Beginning of legume inoculation was made by Nobbe and Hiltner (1895) in Germany by using extracts of crushed root nodules under trade name Nitrogin.
The early method of an application of inoculum involved top dressing of new fields with large quantity of soils transported from places where the legume grew well. This cumbersome soil transfer method was probably the reason for the birth of the present day inoculant production industry. Instead of the bulk transfer of soil which involves spreading of soil borne pathogens, weed seeds, ineffective rhizobia etc. The present method, supplies Rhizobium biofertilizers in small packets commercially.
The first inoculants were pure cultures of Rhizobium growing on solid medium and these were marketed in 8-10 ounces glass bottles. Since then, the methods of producing inoculants commercially have undergone a significant evolution to meet the demands of present day utilization of biological nitrogen fixation technology in agriculture. Rhizobium biofertilizers are commercially produced in both developed where fertilizer N is affordable and developing countries.
The earliest documented evidence of the production of biofertilizers from isolated Rhizobium strains exist in India. India is the largest producer and consumer of biofertilizers in the world, Madhya Pradesh, Tamil Nadu and Gujarat being the major producing states.