Biofertilizers are defined as formulations containing either living or latent cells of efficient strains of microorganisms that facilitate the uptake of nutrients form crop plants. They execute this pivotal role through interactions in the plant rhizosphere when applied through seed or soil. Biofertilizers accelerate certain microbial processes in the soil which supplement nutrients in a form easily assimilated by plants. Biofertilizers supply nutrients through the natural processes of nitrogen fixation, solubilizing phosphorus and stimulating plant growth through the synthesis of growth-promoting substances. Currently, biofertilizers are an important component of the integrated nutrient supply system.
Biofertilizers like Rhizobium, Azotobacter, Azospirillum and blue-green algae (BGA) are in use for decades. However, these microorganisms are very often not as efficient in natural surroundings as desired; thus, application of massively multiplied cultures of selected efficient microorganisms is needed to accelerate the microbial processes in soil. Therefore, the use of biofertilizers is strongly recommended by the competent professionals to guarantee good plant growth and higher production yields.
Biological fertilization (or biofertilization) as a process of application of natural inputs including fertilizers offers significant advantages in the efforts of contemporary agriculture to reduce the use of chemical fertilizers and pesticides. The most important advantages can be summarized as follows:
Biofertilizers are cost effective relative to chemical fertilizers. They differ from chemical and organic fertilizers because they do not directly supply any nutrients to crops and constitute cultures of special bacteria and fungi with relatively low installation cost. The use of biofertilizers can improve the productivity per unit area in a relatively short time. They have lower manufacturing costs and reduced use costs, especially regarding nitrogen and phosphorus use. Their easy way of application consumes smaller amounts of energy. This means lower costs associated with the process of fertilization that can be directly translated into profitable benefits for farmers. In this sense, application of biological fertilizers can bring benefits from an economic point of view, since biofertilizers are a cost effective and renewable source of plant nutrients to substitute the chemical fertilizers for sustainable agriculture.
Most commonly biofertilizers are in powder, carrier-based form. The carrier usually is lignite. The lignite has high organic matter content and holds more than 200% water. This high water content enhances the growth of the microorganisms. The application method for this type of biofertilizers is preparation of slurry, which is applied to the seeds. This method was considered universal until recently.
At present, however, another method, dry complex fertilizer for direct soil application, has been developed. It consists of granules (1–2 mm) made from tank bed clay (TBC) and baked at 200 °C in a muffle furnace, which helps to sterilize the material and gives porosity to the granules. The baked granules are soaked in a suspension of desired bacteria grown in a suitable medium overnight. The clay granules are air-dried at room temperature under aseptic conditions. They contain about 109 bacteria per gram of granules. These granules are suitable for field application along with seeds. However, the quantity of biofertilizer to be applied is slightly higher than that in seed application.
Biofertilizer is a technological innovation that has the potential to increase crop yield, reduce production cost and improve soil condition.
Biofertilizers can be considered as supplementary to chemical fertilizers. When they are applied as seed or soil inoculants, they multiply and participate in the nutrient cycling, thus benefiting the crop productivity. Biofertilizers have great potential to improve crop yields through environmentally better nutrient supplies. They provide reserve plant nutrients. It is reported that biofertilizers increase crop yields by 20–30% and stimulate plant growth. The efficiency of biofertilizer use is the key characteristic that ultimately contributes to the increase of the crop yield.
There are numerous examples that biofertilizers positively affect the crop yield. For instance, Vital N®, an organic biofertilizer registered with the Philippine FPA, is a powder formulation that induces extensive growth in roots of crops like corn, rice, banana, garlic, orchids and onion. It contains Azospirillium, a beneficial bacterium that produces the plant-growth-stimulating substance indole-3-acetic acid (IAA), resulting in higher growth yield.
There are reports that the overall performance of potato crops is positively influenced by application of green manures (cowpea and Crotolaria sp.): 30% yield improvements. The increased productivity values verify the efficiency of biofertilizers in agricultural production. On the other hand, some physicochemical properties of the soil are improved and environmental impacts due to the prolonged use of chemical fertilizers are gradually mitigated.
Furthermore, 10% increases in the yield per hectare have been observed for crops treated with arbuscular mycorrhizal (AM) fungi, combined with increased resistance of the plants to the action of pathogenic microorganisms. Additionally, when AM is combined with nitrogen-fixing bacteria or compost extracts, this combined use of biofertilizer on crops provides better yield performance, higher by a factor of two, and better physical characteristics of individual plants.
A trial investigating the feasibility of biofertilizers prototypes based on native bacteria from rice crops reported 10% increases in yield production by using the mixtures, from 7,625 kg/ha to 8,500 kg/ha. The main outcomes deal with the importance of biofertilizers to get higher revenues and increase productivity, in order to achieve, progressively, sustainable agricultural development.
The application of the aquatic fern–cyanobacteria symbiotic association Azolla–Anabaena as a biofertilizer in rice paddies of northern Italy allowed obtaining yields close to 40 kg nitrogen/ha during a 3-month period and verifying increases in the growth rate of rice. Furthermore, higher resistance of some of the rice species to the presence of herbicide Propanil was evidenced.
Biofertilizers contribute to the maintenance of stable nitrogen (N) concentrations in the soil. They replace chemical nitrogen by 25%. Thus, nitrogen-fixing microorganisms play an important role in nitrogen supply by converting atmospheric nitrogen into organic forms usable by plants. Use of biological N2-fixation technology can contribute to a decrease in the N fertilizer application and to the reduction of environmental risks. Azotobacter (free-living N2-fixer) plays an important role in the nitrogen cycle in nature due to its diverse metabolic potential. In addition to N2 fixation, this microorganism has the ability to synthesize and secrete considerable amounts of biologically active substances, among which the vitamins thiamine and riboflavin, nicotinic acid, pantothenic acid, biotin; the plant-growth hormones heteroxins, gibberellins. These biologically active substances help in modification of the nutrient uptake by the plants. Another free-living N2-fixer, Azospirillum, is reported to produce plant-growth-promoting substances indole acetic acid (IAA) and indole butyric acid (IBA) and increase the rate of mineral uptake by plant roots, resulting in the enhancement of plant yield.
It is well known that most plants form symbiotic associations with the arbuscular mycorrhizal fungi (AMF) acting as bio-ameliorators. They have the potential to considerably enhance the rhizospheric soil characteristics. This, in turn, leads to improved soil structure and promotes plant growth under normal as well as stressed conditions. The results revealed that the AMF-induced enhancement in nutrient uptake promotes various biologically important metabolites. Among them of special importance are the plant hormones, including GA and auxin, which play a unique role in plant growth regulation under both normal and stress conditions. The activity of phytohormones like cytokinin and IAA is also significantly higher in plants inoculated with AMF. Higher hormone production results in better growth and development of the plant.
The use of biofertilizers is not only cost effective; it also augments the problem of environmental pollution. They are environmentally friendly because their use not only prevents damaging the natural resources but also helps to some extent to free the plants of precipitated chemical fertilizers. Biofertilizers promote the reduction of environmental impacts associated with the excessive use of chemical fertilization. Thus, their use in organic farming, sustainable agriculture, green farming and non-pollution farming contribute to implementation of healthy environment policies at national, regional and global level.
All types of crops grown in different agro-ecologies can benefit from the use of biofertilizers. Continuous use of biofertilizers enables the microbial population to remain and build up in the soil and helps in maintaining soil fertility contributing to sustainable agriculture.
Biofertilizers keep the soil environment rich in all kinds of micro- and macro-nutrients via nitrogen fixation, phosphate and potassium solubilization or mineralization, release of plant-growth-regulating substances, production of antibiotics and biodegradation of organic matter in the soil. Growing crops using biofertilizers is advantageous in protecting the soil from degradation. Biofertilizers can mobilize nutrients that favour the development of biological activities in soils. In this way, they prevent micro-nutrient deficiencies in plants and guarantee better nutrient uptake and increased tolerance to drought and moisture stress, all factors that strongly contribute to soil fertility.
The use of biofertilizers can promote antagonism and biological control of phytopathogenic organisms. Thus, positive effect on soil microbiology is exerted: suppression or control through competition of pathogenic populations of microorganisms present on the soil.
Strategies for biological control of fungal species in crops include application of biofertilizers obtained from biological digestion to control target pests and pathogens. Through the siderophores and antibiotics produced by them, biofertilizers are antagonistic to foliar or rhizosphere pathogenic bacteria, fungi and insects.
Arbuscular mycorrhizal fungi (AMF) have the potential to reduce damage caused by soil-borne pathogenic fungi, nematodes and bacteria. Meta-analysis has shown that AMF generally decrease the effects of fungal pathogens. A variety of mechanisms have been proposed to explain the protective role of mycorrhizal fungi. The major mechanism is nutritional, because plants with a good phosphorus status are less sensitive to pathogen damage. Non-nutritional mechanisms are also important, because mycorrhizal and non-mycorrhizal plants with the same internal phosphorus concentration may still be differentially affected by pathogens. Such non-nutritional mechanisms include activation of plant defense systems, changes in exudation patterns and concomitant changes in mycorrhizosphere populations, increased lignification of cell walls and competition for space for colonization and infection sites.
Recently, several fungal endophytes, like Trichoderma spp. (Ascomycota) and Sebacinales (Basidiomycota, with Piriformospora indica as a model organism), which are distinct from the mycorrhizal species, have attracted scientific attention. These fungi are able to live at least part of their life cycle away from the plant, to colonize its roots and to transfer nutrients to their hosts, using mechanisms that are not clear yet. They are receiving increasing attention, both as plant inoculants easier to multiply in vitro and as model organisms for revealing the mechanisms of nutrient transfer between fungal endosymbionts and their hosts.
Trichoderma spp. have been extensively studied and used for their biopesticidal (mycoparasitic) and biocontrol (inducer of disease resistance) potential, and have been exploited as sources of enzymes by biotechnological industries. Now it is speculated (on the basis of convincing evidence) that Trichoderma spp. also induce many plant responses. Among the most important of them are the increased tolerance to abiotic stress, nutrient use efficiency and organ growth and morphogenesis.
On the basis of these effects, these fungal endophytes may be regarded as both biopesticides and biostimulants.
Biofertilizers contribute to better physical conditions in the soil through improvement of structure and aggregation of soil particles, reducing compaction and increasing the pore spaces and water infiltration. They improve soil structure and allow better tilth; ensure better soil aeration and water percolation, reducing soil erosion. Biofertilizers serve as major food source for microbial populations thus keeping the soil alive. They also contribute to soil chemical conditions through improvement of nutrients availability in the soil, leaving free elements to facilitate their absorption by the root system; improved capacity of nutrients’ exchange in the soil resulting in favourable effects on the physico-chemical stability of soils. As a result of the good structure and improved stability provided to the soil, root growth is promoted.
The maintenance of good soil structure in all ecosystems is largely dependent on mycorrhizal fungi. Formation and maintenance of soil structure is influenced by soil properties, root architecture and management practices. The use of machines and fertilizers are considered to be responsible for soil degradation, which is a key component of soil structure. Mycorrhizal fungi contribute to maintain good soil structure through the following processes:
Biofertilizers increase the water and nutrient holding capacity of the soil and also increase the drainage and absorption of moisture in soils, especially in those with structural deficiencies or lack of nutrients. They increase the tolerance towards drought and moisture stress. In this way, they increase the crop yield even in plantations that lack sufficient natural water supply or irrigation. For instance, AM association improves the hydraulic conductivity of roots at lower soil water potentials and this improvement is one of the factors contributing towards better uptake of water by plants. Moreover, leaf wilting after soil drying does not occur in mycorrhizal plants until the soil water potential is considerably lowered (approx. 1.0 MPa). Mycorrhiza-induced drought tolerance can be related to factors associated with AM colonization such as improved leaf water and turgor potentials and maintenance of stomatal functioning and transpiration, greater hydraulic conductivities and increased root length and development.
The most important and contributing function of biofertilizers is considerable reduction in environmental pollution and improvement of agro-ecological soundness. Biofertilizers are eco-friendly organic agro-input compared to chemical fertilizers. They cause no harm to ecosystems and are valuable to the environment as they enable reduced use of chemical fertilizers in the production of crops worldwide. Namely due to their eco-friendly characteristics, the demand for biofertilizers is on the increase during the last decade. Their activities influence the soil ecosystem and produce supplementary substances for the plants. Providing continuous supply of balanced micronutrients to the plants and eliminating plantar diseases, biofertilizers enhance the maintenance of plant health and contribute to soil ecology. The provided food supply and impelled growth of beneficial microorganisms contribute to sustain the ecological balance. In the long run, biofertilizers are planned to complement and, where appropriate, replace conventional chemical fertilizers, resulting in economic and environmental benefits.
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