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Q15:
How do biofertilisers enrich the fertility of the soil?
Solution :
Definition and Theoretical Foundation
Biofertilizers are defined as preparations containing live or latent cells of efficient strains of nitrogen-fixing, phosphate-solubilizing, or cellulolytic microorganisms. They function by augmenting the nutrient supply of the host plant via biological processes, primarily biological nitrogen fixation ($N_2$-fixation), solubilization of insoluble phosphorus, and the synthesis of growth-promoting substances [Per the principles of Soil Microbiology].
Step 1: Biological Nitrogen Fixation
The primary mechanism by which biofertilizers enrich soil fertility is through the conversion of atmospheric dinitrogen ($N_2$) into ammonia ($NH_3$), a form readily assimilable by plants. This process is mediated by the enzyme nitrogenase [Per the Nitrogenase Catalysis Hypothesis].
- Symbiotic Fixation: Rhizobium bacteria form root nodules in leguminous plants. Within these nodules, the bacteria fix atmospheric $N_2$ and supply it to the plant in exchange for carbohydrates.
- Free-living/Associative Fixation: Organisms such as Azotobacter and Azospirillum live freely in the soil or in the rhizosphere, enriching the soil nitrogen pool independently of a host.
Step 2: Phosphate Solubilization
Phosphorus exists in soil often as insoluble mineral complexes. Biofertilizers enrich the available nutrient pool by secreting organic acids (e.g., citric acid, gluconic acid) that lower the pH and dissolve these complexes [By Chelation and Acidification Mechanisms].
- Bacteria (PSB): Strains such as Bacillus polymyxa and Pseudomonas striata.
- Fungi (PSF): Mycorrhizal fungi (e.g., Glomus) extend their hyphal network, vastly increasing the surface area for phosphorus absorption from the soil matrix.
Step 3: Synthesis of Plant Growth-Promoting Substances
Beyond nutrient availability, many biofertilizers (such as Cyanobacteria/Blue-green algae like Anabaena and Nostoc in paddy fields) contribute to soil health by:
| Mechanism | Effect on Soil/Plant |
|---|---|
| Secretion of Phytohormones | Stimulates root elongation and nutrient uptake efficiency. |
| Synthesis of Vitamins | Promotes microbial diversity in the rhizosphere. |
| Production of Siderophores | Chelates iron, making it bioavailable while suppressing pathogens. |
Summary of Soil Enrichment
Biofertilizers do not replace the soil's organic matter; rather, they catalyze the transformation of inert nutrient stocks into bioavailable forms. By restoring the natural microbial balance, they enhance soil structure and long-term chemical fertility without the deleterious effects of excessive synthetic chemical runoff.
Final Solution: Biofertilizers enrich soil fertility by fixing atmospheric nitrogen via organisms like Rhizobium and Azotobacter, solubilizing insoluble phosphorus through phosphate-solubilizing bacteria and mycorrhizal fungi, and augmenting the soil with essential growth-promoting substances and vitamins, thereby increasing nutrient bioavailability for plant uptake.
More Questions from Class 12 Biology Microbes in Human Welfare EXERCISES
- Q1: Bacteria cannot be seen with the naked eyes, but these can be seen with the help of a microscope. If you have to carry a sample from your home to your biology laboratory to demonstrate the presence of microbes with the help of a microscope, which sample would you carry and why?
- Q1: Bacteria cannot be seen with the naked eyes, but these can be seen with the help of a microscope. If you have to carry a sample from your home to your biology laboratory to demonstrate the presence of microbes with the help of a microscope, which sample would you carry and why?
- Q2: Give examples to prove that microbes release gases during metabolism.
- Q2: Give examples to prove that microbes release gases during metabolism.
- Q3: In which food would you find lactic acid bacteria? Mention some of their useful applications.
- Q3: In which food would you find lactic acid bacteria? Mention some of their useful applications.
- Q4: Name some traditional Indian foods made of wheat, rice and Bengal gram (or their products) which involve use of microbes.
- Q4: Name some traditional Indian foods made of wheat, rice and Bengal gram (or their products) which involve use of microbes.
- Q5: In which way have microbes played a major role in controlling diseases caused by harmful bacteria?
- Q5: In which way have microbes played a major role in controlling diseases caused by harmful bacteria?
- Q6: Name any two species of fungus, which are used in the production of the antibiotics.
- Q6: Name any two species of fungus, which are used in the production of the antibiotics.
- Q7: What is sewage? In which way can sewage be harmful to us?
- Q7: What is sewage? In which way can sewage be harmful to us?
- Q8: What is the key difference between primary and secondary sewage treatment?
- Q8: What is the key difference between primary and secondary sewage treatment?
- Q9: Do you think microbes can also be used as source of energy? If yes, how?
- Q9: Do you think microbes can also be used as source of energy? If yes, how?
- Q10: Microbes can be used to decrease the use of chemical fertilisers and pesticides. Explain how this can be accomplished.
- Q10: Microbes can be used to decrease the use of chemical fertilisers and pesticides. Explain how this can be accomplished.
- Q11: Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled \(A\), \(B\) and \(C\); but the laboratory attendant did not note which was which. The BOD values of the three samples \(A\), \(B\) and \(C\) were recorded as 20mg/L, 8mg/L and 400mg/L, respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?
- Q11: Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled (A), (B) and (C); but the laboratory attendant did not note which was which. The BOD values of the three samples (A), (B) and (C) were recorded as (20mg/L), (8mg/L) and (400mg/L), respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?
- Q12: Find out the name of the microbes from which Cyclosporin \(A\) (an immunosuppressive drug) and Statins (blood cholesterol lowering agents) are obtained.
- Q12: Find out the name of the microbes from which Cyclosporin (A) (an immunosuppressive drug) and Statins (blood cholesterol lowering agents) are obtained.
- Q13(a): Find out the role of microbes in the following and discuss it with your teacher. (a) Single cell protein (SCP)
- Q13(a): Find out the role of microbes in the following and discuss it with your teacher. (a) Single cell protein (SCP)
- Q13(b): Find out the role of microbes in the following and discuss it with your teacher. (b) Soil
- Q13(b): Find out the role of microbes in the following and discuss it with your teacher. (b) Soil
- Q14: Arrange the following in the decreasing order (most important first) of their importance, for the welfare of human society. Give reasons for your answer. Biogas, Citric acid, Penicillin and Curd
- Q14: Arrange the following in the decreasing order (most important first) of their importance, for the welfare of human society. Give reasons for your answer. Biogas, Citric acid, Penicillin and Curd
- Q15: How do biofertilisers enrich the fertility of the soil?
CBSE Solutions for Class 12 Biology Microbes in Human Welfare
Chapters in CBSE - Class 12 Biology
- Reproduction in Organisms
- Human Reproduction
- Reproductive Health
- Principles of Inheritance and Variation
- Molecular Basis of Inheritance
- Evolution
- Human Health and Disease
- Strategies for Enhancement in Food Production
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
- Environmental Issues
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