Biofertilizers is part of Biology. Below are notes on 8 sub-topics, followed by practice MCQs from State PSC and UPSC papers.

Classification of Biofertilizers based on microbial type and function

  • Biofertilizers are microorganisms that enrich soil nutrient content, promoting plant growth.
  • Classification based on Microbial Type and Function:
  • 1. Nitrogen-fixing Biofertilizers: Convert atmospheric N2 into plant-available forms.
  • a. Symbiotic Nitrogen Fixers:
  • Rhizobium: Associates with legumes (e.g., Soyabean, Pea) forming root nodules.
  • Anabaena azollae: Symbiotically lives within the water fern Azolla.
  • Azolla: A water fern acting as a biofertilizer (due to Anabaena azollae), extensively used in rice paddies.
  • b. Associative Symbiotic Nitrogen Fixers:
  • Azospirillum: Associates closely with roots of grasses (e.g., Maize, Sorghum).
  • c. Free-living Nitrogen Fixers:
  • Azotobacter: Aerobic, non-symbiotic soil bacteria, fixes N2 for non-leguminous plants.
  • Cyanobacteria (Blue-green algae, BGA): Photosynthetic prokaryotes (e.g., Anabaena, Nostoc), fix N2, commonly used in rice fields.
  • 2. Phosphorus-mobilizing/solubilizing Biofertilizers: Make insoluble soil phosphorus available to plants.
  • a. Mycorrhiza (Fungi): Form symbiotic associations with plant roots (e.g., Pine, many crops), enhancing phosphorus and water uptake (e.g., VAM fungi like Glomus).
  • b. Phosphate Solubilizing Bacteria (PSB): E.g., Bacillus, Pseudomonas, solubilize insoluble inorganic phosphates.

Definition and Significance of Biofertilizers

  • Biofertilizers: Cultures of microorganisms applied to soil/seed/plants to enhance nutrient availability and promote plant growth.
  • Rhizobium: A type of biofertilizer, known as a symbiotic nitrogen-fixing bacterium for leguminous plants.
  • Saccharomyces cerevisiae (Yeast): Microorganism used in bakery for CO₂ production (leavening) and in beer/wine production for ethanol (fermented end product).
  • Beer production: Primarily uses barley as the substrate.
  • Biogas: An excellent fuel comprising up to 75% methane; burns without smoke and leaves no ash residue; possesses high heating capacity; used as both fuel and a source of light.
  • Koji (Aspergillus oryzae): Microorganism utilized in the fermentation of soybean, for products like Miso.
  • Miso: A fermented condiment derived from soybean.
  • Bioremediation: The use of microorganisms to degrade environmental pollutants.
  • Phytoremediation: A specific type of bioremediation that employs plants to degrade environmental pollutants.
  • Probiotics: Microorganisms that, when consumed, confer beneficial health effects to the host.
  • Natural probiotic sources: Examples include Buttermilk, Kefir, Tempeh, and Sourdough bread.
  • Sauerkraut fermentation (initial stage): Often initiated by Leuconostoc mesenteroides, a type of Lactic Acid Bacteria.

Mechanism of Action of Biofertilizers (Nutrient cycling, phytohormone production)

  • Nitrogen Fixation (Nutrient Cycling):
    • Biofertilizers (e.g., Rhizobium, Azotobacter, cyanobacteria) convert atmospheric nitrogen (N2) into plant-available forms like ammonia (NH3).
    • Fundamental Requirements for Nitrogen Fixation:
      • Nitrogenase enzyme: Catalyzes N2 reduction; highly sensitive to oxygen.
      • ATP: Provides energy for the reaction (high energy demand).
      • Ferredoxin (or other strong reductants): Acts as an electron donor to nitrogenase.
      • Anaerobic or Microaerobic conditions: Crucial to protect the oxygen-sensitive nitrogenase enzyme (e.g., leghemoglobin in legume nodules maintains low O2).
      • Trace elements: Molybdenum (Mo) and Iron (Fe) are cofactors of nitrogenase.
  • Phytohormone Production:
    • Many biofertilizers (e.g., Azotobacter, Bacillus, Pseudomonas) produce plant growth-promoting substances.
    • These include auxins, gibberellins, and cytokinins.
    • These phytohormones stimulate root development, shoot growth, nutrient uptake, and overall plant vigor.
  • Phosphate Solubilization/Mobilization (Nutrient Cycling):
    • Phosphate Solubilizing Bacteria (PSB) like Bacillus, Pseudomonas convert insoluble phosphate into soluble forms usable by plants (e.g., by producing organic acids like citric, lactic acid).
    • Arbuscular Mycorrhizal Fungi (AMF) extend the root system, enhancing uptake of immobile nutrients like phosphorus.
  • Siderophore Production:
    • Some biofertilizers produce siderophores, which chelate iron (Fe) from the soil, making it available to plants and also sequestering it from plant pathogens.
  • Disease Suppression:
    • Some biofertilizers enhance plant immunity or competitively exclude pathogens, leading to disease suppression.

Mycorrhizal Biofertilizers (e.g., Vesicular Arbuscular Mycorrhizae - VAM)

  • Mycorrhiza: Mutualistic symbiotic association between fungi and roots of higher plants.
  • Fungi component: Essential for nutrient (especially phosphorus) and water uptake by plants from soil.
  • Classification: Functions as a biofertilizer, enhancing plant growth and health.
  • VAM (Vesicular Arbuscular Mycorrhizae): A major type where fungal hyphae (e.g., Glomeromycota) penetrate root cells, forming structures like vesicles (storage) and arbuscules (nutrient exchange sites).

Nitrogen-fixing Biofertilizers: Non-symbiotic/Free-living (e.g., Azotobacter, Azospirillum, Cyanobacteria/BGA)

  • Nitrogen-fixing Biofertilizers (Free-living/ Non-symbiotic): Convert atmospheric molecular nitrogen (N₂) into ammonia (NH₃).
  • Azotobacter:
    • Free-living, aerobic bacterium.
    • Fixes atmospheric N₂ into ammonia.
    • Applications: Non-leguminous crops like Wheat, Rice, Maize, Cotton, Mustard, Sorghum, Sugarcane.
  • Azospirillum:
    • Free-living, associative symbiotic bacterium.
    • Fixes atmospheric N₂.
    • Applications: Cereals (Maize, Wheat, Sorghum, Millets), grasses, sugarcane.
  • Cyanobacteria (Blue-Green Algae / BGA):
    • Photosynthetic prokaryotes; free-living nitrogen fixers.
    • Key genera: Nostoc, Anabaena.
    • Primary application: Water-logged rice/paddy fields to supply nitrogen.
    • Associated with biological nitrogen fixation.
  • Azolla:
    • Aquatic fern used as a bio-fertilizer, especially in water-logged rice/paddy fields.
    • Its bio-fertilizer property is due to a symbiotic association with the nitrogen-fixing cyanobacterium Anabaena azollae.
    • Can be used as supplementary food for cattle, particularly milking bovines.
    • Not suitable for maize.

Nitrogen-fixing Biofertilizers: Symbiotic (e.g., Rhizobium, Frankia)

  • Symbiotic Nitrogen-fixing Biofertilizers: Microorganisms forming mutualistic associations with specific plants to convert atmospheric N₂ into a plant-usable form (ammonia).
  • Rhizobium:
    • Gram-negative bacteria forming symbiotic relationships with leguminous plants (e.g., pulses, peas, beans, clover, groundnuts).
    • Resides within root nodules of legumes, where it fixes atmospheric nitrogen.
    • Responsible for nitrogen fixation by legumes.
  • Frankia:
    • Actinomycete (filamentous bacteria) forming symbiotic associations with root nodules of certain nonleguminous plants.
    • Such plants are called actinorrhizal plants, e.g., Casuarina, Alnus, Myrica.
    • Fixes atmospheric nitrogen in the root nodules of these plants.

Phosphate-Solubilizing

and

Mobilizing

Biofertilizers (PSB)

  • Phosphate-Solubilizing Biofertilizers (PSB) are microorganisms that convert insoluble soil phosphorus into forms accessible to plants.
  • Mechanism: Produce organic acids (e.g., gluconic, lactic, citric, succinic, malic) to lower soil pH and chelate cations, and secrete phosphatase enzymes.
  • Key Bacterial PSB: Pseudomonas (e.g., P. striata), Bacillus (e.g., B. megaterium, B. polymyxa), Micrococcus, Flavobacterium, Thiobacillus.
  • Key Fungal PSB: Aspergillus (e.g., A. niger), Penicillium (e.g., P. digitatum).
  • Benefits: Enhances phosphorus uptake by plants, promotes root development, increases nutrient use efficiency (P, Zn, Fe), and improves crop yield.

Role of Biofertilizers in Sustainable Agriculture and Organic Farming

  • Biofertilizers: Living microorganisms applied to soil or seeds, colonize rhizosphere, promote plant growth by increasing nutrient supply (e.g., N, P).
  • Types of Biofertilizers used in Organic Farming:
    • Nitrogen-fixing microorganisms:
      • Symbiotic: Rhizobium (legumes).
      • Free-living: Azotobacter, Azospirillum.
      • Cyanobacteria (Blue-green algae): Anabaena, Nostoc (especially in rice paddies).
    • Phosphate-solubilizing microorganisms (PSM): Bacillus, Pseudomonas, Aspergillus.
  • Organic Farming: System that avoids synthetic inputs like chemical fertilizers, pesticides, GMOs. Emphasizes bio-fertilizers, biopesticides, crop rotation, green manuring, vermicomposting.
  • Organic Manure:
    • Enhances soil water-holding capacity.
    • Provides humus to soil.
    • Improves soil texture.
    • Obtained from both plant (e.g., crop residues, green manure) and animal matter (e.g., farmyard manure).
  • Green Manuring: Practice of incorporating undecomposed green plant material into soil.
    • Goal: Enriches soil with nutrients and organic matter.
    • Most suitable crops: Leguminous plants like Sunhemp (Crotalaria juncea), Dhaincha, Mung.
  • Vermicomposting: Process using earthworms to convert organic waste into nutrient-rich humus (vermicompost).
    • Vermicompost is an organic bio-fertilizer.
    • Commonly used worms: Red worms (Eisenia fetida, Lumbricus rubellus).
    • Materials used: Green leaves, old newspapers, cardboard pieces, agricultural waste.
    • Materials not used: Plastic waste (non-biodegradable).
  • Earthworms:
    • Known as "Nature's ploughman" and "friends of farmers".
    • Increase soil fertility by making soil soft and porous, which improves aeration and water infiltration.
    • Do not reduce soil fertility.

7. Human Anatomy and Physiology

Practice MCQs: Biofertilizers

Classification of Biofertilizers based on microbial type and function

  1. Which of the following is used as Biofertilizer ?

    Asked in: CGPSC Prelims Exam 2019 · UPSC CDS 1 2021

    1. Rhizobium
    2. Blue-green algae
    3. Azolla
    4. All of these
    Show answer

    Answer: D. All of these

    Ans: B To determine the genotype of a plant showing a dominant phenotype (tall), the standard method is a test cross. A test cross involves cross-pollinating the plant with a homozygous recessive plant (short). If the progeny includes any short plants, the tall parent must be heterozygous (Tt). If all progeny are tall, the tall parent is likely homozygous dominant (TT). This method allows differentiation between potential parental genotypes (Tall x Tall vs. Tall x Short leading to different progeny genotypes).

  2. Test cross is the cross of an individual with -

    Asked in: Head Master Praveshika School (Sanskrit Edu.) 2021

    1. An individual heterozygous for the characters involved
    2. Both the parents (homozygous recessive and homozygous dominant)
    3. A homozygous recessive parent
    4. An individual with similar gene type
    Show answer

    Answer: C. A homozygous recessive parent

    The key feature of a test cross is crossing with a homozygous recessive individual to reveal the genotype of the other parent based on the phenotypes of the offspring. - Biofertilizers are substances that contain microorganisms which promote plant growth by increasing the supply of essential nutrients to the host plant when applied to seed, plant surface, or soil. - They are considered environmentally friendly alternatives or supplements to chemical fertilizers.

  3. Duckweed or Azolla is used as a/an

    Asked in: UPSC Geoscientist 2023

    1. pesticide
    2. biofertilizer
    3. arsenic indicator
    4. antiseptic
    Show answer

    Answer: B. biofertilizer

    - Azolla is a symbiotic host for nitrogen-fixing cyanobacteria (Anabaena azollae). - Nitrogen fixation makes Azolla a valuable biofertilizer, especially in agriculture. - Duckweed can also be used as a source of organic matter and nutrients.

  4. What kind of micro-organisms are there in nitrogen fixing bacteria ?

    Asked in: HAS Prelims 2019

    1. Rhizobium
    2. Procaryotic
    3. Azotobacter
    4. Cyanobacteria
    Show answer

    Answer: B. Procaryotic

    Nitrogen-fixing bacteria like Rhizobium, Azotobacter, and Cyanobacteria are classified as prokaryotic organisms because their cells lack a membrane- bound nucleus and other membrane-bound organelles.

  5. Match List I with List II and select the correct answer using the code given below the lists : List I List II (Plant) (Biofertilizer)

    Asked in: UPSC CAPF 2011

    1. Soyabean 1. Anabaena
    2. Azolla 2. Rhizobium
    3. Pine 3. Azospirillum
    4. Maize 4. Mycorrhiza A. 3 4 1 2 B. 2 1 4 3 C. 2 4 1 3 D. 3 1 4 2
    Show answer

    Answer: B. Azolla 2. Rhizobium

    Different plants form symbiotic relationships with specific microorganisms that act as biofertilizers, enhancing nutrient uptake, most notably nitrogen fixation. Legumes like soybean associate with Rhizobium. The aquatic fern Azolla hosts the cyanobacterium Anabaena azollae. Many trees, including pine, form mutualistic associations with fungi known as Mycorrhizae. Cereals like maize can associate with bacteria like Azospirillum in their root zone.

Definition and Significance of Biofertilizers

  1. Rhizobium is a

    Asked in: UPSC CISF AC EXE 2024

    1. biopesticide
    2. biofertilizer
    3. bioherbicide
    4. mycoherbicide
    Show answer

    Answer: B. biofertilizer

    Rhizobium bacteria fix atmospheric nitrogen, acting as a natural fertilizer for plants, especially legumes.

  2. Yeast are used in bakery industry for production of –

    Asked in: Agri. Officer 2020

    1. CO₂
    2. Alcohol
    3. Protein
    4. Flavour
    Show answer

    Answer: A. CO₂

    Yeast's primary role in baking is the production of CO₂ through fermentation, which leavens the dough.

  3. Which of the following statements is incorrect for Bio-gas ? (1) It is an excellent fuel as it contains upto 75% methane. (2) It burns with smoke and leaves residue like ash. (3) Its heating capacity is high. (4) Bio-gas is also used as source of light. (5) Question not attempted

    Asked in: Deputy Jailor 2024

    1. Option text
    2. Option text
    3. Option text
    4. Option text
    Show answer

    Answer: B. Option text

    Bio-gas burns cleanly without smoke and does not leave residue like ash.

  4. The micro-organism _______ is responsible for the formation of beer and wine and its fermented end product is ________.

    Asked in: Food Safety Officer 2019

    1. Saccharomyces cerevisiae, ethanol
    2. Saccharomyces cerevisiae, acetic acid
    3. Lactobacillus plantarum, ethanol
    4. Lactobacillus plantarum, lactic acid
    Show answer

    Answer: A. Saccharomyces cerevisiae, ethanol

    - Saccharomyces cerevisiae is a key yeast strain in brewing and winemaking. - It performs alcoholic fermentation, converting sugars into ethanol and carbon dioxide. - Ethanol is the primary alcohol in alcoholic beverages.

  5. Which of the following is fermented by Koji ?

    Asked in: Food Safety Officer 2019

    1. Barley
    2. Millet
    3. Soyabean
    4. Zea mays
    Show answer

    Answer: C. Soyabean

    - Koji is a mold culture, most commonly Aspergillus oryzae. - It is used to ferment soybeans and/or grains. - Koji production is crucial for making fermented products like soy sauce and miso. - Soybeans are a common substrate for Koji production, especially for savory fermented products.

  6. The use of micro-organisms to degrade environmental pollutants is called

    Asked in: Food Safety Officer 2019

    1. Microremediation
    2. Nanoremediation
    3. Bioremediation
    4. Biocoenose
    Show answer

    Answer: C. Bioremediation

    Bioremediation is a process that uses naturally occurring or deliberately introduced microorganisms to consume and break down environmental pollutants, transforming them into less harmful substances. It is a cost-effective and environmentally friendly approach to pollution control.

  7. Microbes that have beneficial effect on health of the consumers are called

    Asked in: Food Safety Officer 2019

    1. Prebiotics
    2. Probiotics
    3. Synbiotics
    4. Antibiotics
    Show answer

    Answer: B. Probiotics

    Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. They are often beneficial bacteria or yeasts found in foods like yogurt and supplements.

  8. Nitrate rich foods favour the luxurious growth of one of the following :

    Asked in: Food Safety Officer 2022

    1. Mucor
    2. Lactobacilli
    3. Pseudomonas
    4. Staphylococcus
    Show answer

    Answer: C. Pseudomonas

    Pseudomonas are nitrate-reducing bacteria, and nitrate-rich environments favour their growth.

  9. All are rich natural sources of probiotic except

    Asked in: Food Safety Officer 2022

    1. Buttermilk
    2. Kefir
    3. Tempeh
    4. Sourdough bread
    Show answer

    Answer: D. Sourdough bread

    Probiotic foods are fermented products containing viable microorganisms beneficial to health. - Buttermilk Cultured buttermilk is made by fermenting milk with live bacterial cultures, often Lactococcus lactis, Lactococcus cremoris, and Leuconostoc species, which can have probiotic properties. - Kefir A fermented milk drink made with kefir grains, containing a diverse range of bacteria and yeasts, many of which are considered probiotic. - Tempeh A traditional Indonesian fermented soybean product where soybeans are fermented by a mold, Rhizopus oligosporus. While beneficial, the probiotic nature is often attributed to the fermentation byproducts and the presence of live cultures if not cooked extensively. - Sourdough bread While made using a starter culture (wild yeasts and bacteria like Lactobacillus species), the baking process is lethal to these microorganisms, meaning it lacks live probiotic bacteria.

  10. Which substrate is used for the production of Beer ?

    Asked in: Food Safety Officer 2022

    1. Molasses
    2. Sugarcane Juice
    3. Grapes
    4. Barley
    Show answer

    Answer: D. Barley

    The brewing process involves: 1. Malting Barley is malted to activate enzymes that break down starches into sugars. 2. Mashing The malted barley is mixed with hot water to extract these sugars, creating a wort. 3. Boiling The wort is boiled, often with hops for flavor and preservation. 4. Fermentation Yeast is added to the cooled wort to ferment the sugars into alcohol and carbon dioxide. Barley is the essential grain that provides the fermentable sugars for beer.

  11. One of the following is a fermented condiment :

    Asked in: Food Safety Officer 2022

    1. Idli
    2. Kombucha
    3. Miso
    4. Ang-khak
    Show answer

    Answer: C. Miso

    - Fermented Condiment A condiment is a substance such as sauce, relish, or spice that is added to food, typically after cooking, to enhance its flavor. A fermented condiment has undergone microbial fermentation. - Miso A Japanese paste made from fermented soybeans, rice, barley, or other grains. It is a savory umami-rich seasoning. - Idli A type of savory cake popular in South India, made by fermenting rice and lentil batter. While fermented, it's usually considered a staple food or breakfast item, not primarily a condiment. - Kombucha A fermented tea drink, not a condiment. - Ang-khak This term often refers to red yeast rice (Monascus purpureus), which is used as a food coloring and in traditional medicine. While it involves fermentation, it's not typically classified as a condiment in the same way as miso.

  12. Initial Lactic acid bacteria utilizing sugars of cabbage in sauerkraut fermentation is :

    Asked in: Food Safety Officer 2022

    1. Lactobacillus Plantarum
    2. Leuconostoc mesenteroides
    3. Pediococcus pentosaceus
    4. None of these
    Show answer

    Answer: B. Leuconostoc mesenteroides

    Leuconostoc mesenteroides is the primary lactic acid bacterium in the initial phase of sauerkraut fermentation.

Mechanism of Action of Biofertilizers (Nutrient cycling, phytohormone production)

  1. The fundamental requirements for nitrogen fixation are : (a) Nitrogenase (b) Ferredoxin (c) ATP (d) Aerobic conditions

    Asked in: MPSC CS Prelims 2014

    1. All the above
    2. (a), (b), (c)
    3. (a), (b), (d)
    4. (a), (c), (d)
    Show answer

    Answer: B. (a), (b), (c)

    Nitrogenase, ATP, and reducing agents are essential components for nitrogen fixation. Anaerobic conditions are generally required for the functioning of the nitrogenase enzyme due to its sensitivity to oxygen.

Mycorrhizal Biofertilizers (e.g., Vesicular Arbuscular Mycorrhizae - VAM)

  1. Which one of the following is a useful functional association between fungi and the roots of higher plants?

    Asked in: UPPCS Prelims 1999

    1. Biofertilizer
    2. Coralloid root
    3. Lichen
    4. Mycorrhiza
    Show answer

    Answer: D. Mycorrhiza

    - Mycorrhizal associations are widespread and occur in most plant families. - They enhance nutrient uptake and drought tolerance in plants.

  2. Mycorrhiza is associated with which organism?

    Asked in: lower devision clerk 2025

    1. Algae
    2. Fungi
    3. Bacteria
    4. Bryophytes
    Show answer

    Answer: B. Fungi

    Mycorrhiza is a symbiotic association between a fungus and the roots of a plant. The fungus colonizes the root system of a host plant, providing increased water and mineral absorption to the plant. In return, the plant provides the fungus with carbohydrates produced during photosynthesis.

Nitrogen-fixing Biofertilizers: Non-symbiotic/Free-living (e.g., Azotobacter, Azospirillum, Cyanobacteria/BGA)

  1. Azotobacter and Clostridium convert

    Asked in: UPSC Geoscientist 2022

    1. gaseous molecular nitrogen into ammonia
    2. ammonia present in soil to gaseous nitrogen
    3. humus present in soil to mineralized constitu‐ ents
    4. normal healthy soil to toxic soil
    Show answer

    Answer: A. gaseous molecular nitrogen into ammonia

    - Azotobacter and Clostridium are free-living nitrogen-fixing bacteria. - Nitrogen fixation is the conversion of N₂ gas into ammonia (NH₃). - This process is essential for making nitrogen available in ecosystems. - Ammonia can be further converted into other nitrogen compounds like nitrates by other bacteria (nitrification).

  2. Azolla is used as a bio-fertilizer as it possesses _______

    Asked in: GPSC Prelims CSP 2 2024

    1. Humus in large quantities
    2. Rhizobium
    3. Mycorrhiza
    4. Cyanobacteria
    Show answer

    Answer: D. Cyanobacteria

    - Azolla is an aquatic fern that harbors a symbiotic cyanobacterium, Anabaena azollae, within cavities in its fronds. - The cyanobacteria are capable of fixing atmospheric nitrogen (N2) into ammonia (NH3), a form usable by plants. - This fixed nitrogen is utilized by the Azolla fern for its growth, and when Azolla decomposes or is incorporated into the soil (particularly in rice paddies), the fixed nitrogen becomes available to the crop plants.

  3. The bio-fertilizer used as a supplementary food for the cattle, especially for milking bovines is

    Asked in: RPSC RAS Prelims 2016

    1. Azotobacter
    2. Azospirillum
    3. Rhizobium
    4. Azolla
    Show answer

    Answer: D. Azolla

    Azolla is a symbiotic aquatic fern used as a bio- fertilizer (green manure) and a nutritious feed supplement for livestock.

  4. Which of the following is associated with biological nitrogen fixation?

    Asked in: UPPCS Prelims 2019

    1. Red algae
    2. Brown algae
    3. Green algae
    4. Blue green algae
    Show answer

    Answer: D. Blue green algae

    Cyanobacteria (Blue-green algae) are important biological nitrogen fixers.

  5. Which one of the following is not correctly matched? Nitrogen fixing agent - Crop

    Asked in: UPPCS Prelims 2017

    1. Blue green algae - Paddy
    2. Rhizobium Leguminisarum - Pea
    3. Azotobacter - Wheat
    4. Azolla - Maize
    Show answer

    Answer: D. Azolla - Maize

    Azolla is an aquatic fern that lives in symbiotic association with the nitrogen-fixing cyanobacterium Anabaena azollae. This symbiosis enables Azolla to fix atmospheric nitrogen. Azolla is widely used as a green manure or biofertilizer primarily in paddy (rice) cultivation due to the waterlogged conditions required for its growth. It is not typically associated with maize cultivation.

  6. The aquatic plant commonly used as bio fertilizer in the water-logged rice field is -

    Asked in: UPPCS Prelims 2023

    1. Wolffia
    2. Trapa
    3. Azolla
    4. Lemna
    Show answer

    Answer: C. Azolla

    Azolla is an aquatic fern that lives in symbiosis with a nitrogen-fixing cyanobacterium, Anabaena azollae. This symbiosis allows Azolla to fix atmospheric nitrogen, which is then released into the soil upon its decomposition, acting as a natural fertilizer for rice crops in flooded conditions.

  7. Consider the following organisms : 1. Agaricus 2. Nostoc 3. Spirogyra Which of the above is/are used as biofertilizer/ biofertilizers?

    Asked in: UPSC IAS 2013

    1. 1 and 2
    2. 2 only
    3. 2 and 3
    4. 3 only
    Show answer

    Answer: B. 2 only

  8. Agaricus is a genus of fungi (mushrooms) and is not generally used as a biofertilizer. Some fungi like mycorrhizal fungi are used as biofertilizers as they enhance nutrient uptake, but Agaricus is not primarily used for this purpose. 2. Nostoc is a genus of cyanobacteria (blue-green algae) known for its ability to fix atmospheric nitrogen. It is widely used as a biofertilizer, especially in paddy fields. 3. Spirogyra is a genus of filamentous green algae. It is photosynthetic but does not fix nitrogen and is not used as a biofertilizer. 30. Which of the following is a source of biofertilizer?

    Asked in: UPPSC Upper Mains 2009

    1. Yeast
    2. Chlorella
    3. Azolla
    4. Mold
    Show answer

    Answer: C. Azolla

    Azolla is a genus of small aquatic ferns that live in symbiotic association with the nitrogen-fixing cyanobacterium Anabaena azollae within cavities in their leaves. Anabaena is capable of fixing atmospheric nitrogen (N₂) into a form usable by plants (like ammonia), making Azolla an excellent natural source of nitrogen fertilizer, particularly for rice cultivation in flooded fields.

  9. Blue green algae is used to supply nitrogen to

    Asked in: UPPSC Upper Mains 2010

    1. Bajra
    2. Maize
    3. Paddy
    4. Wheat
    Show answer

    Answer: C. Paddy

    Certain species of blue-green algae are nitrogen- fixing organisms and can convert atmospheric nitrogen into usable forms for plants.

  10. Azolla is often used as biofertilizer, because it is associated with

    Asked in: UPPSC Upper Mains 2011

    1. Blue Green Algae
    2. Red Algae
    3. Fungus
    4. Moss
    Show answer

    Answer: A. Blue Green Algae

    Azolla is widely used as a biofertilizer in rice cultivation, particularly in Asian countries, due to its ability to fix atmospheric nitrogen and its rapid growth rate.

  11. Which of the following micro organisms are used as biofertilizer?

    Asked in: UPPSC Upper Mains 2012

    1. Cyano bacteria
    2. Protozoa
    3. Virus
    4. None of above
    Show answer

    Answer: A. Cyano bacteria

    - Biofertilizers are substances containing living microorganisms that, when applied to seeds, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promote growth by increasing the supply or availability of primary nutrients to the host plant. - Cyanobacteria act as biofertilizers by fixing atmospheric nitrogen.

  12. Which one of the following organisms can serve as a biofertilizer for rice crop?

    Asked in: UPPCS Prelims 2000

    1. Blue-green algae
    2. Rhizobium sp
    3. Mycorrhizal fungi
    4. Azotobacter sp
    Show answer

    Answer: A. Blue-green algae

    Blue-green algae, particularly nitrogen-fixing species like Anabaena and Nostoc, are commonly used as biofertilizers in rice paddies. They perform nitrogen fixation, converting atmospheric nitrogen (N₂) into forms usable by plants (like ammonia), enriching the soil with this essential nutrient in the aquatic environment of rice fields.

Nitrogen-fixing Biofertilizers: Symbiotic (e.g., Rhizobium, Frankia)

  1. The root nodules of Casuarina which are capable of symbiotic nitrogen fixation contain the microbe

    Asked in: TNPSC CS Prelims 2017

    1. Rhizobium
    2. Frankia
    3. Anabaena
    4. Azotobacter
    Show answer

    Answer: B. Frankia

    Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into ammonia (NH₃), which can then be used by plants. Symbiotic nitrogen fixation occurs when microorganisms live in association with plants and fix nitrogen for them in exchange for carbohydrates. While Rhizobium bacteria are well-known for forming root nodules and fixing nitrogen symbiotically with leguminous plants (like peas, beans, soybeans), other microbes form symbiotic associations with non-leguminous plants. Casuarina is a genus of trees (non-leguminous) that form root nodules capable of nitrogen fixation. The microbe responsible for this symbiosis in Casuarina is Frankia, a filamentous, nitrogen- fixing bacterium belonging to the group known as actinomycetes.

  2. Nitrogen fixing bacteria make combination with cells of the roots of

    Asked in: UPPCS Prelims 2019

    1. Pulses
    2. Rice
    3. Wheat
    4. Sugarcane
    Show answer

    Answer: A. Pulses

    Nitrogen-fixing bacteria, particularly species of Rhizobium, form a symbiotic relationship with leguminous plants, which include pulses. These bacteria live in nodules on the roots of the plants and convert atmospheric nitrogen into a form usable by the plant (ammonia), while the plant provides the bacteria with carbohydrates and a protected environment.

  3. The type of crop which is able to fix nitrogen from air is:

    Asked in: UPPSC Upper Mains 2006

    1. Wheat
    2. Legumes
    3. Coffee
    4. Rubber
    Show answer

    Answer: B. Legumes

    Legumes (plants belonging to the family Fabaceae) have a symbiotic relationship with certain bacteria, primarily Rhizobium species. These bacteria form nodules on the roots of leguminous plants and can convert atmospheric nitrogen gas (N2), which plants cannot directly use, into ammonia (NH3), a form of nitrogen usable by the plant. This process is called nitrogen fixation.

  4. What is responsible for the Nitrogen fixation by legumes?

    Asked in: Assistant Engineer (AE) Exam 2013

    1. Mycorrhiza
    2. Rhizospheric microflora
    3. Rhizombia
    4. Soil fungi
    Show answer

    Answer: C. Rhizombia

    Legumes have a symbiotic relationship with Rhizobium bacteria. The bacteria live in root nodules and fix atmospheric nitrogen. Mycorrhiza (A) are fungi that form symbiotic relationships with plant roots and help in nutrient uptake, particularly phosphorus. Rhizospheric microflora (B) is a general term for microorganisms in the soil around plant roots, which may include nitrogen fixers but Rhizobium is the specific and most significant for legumes. Soil fungi (D) is a broad category and does not specifically refer to nitrogen fixation in legumes.

  5. Which of the following crops contains Nitrogen fixing bacteria ?

    Asked in: Executive Officer and Tax & Revenue Inspector Exam 2023

    1. Wheat
    2. Paddy
    3. Maize
    4. Peas
    Show answer

    Answer: D. Peas

    Peas, being a legume, have symbiotic nitrogen- fixing bacteria (Rhizobium) in their root nodules, which enrich the soil with nitrogen.

Phosphate-Solubilizing and Mobilizing Biofertilizers (PSB)

  1. Which of the following is a phosphate soluble bacteria?

    Asked in: STAFF NURSE AYURVEDA 2023

    1. Azolla
    2. Azotobacter
    3. Rhizobium
    4. Pseudomonas
    Show answer

    Answer: D. Pseudomonas

    STAFF NURSE AYURVEDA -2023 Pseudomonas species are recognized for their role in phosphate solubilization, which is beneficial for soil fertility and plant growth.

Role of Biofertilizers in Sustainable Agriculture and Organic Farming

  1. With reference to the role of biofilters in Recirculating Aquaculture System, consider the following statements: 1. Biofilters provide waste treatment by removing uneaten fish feed. 2. Biofilters convert ammonia present in fish waste to nitrate. 3. Biofilters increase phosphorus as nutrient for fish in water. How many of the statements given above are correct?

    Asked in: UPSC IAS 2023

    1. Only one
    2. Only two
    3. All three
    4. None
    Show answer

    Answer: A. Only one

    Recirculating Aquaculture Systems (RAS) use biofilters as a key component for water treatment. Statement 1 is incorrect. Biofilters primarily treat dissolved waste products, particularly nitrogenous compounds. Solid wastes, such as uneaten fish feed and feces, are typically removed by mechanical filters (like drum filters or settling tanks) before the water reaches the biofilter. While the decomposition of uneaten feed produces substances that the biofilter treats, the biofilter itself does not remove the solid feed. Statement 2 is correct. The main function of a biofilter in RAS is nitrification, which is the biological process where beneficial bacteria convert toxic ammonia (excreted by fish and from decomposition) first into nitrite and then into much less toxic nitrate. This is essential for maintaining water quality suitable for fish. Statement 3 is incorrect. Biofilters are designed to remove or convert waste products, not increase nutrients for fish. While phosphorus is present in fish feed and waste, biofilters are primarily focused on managing nitrogen. Systems for nutrient addition for fish involve providing high-quality feed, not manipulating waste products in the water circuit.

  2. The most suitable crop for green manuring is

    Asked in: UPPSC Upper Mains 2015

    1. Potato
    2. Sugarcane
    3. Sunhemp
    4. Gram
    Show answer

    Answer: C. Sunhemp

    Green manuring involves growing a crop and then incorporating it into the soil to improve its fertility and structure. Leguminous crops are particularly suitable for green manuring because they host nitrogen-fixing bacteria in their root nodules, which convert atmospheric nitrogen into a form usable by plants. Sunhemp (Crotalaria juncea) is a widely used and highly effective green manure crop in many parts of the world, known for its fast growth, significant biomass production, and high nitrogen- fixing capacity.

  3. Which of the following is not true about organic manure ?

    Asked in: Junior Accountant and TRA 2013

    1. It enhances water holding capacity of soil.
    2. It is obtained only from animal matter.
    3. It provides humus to soil.
    4. It improves the texture of soil.
    Show answer

    Answer: B. It is obtained only from animal matter.

    Organic manure has diverse sources, including both animal and plant matter.

  4. Which of the following is known as Natures ploughman ?

    Asked in: Junior Accountant and TRA 2013

    1. Neries
    2. Cattle Leech
    3. Earthworm
    4. Polychordius
    Show answer

    Answer: C. Earthworm

    Earthworms' subterranean activities significantly improve soil structure and fertility, earning them the nickname "Nature's ploughman."

  5. Ǔनम्नͧलͨखत में से कौन सा कृ ͧमकं पोस्ट तैयार करने के ͧलए काम में नहीं ͧलया जाता है ?

    Asked in: JUNIOR ACCOUNTANT AND TRA 2013

    1. हरी पlत्तियाँ
    2. पुराने समाचार-पत्र
    3. गत्तिे के टुकड़े
    4. प्लिाlÖटक अपlशष्ट
    Show answer

    Answer: D. प्लिाlÖटक अपlशष्ट

    Plastic waste is not suitable for vermicomposting.

  6. Which of the following are used for vermicomposting ?

    Asked in: Protection Officer 2018

    1. (1) Red worm
    2. (2) Flat worm
    3. (3) Round worm
    4. (4) Tape worm
    Show answer

    Answer: A. (1) Red worm

    Red worms are ideal for vermicomposting.

  7. Assertion (A) : The earthworms reduce the fertility of the soil. Reason (R) : They make the soil soft and porous.

    Asked in: Assistant Conservator of Forest 2025

    1. (A) is true but (R) is false.
    2. (A) is false but (R) is true.
    3. Both (A) and (R) are true.
    4. Both (A) and (R) are false.
    Show answer

    Answer: B. (A) is false but (R) is true.

    Earthworms are vital for soil health. They improve soil structure, aeration, and drainage, and their castings are rich in nutrients, thus increasing soil fertility. Therefore, they enhance, not reduce, soil fertility.

  8. Biofertilizers used in organic farming include :

    Asked in: mineral dev officier 2025

    1. Symbiotic and free living nitrogen fixing bac‐ teria
    2. Nitrogen fixing Cynobacteria
    3. Phosphate solubilising microorganisms
    4. All of these
    Show answer

    Answer: D. All of these

    Biofertilizers are natural materials containing living microorganisms which, when applied to seeds, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promote growth by increasing the supply or availability of primary nutrients to the host plant.

  9. Organic farming does not include

    Asked in: mineral dev officier 2025

    1. Bio-fertilizers
    2. Chemical Fertilizers
    3. Bio-pesticides
    4. Crop rotation
    Show answer

    Answer: B. Chemical Fertilizers

    The core principle of organic farming is to work with natural ecosystems and avoid synthetic inputs that can harm the environment and human health.

  10. Vermicompost is a/an

    Asked in: TOURIST INFORMATION OFFICER(TIO) 2017

    1. Inorganic fertilizer
    2. Toxic substance
    3. Organic bio-fertilizer
    4. Synthetic fertilizer
    Show answer

    Answer: C. Organic bio-fertilizer

    Vermicompost is produced from organic matter decomposed by earthworms, making it an organic fertilizer that enhances soil fertility and health.