SOIL SCIENCE (100 Questions)
Soil Physics & Classification
- Which instrument is used to measure soil moisture tension in situ?
a) Hygrometer
b) Tensiometer
c) Permeameter
d) Piezometer
Answer: b) Tensiometer
Explanation:A tensiometer directly measures soil water matric potential (tension). It consists of a porous ceramic cup connected to a vacuum gauge via a water-filled tube. When soil dries, water moves out creating suction measured in centibars.
- The soil moisture content at which plants permanently wilt and cannot recover even when placed in saturated atmosphere is called:
a) Field capacity
b) Hygroscopic coefficient
c) Permanent wilting point
d) Saturation capacity
Answer: c) Permanent wilting point
Explanation:PWP occurs at approximately -15 bars soil moisture tension. At this point, soil holds water so tightly that plant roots cannot extract it, leading to irreversible wilting.
- Which soil structure is considered ideal for crop growth?
a) Platy
b) Prismatic
c) Granular
d) Massive
Answer: c) Granular
Explanation:Granular structure (small, rounded aggregates) provides optimal pore space distribution – 50% macro pores for aeration and drainage, 50% micro pores for water retention.
- The soil order covering maximum area in India is:
a) Aridisols
b) Vertisols
c) Inceptisols
d) Alfisols
Answer: c) Inceptisols
Explanation:Inceptisols cover about 39% of India’s geographical area. These are young soils with beginning of horizon development, found in river valleys, deltas, and foothills.
- Black soils are also known as:
a) Laterite soils
b) Regur soils
c) Podzolic soils
d) Chernozem soils
Answer: b) Regur soils
Explanation:Black cotton soils are called Regur, derived from Telugu word “Regada”. They contain montmorillonite clay which swells when wet and shrinks when dry, forming characteristic cracks.
Soil Chemistry & Fertility
- Cation Exchange Capacity is highest in which clay mineral?
a) Kaolinite
b) Illite
c) Montmorillonite
d) Chlorite
Answer: c) Montmorillonite
Explanation:CEC values: Montmorillonite (80-150 meq/100g) > Illite (20-40) > Kaolinite (3-15). Higher CEC indicates better nutrient holding capacity.
- Phosphorus fixation is maximum in soils with pH:
a) Below 5.5
b) 6.0-6.5
c) 7.0-7.5
d) Above 8.5
Answer: a) Below 5.5
Explanation:In acidic soils (pH<5.5), phosphorus forms insoluble complexes with iron and aluminum (FePO₄, AlPO₄). In alkaline soils (pH>7.5), it fixes with calcium (Ca₃(PO₄)₂).
- The critical pH for aluminum toxicity in soils is:
a) 4.5
b) 5.5
c) 6.5
d) 7.5
Answer: b) 5.5
Explanation:Below pH 5.5, aluminum becomes soluble and toxic to plants, damaging root tips and inhibiting nutrient uptake.
- Which bio-fertilizer is recommended for phosphorus solubilization?
a) Rhizobium
b) Azotobacter
c) PSB (Phosphate Solubilizing Bacteria)
d) Azospirillum
Answer: c) PSB
Explanation:PSB like Bacillus megaterium, Pseudomonas striata produce organic acids (citric, oxalic) that solubilize insoluble phosphates.
- Denitrification is favored by:
a) Aerobic conditions
b) Waterlogged conditions
c) Alkaline pH
d) Low temperature
Answer: b) Waterlogged conditions
Explanation:Denitrification (NO₃⁻ → N₂ gas) is an anaerobic process carried out by bacteria like Pseudomonas and Bacillus in oxygen-deficient environments.
Soil Conservation & Management
- The Universal Soil Loss Equation is A = R × K × L × S × C × P. What does ‘C’ represent?
a) Crop management factor
b) Conservation practice factor
c) Climate factor
d) Soil erodibility factor
Answer: a) Crop management factor
Explanation:In USLE: A=Soil loss, R=Rainfall erosivity, K=Soil erodibility, L=Slope length, S=Slope steepness, C=Cover management, P=Conservation practices.
- Contour bunding is recommended for slopes of:
a) 0-2%
b) 2-7%
c) 7-15%
d) Above 15%
Answer: b) 2-7%
Explanation:Contour bunds (earthen embankments along contours) are effective on gentle slopes (2-7%). For steeper slopes, terracing is recommended.
- Which grass is commonly used for controlling water erosion on bunds?
a) Napier grass
b) Guinea grass
c) Khus grass (Vetiveria zizanioides)
d) Doob grass
Answer: c) Khus grass
Explanation:Vetiver grass has deep, dense root system (3-4m depth) that binds soil effectively. It’s used in vetiver hedgerows for erosion control.
- The process of accumulation of soluble salts in surface soil is called:
a) Leaching
b) Salinization
c) Laterization
d) Podzolization
Answer: b) Salinization
Explanation:Salinization occurs in arid/semi-arid regions due to high evaporation, poor drainage, and irrigation with saline water. ECe > 4 dS/m defines saline soils.
- Gypsum requirement for reclamation of saline-sodic soil is calculated based on:
a) Soil pH
b) Exchangeable Sodium Percentage
c) Electrical Conductivity
d) Organic carbon content
Answer: b) Exchangeable Sodium Percentage
Explanation:Gypsum (CaSO₄.2H₂O) replaces exchangeable sodium. Requirement (tons/acre) = (ESP initial – ESP desired) × CEC × 0.086.
Soil Biology & Organic Matter
- The C:N ratio of ideal compost is:
a) 10:1
b) 20:1
c) 30:1
d) 40:1
Answer: c) 30:1
Explanation:C:N ratio of 25-30:1 is optimal for microbial decomposition. Lower ratios cause N loss as ammonia, higher ratios cause N immobilization.
- Which microorganism is responsible for nitrification?
a) Rhizobium
b) Nitrosomonas and Nitrobacter
c) Azotobacter
d) Thiobacillus
Answer: b) Nitrosomonas and Nitrobacter
Explanation:Nitrification is two-step process: NH₄⁺ → NO₂⁻ (by Nitrosomonas), NO₂⁻ → NO₃⁻ (by Nitrobacter). Requires aerobic conditions.
- The population of earthworms per cubic meter in good agricultural soil is approximately:
a) 10-50
b) 50-100
c) 100-200
d) 200-500
Answer: c) 100-200
Explanation:Earthworms improve soil structure through burrowing and cast formation. Populations >100/m³ indicate healthy soil biology.
- Humus content in soil is generally what percentage of organic matter?
a) 20%
b) 40%
c) 60%
d) 80%
Answer: d) 80%
Explanation:Humus is the stable, decomposed fraction of organic matter. Typically, organic matter contains 80% humus and 20% active fraction.
- Mycorrhizal association helps plants in:
a) Nitrogen fixation
b) Phosphorus uptake
c) Iron absorption
d) Potassium mobilization
Answer: b) Phosphorus uptake
Explanation:Mycorrhizal fungi (VAM) extend hyphae beyond root zone, increasing phosphorus absorption area by 10-100 times.
Nutrient Management
- “Chlorosis in young leaves” indicates deficiency of:
a) Nitrogen
b) Phosphorus
c) Iron
d) Potassium
Answer: c) Iron
Explanation:Iron is immobile in plants, so deficiency appears first in young leaves as interveinal chlorosis. Common in calcareous soils with high pH.
- “Little leaf” symptom in citrus is due to deficiency of:
a) Zinc
b) Copper
c) Boron
d) Molybdenum
Answer: a) Zinc
Explanation:Zinc deficiency causes reduced internodal growth resulting in rosette appearance and little leaf. Common in alkaline soils.
- “Whiptail” in cauliflower is caused by deficiency of:
a) Boron
b) Molybdenum
c) Copper
d) Manganese
Answer: b) Molybdenum
Explanation:Mo deficiency causes narrowing of leaf blades (whiptail) and poor curd formation in cauliflower. Occurs in acidic soils.
- “Internal cork” in apple is due to deficiency of:
a) Boron
b) Calcium
c) Magnesium
d) Sulfur
Answer: a) Boron
Explanation:Boron deficiency causes cork formation inside fruits, cracked stems, and death of growing points.
- “Reclamation disease” of cereals is actually deficiency of:
a) Copper
b) Zinc
c) Manganese
d) Iron
Answer: a) Copper
Explanation:On newly reclaimed organic soils (peat/muck), copper becomes unavailable causing “reclamation disease” – wilting and dieback.
Fertilizers & Amendments
- Neem coated urea was introduced to reduce losses from:
a) Leaching
b) Volatilization
c) Denitrification
d) Fixation
Answer: b) Volatilization
Explanation:Neem oil contains triterpenoids that inhibit urease enzyme, reducing ammonia volatilization from urea by 20-30%.
- Which fertilizer is hygroscopic and requires special storage?
a) Urea
b) DAP
c) CAN
d) Ammonium nitrate
Answer: d) Ammonium nitrate
Explanation:Ammonium nitrate (33-34% N) is highly hygroscopic and explosive when contaminated. Requires careful storage.
- The percentage of nitrogen in ammonium sulfate is:
a) 20.5%
b) 21.0%
c) 33.0%
d) 46.0%
Answer: b) 21.0%
Explanation:(NH₄)₂SO₄ contains 21% N and 24% S. It’s an acid-forming fertilizer suitable for alkaline soils.
- Single Super Phosphate contains:
a) 16% P₂O₅
b) 20% P₂O₅
c) 32% P₂O₅
d) 46% P₂O₅
Answer: a) 16% P₂O₅
Explanation:SSP (16% P₂O₅, 12% S, 21% Ca) is produced by treating rock phosphate with sulfuric acid.
- Muriate of Potash is chemically:
a) KCl
b) K₂SO₄
c) KNO₃
d) K₂O
Answer: a) KCl
Explanation:MOP is potassium chloride (60% K₂O equivalent). Not suitable for chloride-sensitive crops like tobacco, potato.
Soil Testing & Analysis
- For soil pH determination, soil:water ratio used is:
a) 1:1
b) 1:2
c) 1:2.5
d) 1:5
Answer: c) 1:2.5
Explanation:Standard method: 10g soil + 25ml distilled water (1:2.5), stir, let stand 30 minutes, measure with pH meter.
- Walkley and Black method is used for estimation of:
a) Available N
b) Available P
c) Organic carbon
d) CEC
Answer: c) Organic carbon
Explanation:Chromic acid wet oxidation method using potassium dichromate and sulfuric acid, with external heat.
- Olsen’s method is used for estimation of:
a) Available phosphorus in alkaline soils
b) Available potassium
c) Micronutrients
d) CEC
Answer: a) Available phosphorus in alkaline soils
Explanation:Uses 0.5M NaHCO₃ extractant (pH 8.5). For acidic soils, Bray’s method is used.
- The critical limit of available Zn in soil (DTPA extractable) is:
a) 0.2 ppm
b) 0.6 ppm
c) 1.0 ppm
d) 2.0 ppm
Answer: b) 0.6 ppm
Explanation:Below 0.6 ppm Zn (DTPA extractable), response to zinc fertilization is expected in most crops.
- Cation Exchange Capacity is expressed in:
a) ppm
b) meq/100g
c) kg/ha
d) dS/m
Answer: b) meq/100g
Explanation:CEC is milliequivalents of charge per 100g oven-dry soil. It indicates soil’s ability to retain cations.
Soil-Water Relationships
- Field capacity of a loam soil is approximately:
a) 5-10%
b) 10-15%
c) 15-20%
d) 25-30%
Answer: c) 15-20%
Explanation:Field capacity (% moisture by weight): Sand 5-10%, Loam 15-20%, Clay 25-30%.
- The amount of water required to bring soil from permanent wilting point to field capacity is called:
a) Available water
b) Gravitational water
c) Hygroscopic water
d) Capillary water
Answer: a) Available water
Explanation:Available water = FC – PWP. It’s the water plants can extract effectively.
- Infiltration rate is highest in:
a) Clay soil
b) Loam soil
c) Sandy soil
d) Silt soil
Answer: c) Sandy soil
Explanation:Sandy soils have large pores allowing rapid infiltration (5-30 cm/hr). Clay soils have slow infiltration (0.1-0.5 cm/hr).
- The process by which water moves upward against gravity in soil is called:
a) Infiltration
b) Percolation
c) Capillarity
d) Evaporation
Answer: c) Capillarity
Explanation:Capillary rise occurs in small pores due to adhesive forces between water and soil particles.
- Hydraulic conductivity is maximum for:
a) Clay
b) Silt
c) Sand
d) Loam
Answer: c) Sand
Explanation:Hydraulic conductivity (saturated): Sand 10⁻²-10⁻³ cm/sec, Clay 10⁻⁷-10⁻⁹ cm/sec.
- Soil temperature is measured using:
a) Thermometer
b) Thermocouple
c) Soil thermometer
d) All of above
Answer: d) All of above
Explanation:Different instruments used: Mercury thermometers (simple), thermocouples (accurate), infrared thermometers (remote).
- The optimum soil temperature for germination of most crops is:
a) 10-15°C
b) 15-20°C
c) 25-30°C
d) 35-40°C
Answer: c) 25-30°C
Explanation:Most crops germinate best between 25-30°C. Below 10°C and above 40°C inhibits germination.
- Thermal conductivity is highest for:
a) Water
b) Air
c) Mineral particles
d) Organic matter
Answer: c) Mineral particles
Explanation:Quartz has high thermal conductivity (8.4 W/mK), while water (0.6) and air (0.025) are poor conductors.
- Albedo refers to:
a) Soil color
b) Reflectivity of soil surface
c) Water holding capacity
d) Nutrient content
Answer: b) Reflectivity of soil surface
Explanation:Albedo = reflected radiation/incident radiation. Dark soils have low albedo (0.05-0.15), light soils high (0.25-0.45).
- Gravitational water moves through pores larger than:
a) 0.05 mm
b) 0.1 mm
c) 0.5 mm
d) 1.0 mm
Answer: a) 0.05 mm
Explanation:Pores >0.05 mm diameter drain freely by gravity. Smaller pores hold water against gravity.
- Bulk density of mineral soils ranges from:
a) 0.5-0.8 g/cc
b) 1.0-1.6 g/cc
c) 2.0-2.5 g/cc
d) 2.6-2.7 g/cc
Answer: b) 1.0-1.6 g/cc
Explanation:Bulk density: Sands 1.5-1.6, Loams 1.3-1.4, Clays 1.1-1.2, Organic soils 0.2-0.3 g/cc.
- Particle density of most mineral soils is about:
a) 1.0 g/cc
b) 2.65 g/cc
c) 3.5 g/cc
d) 5.0 g/cc
Answer: b) 2.65 g/cc
Explanation:Average particle density of mineral soils is 2.65 g/cc (density of quartz). Organic matter has 1.4 g/cc.
- Porosity can be calculated from bulk and particle density using formula:
a) (1 – BD/PD) × 100
b) (BD/PD) × 100
c) (PD – BD) × 100
d) (PD/BD) × 100
Answer: a) (1 – BD/PD) × 100
Explanation:Porosity % = [1 – (Bulk density/Particle density)] × 100.
- Soil consistency refers to:
a) Soil color
b) Soil texture
c) Soil strength and stickiness
d) Soil structure
Answer: c) Soil strength and stickiness
Explanation:Consistency describes soil behavior at different moisture contents: hard, friable, plastic, sticky.
- Atterberg limits are used to determine:
a) Soil fertility
b) Soil engineering properties
c) Soil biological activity
d) Soil color
Answer: b) Soil engineering properties
Explanation:Liquid limit, plastic limit, plasticity index determine soil behavior for construction.
- Soil air contains CO₂ concentration of about:
a) 0.03%
b) 0.3%
c) 3%
d) 30%
Answer: b) 0.3%
Explanation:Soil air has 0.3-1% CO₂ (vs 0.03% in atmosphere) due to root and microbial respiration.
- The optimum air-filled porosity for plant growth is:
a) 5-10%
b) 10-15%
c) 20-30%
d) 40-50%
Answer: c) 20-30%
Explanation:Plants need 20-30% air space for root respiration. Below 10% causes oxygen deficiency.
- Redox potential (Eh) below +350 mV indicates:
a) Aerobic conditions
b) Anaerobic conditions
c) Neutral conditions
d) Alkaline conditions
Answer: b) Anaerobic conditions
Explanation:Eh >+350 mV: aerobic, +350 to -100 mV: moderately reduced, <-100 mV: strongly reduced (anaerobic).
- Soil color is determined using:
a) pH meter
b) Munsell color chart
c) Spectrophotometer
d) Colorimeter
Answer: b) Munsell color chart
Explanation:Munsell system: Hue (spectral color), Value (lightness), Chroma (intensity).
- Red color in soil indicates presence of:
a) Iron oxides
b) Manganese oxides
c) Organic matter
d) Calcium carbonate
Answer: a) Iron oxides
Explanation:Hematite (Fe₂O₃) gives red color, Goethite (FeOOH) gives yellow/brown.
- White color in soil may indicate presence of:
a) Silica
b) Calcium carbonate
c) Salt accumulation
d) All of above
Answer: d) All of above
Explanation:White colors from: silica/sands (light), carbonates (chalky), salts (efflorescence).
- Buffering capacity of soil is highest for:
a) Sandy soil
b) Loamy soil
c) Clay soil
d) All equal
Answer: c) Clay soil
Explanation:Clay and organic matter provide buffering through cation exchange and pH-dependent charges.
- The pH at which soil has zero net charge is called:
a) Isoelectric point
b) Zero point charge
c) Both a and b
d) None
Answer: c) Both a and b
Explanation:ZPC/pI: pH where positive and negative charges balance. For most soils, between 2-4.
- Specific surface area is highest for:
a) Sand
b) Silt
c) Clay
d) Gravel
Answer: c) Clay
Explanation:Surface area (m²/g): Sand 0.01, Silt 1, Clay 10-1000 (montmorillonite 600-800).
- Soil colloids have size less than:
a) 2 mm
b) 0.2 mm
c) 0.02 mm
d) 0.002 mm
Answer: d) 0.002 mm
Explanation:Colloids are particles <0.002 mm (2 μm) that remain suspended in water.
- Flocculation of clay particles is promoted by:
a) Na⁺ ions
b) Ca²⁺ ions
c) High pH
d) Low salt concentration
Answer: b) Ca²⁺ ions
Explanation:Divalent cations (Ca²⁺, Mg²⁺) promote flocculation by reducing electrical double layer. Na⁺ causes dispersion.
- Dispersion of clay is caused by high concentration of:
a) Calcium
b) Magnesium
c) Sodium
d) Potassium
Answer: c) Sodium
Explanation:Na⁺ ions hydrate strongly, expanding clay lattices and causing dispersion, leading to poor structure.
- SAR (Sodium Adsorption Ratio) is calculated using formula:
a) Na⁺/√(Ca²⁺ + Mg²⁺)
b) Na⁺/√(Ca²⁺ × Mg²⁺)
c) Na⁺/(Ca²⁺ + Mg²⁺)
d) √Na⁺/(Ca²⁺ + Mg²⁺)
Answer: a) Na⁺/√(Ca²⁺ + Mg²⁺)
Explanation:SAR = Na⁺/√[(Ca²⁺ + Mg²⁺)/2], where concentrations are in meq/L.
- Gypsum is added to sodic soils to replace:
a) Calcium with sodium
b) Sodium with calcium
c) Magnesium with calcium
d) Potassium with sodium
Answer: b) Sodium with calcium
Explanation:Ca²⁺ from gypsum replaces Na⁺ on exchange sites, which is then leached with irrigation.
- Pyrite is used for reclamation of:
a) Acidic soils
b) Alkaline soils
c) Saline soils
d) Sodic soils
Answer: b) Alkaline soils
Explanation:Pyrite (FeS₂) oxidizes to produce H₂SO₄, lowering pH of alkaline soils.
- Laterite soils are rich in:
a) Iron and aluminum oxides
b) Calcium carbonate
c) Organic matter
d) Soluble salts
Answer: a) Iron and aluminum oxides
Explanation:Formed under high rainfall with leaching of silica, leaving Fe and Al oxides (Bauxite).
- Peat soils have organic matter content greater than:
a) 10%
b) 20%
c) 35%
d) 50%
Answer: c) 35%
Explanation:Organic soils: Muck (decomposed, 20-35% OM), Peat (fibrous, >35% OM).
- The process of accumulation of silica in subsoil is called:
a) Laterization
b) Podzolization
c) Calcification
d) Gleization
Answer: b) Podzolization
Explanation:In cool humid forests, organic acids leach Fe, Al, leaving silica-rich (ashy) E horizon.
- Gleying refers to:
a) Red color development
b) Gray color in waterlogged soils
c) Salt accumulation
d) Organic matter accumulation
Answer: b) Gray color in waterlogged soils
Explanation:Reduction of Fe³⁺ to Fe²⁺ under anaerobic conditions produces gray/blue colors.
- Calcification occurs in:
a) Humid regions
b) Arid/semi-arid regions
c) Cold regions
d) Tropical regions
Answer: b) Arid/semi-arid regions
Explanation:Limited leaching allows accumulation of CaCO₃ in B horizon (caliche layer).
- Soil taxonomy was developed by:
a) FAO
b) USDA
c) ICAR
d) UNESCO
Answer: b) USDA
Explanation:USDA Soil Taxonomy (1975) with 12 orders, used worldwide with modifications.
- The diagnostic surface horizon rich in organic matter is:
a) Argillic
b) Spodic
c) Mollic
d) Oxic
Answer: c) Mollic
Explanation:Mollic epipedon: Thick, dark, high base saturation (>50%), characteristic of grasslands.
- Subsurface accumulation of clay is called:
a) Cambic horizon
b) Argillic horizon
c) Calcic horizon
d) Duripan
Answer: b) Argillic horizon
Explanation:Bt horizon with ≥20% more clay than overlying horizon, formed by illuviation.
- Hardpan cemented by silica is called:
a) Fragipan
b) Duripan
c) Plinthite
d) Petrocalcic
Answer: b) Duripan
Explanation:Silica-cemented hardpan, does not slake in water or acid.
- Which soil order is characterized by shrink-swell properties?
a) Vertisols
b) Aridisols
c) Ultisols
d) Spodosols
Answer: a) Vertisols
Explanation:Vertisols (black cotton soils) have >30% clay, deep cracks when dry, churning (vertisols = to turn).
- Soils of desert regions belong to order:
a) Entisols
b) Aridisols
c) Mollisols
d) Alfisols
Answer: b) Aridisols
Explanation:Aridisols: Dry soils with limited leaching, often saline/alkaline, ochric epipedon.
- “Soil health card” scheme was launched in:
a) 2010
b) 2015
c) 2018
d) 2020
Answer: b) 2015
Explanation:Launched February 19, 2015 to provide soil nutrient status and recommendations to farmers.
- Neelkanth is a variety of:
a) Earthworm
b) Rhizobium
c) Azotobacter
d) Mycorrhiza
Answer: a) Earthworm
Explanation:Eisenia fetida (red worm) used for vermicomposting, Indian variety called Neelkanth.
- The zone of maximum biological activity in soil is:
a) O horizon
b) A horizon
c) B horizon
d) C horizon
Answer: b) A horizon
Explanation:Topsoil (A horizon) has highest organic matter, roots, and microbial activity.
- Rhizosphere is the zone:
a) Around roots
b) Around soil particles
c) In subsoil
d) In water table
Answer: a) Around roots
Explanation:Soil region influenced by root exudates, with 10-100 times higher microbial population.
- Most of soil nitrogen (95-99%) is present in:
a) Mineral form
b) Organic form
c) Gaseous form
d) None
Answer: b) Organic form
Explanation:As proteins, amino acids, humus. Only 1-5% as NH₄⁺, NO₃⁻ available to plants.
- Mineralization of nitrogen is optimal at pH:
a) 4-5
b) 5-6
c) 6-8
d) 8-9
Answer: c) 6-8
Explanation:Microbial conversion of organic N to mineral N is most efficient in neutral to slightly alkaline range.
- Immobilization refers to:
a) Conversion of mineral to organic N
b) Conversion of organic to mineral N
c) Loss of N as gas
d) Fixation of atmospheric N
Answer: a) Conversion of mineral to organic N
Explanation:Microbes use mineral N for growth, temporarily making it unavailable to plants.
- Ammonia volatilization is highest from:
a) Urea on alkaline soils
b) Ammonium sulfate on acid soils
c) CAN on neutral soils
d) DAP on all soils
Answer: a) Urea on alkaline soils
Explanation:Urea hydrolyzes to ammonium carbonate, which decomposes to NH₃ gas at high pH.
- Nitrification inhibitors like nitrapyrin act by inhibiting:
a) Nitrosomonas
b) Nitrobacter
c) Both
d) Neither
Answer: a) Nitrosomonas
Explanation:Blocks first step (NH₄⁺ → NO₂⁻), reducing nitrate leaching and denitrification losses.
- Non-symbiotic N fixation is done by:
a) Rhizobium
b) Azotobacter
c) Frankia
d) Anabaena
Answer: b) Azotobacter
Explanation:Free-living bacteria (Azotobacter, Clostridium) fix N without plant association.
- Blue-green algae fix nitrogen in association with:
a) Rice
b) Wheat
c) Maize
d) Sugarcane
Answer: a) Rice
Explanation:In flooded rice fields, BGA like Anabaena, Nostoc fix 20-30 kg N/ha.
- The enzyme responsible for biological N fixation is:
a) Nitrogenase
b) Nitrate reductase
c) Nitrite reductase
d) Glutamine synthetase
Answer: a) Nitrogenase
Explanation:Nitrogenase complex (Fe and Mo-Fe proteins) converts N₂ to NH₃, sensitive to oxygen.
- Leghaemoglobin in root nodules functions to:
a) Fix nitrogen
b) Transport oxygen
c) Protect nitrogenase from oxygen
d) Produce energy
Answer: c) Protect nitrogenase from oxygen
Explanation:Binds O₂, maintaining low oxygen concentration for nitrogenase activity.
- Phosphate solubilizing microorganisms produce:
a) Organic acids
b) Alkalis
c) Enzymes
d) Hormones
Answer: a) Organic acids
Explanation:Citric, oxalic, gluconic acids chelate Ca, Fe, Al, releasing phosphates.
- Sulfur oxidation in soil is carried out by:
a) Thiobacillus
b) Nitrosomonas
c) Azotobacter
d) Rhizobium
Answer: a) Thiobacillus
Explanation:Thiobacillus thiooxidans oxidizes S to H₂SO₄, used for alkaline soil reclamation.
- Enzyme activity in soil is highest in:
a) Sand
b) Silt
c) Clay
d) Equal
Answer: c) Clay
Explanation:Clay protects enzymes from degradation. Urease, phosphatase activity correlates with clay content.
- CO₂ evolution from soil indicates:
a) Microbial activity
b) Chemical reactions
c) Root respiration
d) All of above
Answer: d) All of above
Explanation:Soil respiration measures biological activity (microbes, roots, fauna).
- Quincunx system of planting is used in:
a) Square planting
b) Rectangular planting
c) Triangular planting
d) Contour planting
Answer: c) Triangular planting
Explanation:Plants at corners and center of square (like 5 on dice), giving equilateral triangle arrangement.
- Hydroponics refers to growing plants in:
a) Soil
b) Water with nutrients
c) Sand with nutrients
d) Gravel with nutrients
Answer: b) Water with nutrients
Explanation:Soilless culture with roots in nutrient solution. Aeroponics: misting roots.
- The process of soil formation is called:
a) Weathering
b) Pedogenesis
c) Erosion
d) Deposition
Answer: b) Pedogenesis
Explanation:Soil formation from parent material under influence of climate, organisms, topography, time.
- Rill erosion occurs when channel depth is:
a) <30 cm
b) 30-100 cm
c) 100-300 cm
d) >300 cm
Answer: a) <30 cm
Explanation:Erosion forms: Sheet (uniform), Rill (<30 cm deep), Gully (>30 cm), Ravine (>100 cm).
- USLE was developed by:
a) USDA
b) FAO
c) Wischmeier and Smith
d) Both a and c
Answer: d) Both a and c
Explanation:Wischmeier and Smith (1965, 1978) for USDA. Revised as RUSLE (1997).
- T value (tolerable soil loss) for India is:
a) 2.5-12.5 t/ha/yr
b) 5-15 t/ha/yr
c) 10-20 t/ha/yr
d) 20-30 t/ha/yr
Answer: a) 2.5-12.5 t/ha/yr
Explanation:Varies with soil depth: Deep soils 11.2, Medium 8.2, Shallow 4.5, Very shallow 2.5 t/ha/yr.
- Soil conservation day is observed on:
a) June 5
b) December 5
c) July 15
d) September 16
Answer: b) December 5
Explanation:World Soil Day, established by FAO, emphasizes importance of soil for food security.
