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RBSE SS Agriculture study materials
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RBSE SS Agriculture practice questions

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Question 1Agronomy and Crop Production

Which practice is the primary purpose of ploughing a field before sowing a cereal crop?

  1. Loosening and inverting the soil to improve aeration and root penetration Correct answer.
  2. Killing all soil microorganisms in the topsoil
  3. Compacting the surface to reduce water infiltration
  4. Increasing the soil pH to neutralize acidity

Explanation

Ploughing breaks up compacted soil, inverts the top layer and improves aeration, water infiltration and root penetration. It does not deliberately sterilize soil or raise pH; those effects come from other operations such as solarization or liming.

Question 2Agronomy and Crop Production

A farmer wants to sow a kharif cereal at the optimum time in a rainfed region. Which criterion best defines the optimum sowing date?

  1. The latest date before the first winter frost arrives
  2. The date when the market price of the produce is expected to peak
  3. The earliest calendar date on which the field becomes tillable after the previous harvest
  4. The date when soil moisture from monsoon rains is adequate for germination and the crop can complete its critical stages before seasonal stress Correct answer.

Explanation

Optimum sowing time balances adequate moisture for germination with the crop's phenology, so that flowering and grain filling escape drought, heat or cold stress. Calendar dates alone or market prices do not define agronomic optimum.

Question 3Agronomy and Crop Production

Which seed quality attribute is measured by a standard germination test in the laboratory?

  1. The proportion of pure live seeds that produce normal seedlings under specified conditions Correct answer.
  2. The moisture content of the seed at harvest
  3. The total weight of the seed lot delivered to the farmer
  4. The percentage of inert matter mixed with the seed

Explanation

A germination test reports the percentage of seeds that develop into normal seedlings under controlled moisture, temperature and light. Purity analysis and moisture determination are separate tests in seed quality evaluation.

Question 4Agronomy and Crop Production

In a crop rotation, a legume is included before a cereal mainly because legumes:

  1. Fix atmospheric nitrogen through rhizobia and leave residual nitrogen for the following crop Correct answer.
  2. Reduce the need for phosphorus fertilizer by dissolving soil minerals
  3. Eliminate the need for any irrigation in the rotation
  4. Prevent all weed emergence through allelopathy

Explanation

Symbiotic rhizobia in legume root nodules fix atmospheric nitrogen, and part of it remains in residues and roots for the next crop. This reduces the nitrogen fertilizer requirement of the succeeding cereal and breaks pest cycles.

Question 5Agronomy and Crop Production

A field has a soil test value of 180 kg/ha available nitrogen and the recommended dose for the crop is 120 kg N/ha. How much fertilizer nitrogen should be applied, assuming the soil contribution is fully credited?

  1. 120 kg N/ha
  2. 300 kg N/ha
  3. 60 kg N/ha
  4. 0 kg N/ha Correct answer.

Explanation

When soil test nitrogen already exceeds the crop requirement, the fertilizer dose is reduced to zero and no additional nitrogen is needed. Applying more would be wasteful and could cause lodging or nitrate leaching.

Question 6Agronomy and Crop Production

Which irrigation method is generally most water-use efficient for a widely spaced row crop such as cotton in a water-scarce area?

  1. Flood irrigation across the whole field
  2. Basin irrigation covering the entire plot
  3. Furrow irrigation with long uncut runs
  4. Drip irrigation delivering water to the root zone Correct answer.

Explanation

Drip irrigation applies water slowly and directly to the root zone, minimizing evaporation, deep percolation and runoff. Flood, basin and poorly designed furrow systems wet large non-root areas and lose more water.

Question 7Agronomy and Crop Production

The critical period for weed competition in most annual field crops is best described as:

  1. The stage of grain filling only
  2. The early growth stage when the crop canopy has not yet closed Correct answer.
  3. The period after harvest when residues remain
  4. The period immediately after physiological maturity

Explanation

Weeds compete most severely during early crop growth before the canopy closes and shades the soil surface. Keeping the crop weed-free during this critical period protects yield with the least herbicide and labour.

Question 8Agronomy and Crop Production

An herbicide is described as selective. This means the herbicide:

  1. Kills every plant species that contacts the spray
  2. Remains active in soil for many seasons
  3. Requires incorporation into the soil before it works
  4. Injures the target weed species while leaving the crop relatively unharmed at the recommended dose Correct answer.

Explanation

Selectivity is the ability of an herbicide to suppress target weeds without unacceptable injury to the crop at the recommended rate. Non-selective herbicides kill a broad range of plants, and soil persistence or incorporation are separate properties.

Question 9Agronomy and Crop Production

Which insect order includes the majority of serious defoliating caterpillars attacking field crops?

  1. Lepidoptera Correct answer.
  2. Orthoptera
  3. Coleoptera
  4. Hemiptera

Explanation

Lepidoptera is the order of moths and butterflies, and their larvae (caterpillars) are major defoliators of field crops. Hemiptera are sucking bugs, Coleoptera are beetles and Orthoptera include grasshoppers and locusts.

Question 10Agronomy and Crop Production

Integrated Pest Management (IPM) recommends that chemical pesticides be used:

  1. As a last resort after monitoring shows the economic threshold has been exceeded Correct answer.
  2. As the first and routine measure at every crop stage
  3. Only after all natural enemies have been removed
  4. At double the label dose to prevent resistance

Explanation

IPM combines monitoring, cultural, biological and mechanical tactics, and reserves pesticides for situations where pest populations exceed the economic threshold. This protects natural enemies and delays resistance.

Question 11Agronomy and Crop Production

A soil with a pH of 5.2 is to be used for a crop sensitive to acidity. Which amendment is most appropriate to raise the pH?

  1. Urea
  2. Ammonium sulphate
  3. Gypsum
  4. Agricultural lime Correct answer.

Explanation

Agricultural lime (calcium carbonate) neutralizes soil acidity and raises pH, improving nutrient availability for acid-sensitive crops. Gypsum supplies calcium and sulphur but does not raise pH, and ammonium fertilizers acidify soil.

Question 12Agronomy and Crop Production

Which soil structure condition most directly limits root growth and water infiltration in a cultivated field?

  1. A well-developed crumb structure
  2. A high proportion of water-stable aggregates
  3. A compacted plough pan below the topsoil Correct answer.
  4. A friable surface tilth

Explanation

A compacted plough pan is a dense layer that resists root penetration and reduces downward water movement, causing perched water and restricted rooting. Friable tilth and stable aggregates are favourable, not limiting, conditions.

Question 13Agronomy and Crop Production

In a rice paddy, continuous standing water is maintained primarily to:

  1. Raise soil temperature above 45 degrees Celsius
  2. Suppress many weeds and support the crop's water requirement during establishment Correct answer.
  3. Eliminate the need for nitrogen fertilizer
  4. Increase the population of aerobic soil fungi

Explanation

Flooding paddy fields supplies the crop's high water demand and suppresses many terrestrial weeds while favouring anaerobic conditions that suit rice growth. It does not remove the need for nitrogen or raise temperature excessively.

Question 14Agronomy and Crop Production

Which process converts atmospheric nitrogen into plant-available forms during the manufacture of nitrogen fertilizers?

  1. Nitrification converting ammonium into nitrate
  2. Photosynthesis fixing carbon dioxide into sugars
  3. The Haber-Bosch process fixing nitrogen with hydrogen into ammonia Correct answer.
  4. Denitrification converting nitrate into nitrogen gas

Explanation

The Haber-Bosch process combines atmospheric nitrogen with hydrogen under high temperature and pressure to make ammonia, the basis of most nitrogen fertilizers. Nitrification and denitrification are soil processes that transform existing nitrogen.

Question 15Agronomy and Crop Production

A crop shows interveinal chlorosis on younger leaves while veins remain green. Which nutrient deficiency is most consistent with this symptom?

  1. Iron Correct answer.
  2. Phosphorus
  3. Nitrogen
  4. Magnesium

Explanation

Iron is immobile in the plant, so deficiency appears on young leaves as interveinal chlorosis. Nitrogen and magnesium deficiencies generally appear first on older leaves, and phosphorus deficiency causes purpling and stunting.

Question 16Agronomy and Crop Production

Which practice best conserves soil moisture in a rainfed field between rainfall events?

  1. Deep ploughing to expose fresh soil
  2. Removing all crop residues after harvest
  3. Increasing surface runoff by smoothing the field
  4. Mulching the soil surface with crop residues Correct answer.

Explanation

A residue mulch reduces evaporation from the soil surface, moderates temperature and improves infiltration. Removing residues or leaving the surface bare increases evaporative loss.

Question 17Agronomy and Crop Production

The main purpose of treating seed with a fungicide before sowing is to:

  1. Supply nitrogen to the young seedling
  2. Increase the thousand-grain weight of the harvested crop
  3. Protect germinating seeds and seedlings from seed- and soil-borne pathogens Correct answer.
  4. Break seed dormancy

Explanation

Seed treatment coats the seed with fungicide to protect it against pathogens such as smut and damping-off during germination and early establishment. It does not supply nutrients or break dormancy.

Question 18Agronomy and Crop Production

Which cropping system is described as intercropping?

  1. Growing crops in sequence within a single year on the same land
  2. Growing one crop in a field for several consecutive seasons
  3. Leaving a field fallow for a full season
  4. Growing two or more crops simultaneously in the same field in a definite row arrangement Correct answer.

Explanation

Intercropping is the simultaneous cultivation of two or more crops in the same field with a definite spatial arrangement. Sequential cropping and monoculture involve time separation or single species.

Question 19Agronomy and Crop Production

A farmer applies 100 kg of urea (46 percent N) to one hectare. Approximately how much nitrogen is applied?

  1. 4.6 kg N
  2. 100 kg N
  3. 46 kg N Correct answer.
  4. 23 kg N

Explanation

Urea contains 46 percent nitrogen, so 100 kg of urea supplies about 46 kg of actual nitrogen. Fertilizer doses are always calculated on the nutrient content, not the product weight.

Question 20Agronomy and Crop Production

Which factor most strongly determines the water-holding capacity of a soil?

  1. The number of earthworm channels on the surface only
  2. The depth of ploughing in the current season
  3. Its colour in the dry state
  4. Its texture, especially the proportion of clay and organic matter Correct answer.

Explanation

Fine-textured soils with high clay and organic matter content hold more water because of greater surface area and micropore space. Colour and surface features are poor predictors of water-holding capacity.

Question 21Agronomy and Crop Production

In hybrid seed production, the purpose of cytoplasmic male sterility (CMS) is to:

  1. Prevent self-pollination in the female parent so that crossed seed is produced without hand emasculation Correct answer.
  2. Make the hybrid seed resistant to all herbicides
  3. Increase the yield of the hybrid beyond its genetic potential
  4. Ensure that the hybrid seed can be saved and reused by farmers

Explanation

CMS lines cannot produce viable pollen, so the female parent is pollinated only by the desired male parent, producing hybrid seed without manual emasculation. It does not confer herbicide resistance or allow reliable seed saving.

Question 22Agronomy and Crop Production

Which agronomic measure is most effective for reclaiming a sodic soil with high exchangeable sodium?

  1. Flooding with saline water and allowing it to evaporate
  2. Applying ammonium sulphate without drainage
  3. Adding more sodium chloride to the soil
  4. Applying gypsum and leaching with good-quality water Correct answer.

Explanation

Gypsum supplies calcium that replaces exchangeable sodium on the clay complex, and the displaced sodium is removed by leaching with good-quality water. Adequate drainage is essential for the reclamation to succeed.

Question 23Agronomy and Crop Production

Which stage of a cereal crop is most sensitive to high-temperature stress that reduces grain set?

  1. Tillering
  2. Flowering and early grain formation Correct answer.
  3. Seedling emergence
  4. Physiological maturity

Explanation

High temperature during flowering damages pollen viability and fertilization, sharply reducing grain set and yield. Vegetative stages are more tolerant, and maturity is past the stage when grain number is determined.

Question 24Agronomy and Crop Production

A crop rotation that alternates a cereal with a broadleaf crop is mainly recommended to:

  1. Eliminate the need for tillage
  2. Guarantee a higher market price for the cereal
  3. Break the life cycles of pests and diseases specific to one crop group Correct answer.
  4. Increase soil salinity

Explanation

Alternating crop groups with different pest and disease complexes interrupts the life cycles of specialized pathogens and weeds. It also diversifies nutrient use and improves soil health.

Question 25Agronomy and Crop Production

Which statement correctly distinguishes a contact herbicide from a systemic herbicide?

  1. A contact herbicide kills only the plant tissue it touches, whereas a systemic herbicide moves within the plant to act at distant sites Correct answer.
  2. A contact herbicide acts only through the soil, whereas a systemic herbicide acts only through the seed
  3. A contact herbicide moves through the phloem, whereas a systemic herbicide stays on the leaf surface
  4. A contact herbicide is always selective, whereas a systemic herbicide is always non-selective

Explanation

Contact herbicides act locally on sprayed tissue and require thorough coverage, while systemic herbicides are absorbed and translocated to untreated plant parts. Selectivity and application route are independent of this classification.

Question 26Agronomy and Crop Production

In a wheat field, the seed rate is expressed in kg per hectare. Which factor most directly determines the appropriate seed rate for a given wheat variety?

  1. The protein content of the harvested grain
  2. The 1000-grain weight of the seed and the target plant population Correct answer.
  3. The colour of the grain at maturity
  4. The height of the straw at harvest

Explanation

Seed rate is calculated so that a target plant population per unit area is achieved, and this depends on seed size, expressed as 1000-grain weight, together with germination percentage and expected field establishment. Grain colour, straw height and grain protein are quality or morphological traits and do not set the number of seeds to sow.

Question 27Agronomy and Crop Production

A grower wants to sow a pulse crop in a field that carried a cereal the previous season. Which benefit is most directly associated with including the pulse in the rotation?

  1. Biological nitrogen fixation that enriches soil nitrogen for the following crop Correct answer.
  2. Complete control of soil-borne cereal pathogens in a single season
  3. Elimination of the need for any phosphorus fertilizer in the rotation
  4. Permanent improvement of soil texture that lasts for many years

Explanation

Grain legumes form symbiotic associations with Rhizobium bacteria that fix atmospheric nitrogen, and part of this nitrogen remains in residues and roots for the next crop. The other listed benefits are overstated: phosphorus is still required, pathogen control is partial, and textural change is gradual rather than permanent.

Question 28Agronomy and Crop Production

A field is prepared for sowing a small-seeded crop. Which seedbed condition is most important for uniform germination and emergence?

  1. A fine, firm seedbed with adequate moisture at sowing depth Correct answer.
  2. A compacted surface pan that seals in moisture
  3. A coarse, cloddy surface that allows free air movement
  4. A loose, deep layer of dry soil above the seed

Explanation

Small seeds have limited stored reserves and must be placed in a fine, firm bed with good seed-soil contact and sufficient moisture for imbibition, which gives even germination and emergence. Coarse clods, dry loose soil and surface compaction all reduce seed-soil contact and moisture availability.

Question 29Agronomy and Crop Production

A crop is irrigated by the furrow method on a field with a gentle slope. Which management measure most reduces deep percolation losses at the lower end of the furrows?

  1. Increasing the furrow stream size throughout the irrigation event
  2. Extending the furrow length to the full field boundary
  3. Delaying irrigation until the soil surface cracks
  4. Shortening the furrow length or using cut-back flow near the end of the run Correct answer.

Explanation

In furrow irrigation, water applied at a constant large stream advances quickly but continues to infiltrate at the tail end, causing deep percolation. Shortening runs or reducing the stream (cut-back) once water reaches the end equalises infiltration opportunity time along the furrow and cuts losses.

Question 30Agronomy and Crop Production

A soil test reports a cation exchange capacity of 25 cmol/kg with 18 percent base saturation and a pH of 5.0. Which interpretation is most appropriate for a field crop?

  1. The soil has adequate base saturation and needs no amendment
  2. The soil is strongly acidic with low base saturation and requires liming before a sensitive crop Correct answer.
  3. The soil is alkaline and needs gypsum to reduce sodium
  4. The soil is sodic and requires leaching with saline water

Explanation

Low base saturation together with a pH of 5.0 indicates that exchange sites are occupied largely by aluminium and hydrogen ions, making the soil strongly acidic. Liming raises pH and increases base saturation, which is necessary before growing an acid-sensitive crop.

Question 31Agronomy and Crop Production

A farmer notices that a contact herbicide applied to a broadleaf weed gives good kill of small seedlings but poor control of larger plants. Which explanation best accounts for this result?

  1. Contact herbicides kill only the tissue they reach, so larger plants can regrow from untreated growing points Correct answer.
  2. Contact herbicides move in the phloem and are diluted in larger plants
  3. Contact herbicides are inactivated by soil and never reach the roots of larger plants
  4. Contact herbicides require higher temperature to become selective

Explanation

Contact herbicides act only where spray droplets land and do not translocate, so meristems and roots of larger weeds escape injury and regrow. Systemic herbicides move to growing points and are less dependent on complete spray coverage of the whole plant.

Question 32Agronomy and Crop Production

In a maize field, a fungal leaf disease appears first on the lower leaves and spreads upward after several humid nights. Which control approach is most appropriate as the first intervention?

  1. Apply a recommended foliar fungicide at the early stage of symptom development Correct answer.
  2. Delay all action until lesions cover most of the upper leaves
  3. Apply a soil insecticide to kill the vector of the fungus
  4. Increase nitrogen top-dressing to outgrow the disease

Explanation

Foliar fungal diseases spread by spores under humid conditions, and a recommended fungicide applied when the first symptoms appear limits epidemic development before the upper canopy is affected. Insecticides target vectors, extra nitrogen can increase susceptibility, and delay allows the disease to become severe.

Question 33Agronomy and Crop Production

A crop is grown under rainfed conditions in a region with a short growing season. Which agronomic strategy best matches the crop to the available moisture?

  1. Increasing plant density greatly to shade the soil surface
  2. Sowing a late-maturing variety after the rains have ended
  3. Selecting a short-duration variety and sowing at the onset of the rains Correct answer.
  4. Selecting a long-duration variety to use residual moisture

Explanation

In short-season rainfed systems, matching crop duration to the period of available moisture is critical; a short-duration variety sown with the first reliable rains completes its cycle before moisture runs out. Long-duration varieties risk terminal drought, and excessive density depletes stored moisture.

Question 34Agronomy and Crop Production

A grower wants to reduce post-harvest losses in stored grain. Which combination of practices is most effective for preventing insect infestation during storage?

  1. Storing freshly harvested moist grain in open bags for ventilation
  2. Drying grain to safe moisture and storing it in clean, sealed containers Correct answer.
  3. Storing grain in a warm, humid room to prevent condensation
  4. Mixing fresh grain with previously infested grain to equalise moisture

Explanation

Insects and moulds require adequate moisture, so drying grain to a safe moisture level before storage and keeping it in clean, sealed structures greatly reduces infestation and spoilage. Moist or mixed grain and warm humid storage conditions favour pest and mould development.

Question 35Agronomy and Crop Production

A wheat crop shows poor tillering and pale lower leaves despite adequate rainfall, and soil analysis shows low available nitrogen. Which action addresses the immediate deficiency most directly?

  1. Flooding the field to dissolve native soil nitrogen
  2. Applying lime to raise the soil pH before any fertilizer
  3. Top-dressing with a readily available nitrogen fertilizer at the recommended stage Correct answer.
  4. Applying a phosphorus fertilizer alone as a basal dose

Explanation

Low available nitrogen limits tillering and causes pale lower leaves, and a top-dressing of readily available nitrogen supplies the nutrient during the active growth stage. Lime corrects acidity, phosphorus alone does not supply nitrogen, and flooding can cause nitrogen losses through denitrification.

Question 36Agronomy and Crop Production

Two fields have the same soil texture, but field A has been under continuous conventional tillage and field B under long-term no-till with residue retention. Which difference in soil behaviour is most expected?

  1. Field A has higher organic matter and better aggregation than field B
  2. Field A has greater earthworm activity and macropore continuity
  3. Field B has lower biological activity because residues are retained
  4. Field B has higher aggregate stability and greater infiltration than field A Correct answer.

Explanation

Retaining residues and avoiding tillage builds soil organic matter and promotes aggregate formation, which improves structure, infiltration and biological activity. Continuous tillage accelerates organic matter oxidation and breakdown of aggregates, so field A would be expected to be poorer on these measures.

Question 37Soil Science and Soil Fertility

In soil science, the term 'soil texture' refers to which property of the fine-earth fraction?

  1. The concentration of soluble salts in the solution
  2. The relative proportion of sand, silt and clay particles Correct answer.
  3. The total volume of pore space present
  4. The arrangement of peds into aggregates

Explanation

Soil texture is defined strictly by the mass proportions of the three primary particle-size separates: sand, silt and clay. It is a stable, largely inherited property of the mineral fraction. Aggregate arrangement is structure, pore volume is porosity, and salt concentration is salinity.

Question 38Soil Science and Soil Fertility

A soil sample contains 40% sand, 40% silt and 20% clay. Using a standard textural triangle, this soil falls in which textural class?

  1. Clay loam
  2. Silt loam
  3. Loam Correct answer.
  4. Sandy clay loam

Explanation

The loam class occupies the central region of the textural triangle, typically around 40% sand, 40% silt and 20% clay. All three separates are present in moderate amounts, giving balanced physical behaviour. Values near 20% clay sit well below the clay-loam band.

Question 39Soil Science and Soil Fertility

Which soil separate is defined as particles with an effective diameter of less than 0.002 mm?

  1. Gravel
  2. Fine sand
  3. Silt
  4. Clay Correct answer.

Explanation

In the internationally used size classification, clay is the fraction finer than 0.002 mm in equivalent diameter. Silt spans roughly 0.002 to 0.05 mm, fine sand about 0.05 to 0.25 mm, and gravel is coarser than 2 mm. The very small size of clay gives it enormous specific surface area.

Question 40Soil Science and Soil Fertility

Why does a clay-dominated soil generally hold more plant-available water than a sandy soil, despite having less total pore space?

  1. Clay soils have larger macropores that store water against gravity
  2. Clay particles have high specific surface area and abundant micropores that retain water strongly Correct answer.
  3. Clay soils have lower bulk density and therefore more total pore volume
  4. Clay minerals dissolve slowly and release water to roots

Explanation

Clay has an extremely large specific surface area and a network of fine micropores, so capillary retention is strong and much water remains after gravity drainage. Sand has large pores that drain freely, leaving little stored water. The key contrast is pore size and surface area, not total pore volume.

Question 41Soil Science and Soil Fertility

Which statement correctly describes a soil with a bulk density of 1.6 g/cm3 and a particle density of 2.65 g/cm3?

  1. It is a waterlogged, compacted clay
  2. It has about 60% total porosity
  3. It has about 40% total porosity Correct answer.
  4. It has no macroporosity at all

Explanation

Total porosity equals one minus the ratio of bulk density to particle density. Here 1 - (1.6/2.65) gives approximately 0.40, or 40% pore space. This value is typical of a somewhat compacted soil and does not by itself indicate waterlogging.

Question 42Soil Science and Soil Fertility

Which soil structure type is most favourable for root growth and water movement in agricultural topsoils?

  1. Granular structure Correct answer.
  2. Massive structure
  3. Columnar structure
  4. Platy structure

Explanation

Granular (crumb) structure consists of small, rounded, porous aggregates that create a good balance of macropores and micropores. This supports root penetration, aeration and infiltration. Platy, columnar and massive forms restrict these processes.

Question 43Soil Science and Soil Fertility

A farmer notices a pale, greyish layer just below the plough layer where water tends to perch. Which process most likely produced this feature?

  1. Intense leaching of clay to deeper horizons
  2. Accumulation of calcium carbonate
  3. Reduction and loss of iron under periodic saturation Correct answer.
  4. Wind deposition of sandy material

Explanation

Pale grey colours indicate reduced iron that has been mobilized and removed under waterlogged, anaerobic conditions, a process called gleying. Perched water above a less permeable layer fits this setting. Carbonate accumulation would give whitish nodules, clay illuviation would darken and enrich a subsoil horizon, and wind deposition produces different textures.

Question 44Soil Science and Soil Fertility

Which cation is held most strongly on negatively charged soil colloids under comparable conditions?

  1. Sodium
  2. Potassium
  3. Aluminium Correct answer.
  4. Calcium

Explanation

In the lyotropic series, trivalent ions such as aluminium are adsorbed far more strongly than divalent or monovalent ions. Charge magnitude dominates the strength of electrostatic attraction to clay surfaces. Sodium, being monovalent, is held most weakly among these.

Question 45Soil Science and Soil Fertility

A soil has a cation exchange capacity of 20 cmol(+)/kg and 15% base saturation. What is the approximate exchangeable acidity in cmol(+)/kg?

  1. 15
  2. 5
  3. 3
  4. 17 Correct answer.

Explanation

Exchangeable acidity equals CEC multiplied by the acid saturation fraction. Acid saturation is 100 minus 15, which is 85%, so 0.85 times 20 gives 17 cmol(+)/kg. Base saturation of 15% corresponds to only 3 cmol(+)/kg of exchangeable bases.

Question 46Soil Science and Soil Fertility

Which practice most directly raises the pH of a strongly acidic mineral soil?

  1. Applying ammonium sulphate
  2. Applying ground limestone Correct answer.
  3. Applying elemental sulphur
  4. Applying gypsum

Explanation

Ground limestone supplies carbonate that neutralizes hydrogen and aluminium ions on the exchange complex, raising pH. Elemental sulphur and ammonium sulphate acidify soil through oxidation and nitrification. Gypsum supplies calcium and sulphate without a liming effect.

Question 47Soil Science and Soil Fertility

Which soil condition is most likely to induce iron chlorosis in a sensitive crop?

  1. Waterlogged acid peat with abundant organic matter
  2. Low pH with high exchangeable aluminium
  3. High pH with free calcium carbonate Correct answer.
  4. Neutral pH with high available phosphorus

Explanation

Iron solubility falls sharply as pH rises, and free calcium carbonate further precipitates iron, so calcareous alkaline soils commonly produce iron chlorosis. Acid soils, by contrast, usually supply adequate iron. This is why sensitive crops yellow on high-pH soils.

Question 48Soil Science and Soil Fertility

Which form of nitrogen is taken up most readily by most crop plants?

  1. Urea as an intact molecule
  2. Nitrate and ammonium Correct answer.
  3. Nitrous oxide
  4. Elemental nitrogen gas

Explanation

Plants absorb nitrogen chiefly as nitrate and ammonium ions from the soil solution. Elemental nitrogen gas is inert to plants, urea is generally hydrolysed to ammonium before uptake, and nitrous oxide is a gaseous loss product.

Question 49Soil Science and Soil Fertility

Which process converts ammonium to nitrate in aerobic soil and is carried out by Nitrosomonas and Nitrobacter?

  1. Denitrification
  2. Nitrogen fixation
  3. Ammonification
  4. Nitrification Correct answer.

Explanation

Nitrification is the two-step aerobic oxidation of ammonium to nitrite and then nitrate by specialized bacteria. Ammonification releases ammonium from organic matter, denitrification reduces nitrate to gaseous nitrogen under anaerobic conditions, and fixation converts atmospheric nitrogen into combined forms.

Question 50Soil Science and Soil Fertility

A waterlogged paddy field shows declining available nitrogen despite adequate ammonium fertilizer. Which process best explains this?

  1. Phosphorus is volatilized as ammonia
  2. Ammonium is fixed irreversibly by sand particles
  3. Nitrification is enhanced by the anaerobic conditions
  4. Denitrification converts nitrate to nitrogen gases under low oxygen Correct answer.

Explanation

In saturated soils oxygen is depleted, and denitrifying bacteria use nitrate as an electron acceptor, releasing nitrogen gases to the atmosphere. Any nitrate formed in oxidized zones is lost when it moves into reduced zones. Nitrification itself requires oxygen and is suppressed, not enhanced, under anaerobic conditions.