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Lok Sewa Agriculture Officer practice questions

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Question 1Agronomy and Crop Production

In the rice-wheat system of the Terai, which practice most directly reduces the problem of terminal heat stress in late-sown wheat?

  1. Advancing sowing to the optimum November window Correct answer.
  2. Flood-irrigating at the milk stage
  3. Increasing nitrogen dose at booting
  4. Delaying the first irrigation to crown root initiation

Explanation

Terminal heat stress arises when grain filling coincides with rising March-April temperatures, and the most effective management is to shift the crop phenology earlier by sowing within the recommended November window. Later sowings compress vegetative growth and push anthesis into the heat period regardless of fertilizer or irrigation adjustments.

Question 2Agronomy and Crop Production

The critical period for irrigation in wheat, in terms of yield response, is:

  1. Crown root initiation and flowering Correct answer.
  2. Tillering and dough stage
  3. Jointing and physiological maturity
  4. Germination and milk stage

Explanation

Crown root initiation (about 20-25 days after sowing) and flowering are the two most moisture-sensitive stages in wheat, and moisture stress at these stages causes the largest yield penalties. Irrigation scheduling in Nepal therefore prioritizes these stages when water is limited.

Question 3Agronomy and Crop Production

A farmer wants to grow a short-duration legume to fit between rice harvest and wheat sowing in the Terai. Which crop best fits this window?

  1. Pigeon pea
  2. Maize
  3. Sugarcane
  4. Lentil Correct answer.

Explanation

Lentil matures in about 100-110 days and is well adapted to residual soil moisture after rice, making it a standard relay or sequential crop in the rice-fallow period. Its short duration allows timely wheat sowing afterward.

Question 4Agronomy and Crop Production

Which soil characteristic is most directly responsible for phosphorus fixation in acidic hill soils of Nepal?

  1. High exchangeable calcium
  2. High organic matter content
  3. High potassium saturation
  4. High exchangeable aluminium and iron oxides Correct answer.

Explanation

In acidic soils, soluble aluminium and iron oxides react with phosphate to form insoluble aluminium and iron phosphates, reducing phosphorus availability. Liming raises pH and reduces this fixation, improving fertilizer phosphorus efficiency.

Question 5Agronomy and Crop Production

The recommended practice of applying half the nitrogen at transplanting and the remainder in split doses in rice is primarily intended to:

  1. Match nitrogen supply to crop demand and reduce losses Correct answer.
  2. Reduce the need for phosphorus application
  3. Shorten the crop duration
  4. Increase total nitrogen dose without yield loss

Explanation

Split nitrogen application synchronizes nutrient supply with peak uptake at tillering and panicle initiation, reducing volatilization, denitrification and leaching losses. It improves nitrogen use efficiency without changing total crop duration.

Question 6Agronomy and Crop Production

Which of the following is the most reliable field indicator that a rice field has reached the appropriate stage for drainage before harvest?

  1. Presence of algae on the water surface
  2. Yellowing of the lower leaves and hardening of grains in the lower panicle Correct answer.
  3. Appearance of the first tillers
  4. Splitting of the flag leaf sheath

Explanation

Draining the field about 10-15 days before harvest is timed when grains on the lower panicle have hardened and lower leaves yellow, indicating physiological maturity. Early drainage during grain filling reduces grain weight.

Question 7Agronomy and Crop Production

A field previously under continuous rice shows declining yield despite adequate fertilizer. Which analysis best explains this trend?

  1. Improved soil structure from continuous flooding
  2. Depletion of micronutrients and declining soil organic matter under continuous puddling Correct answer.
  3. Increased potassium availability from puddling
  4. Excess phosphorus accumulation from rice straw

Explanation

Continuous puddled rice depletes zinc and other micronutrients and reduces soil organic carbon, causing yield stagnation even with adequate NPK. Diversification and organic amendments are recommended remedies.

Question 8Agronomy and Crop Production

Which agronomic measure is most effective for controlling bacterial blight of rice once the disease appears in a field?

  1. Draining the field and avoiding excess nitrogen application Correct answer.
  2. Spraying a systemic fungicide
  3. Applying lime to raise soil pH
  4. Increasing plant density to compensate for losses

Explanation

Bacterial blight is caused by Xanthomonas oryzae, and no curative chemical is available. Management relies on avoiding excess nitrogen, draining fields, and using resistant varieties to slow spread.

Question 9Agronomy and Crop Production

For seed production of a self-pollinated crop like wheat, the recommended isolation distance is primarily to prevent:

  1. Mechanical mixture and natural outcrossing Correct answer.
  2. Insect damage
  3. Bird damage
  4. Soil-borne pathogens

Explanation

Isolation distance in seed production of self-pollinated crops prevents both mechanical contamination during field operations and the limited natural outcrossing that can still occur. It ensures varietal purity of the harvested seed.

Question 10Agronomy and Crop Production

Which practice best conserves soil moisture in rainfed maize grown on sloping land in the mid-hills?

  1. Clean cultivation up and down the slope
  2. Contour planting with mulching Correct answer.
  3. Burning crop residues after harvest
  4. Deep ploughing before the monsoon

Explanation

Contour planting reduces runoff velocity and mulching reduces evaporation, together conserving soil moisture and reducing erosion on sloping land. Clean cultivation up and down the slope accelerates runoff and soil loss.

Question 11Agronomy and Crop Production

The principal reason for treating legume seed with Rhizobium culture before sowing is to:

  1. Establish effective nodulation and biological nitrogen fixation Correct answer.
  2. Increase seed germination percentage
  3. Supply phosphorus to the seedling
  4. Protect seedlings from soil-borne fungi

Explanation

Rhizobium inoculation introduces compatible nitrogen-fixing bacteria that form root nodules, enabling the legume to fix atmospheric nitrogen. It is especially valuable where the field has no history of the specific legume.

Question 12Agronomy and Crop Production

A farmer observes white, cottony growth at the water line of rice tillers with profuse tillering and lodging. Which disease is most likely?

  1. Sheath blight Correct answer.
  2. Tungro
  3. Blast
  4. Brown spot

Explanation

Sheath blight, caused by Rhizoctonia solani, produces white cottony mycelium on leaf sheaths near the water line and is favored by dense tillers and high humidity. It also causes lodging at later stages.

Question 13Agronomy and Crop Production

Which statement about the effect of photoperiod on crop adaptation is correct?

  1. Day-neutral plants flower only under 12-hour days
  2. Long-day plants flower when the day length exceeds a critical minimum Correct answer.
  3. Short-day plants flower when the day length exceeds a critical minimum
  4. Photoperiod has no effect on flowering in cereals

Explanation

Long-day plants require a day length longer than a critical value to initiate flowering, while short-day plants require days shorter than a critical value. This governs varietal adaptation across latitudes.

Question 14Agronomy and Crop Production

In a rice field, zinc deficiency typically appears as:

  1. Brown spots and rusty discoloration on middle leaves with stunting Correct answer.
  2. Purple coloration of the leaf margins
  3. Yellowing of the older leaves starting from the tip
  4. White streaks along the entire leaf length

Explanation

Zinc deficiency in rice (khaira disease) appears as brown or rusty spots on middle leaves, often with stunting and poor tillering, and is common in puddled soils. It is corrected by zinc sulphate application.

Question 15Agronomy and Crop Production

Which crop rotation is most appropriate for breaking the disease cycle of soil-borne pathogens in a vegetable field?

  1. Rotating with a closely related vegetable
  2. Fallow without weed management
  3. Growing the same vegetable family continuously
  4. Rotating with a non-host cereal for two to three seasons Correct answer.

Explanation

Crop rotation with a non-host species starves soil-borne pathogens of their host and reduces inoculum over seasons. Rotation with closely related crops fails because they share susceptibility.

Question 16Agronomy and Crop Production

The main agronomic advantage of transplanting rice compared with direct seeding is that it:

  1. Allows better weed control and uniform crop stand with earlier seedling establishment Correct answer.
  2. Shortens the total field duration by 60 days
  3. Eliminates the need for irrigation
  4. Eliminates the need for nitrogen fertilizer

Explanation

Transplanting gives the crop a head start over weeds and produces a uniform stand, improving weed competitiveness and crop establishment. It does not remove the need for water or fertilizer.

Question 17Agronomy and Crop Production

Which factor most strongly determines the sowing time of maize in the Terai?

  1. Seed size
  2. Wind direction
  3. Onset of monsoon rainfall and temperature Correct answer.
  4. Soil potassium status

Explanation

Maize sowing in the Terai is timed to the onset of monsoon rainfall and adequate temperature for germination and early growth. Sowing too early risks moisture stress and too late risks terminal stress.

Question 18Agronomy and Crop Production

Which of the following is a true statement about the law of diminishing returns in fertilizer application?

  1. Each additional unit of nutrient adds less yield than the previous unit beyond the economic optimum Correct answer.
  2. Total yield decreases as soon as the first unit of fertilizer is applied
  3. Fertilizer efficiency is constant regardless of application rate
  4. Yield increases proportionally with every additional unit of fertilizer

Explanation

The law of diminishing returns states that successive increments of a nutrient produce progressively smaller yield increases, eventually reaching an economic optimum. Applying beyond that point reduces net returns.

Question 19Agronomy and Crop Production

Which measure is the most appropriate first step for managing a field infested with striga in the Terai?

  1. Rotating with a non-host trap crop and hand weeding before flowering Correct answer.
  2. Applying a broad-spectrum fungicide
  3. Increasing nitrogen dose to suppress the weed
  4. Deep flooding for the entire season

Explanation

Striga is a parasitic weed controlled by rotating with non-host trap crops that stimulate suicidal germination, combined with hand weeding before the weed sets seed. Chemical control with fungicides is ineffective.

Question 20Agronomy and Crop Production

In a seed lot, the term 'germination percentage' as reported on a seed label refers to:

  1. The proportion of pure live seed that produces normal seedlings under standard test conditions Correct answer.
  2. The proportion of seeds that emerge in the field
  3. The proportion of seeds that float in water
  4. The weight of seedlings produced per unit seed weight

Explanation

Germination percentage is determined in a standard laboratory test as the proportion of pure seeds producing normal seedlings under specified conditions. Field emergence can differ due to soil and weather.

Question 21Agronomy and Crop Production

A farmer wants to store wheat seed for the next season in a humid Terai environment. Which practice best maintains seed viability?

  1. Mixing seed with moist sand to prevent insect attack
  2. Storing seed in a thin layer exposed to sunlight
  3. Storing freshly harvested seed in jute bags in a ventilated shed
  4. Drying seed to below 12 percent moisture and storing in airtight containers Correct answer.

Explanation

Seed viability in humid conditions is best maintained by drying to low moisture (below about 12 percent) and sealing in airtight containers to prevent moisture uptake. Cool, dry storage slows deterioration.

Question 22Agronomy and Crop Production

Which of the following best describes the role of a catch crop in a cropping system?

  1. A fast-growing crop grown between main crops to utilize residual moisture and nutrients Correct answer.
  2. A crop grown solely to be ploughed into the soil as green manure
  3. A crop grown to provide shade to the main crop
  4. A crop grown to attract pollinators

Explanation

A catch crop is a quick-growing crop inserted between two main crops to capture residual moisture and nutrients, reducing leaching and providing extra income. It differs from a green manure crop, which is incorporated into the soil.

Question 23Agronomy and Crop Production

Which soil test result would most strongly justify applying agricultural lime in a hill field?

  1. Soil pH of 6.8 with adequate exchangeable calcium
  2. Soil pH of 7.5 with free calcium carbonate
  3. Soil pH of 4.8 with high exchangeable aluminium Correct answer.
  4. Soil pH of 8.2 with high sodium

Explanation

Liming is justified when soil pH is low (below about 5.5) and exchangeable aluminium is high, because lime raises pH and precipitates toxic aluminium. Alkaline soils do not require lime.

Question 24Agronomy and Crop Production

Which physiological process is most directly affected when rice plants are subjected to submergence for more than a week during the vegetative stage?

  1. Photosynthesis and carbohydrate availability due to reduced light and gas exchange Correct answer.
  2. Phloem loading of sucrose into grains
  3. Nitrogen fixation in root nodules
  4. Stomatal closure due to high temperature

Explanation

Prolonged submergence limits light and carbon dioxide diffusion, reducing photosynthesis and carbohydrate supply, while respiration continues and depletes reserves. Submergence-tolerant varieties survive by restricting elongation and conserving carbohydrates.

Question 25Agronomy and Crop Production

Which practice is recommended to reduce post-harvest losses in paddy during the monsoon harvest in the Terai?

  1. Mixing fresh and old grain in the same bag
  2. Stacking bags tightly without ventilation
  3. Storing freshly harvested paddy in large heaps for a week
  4. Drying paddy to safe moisture before storage and avoiding heap storage of wet grain Correct answer.

Explanation

Paddy must be dried to safe moisture (about 14 percent) before storage to prevent heating, mould and aflatoxin development. Heaping wet grain traps moisture and accelerates deterioration.

Question 26Agronomy and Crop Production

In the Terai, wheat is normally sown in the first half of November. If sowing is delayed until the last week of December, which agronomic adjustment is most appropriate?

  1. Apply extra nitrogen at sowing and reduce the seed rate
  2. Sow the same variety at the normal seed rate but irrigate less frequently
  3. Delay the first irrigation until the booting stage to avoid lodging
  4. Increase the seed rate and choose an early-maturing variety Correct answer.

Explanation

Late-sown wheat has a shorter vegetative period and fewer effective tillers, so a higher seed rate compensates for reduced tillering and an early-maturing variety escapes terminal heat stress. Nitrogen should not be increased at sowing because it promotes excessive vegetative growth under already rising temperatures.

Question 27Agronomy and Crop Production

Which combination of characters is required for a crop variety to be eligible for certification as a distinct variety under seed certification standards?

  1. Resistance to pests, earliness and good milling quality
  2. Distinctness, uniformity and stability Correct answer.
  3. High yield, wide adaptation and tolerance to lodging
  4. High germination, purity and low moisture content

Explanation

Varietal identity in seed certification rests on distinctness, uniformity and stability, which allow the variety to be identified and maintained unchanged over multiplication cycles. Yield, adaptation and quality traits are desirable but are not the basis for establishing varietal identity.

Question 28Agronomy and Crop Production

A farmer in the inner Terai grows rice followed by lentil on the same land. Which change would most improve the system's nitrogen economy without reducing total cropping intensity?

  1. Replace lentil with potato in the same winter season
  2. Incorporate the lentil residue into the soil before the next rice crop Correct answer.
  3. Remove lentil residue by burning after harvest
  4. Apply farmyard manure only to the lentil crop

Explanation

Lentil is a legume, and incorporating its residue returns biologically fixed nitrogen and organic matter to the soil for the following rice crop. Removing or burning residue exports that nitrogen, while substituting a non-legume loses the fixation benefit.

Question 29Agronomy and Crop Production

Which irrigation schedule is most appropriate for wheat from crown root initiation onward in a sandy loam soil of the Terai?

  1. One heavy irrigation at sowing followed by no further irrigation
  2. Irrigate at crown root initiation, tillering, jointing, flowering and milk stages Correct answer.
  3. Irrigate only when the crop shows permanent wilting in the afternoon
  4. Flood the field continuously as is done for lowland rice

Explanation

Wheat is most sensitive to moisture stress at crown root initiation, and subsequent critical stages are tillering, jointing, flowering and milk development, giving about five to six irrigations on light soils. Continuous flooding is harmful because wheat is not a lowland crop and waterlogging reduces yield.

Question 30Agronomy and Crop Production

Which statement correctly describes the difference between a trap crop and a border crop in pest management?

  1. A trap crop is always a non-host, while a border crop is always a host of the pest
  2. Both are sown to be harvested, differing only in their position in the field
  3. A trap crop attracts the pest and is destroyed, while a border crop physically or visually obstructs pest entry Correct answer.
  4. A trap crop is grown between main crop rows, while a border crop is ploughed into the soil

Explanation

A trap crop is a preferred host planted to concentrate the pest so that it can be treated or destroyed, whereas a border crop is planted around the field to obstruct or divert pest movement into the main crop. The two differ in function, not merely in position.

Question 31Agronomy and Crop Production

A grower wants to produce hybrid seed of a crop that is normally self-pollinated. Which requirement is most essential?

  1. A single elite variety grown in isolation from other varieties
  2. Complete removal of all insects from the field during flowering
  3. Application of growth regulators to induce simultaneous flowering
  4. A male sterile line and a pollen parent planted in a planned arrangement Correct answer.

Explanation

Hybrid seed production in a self-pollinated crop requires a male sterile female parent so that cross-pollination with the pollen parent occurs instead of selfing. Isolation and insect management support purity but cannot by themselves create hybrid seed.

Question 32Agronomy and Crop Production

Which description correctly matches a cropping intensity of 200 percent?

  1. One crop grown on half of the farm each year
  2. Three crops grown on two-thirds of the land in one year
  3. One crop grown on the whole farm for two consecutive years
  4. Two crops grown on the same land in one year Correct answer.

Explanation

Cropping intensity expresses the total cropped area as a percentage of the cultivated area in a year, so two crops on the same land give 200 percent. It counts crops per unit of land per year, not the share of land occupied.

Question 33Agronomy and Crop Production

Which factor most directly limits the productivity of a rainfed upland rice crop grown on a coarse-textured soil in the mid-hills?

  1. High soil temperature at the time of sowing
  2. Low soil pH caused by continuous leaching
  3. Mid-season drought stress during critical growth stages Correct answer.
  4. Excessive soil nitrogen from legume residues

Explanation

On coarse-textured soils with low water-holding capacity under rainfed conditions, intermittent dry spells during panicle initiation and flowering cause the largest yield losses. Other factors such as acidity may occur but are not the most direct and general constraint described.

Question 34Agronomy and Crop Production

Which agronomic practice best reduces nitrate leaching from a heavily fertilized sandy soil under maize?

  1. Irrigating heavily immediately after each nitrogen application
  2. Increasing the nitrogen dose to offset expected leaching losses
  3. Applying the entire nitrogen dose as urea at sowing
  4. Splitting nitrogen application to match crop demand and using a cover crop Correct answer.

Explanation

Split applications synchronise nitrogen supply with crop uptake, and a cover crop scavenges residual nitrate, both reducing the amount left in the soil for leaching. Applying all nitrogen at sowing or irrigating heavily after application increases the nitrate available to move below the root zone.

Question 35Agronomy and Crop Production

Which practice is most appropriate for breaking dormancy in freshly harvested potato tubers intended for planting?

  1. Soak the tubers in water for several days before planting
  2. Treat the tubers with a sprout suppressant before planting
  3. Keep the tubers in airtight bags in a warm room
  4. Store the tubers in cool, ventilated conditions until the dormant period ends Correct answer.

Explanation

Freshly harvested potato tubers have a natural dormant period, and cool, ventilated storage allows it to pass while keeping the tubers sound until sprouting begins. Warm airtight storage encourages rotting, while sprout suppressants act in the opposite direction to what is needed.

Question 36Agronomy and Crop Production

A field of transplanted rice shows poor tillering and pale yellow leaves from about three weeks after transplanting, although phosphorus and potassium were applied basally. Which explanation is most consistent with these symptoms?

  1. Phosphorus was fixed by the soil and became unavailable
  2. Potassium was leached below the root zone
  3. The seedlings were transplanted too deep
  4. Nitrogen was lost through denitrification or leaching before the crop could take it up Correct answer.

Explanation

Pale yellow leaves and poor tillering appearing a few weeks after transplanting point to nitrogen shortage, which in flooded rice commonly results from denitrification of applied nitrogen or leaching in permeable soils. Phosphorus and potassium were applied basally, and their deficiencies show different symptoms.

Question 37Soil Science and Soil Fertility Management

In the USDA soil textural triangle, a soil sample has 40% sand, 40% silt and 20% clay. Which textural class does this sample belong to?

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

Explanation

The loam class sits near the centre of the USDA textural triangle, covering roughly 7–27% clay, 28–50% silt and 23–52% sand. With 40% sand, 40% silt and 20% clay the point falls squarely inside the loam area, well away from the clay-enriched or silt-dominated classes.

Question 38Soil Science and Soil Fertility Management

Which soil structure type is most commonly associated with the surface horizon of a well-managed grassland soil?

  1. Columnar
  2. Granular Correct answer.
  3. Platy
  4. Single grain

Explanation

Granular structure consists of small, rounded, porous aggregates and is typical of A horizons rich in organic matter and root activity. Grassland soils with high biological activity and polysaccharide binding commonly develop this crumb-like structure, which favours aeration and infiltration.

Question 39Soil Science and Soil Fertility Management

A soil has a bulk density of 1.4 g/cm³ and a particle density of 2.65 g/cm³. What is its total porosity?

  1. About 65%
  2. About 47% Correct answer.
  3. About 35%
  4. About 53%

Explanation

Porosity is calculated as 1 minus the ratio of bulk density to particle density: 1 − (1.4/2.65) = 1 − 0.528 ≈ 0.472, or about 47%. This reflects the pore space available for air and water in the soil matrix.

Question 40Soil Science and Soil Fertility Management

Which soil water potential component is dominant in a moist, nonsaline soil at field capacity?

  1. Pressure potential
  2. Osmotic potential
  3. Matric potential Correct answer.
  4. Gravitational potential

Explanation

In nonsaline soils, matric potential dominates because water is held by adsorption to particle surfaces and capillary forces within pores. Osmotic potential is small when salt concentration is low, and gravitational potential becomes negligible once free drainage has ceased at field capacity.

Question 41Soil Science and Soil Fertility Management

Which cation is held most strongly on soil exchange sites and therefore least readily displaced by leaching?

  1. K⁺
  2. Ca²⁺
  3. Na⁺
  4. Al³⁺ Correct answer.

Explanation

Under the lyotropic series, trivalent cations such as Al³⁺ are adsorbed far more strongly than divalent or monovalent cations because of their higher charge and smaller hydrated radius. Consequently Al³⁺ resists displacement by leaching more than Ca²⁺, K⁺ or Na⁺.

Question 42Soil Science and Soil Fertility Management

A soil has a cation exchange capacity of 20 cmol(+)/kg and 4 cmol(+)/kg of exchangeable sodium. What is its exchangeable sodium percentage?

  1. 5%
  2. 4%
  3. 20% Correct answer.
  4. 80%

Explanation

Exchangeable sodium percentage equals exchangeable sodium divided by CEC, multiplied by 100: (4/20) × 100 = 20%. This value is used together with EC and pH to classify sodic soils.

Question 43Soil Science and Soil Fertility Management

Which soil pH range is generally considered optimal for the availability of most plant nutrients in mineral soils?

  1. 6.0 to 7.0 Correct answer.
  2. 7.5 to 8.5
  3. 4.5 to 5.0
  4. 9.0 to 10.0

Explanation

A pH of roughly 6.0 to 7.0 balances the availability of macronutrients and micronutrients while avoiding severe aluminium toxicity and micronutrient precipitation. Below this range, micronutrients become overly soluble and molybdenum becomes scarce; above it, iron, manganese and zinc become less available.

Question 44Soil Science and Soil Fertility Management

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

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

Explanation

Nitrification is the two-step aerobic oxidation of ammonium to nitrite and then nitrate, mediated by Nitrosomonas and Nitrobacter respectively. It requires oxygen and neutral to slightly alkaline pH, and it acidifies the soil as a by-product.

Question 45Soil Science and Soil Fertility Management

Which fertilizer material has the highest nitrogen content among the following?

  1. Ammonium sulphate
  2. Calcium ammonium nitrate
  3. Single superphosphate
  4. Urea Correct answer.

Explanation

Urea, CO(NH₂)₂, contains about 46% nitrogen, making it the most concentrated solid nitrogen fertilizer in common use. Its high analysis reduces transport and storage costs per unit of nitrogen.

Question 46Soil Science and Soil Fertility Management

Urea applied to the soil surface is prone to nitrogen loss through which process when left unincorporated?

  1. Volatilization of ammonia Correct answer.
  2. Fixation of ammonium by clay
  3. Leaching of nitrate
  4. Denitrification

Explanation

Urea hydrolyses to ammonium carbonate through the enzyme urease, raising local pH and releasing ammonia gas that can escape from the soil surface. Surface application without incorporation or irrigation therefore increases volatilization losses.

Question 47Soil Science and Soil Fertility Management

Which soil condition most strongly limits phosphorus availability to plants?

  1. Slightly alkaline pH with adequate calcium
  2. Low pH with high free iron and aluminium oxides Correct answer.
  3. Well-aerated sandy loam at field capacity
  4. Neutral pH with high organic matter

Explanation

In acidic soils, soluble phosphate is precipitated and adsorbed by iron and aluminium oxides and hydroxides, sharply reducing availability. This is why liming acid soils often improves phosphorus uptake even without additional phosphorus fertilizer.

Question 48Soil Science and Soil Fertility Management

In the FAO soil classification, which soil group is characterised by a strong accumulation of illuvial clay in the subsoil?

  1. Fluvisol
  2. Andosol
  3. Histosol
  4. Luvisol Correct answer.

Explanation

Luvisols show a marked clay-enrichment horizon (argic B horizon) formed by illuviation of clay from the surface horizon. They are typical of temperate regions with distinct wet and dry seasons.

Question 49Soil Science and Soil Fertility Management

Which soil order in Soil Taxonomy is dominated by shrink-swell clays that form deep cracks when dry?

  1. Vertisols Correct answer.
  2. Spodosols
  3. Ultisols
  4. Aridisols

Explanation

Vertisols contain more than 30% swelling clays, mainly smectite, and develop wide surface cracks and slickensides as they shrink and swell. This churning process mixes the profile and makes management difficult.

Question 50Soil Science and Soil Fertility Management

A soil test reports 0.20% total nitrogen in the plough layer. If the plough layer mass is 2,000,000 kg per hectare, what is the total nitrogen content in kilograms per hectare?

  1. 200 kg
  2. 400 kg
  3. 4,000 kg Correct answer.
  4. 40,000 kg

Explanation

Total nitrogen equals 0.20% of 2,000,000 kg, which is 0.002 × 2,000,000 = 4,000 kg per hectare. This large pool illustrates that most soil nitrogen is in organic forms and only a small fraction is mineralised each season.