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Assam ASSEB Higher Secondary Skill Subject Agriculture study materials
Assam ASSEB Higher Secondary Skill Subject Agriculture study resources

Assam ASSEB Higher Secondary Skill Subject Agriculture practice questions

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Question 1Agronomy and Crop Production

Which soil property is most directly improved by adding well-decomposed farmyard manure to a sandy loam field?

  1. Depth of the water table
  2. Particle density of the sand fraction
  3. Water-holding capacity and cation exchange capacity Correct answer.
  4. Bulk density of the subsoil horizon

Explanation

Farmyard manure adds colloidal organic matter that increases both the water-holding capacity and the cation exchange capacity of light-textured soils. It does not change the mineral particle density of sand, the depth of the water table, or the bulk density of deeper horizons in any predictable way.

Question 2Agronomy and Crop Production

In Assam, which crop sequence best fits a rainfed lowland field that remains waterlogged during the kharif season?

  1. Cotton followed by groundnut
  2. Rice followed by a short-duration pulse on residual moisture Correct answer.
  3. Maize followed by sunflower
  4. Sugarcane followed by tobacco

Explanation

Rice tolerates standing water and is the standard kharif crop for lowland fields, and a short-duration pulse such as lentil or lathyrus can use residual soil moisture after harvest. Cotton, maize, sunflower, sugarcane and tobacco are all sensitive to waterlogging and do not suit such fields.

Question 3Agronomy and Crop Production

Which practice is the primary method of controlling bacterial blight of rice in the field?

  1. Using resistant varieties and avoiding excess nitrogen Correct answer.
  2. Spraying a copper-based bactericide at weekly intervals
  3. Treating soil with a systemic fungicide
  4. Applying lime to raise soil pH above 8

Explanation

Bacterial blight is managed mainly through host resistance and balanced nitrogen nutrition, because excessive nitrogen makes plants succulent and more susceptible. Chemical control options are limited and fungicides have no effect on a bacterial pathogen.

Question 4Agronomy and Crop Production

The critical period of crop-weed competition in a transplanted rice crop is best described as which stage?

  1. From panicle initiation to maturity
  2. From harvest to the next sowing
  3. From transplanting to about 30 to 45 days after transplanting Correct answer.
  4. From flowering to grain filling

Explanation

Weeds cause the greatest yield loss during the early establishment phase, roughly the first 30 to 45 days after transplanting, when the crop canopy is still open. Once the canopy closes, rice competes strongly and later weed flushes cause much smaller losses.

Question 5Agronomy and Crop Production

A farmer applies 100 kg of urea to one hectare. Approximately how much nitrogen does this supply?

  1. 46 kg Correct answer.
  2. 100 kg
  3. 60 kg
  4. 20 kg

Explanation

Urea contains about 46 percent nitrogen, so 100 kg of urea supplies roughly 46 kg of actual nitrogen. The remaining mass is mainly the carbonyl and amide components that convert to ammonium in soil.

Question 6Agronomy and Crop Production

Which irrigation method is most efficient in water use for a widely spaced row crop such as maize on a well-drained soil?

  1. Basin irrigation around each plant
  2. Flood irrigation of the whole field
  3. Drip irrigation delivered near the root zone Correct answer.
  4. Furrow irrigation along the crop rows

Explanation

Drip irrigation delivers water directly to the root zone and minimises evaporation, deep percolation and runoff, giving the highest application efficiency. Flood and basin methods wet large areas unnecessarily, and furrow irrigation loses water to percolation and evaporation from the wetted furrow surface.

Question 7Agronomy and Crop Production

Which of the following is a leguminous green manure crop commonly grown for ploughing into the soil before rice?

  1. Rapeseed
  2. Berseem
  3. Dhaincha Correct answer.
  4. Napier grass

Explanation

Dhaincha (Sesbania) is a fast-growing legume used as green manure because it fixes atmospheric nitrogen through root nodulation and produces abundant biomass in a short period. Rapeseed and Napier grass are non-legumes, and berseem is a fodder legume usually grazed or cut rather than ploughed in.

Question 8Agronomy and Crop Production

Seed rate for a crop is increased mainly when which condition applies?

  1. Sowing is delayed and tillering time is reduced Correct answer.
  2. The field has been precisely levelled
  3. Only certified seed is available
  4. Seed of very high germination percentage is used

Explanation

Delayed sowing shortens the vegetative period, so plants tiller less and the crop must compensate with a higher plant population, which is achieved by raising the seed rate. High germination and good levelling normally allow a normal or slightly reduced seed rate.

Question 9Agronomy and Crop Production

Which of the following describes the correct way to take a representative soil sample from a field for fertility testing?

  1. Collect one sample from the field corner
  2. Sample only the spots where the crop looks poor
  3. Take a composite of several borings from a zigzag path across the field Correct answer.
  4. Sample from the surface 1 cm of soil

Explanation

A representative sample is made by collecting many small borings along a zigzag or random path and mixing them into one composite. Sampling a single spot, only poor patches, or a very shallow layer gives a biased or unrepresentative result.

Question 10Agronomy and Crop Production

Which cropping pattern is an example of a multiple cropping system?

  1. Monoculture of jute for three years
  2. Rice-rice-pulse Correct answer.
  3. Sole cropping of wheat year after year
  4. Rice-fallow-fallow

Explanation

Multiple cropping means growing two or more crops in the same field in one year, and rice-rice-pulse fits three crops into a single year. A single crop each year, or a field left fallow after one crop, is not multiple cropping.

Question 11Agronomy and Crop Production

In the context of integrated nutrient management, what is the main agronomic benefit of combining organic manure with chemical fertiliser?

  1. It removes the need for any soil testing
  2. It eliminates nutrient losses by leaching
  3. It synchronises nutrient supply and improves soil physical condition Correct answer.
  4. It guarantees higher yield than chemical fertiliser alone in every soil

Explanation

Combining organic and inorganic sources supplies readily available nutrients while the manure releases nutrients slowly and improves soil structure and microbial activity. It does not remove the need for soil testing, cannot guarantee a yield advantage everywhere, and does not eliminate leaching.

Question 12Agronomy and Crop Production

Which of the following is the most appropriate measure to control stored-grain insect pests in a farm godown?

  1. Mixing soil with the grain before storage
  2. Increasing ventilation and humidity in the godown
  3. Sprinkling water over the grain bags
  4. Fumigation with a suitable fumigant in a sealed space Correct answer.

Explanation

Fumigation in a properly sealed space is the standard method for killing insects inside stored grain, including stages hidden within kernels. Moisture, humidity and soil contamination instead promote moulds and deterioration of the grain.

Question 13Agronomy and Crop Production

Which of the following is a characteristic feature of the rice plant that distinguishes it from maize at the vegetative stage?

  1. Fibrous root system
  2. Presence of a ligule and auricles at the leaf collar Correct answer.
  3. Parallel venation of the leaf
  4. Absence of stomata on the leaf blade

Explanation

Rice leaves bear a distinct ligule and hairy auricles at the junction of blade and sheath, which maize lacks. Both crops have parallel venation, stomata on the leaf blade, and fibrous root systems, so these do not separate them.

Question 14Agronomy and Crop Production

Which operation is carried out to break soil clods and prepare a fine seedbed after the first ploughing?

  1. Weeding
  2. Harrowing Correct answer.
  3. Planking
  4. Ridging

Explanation

Harrowing breaks clods, levels the surface and pulverises the soil to produce a fine tilth suitable for sowing. Planking firms and levels the surface, ridging forms raised beds, and weeding removes unwanted plants without improving tilth.

Question 15Agronomy and Crop Production

A crop shows interveinal chlorosis on older leaves while the veins remain green. Which nutrient deficiency is most likely?

  1. Boron
  2. Magnesium Correct answer.
  3. Iron
  4. Calcium

Explanation

Magnesium is mobile in the plant, so deficiency appears first on older leaves as interveinal chlorosis because the nutrient is moved to growing points. Iron deficiency also causes interveinal chlorosis but typically appears on young leaves, while calcium and boron affect meristems and young tissue.

Question 16Agronomy and Crop Production

Which of the following best describes the principle of crop rotation in relation to pest management?

  1. Growing the same crop repeatedly on the same land
  2. Alternating unrelated crops so that host-specific pests and pathogens decline Correct answer.
  3. Leaving the field fallow for a single season only
  4. Growing two varieties of the same crop in alternate years

Explanation

Rotation with unrelated crops interrupts the life cycle of host-specific pests and pathogens by removing their preferred host and breaking the build-up of inoculum. Repeating the same crop or its varieties allows pests to accumulate, and a single fallow season is not a rotation.

Question 17Agronomy and Crop Production

Which of the following is a kharif season crop in Assam?

  1. Wheat
  2. Lentil
  3. Mustard
  4. Jute Correct answer.

Explanation

Jute is sown with the onset of the monsoon and harvested in the kharif season, making it a typical rainy-season crop. Mustard, lentil and wheat are rabi crops grown in the cooler post-monsoon period.

Question 18Agronomy and Crop Production

What is the main purpose of treating legume seed with a suitable Rhizobium culture before sowing?

  1. To supply phosphorus to the seedling
  2. To protect the seed from storage insects
  3. To promote nodulation and biological nitrogen fixation Correct answer.
  4. To increase seed germination percentage

Explanation

Rhizobium inoculation introduces effective strains of the bacterium that form nodules on legume roots and fix atmospheric nitrogen. It does not act as an insecticide, does not raise germination, and does not supply phosphorus.

Question 19Agronomy and Crop Production

A grower wants to reduce soil erosion on a sloping field. Which combination of practices is most effective?

  1. Deep ploughing along the slope direction
  2. Burning crop residues before the rains
  3. Contour cultivation with vegetative barriers Correct answer.
  4. Clean cultivation up and down the slope

Explanation

Cultivating along the contour shortens the slope length and slows runoff, while vegetative barriers trap soil and reduce rill erosion. Cultivating up and down the slope, deep ploughing downslope, and removing residues all accelerate erosion.

Question 20Agronomy and Crop Production

Which of the following is the correct calculation of plant population per hectare when the recommended spacing is 20 cm between rows and 10 cm between plants?

  1. 50,000 plants per hectare
  2. 50,00,000 plants per hectare
  3. 5,00,000 plants per hectare Correct answer.
  4. 5,000 plants per hectare

Explanation

Each plant occupies 20 cm x 10 cm, or 0.2 m x 0.1 m, which is 0.02 square metres. Dividing 10,000 square metres per hectare by 0.02 gives 5,00,000 plants per hectare.

Question 21Agronomy and Crop Production

Which of the following describes the correct stage to harvest a cereal crop for grain purpose?

  1. Immediately after flowering
  2. When the straw is still green and succulent
  3. When the grain is hard and moisture content is around 14 to 18 percent Correct answer.
  4. At the milky stage

Explanation

Cereals are harvested at physiological to field maturity, when grains are hard and grain moisture has fallen to roughly 14 to 18 percent, which allows safe threshing and storage. Harvesting at the milky stage, just after flowering, or while the straw is green gives immature, high-moisture grain.

Question 22Agronomy and Crop Production

Which of the following is the primary reason for practising summer ploughing in rainfed areas?

  1. To encourage weed seed germination before sowing
  2. To expose soil-borne pests and pathogens to heat and to store rain water Correct answer.
  3. To increase the bulk density of the surface soil
  4. To conserve subsoil moisture by exposing it to sun

Explanation

Summer ploughing exposes soil-borne insect stages and pathogen propagules to the sun and also opens the soil so that pre-monsoon showers are stored in the profile. Increasing bulk density and encouraging weed growth are not its objectives.

Question 23Agronomy and Crop Production

Which of the following is the most suitable method of weed control in a crop where herbicide application is not permitted, such as an organic certified field?

  1. Flooding the field throughout the season
  2. Repeated application of a non-selective herbicide
  3. Mechanical cultivation combined with mulching and crop rotation Correct answer.
  4. Increasing the seed rate to smother all weeds

Explanation

In organic systems weeds are managed through cultural and mechanical means such as timely cultivation, mulching, crop rotation and competitive crop canopies. Herbicides are excluded, a high seed rate alone rarely smothers all weeds, and continuous flooding is not possible for most crops.

Question 24Agronomy and Crop Production

Which of the following is a correct statement about the role of potassium in crop plants?

  1. It is essential for nodulation in legumes
  2. It regulates stomatal opening and improves disease resistance Correct answer.
  3. It is a constituent of chlorophyll
  4. It is a component of the amino acids in proteins

Explanation

Potassium is not built into structural compounds but acts as an osmotic regulator, controlling stomatal movement and improving drought tolerance and disease resistance. Chlorophyll contains magnesium and proteins contain nitrogen, while nodulation depends mainly on the Rhizobium symbiosis.

Question 25Agronomy and Crop Production

Which of the following correctly states the main advantage of transplanting rice seedlings rather than direct seeding?

  1. It allows better weed suppression and uniform crop stand in the main field Correct answer.
  2. It removes the need for a nursery
  3. It shortens the total crop duration
  4. It eliminates the need for irrigation

Explanation

Transplanting raises seedlings in a nursery and then establishes a uniform, vigorous stand in the main field, which competes better with weeds and gives more uniform maturity. It requires a nursery, does not shorten total crop duration, and does not remove the need for irrigation.

Question 26Agronomy and Crop Production

In the Assam context, which of the following is the most appropriate time to sow the sali (winter) rice crop in the low-lying plains?

  1. June to July, coinciding with the peak monsoon rains Correct answer.
  2. October to November, after the monsoon has receded
  3. February to March, at the start of the pre-monsoon showers
  4. April to May, immediately before the summer solstice

Explanation

Sali rice is the main winter rice of Assam and is transplanted during the monsoon period, usually June to July, so that the crop grows through the rainy season and matures in the cool post-monsoon months. Sowing at that time matches water availability and the photoperiod requirements of the recommended varieties.

Question 27Agronomy and Crop Production

A farmer wants to grow a pulse crop in rotation with rice on the same bunded land in Assam. Which sequence best uses the residual moisture and improves soil nitrogen for the following rice crop?

  1. Rice followed by transplanted sugarcane
  2. Rice followed by a second transplanted rice crop
  3. Rice followed by lentil or lathyrus grown on residual soil moisture Correct answer.
  4. Rice followed by a long-duration cotton crop

Explanation

Pulses such as lentil and lathyrus (khesari) can be grown on residual moisture after the rice harvest, and their symbiotic nitrogen fixation leaves nitrogen in the soil for the next rice crop. This utera or relay practice is well suited to the receding-moisture situation of lowland rice fields.

Question 28Agronomy and Crop Production

Which of the following describes the main agronomic purpose of puddling and levelling a field before transplanting rice?

  1. To increase soil aeration and improve root respiration
  2. To increase the population of aerobic nitrifying bacteria
  3. To encourage deep root penetration of the seedlings
  4. To reduce percolation losses and create a soft mud for easy transplanting Correct answer.

Explanation

Puddling breaks down soil aggregates and creates a layer of dispersed fine particles that reduces water percolation, while levelling ensures uniform water depth across the field. The soft muddy condition also makes transplanting of seedlings easier and improves establishment.

Question 29Agronomy and Crop Production

A grower in a rainfed upland area of Assam wants to raise a crop on a slope of about 8 percent. Which practice is the most direct measure to check soil loss along the slope?

  1. Removing all crop residue to keep the surface bare
  2. Ploughing the field up and down the slope to speed drainage
  3. Growing crops in rows running straight down the slope
  4. Constructing contour bunds and sowing along the contour Correct answer.

Explanation

Contour bunds and contour-aligned sowing interrupt the downslope flow of runoff, giving water time to infiltrate and reducing the velocity of the flow that carries soil away. On an 8 percent slope these measures directly reduce rill and sheet erosion.

Question 30Agronomy and Crop Production

A crop field shows poor nodulation on the legume crop despite inoculation. Which of the following is the most likely agronomic cause?

  1. The field had a history of the same legume crop
  2. The soil pH was corrected with lime before sowing
  3. The seed was treated with a fungicide at the same time as the Rhizobium culture Correct answer.
  4. The seed was sown in moist soil soon after inoculation

Explanation

Many seed-treatment fungicides are toxic to Rhizobium cells, so applying them together with the inoculum kills the bacteria before they can colonise the roots. Inoculation should be done after fungicide treatment has dried, or the two operations should be separated.

Question 31Agronomy and Crop Production

Which of the following correctly describes the role of phosphorus in crop plants?

  1. It is a structural component of the cell wall in the form of calcium pectate
  2. It regulates stomatal opening and closing as the main osmotic solute
  3. It promotes root development and is a key component of energy-transfer compounds such as ATP Correct answer.
  4. It is the central atom of the chlorophyll molecule and carries the light-harvesting function

Explanation

Phosphorus is a constituent of nucleic acids, phospholipids and the energy-transfer compounds ATP and ADP, and it strongly promotes early root development. That is why phosphorus is placed in the root zone at sowing as a basal application.

Question 32Agronomy and Crop Production

A farmer applies urea to a standing rice crop on a hot afternoon and then floods the field heavily. Which agronomic concern is most directly addressed by splitting the urea into two or three applications instead?

  1. Depletion of soil organic carbon by the urea
  2. Loss of nitrogen through volatilisation and leaching, which lowers nitrogen use efficiency Correct answer.
  3. Immobilisation of potassium by clay minerals
  4. Loss of phosphorus through fixation on soil particles

Explanation

Split application matches nitrogen supply to crop demand and reduces the period during which urea is exposed to volatilisation, denitrification and leaching losses. This raises nitrogen use efficiency and reduces the amount of fertiliser needed.

Question 33Agronomy and Crop Production

In a seed production plot of a self-pollinated cereal, which of the following is the most important reason for roguing?

  1. To improve the germination percentage of the harvested seed
  2. To increase the plant population per unit area
  3. To reduce the dose of fertiliser applied to the plot
  4. To remove off-type and diseased plants so that genetic purity is maintained Correct answer.

Explanation

Roguing removes off-types, volunteers and diseased plants before they can flower and contribute pollen or seed to the produce. This keeps the genetic purity and health status of the seed lot within certification standards.

Question 34Agronomy and Crop Production

A crop shows characteristic symptoms of a micronutrient deficiency that appears first on the young leaves. Which of the following nutrients is most likely deficient, given that it is relatively immobile in the phloem?

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

Explanation

Iron is largely immobile in the phloem, so when it is deficient the plant cannot move it from older to younger tissue. Symptoms therefore appear first on the young leaves, typically as interveinal chlorosis.

Question 35Agronomy and Crop Production

Which of the following is the correct basis for computing the seed rate of a crop when the recommended spacing, germination percentage and test weight are known?

  1. Multiply the plant population per hectare by the test weight and divide by the germination fraction Correct answer.
  2. Divide the test weight by the spacing in centimetres
  3. Divide the field area by the number of seeds per kilogram
  4. Add the germination percentage to the spacing in centimetres

Explanation

Seed rate is set by the target plant population per hectare, the weight per seed (test weight) and the proportion of seeds expected to germinate. Multiplying population by test weight and dividing by the germination fraction gives the weight of viable seed needed per hectare.

Question 36Agronomy and Crop Production

A grower wants to reduce the weed seed bank in a field that has been continuously cropped with rice for many years. Which of the following is the most effective agronomic approach?

  1. Delaying sowing until the weeds have emerged and set seed
  2. Increasing the rice seed rate to shade out weeds
  3. Continuing the same rice monoculture with the same herbicide every season
  4. Introducing a crop rotation with crops of different growth habit and herbicide groups Correct answer.

Explanation

A rotation with crops of different growth habit, canopy structure and herbicide groups disrupts the life cycle of weeds adapted to continuous rice. This prevents any one weed species from building up a dominant seed bank and reduces selection for herbicide resistance.

Question 37Agronomy and Crop Production

Which of the following best explains why a legume in rotation improves the yield of a following cereal crop even when the legume is not fertilised with nitrogen?

  1. The legume leaves residual nitrogen fixed symbiotically and breaks the pest and disease cycle of the cereal Correct answer.
  2. The legume consumes soil nitrogen, which forces the cereal to grow deeper roots
  3. The legume raises the soil pH permanently and unlocks all soil phosphorus
  4. The legume reduces the soil bulk density to zero through its tap root

Explanation

Symbiotic fixation by rhizobia adds nitrogen to the soil-plant system, and part of it remains available for the following cereal. The break in the cereal monoculture also reduces the build-up of cereal-specific pathogens and pests, which contributes to the yield benefit.

Question 38Agronomy and Crop Production

Which of the following is the most appropriate stage to harvest a paddy crop for grain purpose?

  1. When the panicles are still green and the grains are in the milk stage
  2. When about 80 percent of the grains in the panicle have turned straw-coloured and the grain is hard Correct answer.
  3. When the straw has completely dried and the panicle has shattered
  4. When the crop is still flowering and the anthers are fresh

Explanation

Paddy should be harvested when the grains are in the hard dough stage and most of them have turned straw-coloured, which gives maximum grain weight with minimum shattering. Harvesting too early gives chaffy, immature grain, while too late a harvest leads to shattering and breakage.

Question 39Agronomy and Crop Production

A field with a sandy loam soil in a high-rainfall area of Assam is to be planted with a crop sensitive to waterlogging. Which land configuration is most suitable?

  1. Broad bed and furrow with a gentle slope towards a drainage channel Correct answer.
  2. Flat land with raised field borders and no drainage outlet
  3. Deep puddling followed by continuous flooding
  4. Level basins with the water table kept at the surface

Explanation

A broad bed and furrow layout raises the crop root zone above the water table and allows excess rain to drain along the slope to an outlet. This keeps the root zone aerated in a high-rainfall sandy loam where waterlogging is the main risk.

Question 40Agronomy and Crop Production

Which of the following correctly describes the principle of integrated pest management as applied to an insect pest in a field crop?

  1. Applying the highest labelled dose of a broad-spectrum insecticide at the first sign of the pest
  2. Removing all non-crop vegetation around the field to eliminate alternate hosts
  3. Spraying on a fixed calendar schedule regardless of pest numbers
  4. Combining resistant varieties, cultural practices, biological control and need-based pesticides Correct answer.

Explanation

Integrated pest management combines resistant varieties, cultural and mechanical measures, conservation of natural enemies and judicious, need-based use of selective pesticides. Decisions are based on monitoring and economic threshold levels rather than routine spraying.

Question 41Soil Science and Soil Fertility Management

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

  1. The relative proportions of sand, silt and clay particles Correct answer.
  2. The arrangement of soil particles into aggregates
  3. The degree of acidity measured in the soil solution
  4. The total volume of pore spaces present in the soil

Explanation

Soil texture is defined by the relative proportions of the three primary particle-size separates: sand, silt and clay. It is a fixed, largely permanent characteristic of a soil because it depends on the mineral particles present, not on how they clump together. Structure, porosity and pH are separate properties measured by different methods.

Question 42Soil Science and Soil Fertility Management

A soil sample contains 20% clay, 40% silt and 40% sand. Which texture class does this composition correspond to on the standard USDA textural triangle?

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

Explanation

A loam has roughly balanced proportions, commonly given as about 40% sand, 40% silt and 20% clay, which matches the sample exactly. Sandy clay loam and clay loam both require more than 27% clay, and silt loam requires over 50% silt. Placing the three percentages on the triangle confirms the loam class.

Question 43Soil Science and Soil Fertility Management

Which soil structure type is most commonly associated with good aeration and drainage in cultivated surface horizons?

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

Explanation

Granular structure consists of small, rounded, porous aggregates that allow free movement of air and water and favour root penetration. Platy structure is thin and horizontally flattened, impeding vertical water movement. Columnar and massive structures lack the interconnected pore network needed for good aeration.

Question 44Soil Science and Soil Fertility Management

A farmer wants to raise the pH of a strongly acidic soil. Which amendment is the standard choice for this purpose?

  1. Gypsum
  2. Aluminium sulphate
  3. Elemental sulphur
  4. Ground agricultural limestone Correct answer.

Explanation

Ground limestone (calcium carbonate, sometimes with magnesium carbonate) neutralises soil acidity and raises pH. Elemental sulphur, aluminium sulphate and gypsum do not supply the carbonate needed for neutralisation; sulphur and aluminium sulphate actually lower pH. Gypsum adds calcium without changing pH appreciably.

Question 45Soil Science and Soil Fertility Management

Which cation occupies the largest share of exchange sites in a typical fertile, near-neutral agricultural soil in humid regions?

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

Explanation

Calcium normally dominates the exchange complex of near-neutral soils, typically occupying more than half of the base exchange capacity. Potassium and sodium are present in much smaller proportions, and hydrogen dominates only in strongly acidic soils. This dominance of calcium reflects the abundance of calcium carbonate minerals and liming.

Question 46Soil Science and Soil Fertility Management

The cation exchange capacity (CEC) of a soil is expressed in which unit?

  1. Grams per litre
  2. Centimoles of charge per kilogram (cmol(c) kg-1) Correct answer.
  3. Kilograms per cubic metre
  4. Decisiemens per metre

Explanation

CEC measures the quantity of exchangeable cations held per unit mass of soil, expressed in centimoles of charge per kilogram, formerly milliequivalents per 100 g. The other units measure bulk density, electrical conductivity and concentration respectively. CEC is therefore a charge-based quantity, not a mass or conductivity value.

Question 47Soil Science and Soil Fertility Management

A soil has a CEC of 20 cmol(c) kg-1 and its exchange sites are 75% saturated with calcium. How much exchangeable calcium does the soil hold?

  1. 75 cmol(c) kg-1
  2. 20 cmol(c) kg-1
  3. 5 cmol(c) kg-1
  4. 15 cmol(c) kg-1 Correct answer.

Explanation

Exchangeable calcium equals the base saturation fraction multiplied by the total CEC, so 0.75 multiplied by 20 gives 15 cmol(c) kg-1. The remaining 5 cmol(c) kg-1 is occupied by other cations. The value 20 is the total capacity, and 75 is the percentage, not a quantity.

Question 48Soil Science and Soil Fertility Management

Which soil organism group is chiefly responsible for decomposing organic residues and forming stable humus in arable soils?

  1. Nematodes
  2. Bacteria and fungi Correct answer.
  3. Earthworms
  4. Algae

Explanation

Bacteria and fungi are the principal decomposers, breaking down organic residues and synthesising humus through their metabolic activity. Earthworms mix and fragment residues but do not carry out most of the chemical decomposition. Nematodes and algae are minor contributors to humus formation in most arable soils.

Question 49Soil Science and Soil Fertility Management

Which practice most directly increases soil organic carbon in a degraded cultivated field?

  1. Regular addition of composted manure Correct answer.
  2. Continuous removal of crop residues
  3. Repeated deep ploughing
  4. Prolonged bare fallow

Explanation

Adding composted manure supplies stabilised organic carbon and nutrients, directly building soil organic carbon. Repeated ploughing, residue removal and bare fallow all accelerate organic matter loss through oxidation and erosion. The net balance of carbon inputs and losses therefore determines whether organic carbon rises or falls.

Question 50Soil Science and Soil Fertility Management

Which essential plant nutrient is most commonly deficient in acidic, highly weathered soils of the humid tropics because it is strongly fixed by iron and aluminium oxides?

  1. Calcium
  2. Nitrogen
  3. Boron
  4. Phosphorus Correct answer.

Explanation

In strongly weathered acidic soils, phosphorus is adsorbed and precipitated by iron and aluminium oxides, making it largely unavailable to plants. Nitrogen deficiency can occur but is managed through organic matter and fertiliser, while calcium deficiency is less common except in very acidic soils. Phosphorus fixation is the classic constraint in such soils.