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GCE A/L Agricultural Science study materials
GCE A/L Agricultural Science study resources

GCE A/L Agricultural Science practice questions

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Question 1Plant Science and Crop Production

In a seed-testing laboratory, a sample of rice seed is placed in a germination tray and kept under standard conditions. Which combination of external factors is essential for maximum germination of this seed lot?

  1. High salinity, compacted soil and excess carbon dioxide
  2. Complete darkness, waterlogged soil and low temperature
  3. Adequate moisture, suitable temperature and oxygen Correct answer.
  4. Adequate light, high nitrogen supply and alkaline pH

Explanation

Germination requires the uptake of water to activate enzymes, a temperature within the species' cardinal range for enzyme activity, and oxygen for aerobic respiration of the embryo. Light is not essential for most cereals, and waterlogging or salinity restricts oxygen supply and water uptake.

Question 2Plant Science and Crop Production

A student examines a transverse section of a dicotyledonous root. Which tissue arrangement is expected in this section?

  1. Xylem in a central star shape with alternating phloem strands Correct answer.
  2. Scattered vascular bundles throughout a ground tissue of pith
  3. Phloem at the centre surrounded by a ring of xylem
  4. Vascular bundles arranged in a ring with cambium between xylem and phloem

Explanation

In dicot roots the primary xylem forms a solid central star or plate with phloem strands lying between the xylem arms, and there is no central pith. This radial arrangement contrasts with the collateral bundles of dicot stems.

Question 3Plant Science and Crop Production

A farmer wants to propagate a vegetatively produced crop so that the offspring are genetically identical to the parent. Which method best achieves this aim?

  1. Crossing two selected parental lines by hand
  2. Taking stem cuttings and rooting them in a nursery Correct answer.
  3. Raising seedlings from hybrid seed purchased each season
  4. Collecting open-pollinated seed from the crop

Explanation

Vegetative propagation such as rooting stem cuttings involves mitosis only, so the new plants are clones of the parent. Seed formation involves meiosis and fertilisation, which produces genetic recombination and variability.

Question 4Plant Science and Crop Production

Which statement correctly describes the role of Rhizobium bacteria in the nitrogen nutrition of a leguminous crop?

  1. They convert nitrate to nitrogen gas, which is lost from the soil
  2. They decompose soil organic matter and release ammonium ions
  3. They convert soil ammonium into nitrate for direct uptake by roots
  4. They fix atmospheric nitrogen inside root nodules and supply it to the host plant Correct answer.

Explanation

Rhizobium forms symbiotic root nodules in legumes and reduces atmospheric dinitrogen to ammonia using nitrogenase, supplying combined nitrogen to the host. This is why legumes are used in rotations to improve soil nitrogen status.

Question 5Plant Science and Crop Production

A crop field shows water standing on the surface for several days after heavy rain. Which soil property most directly explains this poor drainage?

  1. High organic matter content at the surface
  2. Low bulk density with loose aggregation
  3. High clay content causing low hydraulic conductivity Correct answer.
  4. High proportion of coarse sand particles

Explanation

Clay particles are very small with large surface area and hold water strongly, and clay-rich horizons have few large pores, so water moves slowly and the profile becomes saturated. Standing water reflects low saturated hydraulic conductivity.

Question 6Plant Science and Crop Production

Which of the following is the primary function of stomata in a crop leaf?

  1. Transport of sucrose through the phloem
  2. Absorption of mineral ions from the leaf surface
  3. Synthesis of chlorophyll in the palisade mesophyll
  4. Regulation of gaseous exchange and transpirational water loss Correct answer.

Explanation

Stomata are pores bounded by guard cells that open and close to allow carbon dioxide entry for photosynthesis and to control the escape of water vapour. Their regulated aperture balances carbon gain against water loss.

Question 7Plant Science and Crop Production

A grower needs to apply a complete fertiliser supplying nitrogen, phosphorus and potassium. Which material is a straight nitrogen fertiliser rather than a compound one?

  1. Diammonium phosphate
  2. Monoammonium phosphate
  3. Muriate of potash
  4. Urea Correct answer.

Explanation

Urea contains only nitrogen, so it is a straight fertiliser. The other materials supply two nutrient elements and are therefore compound fertilisers.

Question 8Plant Science and Crop Production

In a crop rotation, a cereal is followed by a legume. Which benefit most directly justifies this sequence?

  1. The legume adds combined nitrogen to the soil through symbiosis Correct answer.
  2. The legume reduces soil organic matter content
  3. The legume requires more nitrogen fertiliser than the cereal
  4. The legume increases soil-borne cereal pathogens

Explanation

Legumes host nitrogen-fixing rhizobia, so the nitrogen fixed in their nodules and returned in residues enriches the soil for the following cereal. Rotating unrelated crops also interrupts pest and disease cycles.

Question 9Plant Science and Crop Production

A maize field is irrigated with water of high electrical conductivity. Which immediate effect on the crop is most likely?

  1. Reduced transpiration caused by closure of the xylem vessels
  2. Increased availability of all nutrient ions in the soil solution
  3. Increased osmotic stress because roots cannot take up water readily Correct answer.
  4. Rapid increase in soil pH to strongly alkaline values

Explanation

Saline water raises the osmotic potential of the soil solution, so the water potential gradient into roots is reduced and the plant suffers physiological drought. This shows as wilting and stunted growth even when soil is wet.

Question 10Plant Science and Crop Production

The net assimilation rate of a crop canopy is calculated over a period. Which change would most directly raise canopy photosynthesis?

  1. Lowering the crop's optimum temperature for photosynthesis
  2. Increasing the proportion of radiation intercepted by green leaf Correct answer.
  3. Reducing leaf area index to a very low value
  4. Applying a growth regulator that closes stomata

Explanation

Canopy photosynthesis depends on the amount of photosynthetically active radiation intercepted, which rises with healthy green leaf area until self-shading dominates. Increasing interception therefore increases dry matter accumulation.

Question 11Plant Science and Crop Production

A farmer applies a systemic insecticide to control a sap-sucking pest on a vegetable crop. Which property best explains why systemic action is useful against this pest?

  1. The chemical remains on the leaf surface and kills insects on contact
  2. The chemical breaks down immediately in sunlight, giving no residue
  3. The chemical is absorbed and translocated within the plant, reaching pests feeding in sheltered sites Correct answer.
  4. The chemical selectively kills only caterpillars that chew leaves

Explanation

Systemic insecticides are taken up by roots or leaves and moved in the vascular system, so they reach pests such as aphids that feed on phloem sap and are hidden from contact sprays. This also gives some protection to new growth.

Question 12Plant Science and Crop Production

A crop shows interveinal chlorosis on older leaves first, with the veins remaining green. Which nutrient deficiency is most consistent with this symptom?

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

Explanation

Magnesium is a mobile nutrient and a central atom of chlorophyll, so when supply is short the plant withdraws it from older leaves, causing interveinal chlorosis there first. Immobile nutrients such as iron show symptoms on young leaves first.

Question 13Plant Science and Crop Production

A seed lot of a cereal has a laboratory germination of 85 percent and a purity of 98 percent. What is the pure live seed percentage?

  1. 87.0 percent
  2. 83.3 percent Correct answer.
  3. 85.0 percent
  4. 98.0 percent

Explanation

Pure live seed is calculated as purity multiplied by germination, expressed as a percentage: 0.98 multiplied by 0.85 equals 0.833, or 83.3 percent. This figure is used to adjust sowing rates to the actual viable seed.

Question 14Plant Science and Crop Production

In a nursery, tomato seedlings are grown under dense shade and become tall, pale and thin-stemmed. Which explanation best accounts for this response?

  1. Increased chlorophyll synthesis caused by shade
  2. Etiolation due to reduced light, with stem elongation and reduced chlorophyll Correct answer.
  3. Increased stomatal density caused by shade
  4. Enhanced root development induced by low light

Explanation

In shade, the plant allocates growth to stem elongation to reach light, and chlorophyll development is reduced, giving pale, spindly seedlings. This etiolation response lowers seedling quality and field establishment.

Question 15Plant Science and Crop Production

A farmer wants to reduce soil erosion on a sloping field before establishing a perennial crop. Which practice is most effective for this purpose?

  1. Ploughing downslope to speed water removal
  2. Leaving the soil bare during the rainy season
  3. Removing crop residues after harvest
  4. Growing a cover crop across the slope along the contour Correct answer.

Explanation

A cover crop established along the contour intercepts raindrop energy, slows runoff and holds soil particles with its roots, greatly reducing erosion. Bare or downslope-ploughed soil loses both topsoil and water.

Question 16Plant Science and Crop Production

A crop field is treated with a herbicide that acts on a specific enzyme present only in the target weed species. Which property of the herbicide is being described?

  1. Volatility based on vapour pressure
  2. Persistence based on soil adsorption
  3. Selectivity based on biochemical mode of action Correct answer.
  4. Residual action based on leaching depth

Explanation

Selectivity describes the ability of a herbicide to kill weeds while sparing the crop, and biochemical selectivity arises when the target enzyme or pathway differs between species. Other properties such as persistence and volatility relate to behaviour in the environment.

Question 17Plant Science and Crop Production

A fruit grower notices that flowers open but very few fruits develop, and bees are absent from the orchard during flowering. Which explanation is most probable?

  1. Excessive pollination by wind
  2. Surplus nitrogen causing flower drop
  3. Over-irrigation causing fruit enlargement
  4. Inadequate pollination leading to poor fertilisation Correct answer.

Explanation

Many fruit crops depend on insect pollinators to transfer compatible pollen, so absence of bees limits fertilisation and ovary development, leading to poor fruit set. Wind may not substitute effectively for crops with heavy or sticky pollen.

Question 18Plant Science and Crop Production

Which practice best reduces post-harvest losses in a freshly harvested leafy vegetable crop?

  1. Stacking the produce in deep heaps without ventilation
  2. Cooling the produce quickly and maintaining high relative humidity Correct answer.
  3. Storing the produce in a warm, dry, well-ventilated shed
  4. Delaying cooling until several days after harvest

Explanation

Leafy vegetables lose water rapidly and respire actively, so prompt cooling reduces respiration and ethylene effects while high humidity limits water loss and wilting. Delayed cooling or warm storage accelerates deterioration.

Question 19Plant Science and Crop Production

A farmer plants a crop at a very high density and observes that individual plants are smaller and yield per plant is reduced, though total yield may rise. Which principle explains this observation?

  1. Increased efficiency of photosynthesis per unit leaf area
  2. Reduced respiration rates in dense plant populations
  3. Intraspecific competition for light, water and nutrients Correct answer.
  4. Absence of mutual shading in dense stands

Explanation

At high density, plants of the same species compete for limited resources, so each individual captures less and grows smaller even if the community yield increases. This is intraspecific competition, and it defines the optimum plant population.

Question 20Plant Science and Crop Production

A grower applies lime to a strongly acidic soil before planting a legume. Which change in soil chemistry is the main reason for this practice?

  1. Increasing exchangeable aluminium in the soil solution
  2. Raising pH to improve rhizobial nodulation and nutrient availability Correct answer.
  3. Increasing phosphorus fixation by iron and aluminium
  4. Lowering calcium supply to the crop

Explanation

Liming neutralises acidity, raising pH towards the range where rhizobia nodulate effectively and where phosphorus, molybdenum and calcium are more available. It also precipitates toxic soluble aluminium.

Question 21Plant Science and Crop Production

Two maize varieties are compared. Variety A has a higher harvest index than variety B, although total dry matter production is similar. Which conclusion is most defensible?

  1. Variety A partitions a greater proportion of dry matter to grain Correct answer.
  2. Variety A has a longer vegetative growth period
  3. Variety A has a lower grain yield than variety B
  4. Variety A respires more than variety B

Explanation

Harvest index is the ratio of economic yield to total biological yield, so a higher index with similar total dry matter means more assimilate is partitioned into grain. This is a key trait in modern cereal breeding.

Question 22Plant Science and Crop Production

An experiment compares the growth of a crop under two irrigation regimes. Which measurement best quantifies water use efficiency?

  1. The total volume of water applied per hectare
  2. The number of irrigation events per season
  3. The depth of the water table below the field
  4. The ratio of dry matter produced to water consumed Correct answer.

Explanation

Water use efficiency is the amount of dry matter or economic yield produced per unit of water used, so it requires both biomass and water consumption measurements. Volumes or event counts alone do not express efficiency.

Question 23Plant Science and Crop Production

A breeder wants to develop a new variety that combines high yield with resistance to a soil-borne pathogen. Which approach most reliably achieves stable resistance?

  1. Screening segregating populations under disease pressure and selecting resistant high-yielding lines Correct answer.
  2. Selecting only for high yield over several generations
  3. Applying a fungicide to the selected high-yielding line
  4. Growing a single elite line in disease-free soil

Explanation

Exposing segregating generations to natural or artificial inoculum allows resistant genotypes to be identified while selection for yield continues, so both traits are combined in the selected lines. Selection in disease-free conditions or reliance on fungicide does not build genetic resistance.

Question 24Plant Science and Crop Production

In a controlled trial, a crop is grown with a complete nutrient solution except that nitrate is replaced by ammonium as the sole nitrogen source. The root zone pH falls markedly. Which explanation is most consistent with this observation?

  1. Ammonium uptake releases hydroxyl ions, raising rhizosphere pH
  2. Nitrate reduction in leaves releases protons into the soil
  3. Ammonium is not absorbed by roots at all
  4. Ammonium uptake is accompanied by proton extrusion, acidifying the rhizosphere Correct answer.

Explanation

When ammonium is the nitrogen source, roots extrude protons to maintain charge balance during cation uptake, so the rhizosphere becomes more acidic. Nitrate nutrition has the opposite tendency because nitrate uptake is accompanied by hydroxyl efflux.

Question 25Plant Science and Crop Production

A researcher studies a C4 crop and a C3 crop grown together at high temperature and high light intensity. Which explanation best accounts for the higher photosynthetic rate of the C4 crop under these conditions?

  1. C4 plants have lower light saturation points than C3 plants
  2. C4 plants have larger stomata that admit more oxygen
  3. C4 plants lack the Calvin cycle and fix carbon only at night
  4. C4 plants concentrate carbon dioxide at Rubisco, reducing photorespiration Correct answer.

Explanation

In C4 plants, carbon dioxide is fixed initially into a four-carbon acid and released around Rubisco in bundle-sheath cells, raising the carbon dioxide concentration there and suppressing photorespiration. This advantage is greatest at high temperature and light.

Question 26Plant Science and Crop Production

A technician tests a seed lot and reports a germination percentage of 80% and a purity of 95%. A grower wants to sow 200 viable seeds per square metre. What weight of the seed lot is needed if 1000 pure seeds of this crop weigh 40 g?

  1. About 9.5 g per square metre
  2. About 8.0 g per square metre
  3. About 12.5 g per square metre
  4. About 10.5 g per square metre Correct answer.

Explanation

Pure live seed percentage equals germination times purity, here 0.80 x 0.95 = 0.76, so 76% of the lot is viable. To obtain 200 viable seeds you must sow 200/0.76 = 263 seeds. At 40 g per 1000 seeds this is 263 x 40/1000 = 10.5 g per square metre.

Question 27Plant Science and Crop Production

During germination of a starchy cereal seed, the scutellum secretes gibberellins that diffuse to the aleurone layer. Which sequence of events follows most directly?

  1. Storage proteins are converted to starch so that osmotic potential in the endosperm falls
  2. The embryo axis absorbs water and splits the coleoptile before any enzyme is produced
  3. Aleurone cells synthesise alpha-amylase, which hydrolyses endosperm starch to sugars for the embryo Correct answer.
  4. The pericarp hardens and blocks oxygen entry until the radicle ruptures it

Explanation

Gibberellin from the scutellum stimulates the aleurone layer to produce hydrolytic enzymes, chiefly alpha-amylase. These enzymes break down endosperm starch into soluble sugars that nourish the growing embryo. This hormone-directed mobilisation is the central event of cereal germination.

Question 28Plant Science and Crop Production

A crop canopy is measured and found to intercept only 55% of incoming solar radiation at midday, although leaf area index is high. Which change would most directly increase the fraction of radiation intercepted?

  1. Selecting a variety with more vertical, erect leaves
  2. Raising the plant density beyond the point where leaves overlap
  3. Arranging leaves more evenly in space and reducing gaps between rows Correct answer.
  4. Increasing the nitrogen rate without altering plant arrangement

Explanation

Canopy interception depends on how completely foliage covers the ground and on leaf arrangement in space. Even distribution of leaves and narrower row gaps reduce the gaps through which radiation passes to the soil. This raises the intercepted fraction without necessarily increasing leaf area.

Question 29Plant Science and Crop Production

A farmer wants to propagate a uniform, disease-free planting stock of a vegetatively propagated crop quickly. Which propagation method combines genetic uniformity with the lowest risk of transmitting a systemic virus from the mother plant?

  1. Suckers separated from the base of an infected-looking mother plant
  2. Stem cuttings taken from the lower canopy of field plants
  3. Seed collected from open-pollinated flowers of the same crop
  4. Meristem tip culture after virus indexing of the mother plant Correct answer.

Explanation

Meristem tips are often free of systemic viruses because virus particles are poorly distributed to the rapidly dividing apex. Combined with indexing of the mother plant, this gives uniform clonal material of high health status. It is a standard tool for rapid multiplication of clean planting stock.

Question 30Plant Science and Crop Production

Which statement correctly describes the role of the bundle sheath cells in a C4 plant such as maize?

  1. They fix atmospheric nitrogen symbiotically within the leaf veins
  2. They store starch produced by the mesophyll cells during the night
  3. They carry out the Calvin cycle using carbon dioxide concentrated by decarboxylation of four-carbon acids Correct answer.
  4. They perform the initial carboxylation of carbon dioxide using phosphoenolpyruvate carboxylase

Explanation

In C4 plants the mesophyll fixes carbon dioxide into four-carbon acids, which are transported to bundle sheath cells. There the acids are decarboxylated, raising carbon dioxide concentration around rubisco, and the Calvin cycle runs in the bundle sheath. This division of labour suppresses photorespiration.

Question 31Plant Science and Crop Production

A soil test shows a cation exchange capacity of 12 cmol per kg and a base saturation of 60%. Which statement best describes the practical significance of these figures for crop nutrition?

  1. The soil can hold 12 cmol per kg of anions such as nitrate and phosphate for plant uptake
  2. Base saturation of 60% means the soil is fully supplied with potassium and calcium
  3. The remaining 40% of exchange sites are permanently empty and cannot retain any cations
  4. About 40% of exchange sites hold acidic cations, so the soil has moderate reserve acidity and liming may be needed Correct answer.

Explanation

Base saturation is the proportion of exchange sites occupied by basic cations such as calcium, magnesium and potassium. The remaining 40% is occupied mainly by aluminium and hydrogen ions, which constitute reserve acidity. A moderate value like this signals that liming may be warranted for acid-sensitive crops.

Question 32Plant Science and Crop Production

Which practice is the most direct way to reduce ammonia volatilisation losses when urea is applied to a calcareous, dry surface soil?

  1. Applying the urea in a single large dose on the dry surface at midday
  2. Incorporating the urea into the soil and irrigating lightly soon after application Correct answer.
  3. Applying the urea together with ammonium sulphate on the surface
  4. Mixing the urea with lime before broadcasting it

Explanation

Urea is hydrolysed by the enzyme urease to ammonium carbonate, and on a calcareous surface soil this releases ammonia gas. Incorporating the material and moistening the soil moves the ammonium below the surface and dilutes it, reducing loss. Surface broadcasting on dry, high-pH soil maximises volatilisation.

Question 33Plant Science and Crop Production

A crop is grown in a field where the water table is 40 cm below the surface and the soil is a fine-textured clay. Which condition is most likely to develop in the root zone?

  1. Waterlogging with oxygen deficiency because drainage is restricted by the fine texture Correct answer.
  2. Rapid leaching of nitrate because macropores conduct water freely downward
  3. Excessive aeration causing accelerated organic matter breakdown
  4. Drought stress because clay holds little available water

Explanation

A high water table combined with fine clay texture limits downward drainage, so pores fill with water and displace air. Roots then suffer oxygen deficiency, which restricts respiration and nutrient uptake. This is a classic waterlogging situation.

Question 34Plant Science and Crop Production

A grower observes that a crop's leaves have a water-soaked appearance with a yellow halo, and bacterial ooze is visible when a stem is cut in water. Which control measure is most appropriate for managing the disease?

  1. Cultivating the soil deeply between rows during the wet season
  2. Increasing nitrogen application to promote rapid leaf growth
  3. Applying a systemic fungicide at weekly intervals
  4. Using pathogen-free seed and avoiding overhead irrigation and working in wet foliage Correct answer.

Explanation

Water-soaking with a yellow halo and bacterial streaming from cut tissue are characteristic of bacterial diseases. Such pathogens spread readily in water films and on contaminated seed, so clean seed and dry canopy management are central. Fungicides do not control bacteria, and wet-season cultivation can spread inoculum.

Question 35Plant Science and Crop Production

Which statement correctly explains why a crop rotation that includes a non-host break reduces damage from a soil-borne pathogen such as a root-infecting nematode?

  1. Rotating crops increases the soil pH so that pathogens cannot survive
  2. Population of the pathogen declines in the absence of a susceptible host and its resting stages lose viability over time Correct answer.
  3. The non-host crop secretes nutrients that make the soil unsuitable for all pathogens
  4. The non-host crop competes with the pathogen for sunlight

Explanation

A soil-borne pathogen depends on a susceptible host to multiply, so a non-host break starves it and reduces its population. Resting structures also decline in viability over successive seasons without a host. This is the basis of rotational control of nematodes and many soil-borne fungi.

Question 36Plant Science and Crop Production

A farmer applies a pre-emergence herbicide to a field and then a heavy rainstorm occurs within a few hours. What is the most likely consequence?

  1. The herbicide will be activated more strongly and give longer residual control
  2. The herbicide will be broken down instantly and have no effect at all
  3. The herbicide may be leached into the crop's root zone and cause injury, or concentrated in depressions where it damages seedlings Correct answer.
  4. The herbicide will volatilise rapidly and drift to neighbouring fields

Explanation

Pre-emergence herbicides form a thin layer at the soil surface, and heavy rain soon after application can move the chemical downward or concentrate it in low spots. This can bring the active ingredient into contact with crop roots or seeds and cause injury. Placement is critical to selectivity in such products.

Question 37Plant Science and Crop Production

In a crop, the harvest index is 0.45 and the total dry matter produced at maturity is 16 t per hectare. What is the approximate economic yield of dry matter?

  1. 16.0 t per hectare
  2. 7.2 t per hectare Correct answer.
  3. 3.6 t per hectare
  4. 11.2 t per hectare

Explanation

Harvest index is the ratio of economic yield to total dry matter. Multiplying total dry matter of 16 t per hectare by 0.45 gives 7.2 t per hectare. The remainder is the dry matter retained in stems, roots and other vegetative parts.

Question 38Plant Science and Crop Production

Which practice would most effectively reduce the incidence of blossom-end rot in a tomato crop grown on a sandy soil?

  1. Increasing the nitrogen supply to encourage rapid vegetative growth
  2. Applying a foliar fungicide at flowering
  3. Mulching heavily with fresh, unrotted manure to raise organic matter
  4. Maintaining steady soil moisture and avoiding heavy applications of ammonium or potassium fertiliser Correct answer.

Explanation

Blossom-end rot is a physiological disorder caused by localised calcium deficiency in the fruit, often triggered by irregular water supply. Fluctuating moisture and high ammonium or potassium can reduce calcium uptake or its movement to fruit. Steady irrigation and balanced nutrition are the appropriate preventive measures.

Question 39Plant Science and Crop Production

A grower wants to estimate the leaf area index of a crop from a sample. Which procedure gives the most reliable estimate?

  1. Harvest all leaves from a known ground area, measure their combined one-sided area, and divide by that ground area Correct answer.
  2. Measure the length and width of the largest leaf and multiply by the plant density
  3. Estimate canopy height and multiply by the number of rows per metre
  4. Count the number of leaves per plant and multiply by the average petiole length

Explanation

Leaf area index is the one-sided leaf area per unit ground area. The reliable method is to collect all leaves from a measured area of ground, determine their total area, and divide by that ground area. Sampling several quadrats improves the estimate.

Question 40Plant Science and Crop Production

Two fields have the same soil type and receive the same fertiliser, but one has been under continuous monoculture while the other has been under a mixed ley. Which explanation best accounts for the higher organic matter and better structure in the ley field?

  1. The ley field receives more rainfall because of its position in the landscape
  2. Continuous monoculture removes more water, leaving less for organic matter formation
  3. Dense root systems and litter from the ley add organic matter that binds soil particles into stable aggregates Correct answer.
  4. The ley field has a lower cation exchange capacity, which prevents organic matter decomposition

Explanation

Grass and legume leys produce dense fibrous root systems and abundant litter, which add organic carbon to the soil. Decomposition products and root exudates bind particles into stable aggregates, improving structure and porosity. Continuous cropping with frequent cultivation depletes organic matter faster.

Question 41Soil Science and Soil Management

Which soil particle size fraction has the greatest specific surface area per unit mass and therefore dominates a soil's capacity to retain water and nutrients?

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

Explanation

Clay particles are smaller than 0.002 mm and are dominated by layered aluminosilicate minerals with very large surface area relative to their mass. This large specific surface, together with surface charge, gives clay its high water-holding capacity and cation exchange capacity. Coarser fractions have far less surface per gram, so they retain much less water and nutrients.

Question 42Soil Science and Soil Management

A soil sample contains 40% sand, 20% silt and 40% clay. Using the standard soil texture triangle, into which textural class does this soil fall?

  1. Sandy loam
  2. Clay loam Correct answer.
  3. Silty clay loam
  4. Sandy clay

Explanation

Plotting 40% clay, 40% sand and 20% silt on the texture triangle places the sample in the clay loam class, which typically spans roughly 27-40% clay with sand and silt contributing the balance. The high combined clay and sand content rules out the more silt-rich classes. Clay loam soils are moderately heavy, with good structure but restricted drainage when wet.

Question 43Soil Science and Soil Management

Which soil structure type is described as small, rounded, porous aggregates with high stability in the surface horizon of well-managed grassland soils?

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

Explanation

Granular structure consists of roughly spherical, porous aggregates (crumbs) typically 1-10 mm across and is characteristic of the A horizon of grassland soils rich in organic matter and biological activity. It favours aeration, infiltration and root penetration. Platy, columnar and blocky structures are angular or flattened forms associated with different horizons and often poorer drainage.

Question 44Soil Science and Soil Management

Which process is the main mechanism by which nitrate is lost from a waterlogged paddy soil?

  1. Leaching of nitrate directly through the profile
  2. Denitrification to gaseous nitrogen by anaerobic bacteria Correct answer.
  3. Volatilisation of nitrate as ammonia gas
  4. Fixation of nitrate onto clay surfaces

Explanation

In waterlogged, oxygen-poor soils, facultative anaerobic bacteria use nitrate as an electron acceptor and reduce it to nitrogen gases (N2 and N2O), a process called denitrification. This gaseous loss is significant in flooded rice soils and reduces nitrogen use efficiency. Nitrate itself is not fixed on clay surfaces and does not volatilise as ammonia.

Question 45Soil Science and Soil Management

A soil has a pH of 4.8 measured in water. Which management practice most directly and durably raises this pH for a subsequent crop?

  1. Applying gypsum
  2. Applying ammonium sulphate fertiliser
  3. Applying elemental sulphur
  4. Applying agricultural lime and incorporating it Correct answer.

Explanation

Agricultural lime (calcium carbonate) neutralises soil acidity by reacting with hydrogen ions and exchangeable aluminium, raising pH and reducing aluminium toxicity. Incorporation ensures contact with the acidic soil volume. The effect is relatively lasting because the carbonate dissolves slowly as acidity is generated.

Question 46Soil Science and Soil Management

Which cation is held most strongly on the exchange complex of a typical temperate soil, assuming equal charge?

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

Explanation

The strength of cation adsorption increases with charge and decreases with the hydrated radius, following the lyotropic series. Aluminium, being trivalent and small when hydrolysed, is held far more strongly than divalent or monovalent cations. This is why aluminium dominates exchange sites in strongly acid soils and must be displaced by lime to improve base saturation.

Question 47Soil Science and Soil Management

Which soil water term describes water held in soil at tensions between field capacity and the permanent wilting point?

  1. Hygroscopic water
  2. Available water Correct answer.
  3. Gravitational water
  4. Capillary fringe water

Explanation

Available water is the portion of soil water held between field capacity (about -33 kPa) and the permanent wilting point (about -1500 kPa), which plants can extract for growth. Water held more tightly than the wilting point is unavailable. Gravitational water drains freely and is not stored for plant use.

Question 48Soil Science and Soil Management

In a field experiment, a soil with 30% clay had a bulk density of 1.2 g/cm3, while a nearby soil with 45% clay had a bulk density of 1.5 g/cm3. Which conclusion is best supported?

  1. The lower-clay soil is more compacted because its density is lower
  2. The higher-clay soil has greater total porosity because clay packs less densely
  3. Bulk density is unrelated to texture and reflects only organic matter
  4. The higher-clay soil is more compacted or more massive in structure, reducing porosity Correct answer.

Explanation

Bulk density reflects the ratio of mass to total volume, so higher values generally indicate less pore space. A soil with more clay would normally have more total porosity and thus lower bulk density, so the higher density in the clay-rich soil suggests compaction or massive structure. This points to a management or structural problem in that field.

Question 49Soil Science and Soil Management

Which practice is the most effective long-term strategy for conserving soil organic matter in a continuously cropped arable field?

  1. Burning crop residues after each harvest
  2. Returning crop residues and adding organic manure while minimising tillage Correct answer.
  3. Leaving the soil bare between cropping seasons
  4. Increasing the frequency and depth of ploughing

Explanation

Soil organic matter is maintained when organic inputs exceed decomposition losses. Returning residues, adding manure and reducing tillage slows the mineralisation of organic carbon and protects aggregates. Bare or repeatedly ploughed soil loses carbon rapidly through oxidation and erosion.

Question 50Soil Science and Soil Management

Which nutrient deficiency symptom is most characteristic of a mobile nutrient such as nitrogen in a growing crop?

  1. Yellowing of the oldest leaves first Correct answer.
  2. Purple discolouration of the youngest leaves
  3. Necrosis confined to the leaf margins of new leaves
  4. Yellowing of the youngest leaves first

Explanation

Mobile nutrients such as nitrogen, phosphorus and potassium can be redistributed from older to younger tissues when supply is limited. Deficiency therefore appears first as chlorosis or senescence in the oldest leaves. Immobile nutrients such as iron or calcium show symptoms on the youngest leaves.