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RBSE SS Agricultural Chemistry study materials
RBSE SS Agricultural Chemistry study resources

RBSE SS Agricultural Chemistry practice questions

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Question 1Soil Chemistry and Soil Fertility

In soil chemistry, which fraction is generally considered the most chemically reactive and dominant in supplying cation exchange capacity?

  1. The clay and humus colloidal fraction Correct answer.
  2. The gravel and stone fraction
  3. The coarse sand fraction
  4. The soluble salt fraction

Explanation

Cation exchange capacity arises mainly from negative charges on clay minerals and humus colloids, which adsorb and release nutrient cations. Sand, gravel and soluble salts contribute little to this exchange pool.

Question 2Soil Chemistry and Soil Fertility

Which soil property is measured by the pH value of a soil-water suspension?

  1. The electrical conductivity of the saturation extract
  2. The organic carbon percentage
  3. The total exchangeable base content
  4. The activity of hydrogen ions in the soil solution Correct answer.

Explanation

Soil pH expresses the negative logarithm of hydrogen ion activity in the soil solution, indicating acidity or alkalinity. It is not a direct measure of base content, salinity or organic matter.

Question 3Soil Chemistry and Soil Fertility

A soil test reports 1.2 percent organic carbon. Using the conventional Van Bemmelen factor of 1.724, what is the approximate organic matter content?

  1. 2.07 percent Correct answer.
  2. 12.0 percent
  3. 0.70 percent
  4. 1.72 percent

Explanation

Organic matter is estimated by multiplying organic carbon by 1.724, since carbon is assumed to be about 58 percent of organic matter. Thus 1.2 multiplied by 1.724 gives approximately 2.07 percent.

Question 4Soil Chemistry and Soil Fertility

Which cation is held most strongly on soil exchange sites under comparable conditions?

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

Explanation

For ions of similar hydration, exchange selectivity increases with charge and with ionic size, so trivalent aluminium is held far more strongly than divalent or monovalent cations. The lyotropic series places Al3+ ahead of Ca2+, K+ and Na+.

Question 5Soil Chemistry and Soil Fertility

A farmer applies ammonium sulphate to a calcareous soil. Which process best explains the acidifying effect of this fertilizer?

  1. Calcium is removed by crop uptake without replacement
  2. Ammonium is leached unchanged below the root zone
  3. Sulphate ions neutralize soil carbonates directly
  4. Nitrification of ammonium releases hydrogen ions into the soil Correct answer.

Explanation

Ammonium undergoes nitrification, converting to nitrate and releasing hydrogen ions, which lower pH and consume carbonates in calcareous soils. This residual acidity is a recognised property of ammonium-based fertilizers.

Question 6Soil Chemistry and Soil Fertility

Which practice most directly increases the cation exchange capacity of a sandy, low-organic-matter soil over several seasons?

  1. Regular incorporation of well-decomposed organic manure Correct answer.
  2. Frequent irrigation with saline water
  3. Deep ploughing to mix subsoil
  4. Annual application of gypsum

Explanation

Organic matter carries a high density of negative charge, so building humus through manure additions raises CEC substantially. Gypsum, tillage and saline irrigation do not add exchange capacity.

Question 7Soil Chemistry and Soil Fertility

In a sodic soil with high exchangeable sodium percentage, which amendment is classically recommended?

  1. Urea
  2. Muriate of potash
  3. Superphosphate
  4. Gypsum Correct answer.

Explanation

Gypsum supplies calcium, which replaces exchangeable sodium on clay surfaces and permits the sodium to be leached. This improves flocculation and hydraulic conductivity in sodic soils.

Question 8Soil Chemistry and Soil Fertility

Which statement correctly describes the relationship between base saturation and soil pH?

  1. Base saturation is independent of soil pH
  2. Higher base saturation generally corresponds to higher soil pH Correct answer.
  3. Base saturation is measured only in saline soils
  4. Higher base saturation always corresponds to lower soil pH

Explanation

As exchange sites become occupied by bases such as calcium and magnesium rather than hydrogen and aluminium, soil pH rises. Base saturation is therefore positively related to pH across mineral soils.

Question 9Soil Chemistry and Soil Fertility

Which form of soil nitrogen is most readily available for direct plant uptake?

  1. Nitrate and ammonium ions in the soil solution Correct answer.
  2. Nitrogen fixed in clay mineral lattices
  3. Organic nitrogen in undecomposed residues
  4. Elemental nitrogen gas adsorbed on surfaces

Explanation

Plants absorb nitrogen mainly as nitrate and ammonium from the soil solution. Organic nitrogen must first be mineralized, and lattice-bound or gaseous nitrogen is unavailable.

Question 10Soil Chemistry and Soil Fertility

A soil has a pH of 5.2 and shows aluminium toxicity symptoms in wheat. Which liming strategy is most appropriate?

  1. Apply gypsum to displace aluminium
  2. Apply agricultural lime to raise pH toward the crop's target range Correct answer.
  3. Apply ammonium sulphate to increase acidity
  4. Flood the field to leach aluminium upward

Explanation

Liming raises pH and precipitates exchangeable aluminium as insoluble hydroxides, removing the toxicity. The rate should be based on a buffer-pH test to reach the target pH for the crop.

Question 11Soil Chemistry and Soil Fertility

Which mechanism best explains why banding phosphate fertilizer near the seed improves phosphorus recovery?

  1. Band placement converts phosphate to nitrate
  2. Phosphorus leaches readily and bands slow the loss
  3. Phosphorus volatilizes rapidly when broadcast
  4. Phosphorus is relatively immobile, so concentrated bands reduce contact with fixing surfaces Correct answer.

Explanation

Phosphate ions move very little in soil and are readily fixed by iron, aluminium and calcium compounds. Placing fertilizer in concentrated bands limits the soil volume that fixes phosphorus and keeps it near roots.

Question 12Soil Chemistry and Soil Fertility

Which soil condition most favours denitrification losses of nitrate?

  1. Low temperature with no organic matter
  2. Waterlogged, anaerobic conditions with available carbon Correct answer.
  3. Well-aerated dry sandy soil
  4. High pH with free calcium carbonate

Explanation

Denitrification is carried out by facultative anaerobes that use nitrate as an electron acceptor when oxygen is scarce. Saturated, carbon-rich soils therefore lose nitrate as nitrogen gases.

Question 13Soil Chemistry and Soil Fertility

Which clay mineral type is characterized by a 1:1 layer structure with little or no interlayer swelling?

  1. Montmorillonite
  2. Kaolinite Correct answer.
  3. Vermiculite
  4. Illite

Explanation

Kaolinite consists of one tetrahedral and one octahedral sheet held tightly by hydrogen bonding, leaving no interlayer space for swelling. Its CEC is correspondingly low.

Question 14Soil Chemistry and Soil Fertility

Which laboratory measurement is used to estimate the lime requirement of an acidic soil?

  1. Walkley-Black organic carbon
  2. Buffer pH after reaction with a buffer solution Correct answer.
  3. Sodium adsorption ratio
  4. Electrical conductivity of the saturation extract

Explanation

Buffer-pH methods measure the reserve acidity that must be neutralized, giving a lime requirement estimate. Electrical conductivity, SAR and organic carbon describe unrelated properties.

Question 15Soil Chemistry and Soil Fertility

Which nutrient is most commonly deficient in calcareous, high-pH soils because it forms insoluble compounds?

  1. Chloride
  2. Zinc Correct answer.
  3. Nitrogen
  4. Sulphur

Explanation

In calcareous soils, zinc is adsorbed and precipitated as sparingly soluble carbonate and hydroxide phases, reducing availability. Zinc deficiency is therefore widespread on high-pH soils.

Question 16Soil Chemistry and Soil Fertility

Which statement about the soil solution is correct?

  1. It is chemically identical to the exchange complex
  2. It consists only of organic acids
  3. It is the aqueous phase from which roots absorb nutrients Correct answer.
  4. It contains no dissolved gases

Explanation

The soil solution is the water film containing dissolved ions and gases, and it is the immediate source of nutrients taken up by roots. Exchange sites replenish this solution as ions are absorbed.

Question 17Soil Chemistry and Soil Fertility

A soil has a sodium adsorption ratio of 18 and an electrical conductivity of 1.5 dS/m in the saturation extract. How should this soil be classified?

  1. Saline-sodic soil
  2. Saline soil
  3. Normal soil
  4. Sodic soil Correct answer.

Explanation

Sodicity is diagnosed when SAR is high, conventionally above about 13, while salinity is low, conventionally below 4 dS/m. With SAR 18 and EC 1.5 dS/m, the soil is sodic rather than saline.

Question 18Soil Chemistry and Soil Fertility

Which process converts ammonium to nitrate in well-aerated soil?

  1. Ammonification by fungi
  2. Denitrification by anaerobic bacteria
  3. Nitrification by chemoautotrophic bacteria Correct answer.
  4. Nitrogen fixation by rhizobia

Explanation

Nitrification is a two-step aerobic oxidation of ammonium to nitrite and then nitrate by specialized bacteria. It requires oxygen and produces hydrogen ions, acidifying the soil.

Question 19Soil Chemistry and Soil Fertility

Which factor most strongly limits the availability of phosphorus to plants in strongly acidic soils?

  1. Leaching of phosphate below the root zone
  2. Volatilization of phosphate gas
  3. Fixation by iron and aluminium oxides Correct answer.
  4. Rapid microbial conversion to nitrate

Explanation

Under acidic conditions, dissolved phosphate reacts with iron and aluminium to form sparingly soluble compounds, sharply reducing availability. Liming can reduce this fixation within an appropriate pH range.

Question 20Soil Chemistry and Soil Fertility

Which soil texture class generally has the greatest total pore space and water-holding capacity?

  1. Sandy loam
  2. Coarse sand
  3. Loamy sand
  4. Clay Correct answer.

Explanation

Clay soils have abundant fine pores and high specific surface, giving high porosity and water retention, although much water may be held too tightly for plants. Sandy soils have larger pores and lower retention.

Question 21Soil Chemistry and Soil Fertility

Which practice would most effectively reduce nitrate leaching to groundwater after harvest?

  1. Leaving the soil bare over winter
  2. Establishing a cover crop to take up residual nitrate Correct answer.
  3. Applying additional nitrogen fertilizer in autumn
  4. Increasing irrigation after harvest

Explanation

A rapidly growing cover crop scavenges residual nitrate from the profile, reducing the amount available to leach during winter recharge. Bare or over-fertilized soils lose more nitrate.

Question 22Soil Chemistry and Soil Fertility

Which statement best describes cation exchange in soil?

  1. Adsorbed cations are replaced by other cations from the soil solution Correct answer.
  2. Anions are permanently bound to clay surfaces
  3. Cations are lost only through plant uptake
  4. Exchange occurs only in dry soil

Explanation

Cation exchange is the reversible replacement of one adsorbed cation by another from solution, maintaining electroneutrality. It is a rapid, concentration-driven process on charged surfaces.

Question 23Soil Chemistry and Soil Fertility

A soil analysis shows a very narrow carbon-to-nitrogen ratio in incorporated residues. What is the most likely short-term effect on soil nitrogen?

  1. Complete conversion of nitrogen to nitrate within hours
  2. Loss of nitrogen as ammonia gas
  3. Net mineralization and release of plant-available nitrogen Correct answer.
  4. Strong net immobilization of nitrate

Explanation

Residues with a narrow carbon-to-nitrogen ratio contain enough nitrogen for microbial decomposition, so microbes release surplus ammonium into the soil. Wide ratios, by contrast, cause immobilization.

Question 24Soil Chemistry and Soil Fertility

Which chemical reaction describes the hydrolysis of urea applied to moist soil?

  1. Urea is converted to ammonium carbonate by the enzyme urease Correct answer.
  2. Urea reacts with clay to form insoluble calcium phosphate
  3. Urea is oxidized directly to nitrate by nitrifying bacteria
  4. Urea decomposes to nitrogen gas in aerobic soil

Explanation

Urease hydrolyses urea to ammonium carbonate, which then dissociates to ammonium and carbonate ions. This reaction raises pH locally and can cause ammonia volatilization if urea is surface-applied.

Question 25Soil Chemistry and Soil Fertility

Which measurement indicates the reserve acidity that is not reflected by pH of the soil-water suspension alone?

  1. Water-stable aggregate percentage
  2. Electrical conductivity
  3. Bulk density
  4. Exchangeable aluminium and hydrogen determined by titration Correct answer.

Explanation

Reserve or exchangeable acidity includes hydrogen and aluminium held on exchange sites, which is measured by extracting and titrating these ions. This reserve must be neutralized for effective liming.

Question 26Soil Chemistry and Soil Fertility

Which soil colloid fraction is chiefly responsible for the pH-dependent (variable) charge that develops in highly weathered tropical soils?

  1. Primary minerals such as quartz and feldspar
  2. Calcium carbonate particles inherited from the parent rock
  3. Soluble salts present in the soil solution
  4. Iron and aluminium oxide clays together with organic matter Correct answer.

Explanation

Variable charge arises from protonation and deprotonation of surface hydroxyl and carboxyl groups, which are abundant on sesquioxide clays and humified organic matter. These surfaces develop net positive charge at low pH and net negative charge as pH rises, so the charge is not fixed. Primary minerals and soluble salts carry little or no such pH-dependent surface.

Question 27Soil Chemistry and Soil Fertility

A fertilizer recommendation calls for 60 kg of P2O5 per hectare. Using single superphosphate (0 to 20 to 0, containing 20% P2O5), what quantity of product is required per hectare?

  1. 120 kg
  2. 30 kg
  3. 300 kg Correct answer.
  4. 600 kg

Explanation

The product requirement is the nutrient rate divided by the fractional nutrient content: 60 kg P2O5 divided by 0.20 equals 300 kg of single superphosphate per hectare. Checking the calculation, 300 kg multiplied by 20% returns the required 60 kg of P2O5.

Question 28Soil Chemistry and Soil Fertility

Two soils have the same measured pH in water, but soil A has a much higher buffer capacity than soil B. Which statement best explains the practical consequence of this difference?

  1. Both soils will require identical lime rates because their pH values are equal
  2. Soil A will reach the target pH with a smaller lime dose than soil B
  3. Soil A will need more lime than soil B to raise pH by the same amount Correct answer.
  4. Soil B will show a larger pH change only if it contains more clay

Explanation

Buffer capacity reflects the reserve acidity held on exchange sites and surfaces, which must be neutralized before pH rises. A soil with high buffer capacity therefore consumes more lime per unit pH change, even when its initial pH matches another soil. Equal pH alone does not indicate equal lime requirement.

Question 29Soil Chemistry and Soil Fertility

In a flooded paddy soil, which sequence of redox-driven chemical changes is most consistent with increasing reducing intensity?

  1. Manganese reduction, then sulfate reduction, then nitrate reduction
  2. Sulfate reduction, then iron reduction, then nitrate reduction
  3. Nitrate reduction, then manganese and iron reduction, then sulfate reduction Correct answer.
  4. Iron reduction, then nitrate reduction, then manganese reduction

Explanation

As oxygen is depleted, microbes use electron acceptors in order of decreasing free energy yield, so nitrate goes first, followed by manganese oxides, iron oxides, and then sulfate. This thermodynamic sequence explains why nitrate disappears early in waterlogged soils while sulfate reduction occurs only under strongly reducing conditions.

Question 30Soil Chemistry and Soil Fertility

A laboratory reports that a soil sample has a base saturation of 45% and an exchangeable sodium percentage of 13. Which interpretation of these two indices is most defensible?

  1. The soil is acidic but not sodic, because the sodium percentage is below 15%
  2. The soil is acidic and sodic, so both a liming and a gypsum amendment may be justified Correct answer.
  3. The soil is sodic but not acidic, because base saturation is below 50%
  4. The soil is alkaline and non-sodic, so no amendment is needed

Explanation

Base saturation below roughly 50% indicates a substantial share of exchange sites occupied by acidic cations, pointing to an acidic soil. An exchangeable sodium percentage above the conventional 15% threshold defines a sodic soil, so a value of 13% is close to that boundary and warrants sodium management alongside liming. The two indices describe different exchange-site populations and must be interpreted together.

Question 31Plant Nutrition and Essential Nutrient Elements

Which element is classified as a macronutrient in plant nutrition because plants require it in relatively large amounts?

  1. Copper
  2. Boron
  3. Phosphorus Correct answer.
  4. Molybdenum

Explanation

Macronutrients are required in comparatively large quantities and include nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. Phosphorus is taken up in amounts far greater than the micronutrients molybdenum, boron or copper.

Question 32Plant Nutrition and Essential Nutrient Elements

Which element is a structural component of the chlorophyll molecule?

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

Explanation

Chlorophyll is a porphyrin ring with a central magnesium ion, so magnesium deficiency causes interveinal chlorosis. Manganese and iron assist chlorophyll synthesis but are not part of the molecule.

Question 33Plant Nutrition and Essential Nutrient Elements

According to the criteria of essentiality, an element is essential if the plant cannot complete its life cycle without it and it has a direct physiological role. Which element best illustrates this because it is required for photosystem II water splitting?

  1. Sodium
  2. Cobalt
  3. Silicon
  4. Manganese Correct answer.

Explanation

Manganese is an essential micronutrient and is part of the oxygen-evolving complex of photosystem II, where water is split. Sodium, silicon and cobalt are beneficial for some species but are not generally essential.

Question 34Plant Nutrition and Essential Nutrient Elements

Which form of nitrogen is the predominant one taken up by most crop roots from the soil solution?

  1. Atmospheric dinitrogen gas
  2. Nitrate and ammonium ions Correct answer.
  3. Elemental nitrogen in organic matter
  4. Nitrous oxide gas

Explanation

Roots absorb nitrogen mainly as nitrate (NO3-) and ammonium (NH4+) ions from the soil solution. Dinitrogen gas and nitrous oxide are not directly assimilated by higher plant roots.

Question 35Plant Nutrition and Essential Nutrient Elements

A crop grown on a soil with a high pH of about 8.5 shows interveinal chlorosis on young leaves. Which micronutrient is most likely deficient under these alkaline conditions?

  1. Iron Correct answer.
  2. Chlorine
  3. Nickel
  4. Molybdenum

Explanation

Iron solubility falls sharply in alkaline soils, so iron deficiency appears as interveinal chlorosis of the youngest leaves because iron is immobile in the plant. Molybdenum availability actually increases at high pH.

Question 36Plant Nutrition and Essential Nutrient Elements

Which process converts ammonium to nitrate in the soil and thus acidifies the rhizosphere during nitrification?

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

Explanation

Nitrification is the microbial oxidation of ammonium to nitrite and then nitrate, releasing hydrogen ions and lowering soil pH. Denitrification reduces nitrate to nitrogen gases under anaerobic conditions.

Question 37Plant Nutrition and Essential Nutrient Elements

Which nutrient is described as mobile in the phloem, so deficiency symptoms first appear on older leaves?

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

Explanation

Phloem-mobile nutrients such as nitrogen, phosphorus, potassium and magnesium are retranslocated from older to younger leaves, so deficiency shows first on older leaves. Calcium, boron and iron are immobile and show symptoms on young tissue.

Question 38Plant Nutrition and Essential Nutrient Elements

In the context of plant nutrition, what does the term 'critical concentration' mean?

  1. The minimum tissue concentration supporting maximum growth Correct answer.
  2. The concentration of nutrient in the fertilizer applied
  3. The nutrient level at which toxicity symptoms begin
  4. The concentration at which leaching starts

Explanation

The critical concentration is the tissue nutrient level just sufficient for maximum growth, below which deficiency reduces yield. Concentrations above this support growth but do not increase it until toxicity occurs.

Question 39Plant Nutrition and Essential Nutrient Elements

A grower applies ammonium-based fertilizer repeatedly to an already acidic soil. Which change in soil chemistry is most likely to result?

  1. A fall in pH and increased risk of manganese toxicity Correct answer.
  2. A rise in pH and improved molybdenum availability
  3. A rise in pH and increased phosphorus fixation
  4. No change in pH because ammonium is neutral

Explanation

Ammonium uptake and nitrification release hydrogen ions, acidifying the soil. In acidic soils manganese and aluminium become more soluble and can reach toxic levels, while molybdenum availability falls.

Question 40Plant Nutrition and Essential Nutrient Elements

Which combination correctly pairs a nutrient with its principal physiological function?

  1. Calcium with chlorophyll formation
  2. Potassium with osmoregulation and stomatal control Correct answer.
  3. Boron with nitrogenase activity
  4. Sulfur with water splitting in photosystem II

Explanation

Potassium is the main cation regulating turgor, stomatal opening and enzyme activation. Calcium serves in signalling and cell wall structure, sulfur in amino acids, boron in cell walls, and manganese in water splitting.

Question 41Plant Nutrition and Essential Nutrient Elements

Legumes form root nodules with rhizobia. Which nutrient is supplied directly by the bacteroid to the plant as a result of nitrogenase activity?

  1. Nitrification inhibitors
  2. Urea
  3. Ammonium Correct answer.
  4. Nitrate

Explanation

Nitrogenase reduces dinitrogen to ammonia, which is protonated to ammonium and assimilated by the plant. Nitrate is a soil-derived form, and urea is a fertilizer or excretion product, not the direct product of nitrogenase.

Question 42Plant Nutrition and Essential Nutrient Elements

A soil test shows adequate total nitrogen but the crop shows nitrogen deficiency after heavy irrigation. Which explanation is most consistent with this observation?

  1. Potassium blocked nitrogen uptake
  2. Denitrification and leaching removed plant-available nitrate Correct answer.
  3. Nitrogen was fixed into unavailable organic forms
  4. Nitrification was inhibited by oxygen

Explanation

Excess water creates anaerobic conditions favouring denitrification and also leaches nitrate below the root zone, reducing plant-available nitrogen despite adequate total nitrogen. Total nitrogen includes organic forms not immediately available.

Question 43Plant Nutrition and Essential Nutrient Elements

Which element is required for the synthesis of the amino acids cysteine and methionine and is therefore associated with protein quality?

  1. Magnesium
  2. Chlorine
  3. Sulfur Correct answer.
  4. Molybdenum

Explanation

Sulfur is incorporated into cysteine and methionine, so sulfur deficiency lowers protein quality and causes symptoms on young leaves. Magnesium is central to chlorophyll, chlorine acts as an osmolyte, and molybdenum functions in nitrate reductase.

Question 44Plant Nutrition and Essential Nutrient Elements

Why is phosphorus deficiency often observed as purple coloration of older leaves and poor root development?

  1. Phosphorus toxicity causes anthocyanin production
  2. Phosphorus is immobile and accumulates in young tissue
  3. Phosphorus deficiency blocks calcium uptake
  4. Phosphorus is mobile and is redistributed from older leaves, disrupting energy metabolism Correct answer.

Explanation

Phosphorus is phloem-mobile, so it is retranslocated from older leaves to growing points, causing symptoms on older leaves. Impaired ATP synthesis and sugar accumulation favour anthocyanin formation, producing purple coloration.

Question 45Plant Nutrition and Essential Nutrient Elements

Which practice is most appropriate for correcting a confirmed boron deficiency in a sensitive crop without risking toxicity?

  1. Increase irrigation to leach boron
  2. Apply lime to raise soil pH
  3. Broadcast large amounts of boron fertilizer
  4. Apply a small, carefully measured borax dose or foliar spray Correct answer.

Explanation

Boron has a narrow sufficiency range between deficiency and toxicity, so small measured amounts applied to soil or as foliar spray are safest. Large broadcast doses and leaching would risk deficiency or excessive loss.

Question 46Plant Nutrition and Essential Nutrient Elements

Which statement best explains why calcium deficiency symptoms appear on young leaves and growing points?

  1. Calcium is highly mobile in the phloem
  2. Calcium is fixed in the soil and unavailable
  3. Calcium is required only in older leaves
  4. Calcium moves mainly in the xylem and is not redistributed from older tissue Correct answer.

Explanation

Calcium is transported in the xylem with the transpiration stream and is largely immobile in the phloem. Tissues with low transpiration, such as young leaves and fruit tips, therefore show deficiency first.

Question 47Plant Nutrition and Essential Nutrient Elements

Which nutrient is a constituent of nitrate reductase and nitrogenase and is therefore needed for efficient nitrogen assimilation?

  1. Molybdenum Correct answer.
  2. Boron
  3. Zinc
  4. Copper

Explanation

Molybdenum is a cofactor in the molybdenum-iron cofactor of nitrogenase and in nitrate reductase. Copper and zinc serve other enzymes, and boron is structural rather than part of these enzymes.

Question 48Plant Nutrition and Essential Nutrient Elements

A farmer notices that applying nitrate fertilizer raises rhizosphere pH while ammonium fertilizer lowers it. Which explanation is correct?

  1. Nitrate uptake is accompanied by release of hydroxyl ions, whereas ammonium uptake releases hydrogen ions Correct answer.
  2. Both forms release hydrogen ions equally
  3. Nitrate uptake releases hydrogen ions and ammonium uptake releases hydroxyl ions
  4. pH changes are caused only by the fertilizer's granule size

Explanation

Plants maintain charge balance by excreting hydroxyl ions during nitrate uptake and hydrogen ions during ammonium uptake. This makes nitrate nutrition alkalinizing and ammonium nutrition acidifying in the rhizosphere.

Question 49Plant Nutrition and Essential Nutrient Elements

Which statement correctly describes the role of chlorine in plant nutrition?

  1. Chlorine is required for nitrogen fixation
  2. Chlorine is involved in water splitting and osmoregulation Correct answer.
  3. Chlorine is a structural component of chlorophyll
  4. Chlorine is a major component of DNA

Explanation

Chloride is an essential micronutrient that participates in the oxygen-evolving complex of photosystem II and contributes to turgor and osmoregulation. It is not a chlorophyll component or a DNA constituent.

Question 50Plant Nutrition and Essential Nutrient Elements

In a nutrient solution experiment, a plant shows stunted growth and interveinal chlorosis on young leaves, and tissue analysis shows low iron. Which experimental change would best confirm iron as the limiting factor?

  1. Add more calcium to the solution
  2. Increase the sodium chloride concentration
  3. Raise the solution pH to 8.5
  4. Add a soluble iron chelate and observe recovery Correct answer.

Explanation

Adding a soluble iron chelate increases available iron and, if iron is limiting, should reverse the chlorosis. Raising pH would decrease iron solubility and worsen the symptom, so it cannot confirm the diagnosis.