UPHESC Assistant Professor Agricultural Chemistry & Soil Science PYQs

Concept: Bremner's fractionation hydrolyses soil with 6 N HCl.

1. Hydrolysable N includes amino-acid N, amino-sugar N, NH₄-N and hydrolysable unknown N.
2. The N left in the residue (acid-insoluble humin N) is the non-hydrolysable fraction.

Short trick: Survives 6 N HCl = insoluble (humin) N.

Concept: Azotobacter is an aerobic, free-living (non-symbiotic) heterotrophic N-fixer found in soil, water and the rhizosphere.

1. It protects nitrogenase through very high respiration and cyst formation.
2. Rhizobium is symbiotic (root nodules); Azospirillum is an associative, microaerophilic N-fixer; Azolla is a fern that fixes N in symbiosis with Anabaena.

Short trick: Aerobic + free-living = Azotobacter.

Concept: Denitrification (NO₃⁻ → NO₂⁻ → NO → N₂O → N₂) is carried out mainly by Pseudomonas, along with Bacillus, Paracoccus and Thiobacillus denitrificans.

1. Pseudomonas uses nitrate as the electron acceptor when O₂ is lacking (strictly, it is facultative).
2. Thiobacillus thioparus oxidises sulphur; Frankia fixes N₂; Nitrosospira is a nitrifier.

Short trick: Denitrifier = Pseudomonas.

Concept: Commensalism (+, 0): one partner benefits and the other is unaffected.

1. Neutralism (0, 0); amensalism (−, 0); parasitism (+, −); mutualism (+, +).

Short trick: Commensal = 'eating at the same table'; only the guest gains.

Concept: J. Ruinen (1956) coined 'phyllosphere' for the leaf surface as a microbial habitat.

1. L. Hiltner (1904) coined 'rhizosphere'.
2. C.N. Acharya is known for composting (Bangalore method); N.S. Subba Rao for biofertilizers.

Short trick: Phyllo (leaf) = Ruinen; Rhizo (root) = Hiltner.

Concept: Soil phosphatases (phosphomonoesterases) have different origins.

1. Acid phosphatase comes from both plant roots and microbes.
2. Alkaline phosphatase is produced by microorganisms (bacteria, fungi, fauna); higher plants are considered devoid of it.

Short trick: Roots make acid phosphatase only; microbes make alkaline phosphatase.

Concept: In the polyphenol theory of Flaig and associates, lignin is the starting material.

1. Lignin freed from cellulose is degraded to phenolic aldehydes and acids.
2. These are oxidised enzymatically to quinones, which polymerise (with amino compounds) into fulvic and then humic acids.
3. Waksman's lignin–protein theory also starts from lignin, but there lignin is modified directly.

Short trick: Flaig: lignin → polyphenols → quinones → humic acids.

Concept: Phosphate-solubilising bacteria (PSB) such as Pseudomonas striata and Bacillus megaterium var. phosphaticum dissolve insoluble Ca, Fe and Al phosphates.

1. They do this with organic acids (gluconic, citric, oxalic) and chelation.
2. Fungi such as Aspergillus awamori and Penicillium also solubilise P.
3. Azotobacter, Rhizobium and Clostridium are N-fixers.

Short trick: PSB = Pseudomonas + Bacillus.

Concept: Thiocystis is a purple sulphur bacterium (Chromatiaceae) that photosynthesises anoxygenically with H₂S. It is a strict (obligate) anaerobe.

1. Citrobacter is a facultative anaerobe (enteric bacterium).
2. Lyngbya and Calothrix are oxygenic cyanobacteria, i.e. aerobic.

Short trick: Purple sulphur bacteria (Thiocystis, Chromatium) = strict anaerobes.

Concept: Basidiomycetes, especially white-rot fungi such as Phanerochaete and Trametes, are the main decomposers of lignin as well as cellulose.

1. They use lignin peroxidase, Mn-peroxidase and laccase.
2. Phycomycetes mainly attack simple sugars and starch; Ascomycetes and Fungi imperfecti mainly attack cellulose.

Short trick: White rot = Basidiomycetes, the lignin breakers.

Concept: Biological N fixation needs a lot of ATP (16 ATP per N₂) and good root and nodule growth, and both depend on phosphorus.

1. P application increases nodulation and nitrogenase activity in legumes and free-living fixers.
2. Mineral N fertilizer suppresses nodulation and N fixation.

Short trick: Legumes need P, not N (P is the 'legume fertilizer').

Concept: Bacteria are the most numerous soil organisms, but fungi have the largest biomass because their hyphae are large and extensive.

1. Fungal biomass is about 1,000–15,000 kg ha^-1 (furrow slice), against about 400–5,000 kg ha^-1 for bacteria in well-aerated soils.

Short trick: Number: bacteria first; mass: fungi first.

Concept: TDS (mg L^-1) ≈ 640 × EC (dS m^-1).

1. TDS = 2.4 g L^-1 = 2400 mg L^-1.
2. EC = 2400 ÷ 640 = 3.75 dS m^-1 (= 3750 µmhos cm^-1).
3. 3.75 µmhos/cm would be almost pure water, so options (A) and (C) are far off.

Short trick: TDS ÷ 640 = EC (dS/m).

Concept: The cationic micronutrients Fe, Mn, Zn and Cu become more soluble as pH falls, so acid soils are rich in them (sometimes to toxic levels for Fe and Mn).

1. Mo is an anion (MoO₄²⁻) and is least available in acid soils.
2. B is leached from acid, light soils.

Short trick: Acid soils: 'Fe-Mn-Zn-Cu' plenty, Mo scarce.

Concept: Kamprath's (1970) approach: lime requirement (cmol_c CaCO₃ kg^-1) = factor × exchangeable Al.

1. The factor rises as the crop's tolerance to Al falls: about 1.0 for tolerant crops, 1.5 for moderately tolerant crops and 2.0 for the least Al-tolerant (sensitive) crops.
2. The extra lime also neutralises the H⁺ released from hydrolysis of Al³⁺ and raises pH further.

Short trick: More sensitive crop, bigger factor: 1.0, then 1.5, then 2.0.

Concept: Soil acidity has three pools: active, exchangeable and residual (reserve).

1. Active acidity = H⁺ in the soil solution (measured as pH).
2. Exchangeable (salt-replaceable) acidity = Al³⁺ and H⁺ held on exchange sites, displaced by a neutral salt such as 1 N KCl.
3. Residual acidity = Al-hydroxy and organic-bound acidity that is not KCl-exchangeable.

Short trick: KCl-extractable Al + H = exchangeable acidity.

Concept: The rhizosphere (root zone) has 10–100 times more microbes than bulk soil, because root exudates and sloughed cells supply carbon.

1. Biological transformation of wastes and pollutants (rhizodegradation, a form of phytoremediation) therefore happens mainly in the root zone.

Short trick: Microbial hot-spot = rhizosphere (root zone).

Concept: Acid soils occupy about 90 million ha in India (roughly 25–30% of the geographical area).

1. They occur mainly in the NE states, Kerala, Odisha, Jharkhand, West Bengal and the Himalayan region.
2. About 25 million ha of this has pH below 5.5 and needs liming for good crop yields.

Short trick: Acid soils ≈ 90 Mha, of which strongly acid (<5.5) ≈ 25 Mha.

Concept: pE = −log(electron activity); Eh = redox potential in volts.

1. Eh = (2.303 RT/F) pE = 0.0591 pE at 25 °C.
2. So pE = Eh ÷ 0.0591 (Eh in V), or pE ≈ 16.9 Eh.

Short trick: pE ≈ 17 × Eh (V).

Concept: In Soil Taxonomy, salt-affected soils fall mainly in Aridisols, with salic and natric horizons (Salids, Natrargids).

1. Sodic soils with a dark surface fall in Mollisols (Natrustolls, Natraquolls, Natralbolls).
2. Histosols are organic soils and Ultisols are acid, highly leached soils, so they cannot hold salt-affected soils.

Short trick: Salt in dry lands means Aridisols; natric with a dark epipedon means Mollisols.