UPHESC Assistant Professor Agricultural Chemistry & Soil Science PYQs

Concept: ZPC (zero point of charge) is the pH at which the net surface charge is zero.

1. Below ZPC, surface OH groups take up H⁺ (–OH₂⁺), so the colloid is net positive and holds anions (AEC).
2. Above ZPC the H⁺ dissociates (–O⁻) and the colloid is net negative (CEC).

Short trick: Below ZPC = proton-rich = positive.

Concept: Langbeinite is ${\mathrm{K}}_2{\mathrm{SO}}_4\cdot 2{\mathrm{MgSO}}_4$ (sulphate of potash-magnesia, about 22% K₂O).

1. Carnallite = $\mathrm{KCl}\cdot {\mathrm{MgCl}}_2\cdot 6{\mathrm{H}}_2\mathrm{O}$; sylvinite = KCl + NaCl.
2. Kainite = $\mathrm{KCl}\cdot {\mathrm{MgSO}}_4\cdot 3{\mathrm{H}}_2\mathrm{O}$: K is present as chloride, not as K₂SO₄.

Short trick: Langbeinite = K₂SO₄·2MgSO₄ (K-Mag).

Concept: Katherine Warington (1923) showed that boron is essential for broad bean (Vicia faba).

1. Others: Mn – McHargue (1922); Zn – Sommer and Lipman (1926); Cu – Lipman and MacKinney (1931); Mo – Arnon and Stout (1939); Cl – Broyer et al. (1954); Ni – Brown et al. (1987).
2. Sachs and Knop developed solution culture; de Saussure studied plant ash and CO₂.

Short trick: B for Bean, Warington 1923.

Concept: Dicyandiamide (DCD, C₂H₄N₄, about 66% N) blocks Nitrosomonas (the NH₄⁺ → NO₂⁻ step).

1. DCD itself mineralises slowly, so it also acts as a slow-release N source.
2. Oxamide and CDU (crotonylidene diurea) are slow-release N fertilizers only.
3. DCS (sulphathiazole) is another nitrification inhibitor, but not an N source.

Short trick: DCD = inhibitor + 66% N, so it does both jobs.

Concept: Calcium is needed for cell-wall (Ca-pectate) formation and cell division at meristems, and it is immobile in the phloem.

1. Ca deficiency kills root tips and growing points, so the root system becomes short, stubby and poorly branched (stunted).
2. Mg deficiency causes interveinal chlorosis of older leaves; Mo deficiency causes whiptail.

Short trick: Ca = cell-wall cement; without it root tips die.

Concept: Boron and calcium act together in cell-wall formation (B cross-links pectins, Ca forms pectates) and in Ca utilisation.

1. Classical texts state that the primary role of B is associated with Ca metabolism; the Ca/B ratio is used to diagnose B status.
2. Cu is linked to oxidases, Mn to photolysis of water, and Mo to nitrate reductase and nitrogenase.

Short trick: B and Ca are partners in the cell wall.

Concept: Nitrification is carried out by chemo-autotrophic bacteria: Nitrosomonas (NH₄⁺ → NO₂⁻) and Nitrobacter (NO₂⁻ → NO₃⁻).

1. They get energy from oxidising N and carbon from CO₂.
2. Aminization and ammonification are done by a wide range of heterotrophs (bacteria, fungi, actinomycetes).

Short trick: Nitri-fiers are auto-trophs; mineralisers are hetero-trophs.

Concept: Ammonium polysulphide, (NH₄)₂S_x (about 20% N and 40–45% S), is a liquid supplied as a solution.

1. It can be applied through irrigation water or mixed with liquid N fertilizers.
2. Gypsum, phosphogypsum and pyrite are solids.

Short trick: 'Poly-sulphide' solution = liquid S.

Concept: NH₃ volatilisation is a major gaseous N loss from surface-applied urea and ammonium fertilizers.

1. Under normal field conditions it usually accounts for about 5–20% of the applied N.
2. Losses can be higher on calcareous or alkaline soils, with high temperature and with surface application.

Short trick: Normal NH₃ loss is about 5–20%; deep placement reduces it.

Concept: Ammonia/ammonium fixation by organic matter is the chemical reaction of NH₃ with phenolic and quinone groups of humus.

1. It needs oxidation of phenols, which is favoured at high pH.
2. At high pH, NH₄⁺ also shifts towards NH₃, which reacts more readily.
3. So fixation increases with pH and is most rapid in alkaline soils.

Short trick: Organic NH₃ fixation rises with pH.

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.