UPHESC Assistant Professor Agricultural Chemistry & Soil Science PYQs

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.

Concept: In routine soil-testing flame photometers (LPG–air flame), the compressor air pressure for the atomiser is kept at about 0.4–0.6 kg cm^-2 (6–8 psi) for K and Na.

1. This gives a stable, non-luminous blue flame. Too high a pressure blows the flame off; too low a pressure gives poor aspiration.
2. K is read at 766–768 nm and Na at 589 nm.

Short trick: Flame photometer air: 0.4–0.6 kg/cm².

Concept: Jackson's tri-acid mixture for wet digestion of plant samples is HNO₃ : H₂SO₄ : HClO₄ = 10 : 1 : 4.

1. HNO₃ first oxidises the easily oxidised organic matter; HClO₄ completes the oxidation; a little H₂SO₄ raises the boiling point.
2. The digest is used for P, K, Ca, Mg, S and micronutrients. The di-acid mixture is HNO₃ : HClO₄ = 9 : 4.

Short trick: Tri-acid = 10 : 1 : 4 (nitric : sulphuric : perchloric).

Concept: ICP-OES uses an argon plasma (about 6,000–10,000 K) sustained by a radio-frequency induction coil.

1. Argon is inert, cheap and has a high ionisation energy, so it efficiently atomises and excites most elements.
2. Many elements can then be read at once from their emission lines.

Short trick: ICP torch = argon.

Concept: Bragg's law: $n\lambda =2d\sin \theta$. X-rays diffracted by crystal planes reinforce each other when this is satisfied.

1. An X-ray diffractometer measures θ and calculates the basal spacing d. For example, kaolinite is 0.72 nm, illite 1.0 nm, smectite 1.4–1.8 nm (glycolated) and chlorite 1.4 nm. This identifies clay minerals.
2. AAS works on atomic absorption; HPLC on chromatography; mass spectrometry on m/z separation.

Short trick: Bragg = $2d\sin \theta$, i.e. XRD.

Concept: Compare the K content of the pure compounds (K = 39.1).

1. K₂CO₃ (M = 138.2): 78.2 ÷ 138.2 = 56.6% K = 68% K₂O.
2. KCl (74.6): 52.4% K = 63% K₂O (commercial MOP 60% K₂O).
3. K₂SO₄ (174.3): 44.9% K = 54% K₂O (commercial SOP 48–50% K₂O).
4. KNO₃ (101.1): 38.7% K = 46% K₂O (commercial 44% K₂O).
5. So potassium carbonate (about 66–68% K₂O) has the most K. MOP is the richest common potash fertilizer, but it has less K than K₂CO₃.

Short trick: The lighter the partner anion (CO₃ is shared by 2 K), the higher the % K.

Concept: Barber's three mechanisms of nutrient supply to roots are root interception, mass flow and diffusion.

1. Diffusion supplies most of the P (about 90%) and K (about 75–80%), because their solution concentrations are low and they are strongly buffered.
2. N (NO₃⁻), S, Ca and Mg reach the root mainly by mass flow.

Short trick: Diffusion = P and K; mass flow = N, Ca, Mg, S.

Concept: Higher plants, especially hyperaccumulators, take up heavy metals from soil through their roots and store them in their shoots.

1. Examples: Thlaspi caerulescens (Zn, Cd), Brassica juncea (Pb, Cd, Cr), Alyssum (Ni), Pteris vittata (As).
2. This is the basis of phytoextraction, the most widely used phytoremediation technique for metal-contaminated soils.

Short trick: Phyto-extraction = higher plants accumulate metals.

Concept: Climate-smart agriculture practices are grouped as water-, energy-, nutrient-, carbon-, weather- and knowledge-smart.

1. Knowledge-smart practices include capacity development, ICT-based advisories, seed/fodder banks and new crop rotations or diversification.
2. Crop residue management (incorporation or retention instead of burning) is a carbon-smart practice, so it is not knowledge-smart.

Short trick: Residues are about soil carbon, so they are carbon-smart.

Concept: D.J.D. Nicholas (1961) proposed the term 'functional or metabolic nutrient': any element that functions in plant metabolism, whether or not its action is specific.

1. This broadened Arnon and Stout's (1939) strict criteria of essentiality.
2. E. Gris (1844) first showed that Fe is essential; de Saussure studied plant ash and the uptake of CO₂.

Short trick: Arnon = essential element; Nicholas = functional nutrient.

Concept: Doran and Zeiss (2000) wrote that the term 'soil health' portrays soil as a finite, non-renewable and dynamic living resource.

1. They defined soil health as the capacity of soil to function as a vital living system.
2. Karlen et al. (1997) and Doran and Parkin (1994) are associated with definitions of soil quality.

Short trick: 'Living resource' = Doran & Zeiss (2000).