UPHESC Assistant Professor Agricultural Chemistry & Soil Science PYQs

Concept: In the standard Indian textbook view, Indian soils fall in Entisols, Inceptisols, Alfisols, Vertisols, Aridisols, Ultisols, Oxisols, Mollisols and (small areas) Histosols; Spodosols, Andisols and Gelisols are listed as not found in India.

1. Spodosols need a spodic horizon formed by podzolisation under cool, humid coniferous forest on sandy acid material — not mapped in India.
2. Gelisols (permafrost within 2 m) are likewise listed as absent from India.
3. Aridisols (western Rajasthan) and Inceptisols (the most extensive order) are major Indian orders.
4. So both (B) Spodosols and (C) Gelisols fit the question — no single correct option.

Short trick: Not in India: Spodosols, Andisols, Gelisols.

Concept: Jackson and Sherman's weathering stages for clay-size minerals run from 1 (least weathered) to 13 (most weathered).

1. 1 gypsum, 2 calcite, 3 olivine/hornblende, 4 biotite, 5 albite, 6 quartz, 7 muscovite/illite, 8 vermiculite, 9 montmorillonite, 10 kaolinite, 11 gibbsite, 12 hematite, 13 anatase.
2. So biotite is at stage 4.

Short trick: "Gypsum-Calcite-Olivine-Biotite": the 4th is biotite.

Concept: In Brewer's micromorphology, the s-matrix is the fine fabric. Nodules, concretions and septaria (glaebules) embedded in it are pedological features formed by soil-forming processes.

1. When such pedogenic bodies are inherited from an earlier soil, they are called pedorelicts.
2. Lithorelicts are inherited rock fragments; 'sedimentary' and 'volcanic' relicts are not Brewer's categories for glaebules.

Short trick: Nodules and concretions are formed by soil (pedo) processes, so they are pedo relicts.

Concept: Lessivage is the mechanical washing of fine clay from A/E to B in suspension. The clay deposits as clay skins (cutans) and forms an argillic (Bt) horizon.

1. Eluviation only removes material from the upper horizon; the enrichment of B is due to lessivage (with illuviation).
2. Leucinization is the paling (lightening) of horizons when dark organic matter is removed.

Short trick: Lessivage = 'clay washing', which gives the Bt horizon.

Concept: Soil Taxonomy has diagnostic surface horizons (epipedons, e.g. ochric, mollic, umbric, histic) and diagnostic subsurface horizons (e.g. argillic, natric, spodic, cambic, oxic).

1. The English question asks simply for 'a diagnostic horizon': spodic, natric and argillic are diagnostic subsurface horizons and ochric is a diagnostic surface horizon — all four options are diagnostic horizons.
2. The Hindi version asks for a diagnostic surface horizon (सतही संस्तर), for which only Ochric fits.
3. The two language versions ask different things, and the English one has four correct options, so no single option can be marked.

Short trick: Epipedons: Mollic, Umbric, Ochric, Histic, Anthropic, Plaggen, Melanic, Folistic; the rest are subsurface.

Concept: Goldich (1938) proposed the mineral stability series for weathering of primary minerals. It is the reverse of Bowen's reaction series.

1. Olivine weathers fastest, followed by pyroxene, amphibole and biotite, while K-feldspar, muscovite and quartz are the most resistant.
2. Minerals that crystallise first at high temperature are least stable at the surface.

Short trick: Goldich = 'Bowen in reverse'.

Concept: Cryoturbation is frost churning (mixing by freeze–thaw), typical of permafrost-affected soils.

1. Such soils belong to the Gelisols (permafrost within 100 cm); the suborder Turbels is defined by cryoturbation.
2. Gelisols were added as the 12th order in 1998.

Short trick: Gel = frozen, so cryoturbation means Gelisols (Turbels).

Concept: Peptization (dispersion) of clay happens when the diffuse double layer is thick.

1. Na⁺ is monovalent and strongly hydrated, so it gives a thick double layer. Clay particles repel and disperse, as in sodic soils with ESP > 15.
2. Ca²⁺ and Mg²⁺ are flocculating cations.

Short trick: Na disperses, Ca flocculates (that is why gypsum is used for sodic soils).

Concept: In sodic ('black alkali') soils with pH > 8.5 and high exchangeable Na, organic matter dissolves as sodium humate.

1. The dispersed sodium humate moves up with water and dries as a dark film on the surface, which gives the black colour.
2. Calcium humate is insoluble and flocculated, so it does not cause this colouring.

Short trick: Black alkali = Na-humate; white alkali = salts.

Concept: Middleton's erosion ratio = dispersion ratio ÷ (colloid or clay content ÷ moisture equivalent).

1. Soils with an erosion ratio above 10 are considered erodible.
2. Clay ratio = (sand + silt) ÷ clay (Bouyoucos); dispersion ratio is only the numerator here.

Short trick: Dispersion ratio on top, clay/ME below, gives the erosion ratio (Middleton).

Concept: A neutron probe counts fast neutrons slowed (thermalised) by hydrogen atoms and assumes almost all soil hydrogen is in water.

1. Organic soils contain a lot of non-water hydrogen in organic matter, so the reading overestimates moisture and the probe is unsuitable.
2. High organic matter is the classic limitation, along with the uncertain reading near the surface.

Short trick: Neutron probe counts hydrogen; organic matter also has hydrogen, so readings go wrong.

Concept: The oxygen diffusion rate (ODR, measured with a Pt micro-electrode) is the standard index of soil aeration.

1. Root growth of most plants stops when ODR falls below about 0.2 µg cm^-2 min^-1.
2. 0.2 µg = $0.2\times {10}^{-6}$ g = $20\times {10}^{-8}$ g cm^-2 min^-1.

Short trick: Critical ODR = 0.2 µg/cm²/min = $20\times {10}^{-8}$ g.

Concept: Specific heats (cal g^-1 °C^-1): water 1.0, humus about 0.45, clay about 0.22, quartz/sand about 0.19, mica about 0.2.

1. Organic matter has the highest specific heat among the soil solids, so humus-rich soils warm up slowly.

Short trick: After water, humus has the highest specific heat (~0.45).

Concept: Adhesion and cohesion together produce the matric (capillary) potential.

1. At full saturation all pores are filled, there are no air–water menisci, and the matric potential is 0.
2. In saturated flow, water moves under gravity and pressure only.

Short trick: Saturated soil: matric suction = 0.

Concept: Olivine, (Mg,Fe)₂SiO₄, contains ferrous (Fe²⁺) iron.

1. Fe²⁺ is readily oxidised to Fe³⁺, which is why olivine is the least stable common mineral (Goldich).
2. Muscovite is a K-Al mica, and albite and andesine are Na/Ca feldspars; none of them has significant iron.

Short trick: Olivine = Fe²⁺, so it rusts first.

Concept: P fixation depends on pH.

1. In acid soils phosphate precipitates as Fe and Al phosphates (strengite, variscite) and is adsorbed on Fe/Al oxides.
2. In alkaline and calcareous soils it forms Ca phosphates (di- and tricalcium phosphate, apatite).
3. So the key binding ions are Al, Fe and Ca. Phosphate availability is highest at pH 6–7.

Short trick: Acid soil: Fe-Al; alkaline soil: Ca.

Concept: The carrier concept explains active, selective ion uptake through carriers that bind an ion, ferry it across the membrane and release it (enzyme-like kinetics).

1. It was developed by Epstein (1953) and later gave the dual-mechanism (Michaelis–Menten) model.
2. Lundegårdh proposed the cytochrome pump theory; Jenny and Overstreet proposed contact exchange; Mengel and Kirkby wrote a plant nutrition textbook.

Short trick: Carrier = Epstein; Cytochrome pump = Lundegårdh; Contact exchange = Jenny.

Concept: Permanent (isomorphous-substitution) charge per unit mass = layer charge per formula unit ÷ formula mass.

1. ${\sigma }_p=-X/M_r$ (mol_c per g; multiply by 10⁵ for cmol_c kg^-1, or by Faraday's constant F for C kg^-1).
2. The negative sign shows the clay layer is negatively charged.
3. Example: smectite with X = 0.6 and M_r ≈ 367 g mol^-1 gives about 0.0016 mol_c g^-1 ≈ 160 cmol_c kg^-1 (per half unit cell; real CEC is lower).

Short trick: Charge per mass = charge ÷ mass.

Concept: Giles et al. (1960) classified isotherms into S, L, H and C types.

1. H (high-affinity) type: the adsorbate is almost completely adsorbed at low concentration, so the curve starts with a near-vertical rise.
2. L = Langmuir (decreasing slope); S = sigmoid (cooperative); C = constant partition (linear).

Short trick: H = High affinity.

Concept: Silicates are classified by how the SiO₄ tetrahedra are linked.

1. Tectosilicates have all 4 oxygens shared, giving a 3-D framework (quartz, feldspars, zeolites).
2. Phyllosilicates are sheets (micas, clays); nesosilicates are isolated tetrahedra (olivine); cyclosilicates are rings (tourmaline, beryl).

Short trick: Tecto = builder, so a 3-D framework.