UPHESC Assistant Professor Agricultural Chemistry & Soil Science PYQs

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.

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.