ICAR AICE-JRF/SRF (PhD) — Agricultural Engineering subjects 38–43 · India · National Testing Agency for ICAR
Original practice questions written to the published syllabus · not past papersThe soil an engineer tills, irrigates, drains and builds on: its physical, water and chemical properties.
In the exams: ICAR AIEEA-PG Unit VII; ICAR AICE-JRF/SRF subjects 40 and 43; NCEES PE soil and water
ICAR AICE-JRF/SRF — subjects 40 (SWCE) and 43 (IDE)
PG-depth questions on soil water potential and retention curves, unsaturated conductivity and the Richards equation, infiltration theory (Green-Ampt, Philip sorptivity), hysteresis, reclamation chemistry and gypsum requirement, and land capability classification.
How our questions for ICAR AICE-JRF/SRF spread across the units of Soil Science for Engineers. Difficulty: 2 easy, 50 medium, 31 hard.
| Problem soils: saline, sodic and acid | 12 | |
| Bulk density, particle density and porosity | 11 | |
| Soil chemistry: pH and electrical conductivity | 11 | |
| Soil formation and profile | 10 | |
| Infiltration | 9 | |
| Hydraulic conductivity and Darcy's law | 8 | |
| Sodicity: SAR and ESP | 8 | |
| Soil texture and structure | 7 | |
| Soil water: field capacity, wilting point and available water | 7 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
Which master horizon designation is used for hard, consolidated bedrock beneath the soil?
A. R
R is hard bedrock. C is unconsolidated, little-altered parent material that can be dug with a spade, O is an organic layer at the surface, and B is the subsoil horizon of illuviation or alteration.
Further reading: Soil Survey Manual, USDA Handbook 18
Using Stokes' law at 20 °C (viscosity 1.002 × 10⁻³ Pa s, Δρ = 1650 kg/m³), what is the settling velocity of a 0.02 mm silt particle?
D. 0.359 mm/s
v = 9.81 × (2 × 10⁻⁵)² × 1650/(18 × 1.002 × 10⁻³) = 3.590 × 10⁻⁴ m/s = 0.359 mm/s. 0.090 puts the 0.01 mm radius in place of the diameter; 0.577 uses particle density 2650 instead of the density difference; 0.718 uses 9η in place of 18η.
Further reading: Stokes' law
Why does a clay soil usually have a higher total porosity than a sandy soil, yet drain and aerate more poorly?
C. Most of its pores are fine and stay water-filled, with few large drainable pores
Clays have many tiny pores, so total porosity is high, but those pores stay water-filled at field suctions. Clay particle density is similar to sand, measured total porosity is usually higher in clays, and organic matter does not fill the large pores.
Further reading: Brady and Weil, The Nature and Properties of Soils
In a constant-head permeameter test, a soil column 15 cm long and 10 cm in diameter under a head difference of 20 cm passes 120 cm³ of water in 30 minutes. What is the saturated hydraulic conductivity?
C. 2.29 cm/h
A = π × 10²/4 = 78.5 cm²; K = V L/(A h t) = 120 × 15/(78.5 × 20 × 1,800) = 6.37 × 10⁻⁴ cm/s = 2.29 cm/h. Using the diameter as the radius gives 0.57; swapping L and h gives 4.07; converting s to h with 60 instead of 3,600 gives 0.038.
Further reading: Darcy's law
For the Kostiakov equation I = 2.5 t^0.6 (I in cm, t in hours), what is the infiltration rate at t = 2 hours?
D. 1.14 cm/h
Rate = dI/dt = a b t^(b−1) = 2.5 × 0.6 × 2^(−0.4) = 1.14 cm/h. 3.79 is the cumulative depth; 1.89 is the average rate over the first 2 hours, which is higher than the instantaneous rate; 1.50 is the rate at 1 hour.
Further reading: Kostiakov (1932)
The permanent negative charge of 2:1 clay minerals such as montmorillonite arises mainly from:
B. Isomorphous substitution, such as Mg²⁺ for Al³⁺ in the octahedral sheet
Substitution of a lower-valence cation during crystal formation (Al³⁺ for Si⁴⁺, Mg²⁺ for Al³⁺) leaves a permanent negative charge independent of pH. Carboxyl dissociation is the pH-dependent charge of humus; protonation at low pH creates positive, not negative, charge; and anion adsorption is not the source.
Further reading: Brady and Weil, The Nature and Properties of Soils
A crop has a salinity threshold ECe of 2.0 dS/m and a yield decline of 7% per dS/m above it (Maas-Hoffman model). What is the expected relative yield at ECe 5.0 dS/m?
D. 79%
Relative yield = 100 − b(ECe − a) = 100 − 7 × (5.0 − 2.0) = 79%. Ignoring the threshold gives 65%; 21% is the yield loss, not the yield; adding the threshold gives 51%.
Further reading: Maas and Hoffman (1977); FAO Paper 29
How many tonnes of gypsum dihydrate (equivalent weight 86.09 g/eq) are equivalent in reclamation value to 1 t of elemental sulphur (equivalent weight 16 g/eq)?
D. 5.38 t
Sulphur oxidises to H₂SO₄, which supplies 2 equivalents per mole of S (32 g), so its equivalent weight is 16 g/eq. One tonne of S therefore carries 10⁶/16 equivalents, and the same equivalents of gypsum weigh 10⁶/16 × 86.09 g: 1 t S ≈ 86.09/16 = 5.38 t gypsum. 0.19 t is the inverse (tonnes of S per tonne of gypsum); 2.69 t uses the molar mass of S with the equivalent weight of gypsum; 10.76 t uses the molar mass of gypsum with the equivalent weight of S.
Further reading: USDA Agriculture Handbook 60 (1954)
An acid soil has 2 cmol(+)/kg of exchangeable acidity to be neutralised in the top 15 cm, bulk density 1.3 g/cm³. How much pure CaCO₃ (equivalent weight 50 g/eq) is required per hectare?
D. 1.95 t/ha
2 cmol(+)/kg = 0.02 eq/kg; CaCO₃ = 0.02 × 50 = 1.0 g/kg. Soil mass = 10,000 × 0.15 × 1,300 = 1.95 × 10⁶ kg/ha, so lime = 1.95 t/ha. Using the molar mass 100 doubles it; using bulk density 1.5 gives 2.25; treating 2 cmol as 0.002 eq gives the tenfold-low answer.
Further reading: Brady and Weil, The Nature and Properties of Soils
A laboratory reports a sodic soil as calcareous (free CaCO₃ present). Which amendment option becomes available that would not work in a non-calcareous sodic soil?
A. Elemental sulphur or sulphuric acid, which dissolve native CaCO₃ to release calcium
Acid formers work only where CaCO₃ is present to be dissolved. Calcium chloride, gypsum and organic manures work in both calcareous and non-calcareous sodic soils, so none of them is the option that calcareousness newly makes possible.
Further reading: USDA Agriculture Handbook 60 (1954)
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