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Tamil Nadu PSC Soil Science for Engineers: practice questions

Tamil Nadu PSC — Agricultural Engineering (Code 164, degree standard) · India · Tamil Nadu Public Service Commission

Original practice questions written to the published syllabus · not past papers
10worked here, free
31written to this exam in this subject
9syllabus units

About Tamil Nadu PSC

Tamil Nadu PSC — Agricultural Engineering (Code 164, degree standard) is a government recruitment exam run by Tamil Nadu Public Service Commission, for candidates in India. Degree-standard subject paper (for posts such as Assistant Engineer (Agri)).

Source: tnpsc.gov.in.

Checked against the exam body's published information. Formats and fees change; confirm the current edition there before you book.

Soil Science for Engineers in Tamil Nadu PSC

The soil an engineer tills, irrigates, drains and builds on: its physical, water and chemical properties.

Our 31 Tamil Nadu PSC questions in this subject: 15 easy, 13 medium, 3 hard, across 9 units.

10 practice questions, worked

Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.

These 10 were written to Tamil Nadu PSC and are not shown on any other exam's page.

Question 1 · easy

In Jenny's equation of soil formation, which set lists the five soil-forming factors?

  1. Climate, vegetation, rainfall, temperature and slope gradient
  2. Texture, structure, porosity, organic matter and soil reaction
  3. Weathering, leaching, erosion, deposition and compaction
  4. Climate, organisms, relief, parent material and time
Answer and working

D. Climate, organisms, relief, parent material and time

Jenny (1941) wrote soil as a function of cl, o, r, p, t: climate, organisms, relief, parent material and time. Weathering, leaching and erosion are processes, not factors; texture and structure are soil properties that result from the factors.

Question 2 · medium

Under the USDA land capability classification, a field on a 10 percent slope that is cultivable but needs terracing and contour farming to control erosion would be placed in:

  1. Class II with subclass c
  2. Class III or IV with subclass e
  3. Class I with subclass w
  4. Class VIII with subclass s
Answer and working

B. Class III or IV with subclass e

Land capability classes I to IV are cultivable with increasing limitations, and the subclass letter names the limitation: e for erosion. Class I land has few limitations and no subclass; class VIII is not usable for cultivation, grazing or forestry; c is climate and w wetness, neither of which is the stated problem. The exact class (III or IV) depends on local criteria for slope and erodibility.

Question 3 · easy

Which textural group usually has the highest available water capacity (field capacity minus wilting point)?

  1. Silt loams and loams
  2. Coarse sands
  3. Gravelly loamy sands
  4. Heavy clays
Answer and working

A. Silt loams and loams

Silt loams and loams hold a lot of water at field capacity but release most of it before wilting point. Sands hold little at field capacity; heavy clays hold the most at field capacity but also a large amount at wilting point, so their available range is often no larger than a loam's.

Question 4 · easy

In a double-ring infiltrometer test, the water level in the inner ring falls 1.5 cm in 15 minutes during the steady period. The infiltration rate is:

  1. 1.5 cm/h
  2. 22.5 cm/h
  3. 0.1 cm/h
  4. 6.0 cm/h
Answer and working

D. 6.0 cm/h

Rate = 1.5 cm/15 min = 0.1 cm/min = 6.0 cm/h. 1.5 cm/h ignores the time conversion, 0.1 is the rate per minute mislabelled as per hour, and 22.5 multiplies 1.5 by 15.

Question 5 · easy

Soil A has pH 5.0 and soil B has pH 7.0. The hydrogen-ion activity in soil A is:

  1. 100 times that in soil B
  2. 20 times that in soil B
  3. 0.01 times that in soil B
  4. 2 times that in soil B
Answer and working

A. 100 times that in soil B

pH is −log₁₀(H⁺ activity); two units lower means 10² = 100 times more H⁺. Treating pH as linear gives the small ratios, and 0.01 is the inverse.

Question 6 · medium

Which sequence correctly ranks cations by their strength of adsorption on soil exchange sites (lyotropic series)?

  1. Na⁺ > K⁺ > Mg²⁺ > Ca²⁺ > Al³⁺
  2. K⁺ > Na⁺ > Al³⁺ > Ca²⁺ > Mg²⁺
  3. Ca²⁺ > Al³⁺ > Na⁺ > Mg²⁺ > K⁺
  4. Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ > Na⁺
Answer and working

D. Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ > Na⁺

Higher charge and smaller hydrated radius give stronger adsorption: Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ ≈ NH₄⁺ > Na⁺. Sodium is held weakest, which is why calcium from gypsum can replace it. The fully reversed series is the common confusion.

Question 7 · easy

A soil has a CEC of 20 cmol(+)/kg and exchangeable Na⁺ of 3.6 cmol(+)/kg. What is its exchangeable sodium percentage?

  1. 3.6%
  2. 18%
  3. 5.6%
  4. 72%
Answer and working

B. 18%

ESP = exchangeable Na/CEC × 100 = 3.6/20 × 100 = 18%. 3.6% confuses cmol with percent, 5.6% is CEC/Na, and 72% is Na × CEC.

Question 8 · hard

A saline-sodic soil is leached with good-quality canal water without any amendment. What is the most likely result?

  1. It becomes sodic, with higher pH and lower permeability, as salts leave but exchangeable sodium stays
  2. It becomes more saline, because leaching water dissolves calcium carbonate and adds salt to the root zone
  3. It becomes a normal soil, because leaching removes the soluble salts and the exchangeable sodium together
  4. It becomes acid, because leaching removes all the base cations from the exchange complex quickly
Answer and working

A. It becomes sodic, with higher pH and lower permeability, as salts leave but exchangeable sodium stays

Leaching removes soluble salts but not exchangeable sodium; with electrolyte gone, the clay disperses and pH rises, so the soil turns sodic. Calcium must be supplied (gypsum or acid formers in calcareous soils) before or during leaching. Leaching does not remove exchangeable Na without a replacing cation, and it does not make the soil acid in one reclamation.

Question 9 · medium

A farmer reclaiming a sodic soil with gypsum in a monsoon region asks which crop to grow in the first season. Which choice is most often recommended and why?

  1. A sodicity-sensitive pulse such as chickpea, because legumes fix nitrogen and repair structure
  2. Any deep-rooted fruit tree, because perennial roots break up the dense sodic subsoil quickly
  3. A sodicity-tolerant crop such as rice, whose ponding also helps leach the displaced sodium
  4. Leaving the field fallow for three years so that rainfall alone leaches the sodium completely
Answer and working

C. A sodicity-tolerant crop such as rice, whose ponding also helps leach the displaced sodium

Rice tolerates moderate sodicity, and the standing water during its growth helps move the displaced sodium down. Chickpea and most pulses are sensitive to sodicity; tree roots grow poorly in dense sodic subsoils; and rain alone without calcium does not remove exchangeable sodium. Local recommendations vary by region and crop.

Question 10 · easy

A white crust of salts on the surface of a flocculated, permeable soil with pH 8.0 is typical of a:

  1. Saline soil
  2. Peat soil
  3. Sodic soil
  4. Acid sulphate soil
Answer and working

A. Saline soil

Saline soils often show white efflorescence and remain flocculated with pH below 8.5. Sodic soils are dispersed and dense with pH above 8.5 and may look dark from dispersed organic matter; acid sulphate soils are strongly acid; peat soils are organic.

A study plan for Soil Science for Engineers in Tamil Nadu PSC

This plan follows where our questions actually fall, so the time goes where the marks are likely to be. It is our reading of the syllabus, not the exam body's weighting.

  1. First, the heavy units (68% of the 31 questions): Problem soils: saline, sodic and acid (12, 39%); Soil formation and profile (5, 16%); Sodicity: SAR and ESP (4, 13%).

    Read the manual chapters for these first and do every worked question you can find on them.

  2. Then the middle band: Infiltration (3, 10%); Soil chemistry: pH and electrical conductivity (2, 6%); Soil water: field capacity, wilting point and available water (2, 6%).

    Aim for one sitting per unit, then mixed questions across them.

  3. Then the lighter units: Bulk density, particle density and porosity (1, 3%); Hydraulic conductivity and Darcy's law (1, 3%); Soil texture and structure (1, 3%).

    Learn the definitions, classifications and standard numbers; these units are more often recalled than worked.

  4. Leave time for the hard ones: Problem soils: saline, sodic and acid (3 hard of 12).

    Come back to these after a first pass, untimed, and read every explanation, including for the options you rejected.

  5. Bank the easy marks: Infiltration (3 easy of 3); Soil water: field capacity, wilting point and available water (2 easy of 2).

    Do these early in revision so they are fast on the day.

  6. Finish timed. Sit the subject as a timed set without notes, mark it, and send each wrong answer back to its unit above.

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