LAISHA
Open Laisha

Irrigation Association Certified Agricultural Irrigation Specialist Soil Science for Engineers: practice questions

Irrigation Association Certified Agricultural Irrigation Specialist (CAIS) · USA · Irrigation Association

Original practice questions written to the published syllabus · not past papers
10worked here, free
25written to this exam in this subject
6syllabus units

About Irrigation Association Certified Agricultural Irrigation Specialist

Irrigation Association Certified Agricultural Irrigation Specialist (CAIS) is a professional certification run by Irrigation Association, for candidates in USA. 100 MCQs, 3 hours.

Source: irrigation.org.

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 Irrigation Association Certified Agricultural Irrigation Specialist

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

Our 25 Irrigation Association Certified Agricultural Irrigation Specialist questions in this subject: 9 easy, 12 medium, 4 hard, across 6 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 Irrigation Association Certified Agricultural Irrigation Specialist and are not shown on any other exam's page.

Question 1 · medium

A drip designer has a clay loam field and a loamy sand field. For the same emitter discharge, in which field should emitter spacing along the lateral generally be closer to give a continuous wetted strip?

  1. Neither, because lateral spread depends only on the emitter discharge rate
  2. The loamy sand, because water spreads less laterally and moves more vertically
  3. The loamy sand, because its higher bulk density forces water sideways from the emitter
  4. The clay loam, because water spreads less laterally and moves more vertically
Answer and working

B. The loamy sand, because water spreads less laterally and moves more vertically

In coarse soils gravity dominates over capillarity, so the wetted bulb is narrow and deep and emitters must be closer. Fine soils spread water further sideways. Lateral spread depends on soil as well as discharge, and the reason given in the bulk-density option is wrong.

Question 2 · easy

A moist soil sample weighs 150 g and 125 g after oven drying at 105 °C. What is its gravimetric water content?

  1. 25.0%
  2. 83.3%
  3. 16.7%
  4. 20.0%
Answer and working

D. 20.0%

w = mass of water/mass of dry soil = (150 − 125)/125 = 25/125 = 20.0%. Dividing by the wet mass gives 16.7%, the classic slip; 25.0% confuses grams of water with percent; 83.3% is the dry-to-wet mass ratio.

Question 3 · medium

A soil has field capacity 28% and permanent wilting point 12% (both by mass) and bulk density 1.40 g/cm³. How much available water does a 1.0 m root zone hold?

  1. 224 mm
  2. 392 mm
  3. 160 mm
  4. 114 mm
Answer and working

A. 224 mm

Available water = (0.28 − 0.12) × 1.40 × 1,000 mm = 224 mm. 160 mm forgets the bulk density (treats gravimetric as volumetric); 114 mm divides instead of multiplying; 392 mm is the total water at field capacity, not the available part.

Question 4 · easy

The permanent wilting point is conventionally estimated as the soil water content at a matric potential of:

  1. −33 kPa (−1/3 bar)
  2. −3,100 kPa (−31 bar)
  3. −1,500 kPa (−15 bar)
  4. −10 kPa (−0.1 bar)
Answer and working

C. −1,500 kPa (−15 bar)

PWP is conventionally taken at −1,500 kPa. −33 kPa is the usual laboratory field capacity for medium and fine soils, −10 kPa is used for field capacity of sands, and about −3,100 kPa is the hygroscopic coefficient.

Question 5 · medium

Time-domain reflectometry (TDR) estimates soil water content mainly because:

  1. Water slows fast neutrons emitted by a radioactive source, which a detector then counts
  2. Water absorbs gamma rays far more strongly (about tenfold) than soil minerals do per unit of mass
  3. Water's dielectric permittivity (about 80) far exceeds that of solids and air
  4. The electrical resistance of a gypsum block falls as it absorbs water from the soil
Answer and working

C. Water's dielectric permittivity (about 80) far exceeds that of solids and air

TDR measures the travel time of an electromagnetic pulse, which depends on the bulk dielectric permittivity; water (about 80) dominates over solids (3–5) and air (1). Neutron thermalisation is the neutron probe, electrical resistance is the gypsum block, and gamma attenuation is the gamma densitometer.

Question 6 · easy

One dS/m of electrical conductivity is equal to:

  1. 1 mho/m
  2. 1 mmho/cm
  3. 1 µmho/cm
  4. 10 mmho/cm
Answer and working

B. 1 mmho/cm

1 dS/m = 0.1 S/m = 1 mS/cm = 1 mmho/cm. 1 µmho/cm is a thousandth of that; 1 mho/m is 10 dS/m.

Question 7 · easy

An irrigation water contains Na⁺ 20, Ca²⁺ 4 and Mg²⁺ 2 meq/L. What is its sodium adsorption ratio?

  1. 14.14
  2. 8.16
  3. 11.55
  4. 6.67
Answer and working

C. 11.55

SAR = Na/√((Ca + Mg)/2) = 20/√(6/2) = 20/√3 = 11.55. Omitting the division by 2 gives 8.16; forgetting the square root gives 6.67; 14.14 leaves magnesium out and uses calcium alone.

Question 8 · medium

A tubewell water has CO₃²⁻ 0.5, HCO₃⁻ 5.5, Ca²⁺ 1.8 and Mg²⁺ 1.2 meq/L. Under the US Salinity Laboratory guidance, how is it rated for residual sodium carbonate?

  1. RSC 3.0 meq/L, unsuitable (above 2.5)
  2. RSC 3.0 meq/L, marginal (1.25 to 2.5)
  3. RSC −3.0 meq/L, probably safe (below 1.25)
  4. RSC 2.0 meq/L, marginal (1.25 to 2.5)
Answer and working

A. RSC 3.0 meq/L, unsuitable (above 2.5)

RSC = (0.5 + 5.5) − (1.8 + 1.2) = 3.0 meq/L, above 2.5, so unsuitable without treatment (for example gypsum or blending). The same value rated marginal misreads the class limits; −3.0 reverses the subtraction; 2.0 is a ratio, not RSC.

Question 9 · hard

A soil at field capacity contains 0.32 volumetric water content and at wilting point 0.14. Root depth is 0.9 m. With p = 0.4 (allowable depletion fraction), what is the readily available water?

  1. 162 mm
  2. 97.2 mm
  3. 115.2 mm
  4. 64.8 mm
Answer and working

D. 64.8 mm

TAW = (0.32 − 0.14) × 900 mm = 162 mm; RAW = p × TAW = 0.4 × 162 = 64.8 mm. 162 mm is total available water; 115.2 uses field capacity instead of the available range; 97.2 uses 1 − p.

Question 10 · hard

A drip-irrigated field is monitored with a 1:5 soil:water EC meter, and a consultant converts the readings to ECe with a single fixed factor taken from another region. What is the main problem?

  1. The conversion factor is fixed by an international standard and must not be changed for any soil
  2. The EC1:5-to-ECe conversion depends on texture and salt type and must be calibrated locally
  3. EC1:5 cannot be measured with field meters, so all of the readings are invalid from the start
  4. ECe is always lower than EC1:5, so any multiplying factor overstates the salinity of the field
Answer and working

B. The EC1:5-to-ECe conversion depends on texture and salt type and must be calibrated locally

Because the 1:5 ratio dilutes the solution far more in a sand than in a clay relative to its saturation water content, and dissolves sparingly soluble salts such as gypsum, the factor varies by soil and must be calibrated against saturated paste extracts. EC1:5 is easily measured, ECe is generally higher, and there is no single universal factor.

A study plan for Soil Science for Engineers in Irrigation Association Certified Agricultural Irrigation Specialist

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 (60% of the 25 questions): Soil water: field capacity, wilting point and available water (9, 36%); Sodicity: SAR and ESP (6, 24%).

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

  2. Then the middle band: Soil chemistry: pH and electrical conductivity (6, 24%); Infiltration (2, 8%).

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

  3. Then the lighter units: Bulk density, particle density and porosity (1, 4%); Soil texture and structure (1, 4%).

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

  4. Leave time for the hard ones: Sodicity: SAR and ESP (2 hard of 6), Soil chemistry: pH and electrical conductivity (1 hard of 6), Soil water: field capacity, wilting point and available water (1 hard of 9).

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

  5. Bank the easy marks: Bulk density, particle density and porosity (1 easy of 1); Infiltration (1 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.

Last updated · How Laisha writes and checks its pages · Report a mistake

Ask Laisha about this

Opens Laisha's chat with your question ready to send. Sign in or start free.

Unlock every question — Irrigation Association Certified Agricultural Irrigation Specialist Soil Science for Engin

US$10 a month · one price worldwide · cancel any time

  • Every practice question in this subject, with full working
  • Timed exam papers and progress by unit
  • Ask Laisha to explain any answer — up to 1,000 questions a month
Free or Smarter
FreeSmarter
Grow guides, pest pages, articlesRead allRead all
Ask Laisha5 a dayUp to 1,000 a month
Exam practiceThe sample on each pageEvery question, timed papers
Crop calendar and pest alerts—For your own fields
Calculators and tools—All
Uploads and reports—100 a month, real files
Try Smarter free for 10 daysOr subscribe now — US$10/month
10 days free for every new account, no card.