Agricultural and Biosystems Engineers Licensure Examination (ABELE) · Philippines · Professional Regulatory Board of Agricultural and Biosystems Engineering, PRC
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
How our questions for Agricultural and Biosystems Engineers Licensure Examination spread across the units of Soil Science for Engineers. Difficulty: 34 easy, 13 medium, 4 hard.
| Problem soils: saline, sodic and acid | 9 | |
| Soil formation and profile | 8 | |
| Soil texture and structure | 6 | |
| Bulk density, particle density and porosity | 6 | |
| Soil chemistry: pH and electrical conductivity | 6 | |
| Soil water: field capacity, wilting point and available water | 5 | |
| Infiltration | 5 | |
| Hydraulic conductivity and Darcy's law | 3 | |
| Sodicity: SAR and ESP | 3 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
In a soil profile, the pale horizon from which clay, iron, aluminium and humus have been washed out is designated:
C. E horizon
The E (eluvial) horizon loses clay, iron, aluminium and organic matter and is therefore pale. The Bt horizon is where clay accumulates (illuviation); C is little-altered parent material; Ap is a ploughed surface horizon.
Further reading: Soil Survey Manual, USDA Handbook 18
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
For designing subsurface drain spacing, which field method is standard for measuring saturated hydraulic conductivity below a shallow water table?
C. Auger-hole method
The auger-hole method measures the rate of rise of water in a hole bored below the water table and gives K of a large soil volume at the depth that matters for drains. Ring and tension infiltrometers measure surface infiltration above the water table, and small cores in a laboratory permeameter often miss the macropores that dominate field flow.
Further reading: ILRI Publication 16
What is the purpose of the outer ring in a double-ring infiltrometer?
B. It buffers lateral spread so the inner ring measures nearly vertical infiltration
Water from the outer ring wets the soil around the inner ring, so water from the inner ring is not drawn sideways and its fall represents one-dimensional vertical infiltration. Readings are taken from the inner ring only. The outer ring is not an evaporation pan, a reservoir or an anchor.
Further reading: ASTM D3385
Using the common approximation TDS (mg/L) ≈ 640 × EC (dS/m), what is the total dissolved solids of an irrigation water with EC 2.5 dS/m?
C. 1600 mg/L
TDS ≈ 640 × 2.5 = 1600 mg/L. Dividing gives 256; 25 is the approximate total cations in meq/L (10 × EC), not mg/L; 16000 is a decimal slip. The factor varies with salt type (about 550–700).
Further reading: FAO Irrigation and Drainage Paper 29 Rev. 1
Which of the following is a micronutrient essential for higher plants?
D. Boron
Boron is one of the micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl, Ni). Sulphur, magnesium and calcium are secondary macronutrients, required in larger amounts.
Further reading: Brady and Weil, The Nature and Properties of Soils
Under the US Salinity Laboratory (Handbook 60) classification, a soil with ECe 9 dS/m, ESP 8 and pH 7.9 is:
D. Saline
Saline soils have ECe above 4 dS/m, ESP below 15 and pH usually below 8.5. Sodic soils need ESP above 15 with ECe below 4; saline-sodic need both ECe above 4 and ESP above 15; acid sulphate soils are strongly acid, not pH 7.9.
Further reading: USDA Agriculture Handbook 60 (1954)
A sodic soil with GR 7.75 t/ha of pure gypsum is to be reclaimed with agricultural gypsum of 80% purity. How much product is needed?
B. 9.69 t/ha
Product = 7.75/0.80 = 9.69 t/ha. Multiplying by purity gives 6.20, the common slip; 7.75 ignores purity; multiplying by 1.2 treats 80% purity as a 20% surcharge.
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
The steady infiltration rate that a soil approaches after prolonged wetting is called the:
A. Basic infiltration rate
The basic (final or steady-state) infiltration rate is the near-constant rate reached after long wetting; it approaches the saturated conductivity of the surface layer. Initial capacity is the high early rate, sorptivity describes early suction-driven uptake, and percolation is movement within the profile.
Further reading: Michael, Irrigation: Theory and Practice
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