LEA Reviewer: Soil Science MCQs with Answers

1. Which of the following factors is NOT part of the CLORPT model of soil formation?

• a) Climate
• b) Organisms
• c) Parent material
• d) Tillage

2. In a soil profile, which horizon is typically richest in organic matter and is the topmost mineral horizon?

• a) O horizon
• b) A horizon
• c) B horizon
• d) C horizon

3. Which soil texture class contains the highest proportion of clay-sized particles?

• a) Sandy loam
• b) Silty loam
• c) Clay
• d) Loamy sand

4. Which of the following is the primary cause of soil acidity in many tropical upland soils?

• a) High carbonate content of parent material
• b) Leaching of basic cations under high rainfall
• c) Excessive liming
• d) High sodium adsorption ratio (SAR)

5. Which of the following measurements expresses the soil’s ability to retain cations?

• a) Bulk density
• b) Cation exchange capacity (CEC)
• c) Electrical conductivity (EC)
• d) Percent base saturation

6. Which form of phosphorus is commonly measured as “available P” in alkaline soils using the Olsen method?

• a) H₂PO₄⁻ (dihydrogen phosphate)
• b) PO₄³⁻ (orthophosphate)
• c) Organic P only
• d) Condensed polyphosphates

7. Which of the following soil conservation practices is most effective in reducing gully formation on steep slopes?

• a) Contour plowing
• b) Strip cropping
• c) Construction of terraces
• d) Reduced tillage

8. Which soil order is typically deep, highly weathered, and common in humid tropical regions including parts of the Philippines?

• a) Mollisols
• b) Ultisols
• c) Aridisols
• d) Gelisols

9. Which microorganism group is primarily responsible for biological nitrogen fixation in legume root nodules?

• a) Mycorrhizal fungi
• b) Rhizobium bacteria
• c) Azotobacter
• d) Actinomycetes

10. In land capability classification (LCC), which class indicates land suitable for permanent cultivation with only minor limitations?

• a) Class I
• b) Class II
• c) Class III
• d) Class IV

11. Which of the following best describes soil bulk density?

• a) Mass of soil solids per unit volume of soil (including pore space)
• b) Volume of pore spaces per unit mass of soil
• c) Mass of water held by the soil at field capacity
• d) Percent of organic matter in the soil

12. Which of the following is a correct pairing of deficiency symptom and nutrient in plants?

• a) Interveinal chlorosis of new leaves — Nitrogen
• b) Stunted growth and purpling of older leaves — Phosphorus
• c) Marginal leaf burn on old leaves — Iron
• d) General chlorosis of young leaves — Potassium

13. Which amendment is commonly recommended to reclaim sodic soils by replacing exchangeable Na⁺ with Ca²⁺?

• a) Elemental sulfur
• b) Gypsum (calcium sulfate)
• c) Lime (calcium carbonate)
• d) Organic manure only

14. In the context of soil water, which term describes the water content remaining in the soil after excess gravitational water has drained away?

• a) Wilting point
• b) Saturation
• c) Field capacity
• d) Permanent wilting point

15. Which test method is commonly used for routine soil pH measurement in a laboratory?

• a) Potentiometric measurement with a glass electrode
• b) Flame photometry
• c) Kjeldahl digestion
• d) Atomic absorption spectrophotometry

16. Which of the following best explains why organic matter improves soil structure?

• a) Organic matter increases soil pH
• b) Organic matter acts as a binding agent, promoting aggregation
• c) Organic matter replaces clay particles
• d) Organic matter reduces cation exchange capacity

17. Which pesticide-related practice most directly contributes to long-term buildup of pesticide residues in soils?

• a) Use of biodegradable pesticides only
• b) Frequent use of persistent, lipophilic pesticides
• c) Alternating pesticides with different modes of action
• d) Integrated pest management (IPM)

18. Which indicator is commonly used to assess soil salinity for irrigation suitability?

• a) Soil pH
• b) Electrical conductivity (EC)
• c) Cation exchange capacity (CEC)
• d) Organic carbon content

19. Which of the following is a commonly used biological indicator of good soil health?

• a) Low earthworm population
• b) High abundance of beneficial soil fauna (e.g., earthworms)
• c) High bulk density
• d) Absence of microbial activity

20. Which remote-sensing tool is often used to assist soil mapping and identifying land-use suitability?

• a) Sonar imaging
• b) Satellite multispectral imagery
• c) X-ray diffraction
• d) Gas chromatography

21. Which of the following soil properties most directly limits root penetration and growth?

• a) Soil color
• b) High bulk density / compaction
• c) High organic carbon
• d) Low cation exchange capacity

22. Which nutrient is most commonly associated with promoting early root development and energy transfer in plants?

• a) Nitrogen (N)
• b) Phosphorus (P)
• c) Potassium (K)
• d) Calcium (Ca)

23. Which soil testing extractant is commonly used for available phosphorus determination in acid soils (Bray method)?

• a) Sodium bicarbonate
• b) Bray-1 (acid fluoride mixture)
• c) Ammonium acetate
• d) DTPA

24. Which of the following is an example of a conservative nutrient management practice?

• a) Sole reliance on synthetic fertilizers without soil testing
• b) Integrated nutrient management (combining organic and inorganic sources)
• c) Excessive broadcast application of nitrogen
• d) Continuous monocropping with no residue return

25. Which process describes the conversion of organic nitrogen into mineral forms (ammonium and nitrate) available to plants?

• a) Immobilization
• b) Mineralization (ammonification and nitrification)
• c) Denitrification only
• d) Volatilization

26. Which of the following best describes the SAR (Sodium Adsorption Ratio) in irrigation water assessment?

• a) Ratio of chloride to bicarbonate
• b) Ratio indicating relative concentration of sodium to calcium and magnesium
• c) Measure of total dissolved solids only
• d) Indicator of soil organic matter content

27. Which of the following best describes the role of mycorrhizae in soils?

• a) They antagonize plant roots and reduce nutrient uptake
• b) They form symbiotic associations that enhance phosphorus uptake
• c) They fix atmospheric nitrogen in root nodules
• d) They primarily increase soil pH

28. Which soil color notation system is commonly used in field soil descriptions?

• a) Pantone color system
• b) Munsell color system
• c) RGB color model
• d) CMYK color model

29. Which of the following statements about peat soils is CORRECT?

• a) Peat soils are highly decomposed and have very high bulk density
• b) Peat soils are rich in organic matter and are prone to subsidence when drained
• c) Peat soils typically have very high mineral nutrient reserves
• d) Peat soils are common in arid regions

30. Which of the following is a direct consequence of long-term continuous monocropping on soil?

• a) Increased crop biodiversity
• b) Build-up of specific pests and nutrient depletion
• c) Immediate increase in soil organic matter
• d) Reduced soil erosion automatically

31. Which of the following is the BEST immediate measure to reduce surface runoff and soil loss on a newly planted sloping field?

• a) Install tile drains
• b) Establish cover crops or mulches
• c) Apply high doses of nitrogen fertilizer
• d) Increase tillage intensity

32. Which Philippine soil order or major soil group is commonly associated with clay soils that swell when wet and crack when dry (vertisols)?

• a) Oxisols
• b) Vertisols
• c) Histosols
• d) Entisols

33. Which of the following is an advantage of using green manures in cropping systems?

• a) They increase soil erosion
• b) They add organic matter and can fix atmospheric nitrogen
• c) They permanently eliminate weeds
• d) They reduce soil moisture retention

34. Which laboratory method is typically used to determine soil texture by separating sand, silt and clay?

• a) Hydrometer or sieve–pipette method
• b) Loss on ignition
• c) Kjeldahl digestion
• d) DTPA extraction

35. What is the major environmental risk associated with excessive application of nitrogen fertilizers in wet tropical systems?

• a) Increased phosphorus fixation
• b) Leaching to groundwater and denitrification losses (nitrate pollution)
• c) Immediate increase in soil organic carbon
• d) Reduced soil pH to neutral

36. Which of the following practices helps increase soil organic carbon over time?

• a) Burning crop residues
• b) Incorporation of crop residues and use of manures/compost
• c) Continuous deep plowing with residue removal
• d) Repeated fallowing without vegetation

37. Which of the following statements about soil texture and water-holding capacity is TRUE?

• a) Sandy soils typically have higher available water capacity than clay soils
• b) Clay soils retain more water overall but much of it may be at tensions unavailable to plants
• c) Texture has no influence on water-holding capacity
• d) Loamy sand holds the most plant-available water

38. Which of the following is a common practice to reclaim saline soils for crop production?

• a) Leaching salts below the root zone with good-quality water and improving drainage
• b) Increasing salt inputs through irrigation
• c) Removing topsoil only
• d) Avoiding any irrigation permanently

39. Which soil testing extractant is commonly used to estimate available micronutrients like Fe, Mn, Zn, and Cu?

• a) Olsen P
• b) DTPA (diethylenetriaminepentaacetic acid)
• c) Bray-1
• d) Ammonium acetate

40. Which Indonesian/Philippine-relevant soil limitation would most likely require both drainage improvement and salt leaching for reclamation?

• a) Acid sulfate soils with secondary salinization
• b) Well-drained upland Ultisols
• c) Sandy coastal dunes with low salt
• d) Rocky shallow Entisols

41. Which of the following is an integrated practice to reduce greenhouse gas emissions from rice paddies?

• a) Continuous flooding throughout the season
• b) Alternate wetting and drying (AWD) combined with optimized nitrogen management
• c) Excessive organic amendments without drainage
• d) Burning rice straw in the field

42. Which tool or practice helps farmers determine the right lime requirement for acid soils?

• a) Measuring electrical conductivity (EC)
• b) Soil buffering capacity tests and pH measurement
• c) Estimating from bulk density alone
• d) Visual soil color only

43. Which of the following is a primary reason for conducting a soil survey at the farm or watershed level?

• a) To determine farmers’ cultural practices
• b) To map soil types and their suitability for different land uses
• c) To set commodity market prices
• d) To measure daily weather variations

44. Which practice helps reduce phosphorus runoff from agricultural fields?

• a) Broadcasting high rates of P fertilizer just before heavy rains
• b) Band placement of phosphorus near seed and buffer strips along waterways
• c) Removing vegetation from riparian zones
• d) Increasing soil erosion intentionally

45. Which of the following is TRUE regarding conservation tillage?

• a) It always increases immediate crop yields regardless of context
• b) It reduces soil disturbance and can increase residue cover, improving erosion control
• c) It eliminates the need for crop rotation
• d) It is synonymous with burning residues

46. Which of the following best describes soil buffering capacity?

• a) Soil’s resistance to change in temperature
• b) Soil’s resistance to change in pH when acids or bases are added
• c) Soil’s ability to hold water at field capacity
• d) Soil’s color variation range

47. Which of the following management options is most appropriate for a farmer dealing with iron deficiency chlorosis in calcareous soils?

• a) Apply phosphate fertilizers only
• b) Use iron chelates or foliar iron sprays and consider soil amendments to lower pH locally
• c) Apply more lime
• d) Increase sodium in the soil

48. Which statement about soil microbial activity is CORRECT?

• a) Microbial activity is unaffected by soil moisture and temperature
• b) Microbial activity generally increases with optimal moisture and moderate temperatures
• c) High salinity always increases microbial activity
• d) Microbial activity is highest in sterile soils

49. Which of the following is an example of an in-situ remediation technique for soils contaminated with petroleum hydrocarbons?

• a) Excavation and landfilling off-site
• b) Bioventing (enhancing microbial degradation in place)
• c) Incineration off-site
• d) Exporting soil to industrial dumps

50. Which farming measure helps both soil conservation and improvement of on-farm nutrient cycling simultaneously?

• a) Continuous removal of crop residues
• b) Agroforestry or alley cropping with periodic incorporation of prunings
• c) Sole reliance on synthetic N at high rates
• d) Replacing perennial vegetation with bare fallow