Soil science MCQ, with answers.
25 soil science questions on soil reaction, texture, clay minerals, fertilisers and soil water. Pick an answer to see if you are right and why. The full answer key is below the quiz.
Q1. A soil with pH 7.0 is:
Q2. Acidic soils are usually corrected by adding:
Q3. Sodic (alkali) soils are usually reclaimed with:
Q4. Under the USDA system, clay particles are smaller than:
Q5. Under the USDA system, sand particles range from:
Q6. The soil textural triangle is based on the percentages of:
Q7. Black cotton soils swell and crack because they are rich in:
Q8. The red colour of red soils is due to:
Q9. Kaolinite is a:
Q10. Laterite soils form under:
Q11. The primary plant nutrients are:
Q12. Which micronutrient is part of the nitrogenase enzyme used in nitrogen fixation?
Q13. Khaira disease of rice is caused by a deficiency of:
Q14. Urea contains about what percentage of nitrogen?
Q15. The nutrient grade of diammonium phosphate (DAP) is:
Q16. Muriate of potash (MOP) contains about what percentage of K₂O?
Q17. Single super phosphate (SSP) contains about what percentage of P₂O₅?
Q18. The average particle density of mineral soils is about:
Q19. Field capacity is the soil water held at a tension of about:
Q20. The permanent wilting point corresponds to a tension of about:
Q21. Available water capacity of a soil is:
Q22. To convert soil organic carbon to organic matter, the usual factor (Van Bemmelen) is:
Q23. Ammonium is oxidised to nitrite in soil by:
Q24. A sodic soil has an exchangeable sodium percentage (ESP) above:
Q25. Nitrogen deficiency symptoms appear first on:
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Soil science MCQ answers and reasons.
Q1. A soil with pH 7.0 is:
Answer: B, Neutral. pH 7 is neutral; below 7 is acidic and above 7 is alkaline.
Q2. Acidic soils are usually corrected by adding:
Answer: B, Lime. Agricultural lime (calcium carbonate) neutralises soil acidity.
Q3. Sodic (alkali) soils are usually reclaimed with:
Answer: B, Gypsum. Calcium from gypsum replaces exchangeable sodium, which is then leached out with water.
Q4. Under the USDA system, clay particles are smaller than:
Answer: A, 0.002 mm. USDA classes: clay < 0.002 mm, silt 0.002–0.05 mm, sand 0.05–2.0 mm.
Q5. Under the USDA system, sand particles range from:
Answer: B, 0.05 to 2.0 mm. Sand is 0.05 to 2.0 mm; anything over 2 mm is gravel.
Q6. The soil textural triangle is based on the percentages of:
Answer: A, Sand, silt and clay. Soil texture classes are read from the proportions of sand, silt and clay.
Q7. Black cotton soils swell and crack because they are rich in:
Answer: B, Montmorillonite (smectite). Montmorillonite is a 2:1 expanding clay that swells when wet and shrinks when dry.
Q8. The red colour of red soils is due to:
Answer: B, Iron oxides. Red soils owe their colour to oxidised iron (ferric oxides).
Q9. Kaolinite is a:
Answer: A, 1:1 clay mineral. Kaolinite has one silica sheet and one alumina sheet per layer, so it is a 1:1 non-expanding clay.
Q10. Laterite soils form under:
Answer: B, High rainfall and high temperature. Heavy rain and heat leach out silica and bases, leaving iron and aluminium oxides.
Q11. The primary plant nutrients are:
Answer: B, N, P, K. Nitrogen, phosphorus and potassium are the primary macronutrients. Ca, Mg and S are secondary nutrients.
Q12. Which micronutrient is part of the nitrogenase enzyme used in nitrogen fixation?
Answer: A, Molybdenum. Nitrogenase contains molybdenum and iron, so legumes need Mo to fix nitrogen.
Q13. Khaira disease of rice is caused by a deficiency of:
Answer: B, Zinc. Khaira is zinc deficiency in rice, seen as brown patches on leaves; it is corrected with zinc sulphate.
Q14. Urea contains about what percentage of nitrogen?
Answer: C, 46%. Urea has 46% N, the highest of the common solid nitrogen fertilisers.
Q15. The nutrient grade of diammonium phosphate (DAP) is:
Answer: B, 18-46-0. DAP supplies 18% N and 46% P₂O₅.
Q16. Muriate of potash (MOP) contains about what percentage of K₂O?
Answer: D, 60%. MOP (potassium chloride) has about 60% K₂O.
Q17. Single super phosphate (SSP) contains about what percentage of P₂O₅?
Answer: A, 16%. SSP has about 16% P₂O₅ and also supplies sulphur and calcium.
Q18. The average particle density of mineral soils is about:
Answer: B, 2.65 g/cm³. Particle density of most mineral soils is about 2.65 g/cm³, close to that of quartz.
Q19. Field capacity is the soil water held at a tension of about:
Answer: A, -1/3 bar. Field capacity is about -1/3 bar (-33 kPa), after gravitational water has drained.
Q20. The permanent wilting point corresponds to a tension of about:
Answer: C, -15 bar. At about -15 bar (-1.5 MPa) plants can no longer draw water and wilt permanently.
Q21. Available water capacity of a soil is:
Answer: A, Field capacity minus permanent wilting point. Plant-available water is the water held between field capacity and the permanent wilting point.
Q22. To convert soil organic carbon to organic matter, the usual factor (Van Bemmelen) is:
Answer: A, 1.724. Organic matter ≈ organic carbon × 1.724, assuming organic matter is 58% carbon.
Q23. Ammonium is oxidised to nitrite in soil by:
Answer: B, Nitrosomonas. Nitrosomonas turns ammonium into nitrite; Nitrobacter then turns nitrite into nitrate.
Q24. A sodic soil has an exchangeable sodium percentage (ESP) above:
Answer: B, 15. Soils with ESP above 15 are sodic; saline soils have EC above 4 dS/m with ESP below 15.
Q25. Nitrogen deficiency symptoms appear first on:
Answer: B, Older (lower) leaves. Nitrogen is mobile in the plant and moves to new growth, so older leaves turn yellow first.
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