Soil Chemistry — Soil Science Reviewer Questions

13 board-style Soil Chemistry items for the Agriculturist Licensure Examination. Try 40 questions free; lifetime access is ₱49. Texture drives water, water drives aeration, and aeration drives nutrient availability. Reason down that chain and most items unlock.

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Sample Soil Chemistry questions with answers and explanations

Board-style items taken from the Soil Science bank. Every answer is explained, which is the part that makes a review question worth doing twice.

A soil test reports exchangeable Ca 8.0, Mg 3.0, K 0.5, Na 0.3 and exchangeable acidity 3.2, all in cmol(+)/kg. What is the effective CEC?
- A. 15.0 cmol(+)/kg correct
- B. 11.8 cmol(+)/kg
- C. 11.5 cmol(+)/kg
- D. 18.2 cmol(+)/kg
Why: Effective CEC is the sum of ALL exchangeable cations including acidity: 8.0 + 3.0 + 0.5 + 0.3 + 3.2 = 15.0 cmol(+)/kg. Omitting exchangeable acidity gives 11.8, which is the sum of bases -- a different quantity used for base saturation, not for CEC.
Using the same report -- Ca 8.0, Mg 3.0, K 0.5, Na 0.3, exchangeable acidity 3.2 cmol(+)/kg -- what is the base saturation?
- A. 21.3%
- B. 78.7% correct
- C. 88.5%
- D. 61.5%
Why: Bases total 8.0 + 3.0 + 0.5 + 0.3 = 11.8; base saturation = 11.8 / 15.0 x 100 = 78.7%. The complement, 21.3%, is acid saturation. A base saturation near 80% is consistent with a soil that needs little or no lime.
Two soils both read pH 5.2. Soil A is a sandy loam with 4 cmol(+)/kg CEC; soil B is a clay with 30 cmol(+)/kg CEC. What follows for liming?
- A. Both need the same lime because pH is the same
- B. Soil A needs more lime because sand is inherently more acidic
- C. Soil B needs far more lime despite the identical pH correct
- D. Neither needs lime because pH 5.2 is optimal for most crops
Why: pH measures ACTIVE acidity in solution; the lime requirement is set by RESERVE acidity held on the exchange complex, which scales with CEC. The high-CEC clay buffers against pH change and so consumes far more lime to shift the same number of pH units.
A soil requires 2.5 cmol(+)/kg of acidity to be neutralised in a furrow slice weighing 2,000,000 kg/ha. Given that 1 cmol(+) of CaCO3 weighs 0.50 g, what is the lime requirement?
- A. 5,000 kg/ha
- B. 1,250 kg/ha
- C. 500 kg/ha
- D. 2,500 kg/ha correct
Why: Per kg of soil: 2.5 cmol(+) x 0.50 g = 1.25 g CaCO3. For 2,000,000 kg: 1.25 g x 2,000,000 = 2,500,000 g = 2,500 kg/ha. The equivalent weight step is what most candidates drop, and dropping it doubles or halves the recommendation.
A liming material is only 80% pure CaCO3 equivalent. If the soil needs 2,400 kg/ha of pure CaCO3, how much of this material must be applied?
- A. 3,000 kg/ha correct
- B. 1,920 kg/ha
- C. 2,400 kg/ha
- D. 2,880 kg/ha
Why: Divide the pure requirement by the purity: 2,400 / 0.80 = 3,000 kg/ha. Multiplying instead (1,920 kg) under-limes by more than a fifth. Calcium carbonate equivalent is the only fair basis on which two liming materials can be compared.
A saturated paste extract reads EC 6.5 dS/m and exchangeable sodium percentage 9%, with pH 7.6. How is this soil classified?
- A. Sodic
- B. Saline correct
- C. Saline-sodic
- D. Normal
Why: Saline soils exceed 4 dS/m but stay below 15% ESP and below pH 8.5. Here EC is high while ESP is low, so it is saline, not sodic. The distinction governs the remedy: a saline soil is leached, whereas a sodic soil must first receive calcium to displace sodium or leaching only makes structure worse.
Rice grown on a strongly acidic soil shows stunting and root damage. Which chemical explanation is best supported?
- A. Phosphorus has become excessively available and toxic
- B. Calcium has become soluble and displaced potassium
- C. Aluminium and manganese have become soluble and toxic correct
- D. Nitrogen mineralisation has accelerated beyond crop demand
Why: Below about pH 5.5 aluminium dissolves from clay minerals and, with manganese, injures root tips directly. Phosphorus does the opposite in acid conditions -- it is FIXED by iron and aluminium and becomes less available, so a toxicity explanation for it is not defensible.
A submerged paddy soil is compared with the same soil when drained. Which change is expected upon flooding?
- A. Redox potential rises and iron is oxidised to the ferric form
- B. pH of acid soils falls further as oxygen is depleted
- C. Nitrate accumulates because nitrification accelerates
- D. Redox potential falls and pH of acid soils rises toward neutrality correct
Why: Flooding excludes oxygen, so redox potential falls and iron is REDUCED to the ferrous form, a process that consumes protons and drives acid soils toward neutrality. Nitrification requires oxygen, so nitrate is lost to denitrification rather than accumulating.

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Soil Chemistry — Soil Science Reviewer Questions

Sample Soil Chemistry questions with answers and explanations

A soil test reports exchangeable Ca 8.0, Mg 3.0, K 0.5, Na 0.3 and exchangeable acidity 3.2, all in cmol(+)/kg. What is the effective CEC?

Using the same report -- Ca 8.0, Mg 3.0, K 0.5, Na 0.3, exchangeable acidity 3.2 cmol(+)/kg -- what is the base saturation?

Two soils both read pH 5.2. Soil A is a sandy loam with 4 cmol(+)/kg CEC; soil B is a clay with 30 cmol(+)/kg CEC. What follows for liming?

A soil requires 2.5 cmol(+)/kg of acidity to be neutralised in a furrow slice weighing 2,000,000 kg/ha. Given that 1 cmol(+) of CaCO3 weighs 0.50 g, what is the lime requirement?

A liming material is only 80% pure CaCO3 equivalent. If the soil needs 2,400 kg/ha of pure CaCO3, how much of this material must be applied?

A saturated paste extract reads EC 6.5 dS/m and exchangeable sodium percentage 9%, with pH 7.6. How is this soil classified?

Rice grown on a strongly acidic soil shows stunting and root damage. Which chemical explanation is best supported?

A submerged paddy soil is compared with the same soil when drained. Which change is expected upon flooding?

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