Soil Chemistry — Soil Science Reviewer Questions

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.

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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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How to study soil chemistry for the AgLE

Soil chemistry questions usually test relationships, not isolated definitions. Start with soil pH, cation-exchange capacity, base saturation and buffering, then connect them to nutrient availability and amendment decisions. A change in one measurement should lead to a defensible prediction about the others. For example, liming an acid soil affects exchangeable acidity and calcium supply as well as pH; the best answer explains the whole chain rather than naming only the amendment.

Interpret the measurement before choosing a treatment

Keep units and extraction methods visible when comparing laboratory results. Distinguish salinity from sodicity, total nutrient content from plant-available forms, and a soil's capacity to hold cations from the percentage of sites occupied by particular bases. In calculation items, write the given values, convert units once, and check whether the final magnitude is realistic for a field soil. This prevents a memorised formula from producing an agronomically impossible recommendation.

• Link pH and buffering to lime requirement and nutrient availability.
• Relate clay and organic matter to cation-exchange capacity.
• Separate saline, sodic and saline-sodic conditions before selecting management.
• Use charge balance, percentages and units to check every computed answer.

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