Difficulty: Easy
Correct Answer: Ammonium sulfate
Explanation:
Introduction / Context:
Rice is typically grown in flooded or water-logged soils (paddy fields). Redox conditions in submerged soils alter nitrogen transformations and losses, so the choice of nitrogen source matters. Ammonium-based fertilizers have advantages in flooded conditions, where nitrate forms are prone to denitrification losses.
Given Data / Assumptions:
Concept / Approach:
Ammonium sulfate supplies nitrogen in the ammonium form (NH4+), which is less susceptible to leaching and denitrification than nitrate in flooded soils. Additionally, it provides sulfur, which can be beneficial where S is limiting. Nitrophosphate and single superphosphate are mainly P (with some N in nitrophosphate), not the preferred dedicated N source in paddy. Potassium nitrate supplies nitrate-N, which is vulnerable to denitrification under reduced conditions; it is generally less efficient for flooded rice.
Step-by-Step Solution:
Recognize anaerobic flooded soils favor NH4+ retention on clays and organic matter.Select an NH4+ donor: ammonium sulfate.Exclude nitrate-dominant sources (KNO3) due to denitrification losses.Acknowledge that P sources (nitrophosphate, SSP) address phosphorus nutrition, not the main N requirement.
Verification / Alternative check:
Agronomic recommendations for irrigated lowland rice frequently endorse ammonium-forming fertilizers (e.g., ammonium sulfate, ammonium chloride, urea with placement techniques) to reduce gaseous N losses.
Why Other Options Are Wrong:
Nitrophosphate/SSP: primarily phosphorus fertilizers; insufficient as the main N source.Potassium nitrate: nitrate prone to denitrification in flooded soils, reducing N use efficiency.
Common Pitfalls:
Broadcasting urea on floodwater without proper placement, leading to volatilization; ignoring sulfur nutrition where ammonium sulfate offers an added benefit.
Final Answer:
Ammonium sulfate
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