Rice (paddy) agronomy: which fertilizer is generally most suitable for flooded paddy fields, considering nitrogen form and soil conditions?

Introduction / Context:
Paddy (rice) is typically grown under flooded or waterlogged conditions that create anaerobic, reducing soils. The choice of nitrogen fertilizer affects losses (volatilization, denitrification) and plant availability. Among common N sources, the ammonium form is favored under submerged conditions.

Given Data / Assumptions:

• Fields are flooded; soil is reducing with limited oxygen.
• Farmers seek to minimize nitrogen losses and ensure uptake.
• Comparison is among typical straight fertilizers.

Concept / Approach:
In flooded soils, ammonium (NH4+) is stable and less prone to immediate losses than nitrate, which can denitrify to N2/N2O under anaerobic conditions. Urea rapidly hydrolyzes to ammonium but is susceptible to ammonia volatilization at the surface if not incorporated. Ammonium sulphate directly supplies NH4+ and adds sulphur, often benefiting rice nutrition. Therefore, ammonium sulphate is classically regarded as most suitable for paddy among the given choices.

Step-by-Step Solution:
Identify soil condition: flooded, reducing—favor ammonium over nitrate.Compare N sources: ammonium sulphate supplies NH4+ directly; urea needs hydrolysis and can volatilize if surface-applied.Select ammonium sulphate as the preferred fertilizer for paddy fields.

Verification / Alternative check:
Rice agronomy texts often recommend ammonium-based fertilizers or deep placement of urea briquettes to reduce losses; the straightforward answer in MCQs is ammonium sulphate.

Why Other Options Are Wrong:
Urea is widely used but loss-prone without proper management; superphosphate is a P source, not N; potassium nitrate supplies nitrate-N, which is less suitable in flooded soils; diammonium phosphate is a P + N source used for basal P rather than the main N program.

Common Pitfalls:
Assuming “common use” equals “most suitable”; management practices can make urea effective, but the conceptual best match under flooding is ammonium sulphate.

Final Answer:
Ammonium sulphate