Crop-specific nitrogen sources: For flooded paddy (rice) cultivation, which nitrogenous fertilizer is generally considered most suitable?

Introduction / Context:
Choice of nitrogen fertilizer depends on soil chemistry, crop type, and water management. Paddy fields are waterlogged (anaerobic), which alters nitrogen transformations and losses. The fertilizer should minimize volatilization and fit soil pH management goals.

Given Data / Assumptions:

• Flooded, reduced soil conditions typical of paddy fields.
• Objective: efficient nitrogen use with minimal gaseous losses.
• Common fertilizers: ammonium sulphate, CAN, ammonium nitrate, and phosphorus sources.

Concept / Approach:
Under submerged conditions, urea hydrolyzes to ammonium, and nitrate forms can denitrify to N2/N2O, causing losses. Ammonium sulphate provides nitrogen in the ammoniacal form less prone to immediate denitrification and also acidifies alkaline soils, which can reduce ammonia volatilization from the water surface. Therefore, ammonium sulphate is often preferred for paddy.

Step-by-Step Solution:
Consider nitrogen transformations in flooded soil: nitrification is limited; denitrification risk is high for nitrate.Ammonium-based fertilizers are favored over nitrate-based forms.Ammonium sulphate also helps manage high pH by supplying sulfate acidity.Choose ammonium sulphate as the most suitable option.

Verification / Alternative check:
Field recommendations in many rice-growing regions endorse ammonium sulphate (or split-applied urea) to reduce volatilization and denitrification losses.

Why Other Options Are Wrong:
CAN and ammonium nitrate supply nitrate, which is susceptible to denitrification under flooded conditions.

Superphosphate is a phosphorus fertilizer, not a nitrogen source.

Common Pitfalls:

• Ignoring soil pH; in acidic soils, ammonium sulphate can further acidify and may require liming.
• Assuming a single application; split dosing around crop stages improves nitrogen use efficiency.

Final Answer:
Ammonium sulphate