Use-case identification: Which material serves as a nitrogenous fertiliser, acts as a weed killer in onion fields, and helps correct acidic soils when applied properly?

Introduction / Context:
Some agrochemicals play multiple roles depending on application method and soil conditions. Calcium cyanamide is a classic example: it is a nitrogen source, exhibits selective herbicidal action in certain crops (such as onions), and can reduce soil acidity when managed correctly. Identifying it among common fertilisers prevents misuse and supports integrated nutrient–weed management.

Given Data / Assumptions:

• We consider mainstream field materials with known agronomic uses.
• Onion fields often benefit from selective weed suppression strategies compatible with crop sensitivity.
• Soil acidity correction involves materials that raise pH or buffer acidity.

Concept / Approach:
Calcium cyanamide (CaCN2) hydrolyses in moist soil, gradually releasing plant-available nitrogen while transiently producing alkaline conditions that suppress certain weeds and pathogens. With appropriate timing before planting or at recommended intervals, it can act as a weed killer and soil amendment. Urea, CAN, and ammonium sulphate are valuable nitrogen fertilisers but do not deliver the same combined weed-suppressing and liming action as CaCN2 under typical use patterns.

Step-by-Step Solution:

Verification / Alternative check:
Agronomic guides cite CaCN2 for pre-plant soil conditioning, weed suppression, and supplying nitrogen; safety and timing guidelines are emphasised to protect crop seedlings.

Why Other Options Are Wrong:

• Urea/CAN/AS/DAP: effective nutrients but not standard weed killers nor liming agents.

Common Pitfalls:
Applying CaCN2 too close to seeding/transplanting; underestimating pH and timing effects critical to crop safety.

Final Answer:
Calcium cyanamide