Identify the minimum nitrogen content: Among the following common nitrogen fertilisers, which has the lowest percentage of nitrogen by mass?

Introduction / Context:
Fertiliser selection frequently requires quick comparison of nitrogen analyses to meet target application rates. Among mainstream nitrogen carriers, percentages vary widely from ~21% to ~82% N. This question asks you to choose the product with the lowest nitrogen percentage among widely used solid nitrogen fertilisers.

Given Data / Assumptions:

• Nominal nitrogen analyses: urea ≈ 46% N; ammonium nitrate ≈ 33–35% N; ammonium chloride ≈ 25–26% N; calcium ammonium nitrate ≈ 26–28% N; ammonium sulphate ≈ 21% N.
• We compare on a mass-percentage basis and ignore moisture variation.

Concept / Approach:
Ammonium sulphate carries the lowest nitrogen percentage among the listed mainstream solids because a significant portion of its mass is sulphate sulphur. While urea is the highest-N solid, the ammonium salts vary by anion mass contribution, with sulphate being heavier than nitrate or chloride, thereby diluting the nitrogen fraction in ammonium sulphate to about 21% N.

Step-by-Step Solution:

Verification / Alternative check:
Product datasheets and extension bulletins consistently list ammonium sulphate at about 20–21% N, confirming it as the lowest among these standard solids.

Why Other Options Are Wrong:

• Urea: substantially higher N (≈46%).
• Ammonium nitrate: ≈33–35% N.
• Ammonium chloride: ≈25–26% N.
• Calcium ammonium nitrate: ≈26–28% N.

Common Pitfalls:
Confusing nitrogen percentage with agronomic efficiency; although lower in N%, ammonium sulphate supplies sulphur, which may be beneficial on S-deficient soils.

Final Answer:
Ammonium sulphate (~21% N)