Fertiliser vs. non-fertiliser classification:\nWhich of the following is not classified as a standard fertiliser supplying a primary N, P or K plant nutrient?

Introduction / Context:
Commercial fertilisers are primarily categorized by the primary nutrients they supply: nitrogen (N), phosphorus (P) and potassium (K). Some materials are soil amendments or micronutrient sources rather than primary fertilisers. Distinguishing between these groups helps avoid mislabeling and ensures correct agronomic recommendations.

Given Data / Assumptions:

• Primary fertilisers deliver N, P, or K as their main function.
• Products like CAN, urea, ammonium sulphate, and anhydrous ammonia are standard nitrogen fertilisers.
• Ferrous sulphate is mainly a micronutrient/soil amendment supplying iron (Fe), not a primary N, P, or K fertiliser.

Concept / Approach:
Urea (~46% N), ammonium sulphate (~21% N), calcium ammonium nitrate (~26–28% N), and anhydrous ammonia (~82% N) are straight nitrogen fertilisers. Ferrous sulphate is used to correct iron chlorosis and to adjust soil conditions in turf/horticulture, but it does not supply N, P, or K in significant quantities. Therefore, among the choices, ferrous sulphate is not a standard primary fertiliser.

Step-by-Step Solution:

Verification / Alternative check:
Fertiliser regulations and labels list N–P–K grades for primary fertilisers; ferrous sulphate is sold with Fe percentage and is categorized under micronutrients or soil conditioners, not NPK fertilisers.

Why Other Options Are Wrong:

• CAN, ammonium sulphate, liquid ammonia, and urea are all standard nitrogen fertilisers widely used in field crops.

Common Pitfalls:
Assuming any nutrient source is a fertiliser in the NPK sense; micronutrient salts correct deficiencies but are not NPK carriers.

Final Answer:
Ferrous sulphate (iron(II) sulphate)