Which of the following commercially used fertilisers is not classified as a nitrogenous fertiliser, that is, it does not primarily supply nitrogen to plants?

Introduction / Context:
Plant growth depends on the availability of essential nutrients, especially nitrogen, phosphorus, and potassium. Fertilisers are often classified according to which nutrient they mainly supply, such as nitrogenous, phosphatic, or potassic fertilisers. Understanding these categories helps in agriculture and environmental studies. This question asks you to identify which fertiliser in the list is not a nitrogenous fertiliser, meaning it does not primarily provide nitrogen to crops.

Given Data / Assumptions:

• The fertilisers listed are ammonium sulphate, urea, ammonium nitrate, and superphosphate.
• Ammonium salts and urea are well known nitrogen sources.
• Superphosphate is known as a phosphatic fertiliser.
• We assume standard formulations used in agriculture.

Concept / Approach:
Nitrogenous fertilisers are those that mainly supply nitrogen, often in the form of ammonium ions, nitrate ions, or amide nitrogen from urea. Ammonium sulphate ((NH4)2SO4) contains ammonium nitrogen, ammonium nitrate (NH4NO3) contains both ammonium and nitrate nitrogen, and urea (CO(NH2)2) is a high nitrogen fertiliser. Single superphosphate, on the other hand, is primarily a phosphatic fertiliser, containing calcium dihydrogen phosphate and gypsum, and is used to supply phosphorus, not nitrogen. Therefore, among the options, superphosphate is not a nitrogenous fertiliser.

Step-by-Step Solution:
Step 1: Examine ammonium sulphate. Ammonium sulphate contains ammonium ions (NH4+), which provide nitrogen to plants, making it a nitrogenous fertiliser. Step 2: Examine urea. Urea is an organic compound with a very high nitrogen content and is widely used as a nitrogenous fertiliser. Step 3: Examine ammonium nitrate. Ammonium nitrate contains both ammonium and nitrate forms of nitrogen, so it clearly supplies nitrogen. Step 4: Examine superphosphate. Superphosphate is mainly calcium phosphate based and is used to supply phosphorus, not nitrogen, to the soil. Step 5: Conclude that superphosphate is not a nitrogenous fertiliser.

Verification / Alternative check:
Agricultural chemistry references classify fertilisers into categories such as nitrogenous (urea, ammonium sulphate, ammonium nitrate), phosphatic (single superphosphate, triple superphosphate), and potassic (muriate of potash). Superphosphate is always listed with phosphatic fertilisers and is recommended when soils lack phosphorus. Urea and ammonium based salts are listed under nitrogenous fertilisers and are applied when nitrogen deficiency is diagnosed. This consistent classification confirms that superphosphate is not nitrogenous.

Why Other Options Are Wrong (i.e., they are nitrogenous):
Option A (Ammonium sulphate): Contains ammonium nitrogen and is widely used as a nitrogen source. Option B (Urea): Has one of the highest nitrogen percentages by weight among solid fertilisers, making it a key nitrogenous fertiliser. Option C (Ammonium nitrate): Contains both NH4+ and NO3- nitrogen and is a classic nitrogenous fertiliser.

Common Pitfalls:
Students sometimes focus on the presence of “ammonium” or “nitrate” in the name and correctly identify those as nitrogenous, but then guess wrongly between urea and superphosphate. Another confusion arises from the spelling “superphospohate”, but despite minor spelling variations, the meaning remains the same: it is a phosphatic fertiliser. A useful memory aid is that any fertiliser name with “ammonium” or “nitrate” usually indicates nitrogen content, while “phosphate” points to phosphorus.

Final Answer:
The fertiliser that is not nitrogenous is Superphospohate, which is mainly phosphatic.