Which one of the following calcium compounds is commonly used as a nitrogenous fertilizer in agriculture (often referred to as calcium cyanamide)?

Introduction / Context:
This agricultural chemistry question asks about a calcium compound used as a fertilizer, specifically a nitrogenous fertilizer. Farmers often apply nitrogen containing compounds to improve plant growth, and some of these fertilizers are calcium salts. One such compound is calcium cyanamide, sometimes loosely referred to as calcium cyanide in older texts, which releases nitrogen into the soil. Recognising which calcium compound provides nitrogen helps distinguish it from other calcium salts used for different purposes.

Given Data / Assumptions:
- The options are calcium carbide, calcium carbonate, calcium cyanide (intended to represent calcium cyanamide), and calcium sulphate.
- The question focuses on use as a fertilizer, particularly for nitrogen supply.
- We assume common agricultural uses: nitrogenous fertilizers versus soil conditioners.

Concept / Approach:
Calcium cyanamide, often written as CaCN2, is a nitrogenous fertilizer that slowly releases nitrogen into the soil. Although the option text says calcium cyanide, in many exam contexts it represents calcium cyanamide, which is the agriculturally important compound. Calcium carbonate (limestone) and calcium sulphate (gypsum) are mainly used to correct soil acidity or improve soil structure, not as primary nitrogen sources. Calcium carbide is used to generate acetylene gas, not as a fertilizer. Therefore, the calcium compound that functions as a nitrogenous fertilizer is calcium cyanamide, represented in the options as calcium cyanide.

Step-by-Step Solution:
Step 1: Identify which compounds contain nitrogen. Calcium cyanamide, CaCN2, contains nitrogen, while calcium carbide (CaC2), calcium carbonate (CaCO3), and calcium sulphate (CaSO4) do not contain nitrogen in their formulas.
Step 2: Recognise that nitrogen containing compounds are used as nitrogenous fertilizers because they can release nitrogen in forms that plants can absorb.
Step 3: Calcium carbonate is commonly used to reduce soil acidity (liming) and is considered a soil conditioner rather than a nitrogen fertilizer.
Step 4: Calcium sulphate, or gypsum, is used to improve soil structure and supply calcium and sulphur, but not as a major nitrogen source.
Step 5: Calcium carbide is mainly used industrially to produce acetylene gas and is not applied as a fertilizer.
Step 6: Calcium cyanamide (represented in the option as calcium cyanide) is specifically manufactured and sold as a nitrogenous fertilizer, so this is the correct choice.

Verification / Alternative check:
Agricultural references list calcium cyanamide as a nitrogen fertilizer that slowly breaks down in the soil to release ammonia, which plants can use. Its use is distinct from that of lime (calcium carbonate) and gypsum (calcium sulphate), which are recommended primarily for adjusting soil pH and improving soil structure. Safety data sheets and product labels for calcium cyanamide clearly identify it as a fertilizer, while calcium carbide is labelled as a gas generating chemical and is not applied to fields for plant nutrition. This confirms that the nitrogen supplying calcium compound is calcium cyanamide, reflected in the option about calcium cyanide.

Why Other Options Are Wrong:
- Calcium carbide: Used mainly to generate acetylene gas for welding and other industrial purposes; it is not a standard agricultural fertilizer.

Why Other Options Are Wrong (continued):
- Calcium carbonate: Used to reduce soil acidity and as a soil conditioner, but it does not provide nitrogen to plants.
- Calcium sulphate: Known as gypsum, it supplies calcium and sulphur and helps improve soil physical properties, but it is not primarily a nitrogenous fertilizer.

Common Pitfalls:
Students may confuse calcium carbonate and calcium sulphate because they are both used in soil treatment and contain calcium. Another source of confusion is the wording calcium cyanide, which in many exam questions is intended to represent calcium cyanamide, the actual fertilizer. To avoid mistakes, focus on whether the compound contains nitrogen when the question specifies a nitrogenous fertilizer, and remember that calcium cyanamide is the relevant fertilizer in this group.

Final Answer:
Calcium cyanide (calcium cyanamide) is the calcium compound used as a nitrogenous fertilizer in agriculture.