Fertilizer sources: Which of the following is a natural inorganic (mineral) fertilizer material as obtained from nature and historically used directly?

Introduction / Context:
Fertilizer materials can be categorized as organic (derived from plant/animal matter) or inorganic/mineral (derived from geologic deposits or chemical processing). Understanding classic examples helps in recognizing nutrient analysis, solubility, and application behavior. Chile saltpetre is an iconic natural mineral source of nitrate nitrogen historically mined in Chile’s Atacama desert.

Given Data / Assumptions:

• “Natural inorganic” means mineral sources occurring in nature with minimal processing.
• Organic manures like oilcakes and farmyard manure are biologically derived and carbon-rich.
• We are comparing common textbook examples.

Concept / Approach:
Chile saltpetre is sodium nitrate (NaNO3), a natural mineral deposit used as a direct-application nitrogen fertilizer. By contrast, oilcakes (mustard, groundnut) and gobar manure are organic fertilizers releasing nutrients via mineralization. Bone meal, while mineral-rich in calcium phosphates, is of animal origin and generally classed as organic/mineral-organic rather than “natural inorganic” in the classical agronomy sense used in such MCQs.

Step-by-Step Solution:

Verification / Alternative check:
Historic fertilizer trade records and agronomy texts list sodium nitrate as a natural mineral nitrogen source used before widespread synthetic ammonia production.

Why Other Options Are Wrong:

• Oilcake and gobar manure are organic fertilizers, not inorganic minerals.
• Bone meal is animal-derived and not typically categorized as a pure mineral fertilizer.
• “None of these” is incorrect because Chile saltpetre fits the definition.

Common Pitfalls:
Equating “contains minerals” with “mineral fertilizer.” The classification depends on origin and processing, not just chemical species present.

Final Answer:
Chile saltpetre (sodium nitrate).