Potassium nitrate is not used:

Potassium nitrate (KNO3), commonly known as saltpeter, is a versatile chemical compound with extensive industrial and medical applications. It is widely used as a fertilizer because it provides essential nitrogen and potassium nutrients to plants. In the manufacturing sector, it is a critical component of gunpowder (black powder), acting as a powerful oxidizer [2]. Medically, it is utilized to treat tooth sensitivity and cauterize small wounds [1]. While potassium nitrate is used in the food industry as a preservative and color-protecting agent for cured meats like bacon and luncheon meat [1], it is not typically used as a general preservative for all meat products in the same way that sodium-based salts (like sodium nitrate or sodium nitrite) are preferred. However, since options 1, 2, and 3 are definitive and major uses, the question likely implies a distinction in common usage or a specific context where it is 'not used' compared to other alternatives.

Sources

1. [2] https://www.britannica.com/science/potassium-nitrate
2. [1] https://www.sciencedirect.com/topics/immunology-and-microbiology/potassium-nitrate