Chapter 101: 300 Living Ancestors

Soil nitrate is a form of saltpeter (potassium nitrate, KNO₃) that is extracted from soil.

This is a natural mineral, which can only be formed in nature after a series of complex physical and chemical processes. Generally speaking, in places where organic matter has decomposed over a long time and the environment is rather dry—such as latrines, cesspools, or old corners of walls—the decomposition of nitrogen-containing organic matter and the role of nitrifying bacteria produce nitric acid, which then reacts with potassium salts in the soil, sometimes generating soil nitrate.

Overall, this mineral isn't very eye-catching; you can come across it in daily life, but its uses are broad.