Saltpeter, or potassium nitrate, is used as a food preservative and additive. It is also used for fertilizer and is a large component of gunpowder. It is also used as an oxidizing agent for rockets and fireworks.
Saltpeter is also used topically to ease the pain of sensitive teeth and so is an ingredient in some toothpastes. It is also used to treat asthma. Saltpeter used to be used to treat hypertension and as part of fire extinguishers. It is also used to make glass and to make Concentrating Solar Power plants more efficient. When it is mixed with sodium nitrate, it makes an excellent storage medium for thermal energy. It can store this energy for several days and releases it on demand.
Saltpeter is water-soluble and is free of chlorides in its pure state. This makes it an ideal fertilizer for crops that are intolerant of chlorides, such as potatoes, avocados, beans, lettuce and onions.
As a food preservative, saltpeter also guards against bacteria, such as Clostridium botulinum, and helps foods such as hard cheeses and meats retain an attractive appearance over time. At one point, people believed that saltpeter could be used to suppress the male libido, but there's no credible evidence for this belief.