Potassium nitrate

Potassium nitrate (KNO3), or saltpetre, is a crystalline salt containing potassium and nitrate nitrogen.

Characteristics

Its composition roughly matches an NPK of 13-0-44.
Soluble in water, and far more soluble in hot water.
Supplies two plant macronutrients: nitrogen and potassium.
An oxidiser that can react violently with reducing or combustible materials.

Role in the preparation: Supplies soluble nitrogen and potassium to nutrient solutions and mineral mixes.

Caution: Wear gloves and goggles, do not swallow or inhale the dust, and store it dry, away from fuels, organic matter and heat sources.

Sources: Wikipedia - Potassium nitrate

Documentary information: this is neither a recommendation for use nor a plant-protection or veterinary claim. Check your country's regulations before any use.