Which of the following potassium fertilizers is known to be potassium nitrate?

Potassium nitrate is specifically identified by its chemical formula, KNO3. This compound consists of one potassium ion (K+) combined with one nitrate ion (NO3-). It is widely used in agriculture not only as a source of potassium but also as a nitrogen source, making it a popular choice for crops that require both nutrients.

The other options represent different potassium-containing compounds. KCl, commonly known as potash, primarily supplies potassium without nitrogen. K2SO4, or potassium sulfate, delivers potassium along with sulfur but lacks nitrogen. K2S2O3, which is thiosulfate of potassium, has a different composition and is not commonly used as a primary fertilizer for crops needing both potassium and nitrogen.

Thus, potassium nitrate's unique composition and dual-nutrient benefit designate it clearly as KNO3, distinguishing it from the other potassium sources listed.