Potassium nitrate (KNO₃) is a soluble source of two major essential plant nutrients. It’s commonly used as a fertilizer for high-value crops that benefit from nitrate (NO₃-) nutrition and a source of potassium (K+) free of chloride (Cl⁻).
Manufacturers typically make potassium nitrate fertilizer (sometimes referred to as nitrate of potash or NOP by reacting potassium chloride (KCl) with a nitrate source. Depending on the objectives and available resources, the nitrate may come from sodium nitrate, nitric acid or ammonium nitrate. The resulting KNO3 is identical regardless of the manufacturing process. Potassium nitrate is commonly sold as a water-soluble, crystalline material primarily intended for dissolving and applying with water or in a prilled form for soil application. Traditionally, this compound is known as saltpeter.
Growers value fertilizing with KNO₃ especially in conditions where a highly soluble, chloride-free nutrient source is needed. In such soils, all of the N is immediately available for plant uptake as nitrate, requiring no additional microbial action and soil transformation. Growers of high-value vegetable and orchard crops sometime prefer to use a nitrate-based source of nutrition in an effort to boost yield and quality. Potassium nitrate contains a relatively high proportion of K, with an N to K ratio of approximately one to three. Many crops have high K demands and can remove as much or more K than N at harvest.
Applications of KNO₃ to the soil are made before the growing season or as a supplement during the growing season. A diluted solution is sometimes sprayed on plant foliage to stimulate physiological processes or to overcome nutrient deficiencies. Foliar application of K during fruit development advantages some crops, since this growth stage often coincides with high K demands during the time of declining root activity and nutrient uptake. It’s also commonly used for greenhouse plant production and hydroponic culture.
Both N and K are required by plants to support harvest quality, protein formation, disease resistance and water-use efficiency. Therefore, to support healthy growth, farmers often apply KNO₃ to soil or through the irrigation system during the growing season.
Potassium nitrate accounts for only a small portion of the global K fertilizer market. It’s primarily used where its unique composition and properties can provide specific benefits to growers. Further, it’s easy to handle and apply, and is compatible with many other fertilizers, including specialty fertilizers for many high-value specialty crops, as well as those used on grain and fiber crops.
The relatively high solubility of KNO₃ under warm conditions allows for a more concentrated solution than for other common K fertilizers. However, farmers must carefully manage the water to keep the nitrate from moving below the root zone.
Potassium nitrate has long been used for fireworks and gunpowder. It’s now more commonly added to food to maintain the quality of meat and cheese. Specialty toothpastes often contain KNO₃ to alleviate tooth sensitivity. A mixture of KNO₃ and sodium nitrate (NaNO₃) is used for storing heat in solar energy installations.
Source: Nutrient Source Specifics (No. 11), International Plant Nutrition Institute.