N-P-K (Nitrogen, Phosphorus, and Potassium) Fertilizer: A Guide
Nitrogen, phosphorus, and potassium (N-P-K) are the three major nutrients vital for plant life. Each nutrient has different roles within plants, and different fertilizers contain varying concentrations of each nutrient. Most fertilizers contain small amounts of secondary nutrients and micronutrients, which are needed by plants as well. Each nutrient is available in different applications and formulations.
Understanding fertilizer labels and what each nutrient provides to plants is essential when growing lawns, vegetables, fruits, and ornamental plants.
Reading Fertilizer Labels
Fertilizer labels have three numbers separated by hyphens, commonly called the NPK formula. For example: 10-5-15. The numbers represent nitrogen (N), phosphorus (P), and potassium (K) respectively. So the example above would contain 10% nitrogen, 5% phosphorus, and 15% potassium per bag. The remaining percentage (70%) consists of fillers to make it easy to handle and spread.
There are many other nutrients needed for plant growth, including secondary nutrients and micronutrients. Fertilizers that contain secondary and micronutrients are just as beneficial to plants.
Nitrogen is the most abundant element in the Earth's atmosphere, but atmospheric nitrogen is inert and unavailable to plants. Nitrogen must be combined with other molecules before plants can use it. Microorganisms are essential for transforming organic matter within the soil into useable nitrogen.
Nitrogen Use in Plants
Nitrogen is a major component of chlorophyll, which is the compound that plants use to transform sunlight, water, and carbon dioxide into useable sugars. This process is called photosynthesis and is performed by every plant. Nitrogen is also a major component of amino acids, which are the building blocks of proteins. Some proteins are involved in the structure of plant cells, while others act as enzymes that allow for vital biochemical reactions.
Common nitrogen fertilizers are anhydrous ammonia, urea, solutions, ammonium sulfate, and ammonium nitrate. Anhydrous ammonia contains the most nitrogen but needs to be injected into the soil to prevent dissipation into the air. Urea is the most common and easiest source to obtain, considering it is simply animal urine.
Phosphorus is not found in its elemental state but is bound with other elements to create a useable form. Uptake of phosphorus is enhanced by mycorrhizal fungi that grows in tandem with the roots of plants.
Phosphorus in Plants
Phosphorus plays a large role in the transferring of energy within plant structures. Organic compounds that contain phosphorus are used to transfer energy from one reaction to drive another reaction within plant cells. Processes that require large amounts of energy rely on phosphorus, especially during the production of ATP—a useable form of energy produced through photosynthesis.
Phosphorus stimulates early plant growth and hastens maturity. It is also vital within the building blocks of chromosomes and genes.
Superphosphate of lime is a mixture of two phosphate salts: calcium dihydrogen phosphate and calcium sulfate dihydrate. This is produced by the reaction of sulfuric acid and water with calcium phosphate.
Organic phosphorus sources from animal manures, composts, and biosolids (sewage). Retail biosolid fertilizer (such as Milorganite based in Milwaukee, WI) is treated and sterilized before being sold. Biosolid fertilizer is a very large recycling effort around the world. Many municipalities spread sewage solids on farm fields for free.
Several areas in the U.S. do not allow phosphorus application unless seeding or if the soil is proved to be phosphorus deficient via a soil nutrient test. Check local and regional laws before applying phosphorus. Regulations on phosphorus have been put into place to protect surface waters from algae blooms caused by excessive use. Run-off is a major contributor to high phosphorus levels in surface waters—though applications of organic matter that contain phosphorus are not regulated.
Potassium in nature is only available in ionic salts. It is commonly found dissolved in seawater and is within many minerals.
Potassium helps fight disease and promotes rigorous growth by acting as a catalyst for enzymes during photosynthesis. Potassium is also a major factor in blooming and quality of fruit. Photosynthesis produces a basic energy form called ATP. This source of energy needs to be balanced by potassium. Deficiencies in potassium causes the rate of ATP and photosynthesis to decline.
Potassium plays an essential role in the transport of ATP. Deficiency in potassium causes the transport system to lag and less ATP is available to the plant.
Potassium and Stomates
Stomates are tiny pores that allow plants to exchange carbon dioxide, moisture, and oxygen. Potassium is essential for stomates to function properly. Potassium in cells near stomates causes the cells to swell with water and open. Gases can be exchanged when the stomates are open. Potassium is drawn away from cells during times of drought, which causes the pores to close. Closed pores prevent moisture from escaping and allows plants to live through periods of drought. Potassium deficiency slows down stomate activity, causing the plant to lose vital moisture.
Potassium is commonly sold in the form of potash, which consists of manufactured water-soluble salts that contain potassium. Many years ago, people mixed ashes with water and used the slurry as a fertilizer, hence "pot-ash." This method of mixing hardwood ash and water is still used today.
Compost and kelp meal are also used for potassium fertilizer applications
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.