NPK, also called the "big three", is on nearly every fertilizer label, but what exactly does it mean?
N is for nitrogen, it provides vital nutrients for chlorophyll production.
P stands for phosphorus which transfers nutrients within plants.
K, potassium, strengthens plants' resistance to disease and defenses.
When NPK is on a label, they are in percentages. So, a label that says 4-3-2 is 4% nitrogen, 3% phosphorus and 2% potassium. The remaining 91% of the fertilizer is as "inert ingredients" or "fertilizer." The NPK numbers are what define the fertilizer and its uses.
Nitrogen
Nitrogen (N) is the most important nutrient of the big three. Plants need more nitrogen than any other nutrient. It impacts various functions. It aids in protein formation and is part of chlorophyll, which conducts photosynthesis. Without nitrogen, plants will be unable to get in energy, and the leaves will turn yellow and wither.
Plants with a nitrogen deficiency tend to have pale green to yellow leaves. The veins also lose their color and turn yellow. The older leaves, often towards the bottom of the plant, are the first effected and may die off. The stem may turn yellow, purple or red but it usually takes a while for that to happen.
All plants need nitrogen so most fertilizers will contain some. High nitrogen fertilizers are great for leafy plants such as herbs and young plants.
Phosphorus
Phosphorus (P) is necessary for transporting and storing nutrients in plants. Plants need a good amount of phosphorus to develop a thriving root system.
Phosphorus impacts a wide variety of plant functions. Including root development, stem strength, flower productivity and seed production. Plants with a good amount of phosphorus tend to have improved development.
Plants with a phosphorus deficiency have stunted growth and a weak root system. Plants with a phosphorus deficiency may have purple or otherwise discolored leaves. Tomatoes are especially prone to this, the undersides of their leaves may be the first to turn purple.
If plants can not get enough phosphorus, it may be due to a pH imbalance. A soil pH of 6.5 is ideal for phosphorus intake. If you are unsure of your soil's pH a test from your local extension office is a good place to start.
Looking to help a plant develop a better root system? A higher phosphorus fertilizer may be the way to go. Tomatoes and other plants that may develop blossom end rot benefit extra phosphorus.
Potassium
Potassium (K), sometimes called potash, is the last of the big three. Its chemical symbol represents it to keep it to avoid confusion with phosphorus (P).
Potassium's main job in plants is to fight off diseases. It also aids in the production and yield of crops and the regulation of gasses and water.
One key function of potassium is that it strengthens plants for temperature changes. Giving plants extra potassium is a good way to prepare them for overwintering.
Potassium deficiencies appear as yellow or white coloring around the edges of leaves. The leaves may also curl at their tips. Crops such as corn can enjoy extra potassium. It is beneficial to apply potassium fertilizers when preparing for a temperature shift.