What is NPK?

Photo:  thespruce.com

By Debbie Roland and Emmy Ulmschneider, Master Gardeners

Knowing what N-P-K represents may not be the most exciting gardening knowledge but knowing how to pick a fertilizer and when to use it is important.  Neither of us use many if any fertilizers in our garden or beds.  We go the route of compost most of the time.  When you see N-P-K used in gardening it is shorthand that refers to three macronutrients nitrogen, phosphorus as P2O5, and potassium as K2O.  The numbers on the 10-10-10 bag of fertilizer reflect the percentage by weight of the form that element is in.   So, in a 100 lb. bag of this fertilizer, you have 10 lbs. of nitrogen, 10 lbs. of phosphorous as P2O5 and 10 lbs. of K2O. The rest of the weight is made up of filler, carrier, or the other elements of the pure N-P-K compounds.   

Different types of fertilizers, all of which contain N-P-K work differently in your garden.  Organic fertilizers can be concentrated such as blood meal or bone meal or bulky such as manure or compost.  Bulky fertilizers provide other benefits like improving soil structure or water retention as they break down.   Soluble inorganic fertilizers may be less expensive but can wash away or leach out in heavy rain and should be applied in split applications.  Slow-release fertilizers are soluble fertilizers that are designed to release their nutrients over time.  If you choose to use fertilizer, always remember to read the package directions!  Developers of fertilizer took a great deal of time to test the effectiveness of the product.  More is not better and should be used in harmony with plant cycles.  Our local soil also determines the composition of the fertilizers that work best for us.

So, let’s see how fertilizer works.  Each macronutrient affects different stages of a plant’s growth. In addition to these macronutrients, there are micronutrients which are needed in smaller amounts. 

N stands for nitrogen and is the first number on a bag of fertilizer.  Nitrogen helps the plant put on lush green growth and will also help break down organic matter in compost.  Small leaves and little growth may be signs that your plant lacks nitrogen.  Lawns require quite a bit of nitrogen but be careful in your applications. A lawn should not be fertilized more than three times per year.  The runoff from fertilizers high in nitrogen can harm the water we drink when it is washed away.  Nitrogen in an organic form, can be added to the soil before planting.

P stands for phosphorus and will be the middle number on a fertilizer bag, commonly added as P2O5.  Phosphorus is required for flowering and fruiting.  If a plant is growing slowly or has stunted or disfigured leaves it may indicate a phosphorus deficiency.  To add phosphorus use bat guano, alfalfa meal, and fish meal.

K stands for potassium, commonly added as K2O and is the third and last number.  It helps the plant’s immune system and root development.  It is not needed as much as the other macronutrients.  Sources are guano, potash, and sulfate potassium.  Organic sources include greensand and hardwood ash.

It is a good idea to get your soil tested before you start adding anything to your soil whether it is organic or not.  The test is available through the local extension offices.  A soil test will give you a great deal of information which can be used to correct any deficiencies.

If you have questions, call the AgriLife office in Odessa at 498-4071 or in Midland at 686-4700.   Additional information, and our blog for access to past articles, is available at westtexasgardening.org.  Click on “Resources”.