Explanation of NPK Fertilizer Numbers
Understanding NPK (Nitrogen, Phosphorous, and Potash (Potassium) ratings on lawn and plant fertilizers is an important part of deciding whether or not fertilizers are appropriate or even necessary for your garden and landscaping.
This article is a basic guide to understanding what NPK numbers mean, and what levels of Nitrogen, Phosphorous, and Potash are really appropriate for your garden plants.
Be aware that in most cases, important gardening practices such as aerating and composting are much better for your garden than using chemical fertilizers. Also note that higher NPK levels don’t necessarily mean healthier plants.
What You Should Know about NPK and Fertilizers
Chemical fertilizers and organic fertilizers show their nutrient content with three bold numbers on the package. These numbers represent three different compounds: Nitrogen, Phosphorous, and Potash (Potassium), which we can also describe with the letters N-P-K. The three numbers listed on fertilizer labels correspond to the percentage of these materials found in the fertilizer.
What does each nutrient do? In addition to other properties, Nitrogen helps plant foliage to grow strong. Phosphorous helps roots and flowers grow and develop. Potassium (Potash) is important for overall plant health.
Be aware that high nitrogen fertilizers will make for quick growth but weaker plants that are more susceptible to attacks by diseases and pests. Fast, showy growth is not necessarily the best thing for your plants.
German scientist Justus Von Liebig was responsible for the theory that Nitrogen, Phosphorous, and Potassium levels are the basis for determining healthy plant growth.
However, this theory doesn’t take into account the dozens of other nutrients and elements that are essential to plant growth such as sulfur, hydrogen, oxygen, carbon, magnesium, etc. Nor does the theory talk about the importance of beneficial soil organisms that help your plants to absorb nutrients and fight off pests and diseases.
While Von Liebig’s work was unquestionably important to the science of plant growth, other ways of looking at how plants utilize nutrients have largely been ignored, especially by those companies who produce the chemical fertilizers commonly on the market today.
For example, many of the elements necessary for healthy plants aren’t available from the soil. They are actually available from water and air. Carbon dioxide, oxygen and nitrogen can reach the soil and plants’ roots if you properly aerate the soil. Microbes, earthworms, and other critters will then help your plants make use of these elements found in the air to benefit plant growth.
Additionally, if we look at nature, compost in the form of organic hummus is ever present in natural plant communities, providing lots of the nutrients that plants need to grow and thrive. Compost contains Nitrogen, Phosphorous, Potassium, and a great abundance of other trace elements that will benefit your plants.
It is clear that Nitrogen, Phosphorous, and Potassium are not necessarily the most important elements you need for your plants to grow well. In fact, elements such as carbon, hydrogen, oxygen, sulfur, magnesium, copper, cobalt, sodium, boron, molybdenum, and zinc are just as important to plant development as N-P-K. (Source: http://www.utexas.edu/utpress/excerpts/ exgarhow.html)
Unfortunately, Justus Von Liebig’s theory has become a dominant paradigm for how we grow our ornamental and edible plants. This has lead to a vast amount excess nutrients building up in our streams, lakes, and rivers because fertilizers are over applied.
Organic gardeners can look to the work of Sir Albert Howard for solid research and ideas on how to grow plants more naturally. His ideas consider chemical processes that occur in nature. He then applies them to agriculture and home gardening.
Organic Versus Chemical Fertilizers
When looking at both organic and chemical fertilizer labels, you’ll notice that the NPK numbers don’t add up to 100 percent. So, what is the rest of your fertilizer made up of? That depends on the fertilizer.
Chemical fertilizers can have any number of additional ingredients including dirt, sand, and even materials that are potentially hazardous to your health and to the environment. (Source: http://www.dirtdoctor.com/view_question.php?id=131)
These fillers for chemical fertilizers are useful so that the nutrients aren’t so concentrated that they will damage or “burn” your plants. Organic fertilizers don’t necessarily contain “fillers” as they are made up of a variety of natural components that in one way or another will benefit your plants.
Another thing to be aware of with chemical fertilizers is the kind of nutrients they contain and the way these nutrients are extracted. For example, the kind of nitrogen typically found in chemical fertilizers dissolves very quickly in water. This means that excess nitrogen may find its way into groundwater and freshwater sources and contaminate the water.
Additionally, many chemical fertilizers are now using phosphoric acid to create a high phosphorous content quickly and cheaply. According to Dr. Garrett, this kind of phosphorous essentially neutralizes other important trace minerals from the soil that your plants need. Also, be aware that the potassium found in many chemical fertilizers is a harsh form of potassium that can be potentially harmful to your plants if applied improperly.
Although organic and natural fertilizers usually have a lower NPK number, they are soil amendments that work slowly over time to improve your soil and to help you plants grow strong. They avoid the fast growth and flowering provided by chemical fertilizers that actually weakens plants. Therefore, big NPK numbers don’t necessarily mean a better fertilizer.
Alternatives to Chemical Fertilizers
Many professional gardeners say that little to no additional potassium and phosphorus are needed in our soils as these elements are present already. However, we need to liberate these elements with proper soil aeration, soil drainage, beneficial soil organisms, etc. Therefore, organic fertilizers with NPK labels that show low numbers or even zero for potassium and phosphorus are perfectly adequate for your garden as long as you’re properly caring for your soil.
The thing about fertilizers is that without proper soil aeration, mineral nutrients, and other factors, your plants may not be able to absorb phosphorous and potassium anyway, so loading up your soil with high levels of phosphorous and potassium may not make much difference with the health of your plants.
Nitrogen is typically available to the soil without additional fertilizers. The trick is having healthy soil full of beneficial microorganisms that can make use of the nitrogen that is available in the air. Compost can help, and so can soil additives such as Mycor root builder. Also, companion planting with plants like beans that fix nitrogen will make for happy and healthy soil.
Organic composts and composted manure are excellent sources of all nutrients plants need to grow, including NPK. Professional gardeners who use chemicals and organic gardeners agree that using compost is one of the best things you can do to feed your plants are feed them the nutrients they need.
Other excellent sources of these nutrients are fish meal, bone meal, alfalfa meal, bat guano and earthworm castings.
Resources for Understanding Fertilizers and NPK
Clean Air Gardening’s buyer’s guide to organic fertilizers.
This website from the University of Texas has excerpts from Dr. Howard Garrett’s book about natural gardening in Texas.