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.
See
this article by Howard Garrett for more information on
fertilizer fillers.
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.
Dr.
Howard Garrett addresses questions about Synthetic Versus
Organic Fertilizers.
The
University of Illinois has more information on understanding
NPK numbers.
This website
from the University of Texas has excerpts from Dr.
Howard Garrett’s book about natural gardening in Texas.
