USING THE PSNT TEST TO MANAGE N FERTILIZATION OF VEGETABLE CROPSBy Vern Grubinger, Vegetable and Berry Specialist, University of Vermont Extension
to save some money, protect water quality, and grow better crops?
There’s a pretty easy way to do this that costs just a few dollars and
doesn’t take much time. It’s called the PSNT, or pre-sidedress nitrate
We all know that adequate nitrogen is needed for crops to
produce their best yields and quality. Less well known is the fact that
too much nitrogen can actually reduce the yield of crops like pumpkins,
winter squash, peppers and tomatoes due to an excess of vegetative
growth. This may also promote certain diseases, especially those that
require leaf wetness and thrive when there’s poor air movement in the
So why not just apply exactly the right amount of nitrogen fertilizer for your crops? If only it was that simple…
N is the problem.
Nitrogen is a tricky nutrient for growers to manage because it’s always
changing. Sometimes it’s in the proteins of organic matter, and
sometimes it gets broken down into simpler ‘available’ forms like
ammonium and nitrate. From there, it may leach out of soils or be taken
up by plants or microbes, where it becomes part of organic matter again.
complicate things further, the total amount of nitrogen in the soil is
changing, too. Besides being lost by leaching, nitrogen can also
evaporate, or volatilize, especially when soils are wet. On the other
hand, nitrogen gets added to soil by nodulated legume roots that can
fix it from the air, or by additions of compost, manure, or fertilizer.
Nitrogen can also build up if soil organic matter levels are increased
by the addition of lots of organic residues over time.
matters. The eventual release of available nitrogen from decaying
plants, compost, manure, or soil organic matter is called
mineralization. Think of this process as soil microbes munching up
those materials up and spitting out any extra nitrogen they don’t need
to build their bodies. The microbes involved are especially active when
the soil is warm and moist, and rather sleepy if it’s cold or dry, so
environmental conditions have a big impact on the rate of
What the PSNT does is measure how much available
nitrogen is in the soil at the time when long-season crops are just
about to ‘take off.’ That number is used that to predict how much more
nitrogen will be mineralized and thus made available to the crop while
it’s growing. The significance of that information is it allows you to
fine-tune your nitrogen fertilizer applications to make sure the crop
gets what it needs without putting on extra N that would be a waste of
money and a potential threat to water quality. By using this test, many
growers will find that soil organic matter and previous applications of
compost, legume cover crops and manure can met some or all of their
crop’s nitrogen needs.
Some history. The
PSNT, also called the June Nitrate Test, was originally developed for
field corn in the mid-80s by Dr. Fred Magdoff at the University of
Vermont. The focus of the test at first was on dairy farms where manure
is frequently applied and legumes like alfalfa and clover are often
included in the cropping system, so the goal was to be able to account
for the available nitrogen that would be provided to the corn from
It is now well established that if the nitrate-N
level in the soil is above 25 ppm when field corn is six to twelve
inches tall, adding more N from fertilizer will not increase yield. Not
surprisingly, it turns out that the same is true for sweet corn: 25 ppm
is the threshold for shutting off any additional N fertilization. As
PSNT results get lower, fertilizer recommendations go higher.
It’s not just corn any more. Additional research has been done with other crops over the past
decade, in particular, long-season vegetables that are typically
sidedressed, such as tomatoes, peppers, fall cabbage, butternut squash
and pumpkins. Although the PSNT works to predict the fertilizer needs
of these crops, too, a slightly higher threshold is usually used to
determine whether or not to apply additional nitrogen. That’s because
corn has a deeper and more extensive root system than most vegetables
and is better able to extract N from the soil. Thus, vegetables with
shallower root systems require a higher level of N in their root zones
to meet their needs.
Taking the sample.
To get good PSNT results you need to send a good sample to the soil
test lab for analysis. For each sample, collect 15 to 20 cores or
slices of soil to a depth of 12 inches, and mix together thoroughly to
form a composite sample. (You may want to purchase a soil corer so you
can take samples quickly; they are available from farm supply companies
for under $50.) Avoid sampling fertilizer bands in the rows or
areas that may have received extra N. The sampled area should be
consistent for past crop, soil types and manure applications. If fields
have significant differences sample them separately.
Preparing the sample.
About one cup of the composite sample should be dried to stabilize the
nitrate. Because microbial activity can rapidly change the
concentration of nitrate in soil samples, it’s important to start
drying the sample right away. Either air dry it by spreading out in a
thin layer on a sheet of plastic overnight (using a fan will reduce
drying time) or by placing on a cookie sheet and heating in an oven at
200 degrees F until dry. Samples can also be dried in a microwave by
spreading the cupful of soil thinly on a plate and microwaving at full
power for 5 to 8 minutes, depending on the moisture content of the soil.
drying, don’t place damp soil samples on absorbent material because it
can absorb some of the nitrate. If the soil samples cannot be dried
right away, keeping them cool, less than 50 degrees F to slow down
microbial activity. Storing warm, moist samples in plastic bags is the
worst – this will promote biological activity and changes in soil
You can skip the drying step if you can deliver
the samples to the soil testing lab within a day, or if the lab you are
working with provides you with special cloth mailer bags for this test.
Timing the sampling. The soil should be sampled about a week to 10
days before you expect to sidedress as this will to allow time to
collect the sample, have it analyzed and receive the results. Typically
this is when sweet corn is 6 to 12 inches tall, or just before pumpkin
vines will start to run. Samples taken too early will not be as
accurate because soil releases nitrate continually in the spring. Soil
test labs typically have a very fast turnaround time for these tests
because they understand that PSNT samples taken just a week or so
before sidedress applications of fertilizer will allow the most
accurate assessment of plant-available nitrogen. You might be able to
sample even closer to the date you want to sidedress but check with
your lab about turn-around time and ability to e-mail you the test
Where to send the sample.
Almost all land-grant university soil test labs in the Northeast offer
the PSNT, or June nitrate test, for about $10 per sample. The
University of Vermont Agricultural and Environmental Testing Lab
offers the test for $8. Samples should be sent to: Room 262, 63
Carrigan Drive, University of Vermont, Burlington, VT 05405-1737.
Using the test results.
Your soil test lab will likely provide you with nitrogen fertilizer
recommendations along with your test results. Most labs have
recommendations for field corn and sweet corn but not all have
developed them for other vegetables. Keep in mind that the best way to
use the PSNT test is to submit samples for several years and keep
records of the results as well as your fertilizer applications, growing
conditions, and crop performance. That way you’ll gain the ability to
interpret the results yourself.
An excellent fact sheet for more information is: Soil Nitrate Testing as a Guide to Nitrogen Management for Vegetable Crops
by Joseph R. Heckman, Ph.D., Specialist in Soil Fertility for
Rutgers Cooperative Extension. That publication includes this table:
PSNT test results: ppm NO3-N Interpretation
Very likely N deficient,
sidedress N is recommended.
be sufficient for some crops.
A low rate of
sidedress N may be applied to ensure that N is sufficient.
Sufficient N is available for most crops. Sidedress
N is usually not recommended.
Greater than 30 Sidedress N is not recommended.
Excessive. Indicates excessive application of
manure, compost, or other sources of N.
Note that in years with
unusually dry spring weather, soil nitrate concentrations typically
will be higher than normal; in years with unusually wet spring weather,
soil nitrate concentrations typically will be much lower than normal.
The only way to know what constitutes the normal range of soil nitrate
concentrations for your soils and N management is to test your fields
for a few years in a row and maintain records of the results.
RETURN TO VERMONT VEGETABLE AND