The pre-sidedress soil nitrate
test (PSNT)
for Western Oregon and Western Washington
EM 8650
Reprinted August 1997
E.S. Marx, N.W. Christensen, J. Hart, M. Gangwer, C.G. Cogger,
and A.I. Bary
Do you want to print a copy?
We recommend downloading the print version (110 K).
First download the free Adobe
Reader, if you don't already have it.
Contents
What is PSNT?
Who can benefit?
Why use PSNT?
Preplant N rates
When to test
How much N?
Too much N
Soil sampling
Tbl 1. N fertilization rates
Fig 1. N cycle
Fig 2. Corn N uptake
What is the Pre-sidedress
Soil Nitrate Test (PSNT)?
The Pre-sidedress Soil Nitrate Test (PSNT)
evaluates nitrogen fertilizer needs for field corn. The PSNT
can help ensure maximum yields without excessive nitrogen fertilizer
applications. The benefits are reduced fertilizer purchases and
reduced risk of environmental contamination.
Who can benefit
from the PSNT?
Anyone producing corn for feed can potentially
benefit from the PSNT. Growers who apply manure to their fields
are especially likely to benefit. The organic matter in manure
contributes nitrogen that is not well accounted for by other
soil test methods.
top
Why does the PSNT
tell more about nitrogen availability than early spring soil
tests?
Soil tests performed in early spring do not
tell how much nitrogen might be available later in the growing
season. Most soil nitrogen is held in organic matter and is unavailable
to growing plants. As the soil warms up, increased biological
activity converts organic nitrogen into plant-available inorganic
forms (Figure 1). An early spring soil test does not take this
nitrogen into account. Thus, if you base fertilization on an
early spring test, you might apply too much fertilizer.
top
The PSNT measures soil nitrate (NO3 - -N)
during the growing season, after some nitrogen has become plant-available
and just prior to the crop's period of greatest need.
How much N should
I apply before planting?
If needed, apply no more than 40 lb N/acre
as a starter in spring. Most fields with a history of manure
applications require no starter N fertilizer. If starter N is
needed, it can be supplied by manure or commercial fertilizers.
Delay as much N fertilization as possible
until midseason. If the midseason PSNT shows low N levels, make
a sidedress fertilizer application.
top
When is the best
time to measure soil nitrate?
Corn plants begin their greatest uptake of
nitrogen around the 5-leaf stage, or when the plants are about
12 inches tall at the center of the whorl. The best time to evaluate
soil nitrogen is just prior to this period of rapid uptake (Figure
2).
top
How much N is
enough?
A PSNT value of 25 ppm or greater is sufficient
for maximum silage corn yields. If the PSNT value is below 25
ppm, apply nitrogen at the rates shown in Table 1.
Nitrogen can be supplied by manure or commercial
fertilizer. Most dairies produce enough manure to supply needed
N, making fertilizer purchases unnecessary. See EM 8585, Manure
Application Rates for Forage Production, for more information.
top
What if I apply
more N than the crop needs?
Excess nitrogen will not be used by the crop
and will remain in the soil after harvest. During winter, rain
may wash nitrate from the soil into surface and ground waters.
This poses a threat to human and environmental health and wastes
fertilizer dollars.
top
Prepared
by E.S. Marx, former research assistant in soil science; Neil
W. Christensen, professor of crop and soil science; John Hart,
Extension soil science specialist; Mike Gangwer, Extension agent,
Marion County; Oregon State University; C.G. Cogger, Extension
soil scientist; and A.I. Bary, scientific assistant; Washington
State University.
Reprinted August 1997.
|
