Developing and Using Nitrogen-Rich Strips.
Departments
of Plant & Soil Sciences and Biosystems and Ag
Engineering
Division of
Agricultural Sciences & Natural Resources
PT 2003-7 April 2003 Vol.15, No.7
INTRODUCTION.
Nitrogen-Rich
Strips replace the use of yield goals as a basis for making fertilizer-N
recommendations.
A newly developed nitrogen (N) management strategy addresses
two conditions that greatly affect how much fertilizer N a crop will need. They are
1.
Point-to-point
variability within a field (spatial variability);
2.
Year-to-year
variability over time (temporal variability).
Spatial variability has been the focus of ‘precision
ag’ research for the past decade and has led to development of variable rate
fertilizer applicators. Temporal
variability, was largely ignored until scientists evaluated and described it in
2000, for winter wheat production in
Grain
yield and nitrogen from non-fertilizer sources greatly affect year-to-year
fertilizer need. Most N from
non-fertilizer sources is believed to come from release of soil organic-N just
before and during the growing season.
This N is also called mineralized-N.
An N-Rich Strip is used during the growing season to estimate potential
yield for that field, that year. The
amount of mineralized N supporting the yield potential is gauged by comparing
the field, in general, to the condition of the N-Rich Strip.
Maximum wheat yields vary greatly
from year-to-year, but mineralized-N changes even more. What’s more, the two are unrelated. That is, when a field has higher than average
yield it doesn’t necessarily have higher than average mineralized-N. Consequently, sometimes mineralized-N
accounts for most of the N used by the crop (figure below). These surprising findings by scientists led
them to look for a way to help farmers better manage fertilizer N inputs.
QUESTIONS
ABOUT NITROGEN-RICH STRIPS
What is an
N-Rich Strip? Use of mineralized-N by the crop can be estimated only
if the crop has not already received fertilizer-N to meet its entire N
requirement. Thus, the new strategy is
to apply little or no fertilizer-N preplant, or with the seed, except for a
spreader width the length of the field that receives enough preplant (or early
season) fertilizer-N that the crop will not be limited by lack of N. This spreader width application is the N-Rich
Strip for that field.
Where should
the N-Rich Strip be in the field? The N-Rich Strip should be located through a representative
part of the field. If possible, it is
also useful to place the strip where it will be convenient (after appropriate
marking) to periodically observe whether it looks different from the rest of
the field. The strip should be
re-fertilized each year.
How is the
N-Rich Strip compared to the rest of the field?
The N-Rich Strip, and an
adjacent, similar part of the field that did not receive as much fertilizer,
are each individually “read” using an optical, hand-held GreenSeeker
sensor. The sensor, in the on-position,
is held about 38
inches above the crop canopy while walking about 100 paces to collect an
average reading.
When is the
N-Rich Strip read? The N-Rich Strip is read just before topdressing and
will identify the fertilizer rate to use. Earlier than “normal” topdressing is indicated
when wheat in the N-Rich Strip appears to be in significantly better condition
than the rest of the field. Sensor
readings and decisions about early topdressing should then be made. When the N-Rich Strip looks the same as the
rest of the field, and sensor readings confirm there is no difference, later
sensor readings may be taken to confirm no fertilizer is needed, or identify
that a small amount of fertilizer may be beneficial.
How is wheat
for pasture managed differently? The major difference is that wheat being managed for
pasture is more likely to respond to a low rate (20 to 40 lb N/acre) of
fertilizer. An N-Rich Strip should still
be established to help determine later topdressing needs.
What exactly
do the sensor readings tell us? Sensor readings, together with the number of days the
crop has been growing, are used to estimate grain yield. The yield without topdress-N, how much N
should be topdressed, and the potential yield with topdressing are all
calculated.
What is the
value of using the N-Rich Strip for making fertilizer recommendations? Research
shows an average profit of $10 to $20/acre using the N-Rich Strip and sensor
readings to determine fertilizer rates compared to using yield goals. The increased profit is related to the fact
that yield goals (averages) are easy to calculate, but are only experienced
about 1/3 of the time. Using the N-Rich
Strip and sensor calculations increase the chance of using the correct rate
each year, and only apply N when it is needed so nitrogen use efficiency is
about doubled.
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Cooperative Extension work, acts of May 8 and