Agriculture Reference
In-Depth Information
NUTRIENT DEFICIENCIES OTHER
THAN N
rapidly transformed in soils and, when in the
NO
3
-N form, is highly mobile. Nitrogen fertilizer
is also relatively expensive because of the high
energy costs associated with its manufacture.
Because N is critical for the production of cereal
grains, farmers may resort to one of two extreme
N application strategies: (i) apply N fertilizer in
excess when suffi cient fertilizer is available and
cost is not excessive or (ii) apply little or none if
only limited amounts of N are available and costs
are high. In either case, there is no inexpensive
scientifi cally based device or management system
available for use by farmers to determine nitrogen
fertilizer rates. Examining the evolving science
and technology for managing N fertilizer and how
it can be delivered to a range of environments and
production systems reveals that midseason sensing
technologies will likely play a role. This is espe-
cially evident in Fig. 10.1, where the mean yields
from the fertilized plots (112 kg N ha
−1
) varied
from 1,500 to 6,000 kg ha
−1
. The mean yield of
the fertilized plots and unfertilized plots was
2,862 ± 860 and 1,720 ± 517 kg ha
−1
, respectively,
over this 35-year period. Using these numbers,
the range in grain produced per kilogram of N
applied was 13.4-53.5 kg. The common rule for
winter wheat (2 lb N per bushel of wheat you
hope to grow) translates to 30.0 kg grain per kilo-
gram of N. Clearly, the demand for N is a func-
tion of yield potential, and that is known to change
considerably by year and by fi eld.
Focusing on the ability to predict grain yield
midway through the growing season will become
increasingly more important as we strive to match
N fertilizer needs with fi nal removal (Raun et al.,
2001). Before proceeding, is it possible to predict
yield potential in the middle of the growing
season? Early work by Raun et al. (2001) showed
that winter wheat grain yield potential could be
predicted using multiple NDVI sensor readings
collected midseason. This same approach was
later modifi ed to use one NDVI sensor reading,
adjusted for the number of days from planting to
sensing (Raun et al., 2005). The in-season esti-
mated yield (INSEY) was successful in predicting
yield potential when evaluated across 30 locations
and 6 years. Other researchers have attempted to
predict wheat yield potential, but the procedures
Where N is available, a costly mistake that can be
made is applying this precious resource when it
is not needed, or when yield response is limited
by something else. On-farm trials that clearly
target yield response as a result of the application
of one factor while holding all others in nonlimit-
ing amounts is an especially useful research and
extension tool. Elimination of other confounding
effects, including soil pH, soil test P, K, S, and/or
micronutrients can be easily detected from com-
prehensive laboratory analyses. Submitting a
surface (0-15 or 0-30 cm) soil sample for routine
and/or comprehensive analysis is always recom-
mended to eliminate the possible needs and/or
correction of a defi ciency or toxicity of another
element. Soil testing to a certain extent is very
much a scalable technology that historically
has
been
underutilized,
and
as
such,
underappreciated.
The experimental method requires that we do
our best to remove the infl uence of all possible
factors, excluding the variable of interest. Thus,
a N-rate experiment on P-defi cient soils requires
that we apply a suffi cient rate of P so that P is not
limiting. In this same light, we cannot recom-
mend a N application in a low-vigor area of the
fi eld that may actually be P-defi cient. The low-
vigor area could also be low in soil organic matter,
or it may have a shallow A-horizon, or low pH,
or one of a host of other yield constraints. Simi-
larly, in portions of the fi eld that exhibit high
vigor without some kind of reference, it would be
diffi cult to discern the maximum level of vigor if
N was moderately limiting at that end of the spec-
trum. The only way to know is to have a reference
strip for N response.
PREDICTION OF YIELD POTENTIAL
Nitrogen fertilizer is critical for plant growth and
grain yield of cereal crops, of which maize, wheat,
and rice are the principal crops grown in the
world. It has been very diffi cult to optimize the
application of nitrogen fertilizer because it is
