Agriculture Reference
In-Depth Information
consumes less than 1.5% of the world fertilizer
N. The US consumed 10,878,330 t of total fertil-
izer N in 2005, or 13% of the world total, with
less than 5% of the world population. However,
in either case, farmers likely are inaccurately and
imprecisely estimating the actual nitrogen demand
of the crop.
Malakoff (1998) estimated that excess N fl owing
down the Mississippi River was valued at over
$750 million. With increased N prices, that value
now exceeds $1.0 billion per year. This becomes
increasingly important considering that SSA
spent only $706 million on fertilizer N in 2005 for
88,435,068 ha of cereal production, while the US
spent $5.8 billion on N fertilizer for over
56,404,000 ha of cereal production. While cereal
farmers are making some effort to improve upon
their fertilizer use effi ciency, it is disturbing to
note that the excesses from fertilizer N loss that
end up in the Mississippi River each year exceed
the total amount of N fertilizer applied for cereal
production in SSA.
The underlying message is that existing
technologies for optimizing the use of nitrogen
fertilizers can be increased in all agricultural envi-
ronments. The consequence has been either gross
overapplication of N in regions with high levels
of mechanization and abundant supplies of N fer-
tilizer or gross underapplication in regions of
limited N supply and small farms. The failure of
existing technologies to determine optimal N
application rates has been a compelling incentive
to develop robust, simple, and fl exible methods
for determining N application rates to correct N
defi ciencies.
is determined using chemical analysis of surface-
and subsurface-soils for NH
4
-N and NO
3
-N
levels. If these surface and subsurface levels of
inorganic N are high, the demand for N can be
small (Ferguson et al., 1991, 2002). However,
increased amounts of inorganic N are normally
associated with an increased risk of NO
3
-N leach-
ing (Andraski et al., 2000). Despite the ability to
detect excesses in inorganic N in soil profi les, soil
testing in US wheat production is not routine. As
a result, future broad adoption of soil testing for
N in the developed or developing world is not
expected, nor is it expected to be a future solution
of N management in wheat. In this regard, the
identifi cation of any technology that improves
upon poor N-use effi ciency present today will
help in the long term.
Two fundamental reasons for the shortfall in
using soil chemical analysis to determine N levels
is the high mobility of nitrate N and the variation
in available nitrate N as a consequence of changes
in the soil environment from year to year. This
problem is further compounded by variable avail-
ability of N throughout a given fi eld. The former
is termed
temporal variability
and the latter
spatial
variability
.
Importance of spatial variability on
N requirement
The applied question of whether or not N should
be applied can be answered by applying a N-rich
strip (NRS) in each fi eld and each year. The NRS
is essentially a rate of preplant fertilizer that will
ensure no N defi ciency is encountered in the crop
throughout the growth cycle. Preplant fertilizer is
by far the most ineffi cient method of applying N
and that should be discouraged. Any methodol-
ogy that works to partition the total amount of N
to be applied will provide improved NUE versus
preplant N applications. The NRS, as it is applied
in a portion or strip of the fi eld as the name
implies, serves simply as a guide for midseason N
determination. Having the NRS in no way
replaces the need to apply a modest rate of pre-
plant N.
As was reported by Ortiz-Monasterio and Raun
(2007), many farmers worldwide have historically
IS NITROGEN NEEDED
The fundamental question that must be answered
for all nutrients is, “Is it needed?” The appropri-
ate application rate is consequential to determin-
ing whether or not added amounts of the nutrient
in question are needed. While simple and seem-
ingly straightforward, this question is not always
properly addressed, regardless of productivity
level of a particular operation. With current
approaches to soil testing, the decision to apply N
