Agriculture Reference
In-Depth Information
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Functions of Nitrogen
in Crop Plants
1.1 INTRODUCTION
Nitrogen (N) is one of the most abundant elements in nature. A large amount of N is present in
the atmosphere, in the lithosphere, and in the hydrosphere. Its role as an essential plant nutrient
is indisputable. N is one of the most yield-limiting nutrients in crop production worldwide. The
increased use of fertilizer has been a major factor explaining perhaps one-third to one-half of the
yield growth in developing countries since the
green revolution
(Bruinsma, 2003; Fischer et al.,
2009). Developing countries now account for 68% of the total fertilizer use (Fischer et al., 2009). Its
use has continued to increase by 3.6% per year over the past decade, which would still account for
a significant share of yield growth. Using a measure of agricultural area standardized for land qual-
ity, the amount of fertilizer used per irrigated equivalent hectare is also now higher in developing
countries than in industrial countries (Fischer et al., 2009).
Globally, fertilizer use has plateaued due to a decline in its use in industrial countries and a dra-
matic fall in the countries of the former Soviet Union after those countries moved toward a market
economy. In developing countries, the increase in fertilizer use has been surprisingly consistent
across most regions. Asia still has the highest and the fastest increase, but fertilizer use intensity is
comparable in Latin America and the Middle East/North Africa too. However, fertilizer use per ha
in sub-Saharan Africa is abysmally low due to reasons such as high prices and poor markets, which
have been well documented (Morris et al., 2007). Low fertilizer use explains in large part the lag-
ging productivity growth in that region (Fischer et al., 2009).
Sainju (2013) reported that a major nutrient required in ample amount to sustain crop yield and
quality is N. N along with water is the key to the realization of the yield potential of modern crop
cultivars or genotypes. In the absence of N inputs, modern crop cultivars yield little more than tra-
ditional counterparts. N was the key component of increasing the yield of most annual crops at an
unprecedented rate, mainly that of rice, corn, barley, and wheat during the 1960s, when a term
green
revolution
(1960-1980) was used. A dwarf gene was incorporated into the modern cultivars of these
crops to reduce the height. This made sure that these cultivars did not fall down with the addition
of high rate of fertilizers, especially N. Frey (1971) reported that modern high-yielding crop culti-
vars have been bred to respond to the high fertilizer rates (particularly N) necessary to support the
increased demands of cropping systems. As a result, agricultural production is more dependent than
ever before on heavy fertilization (Radin and Lynch, 1994). These newly developed cultivars were
also having high yield potential and were responsive to a higher rate of N. In addition, the impor-
tance of N has increased in the recent years due to the high energy requirement for the production
of chemical fertilizers containing N. As long as energy costs were low, N was cheap and there were
no concerted efforts to increase the efficiency of its use by crop plants or to seek alternate sources of
supply. The high cost of energy today changed this situation and increased attention has been given
to the efficient use of N in crop production.
The deficiency of N is reported in most crops worldwide (Fageria, 2009, 2013, 2014; Fageria
et al., 2011a). Figure 1.1 shows the response of lowland rice genotypes to N fertilization grown
on a Brazilian Inceptsol. Similarly, Figure 1.2 shows the linear increase in dry matter yield of
three lowland rice genotypes. Figure 1.3 shows the improvement in grain yield, shoot dry weight,
and the number of pods with the application of N to dry bean grown on a Brazilian Oxisol. The
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