Agriculture Reference
In-Depth Information
in mid-2008 due to high fertilizer demand and the inability of manufacturers to increase production
levels (Huang et al., 2008). Furthermore, inorganic N fertilizers make up >20% of the operating
expenses in maize production in Canada (Tollenaar, 1989), and it is imperative that the NUE be
improved to increase the net income for growers. Similarly, N is one of the most limiting nutrients
for cereal production in many West African countries (Pandey et al., 2001).
N deficiency is also responsible for the low yield of most food crops in the African continent.
For example, corn is an important crop for the smallholder farmers in sub-Saharan Africa, but the
yield has not increased significantly and per capita food production has declined since the 1980s
(Greenland et al., 1994; Muchena et al., 2005). The main contributing factors are poor inherent soil
fertility, particularly N and P deficiencies (Bekunda et al., 1997) exacerbated by soil fertility deple-
tion (Vlek, 1993; Lynam et al., 1998), and other biophysical factors (Kaizzi et al., 2012a).
By 2050, the world's population is likely to be 9.1 billion, the CO
2
concentration 550 ppm, the
ozone concentration 60 ppb, and the climate warmer by an average of 1.8°C (Jaggard et al., 2010). The
global demand for crops is projected to increase 100-110% from 2005 to 2050, resulting in the expan-
sion and intensification of agricultural land, with greater N inputs to increase yields (Tilman et al.,
2011; Wilson et al., 2013). At the same time, there is added pressure on agricultural lands to produce
energy, for example, U.S. energy legislation requires that 136 billion L of biofuel be used per year by
2022, 60 billion of which must be produced from nonfood feedstocks (Wilson et al., 2013). In addition,
in the future, the demand of N will certainly increase because of an increasing demand by the world's
population for food and fiber and its superb role in increasing crop yields. Today, commercial fertilizer
N supplies approximately 45% of the total N input for global food production, and the world use is
approximately 100 million metric tons (FAO, 2010). It is projected that annual total global N use will
grow to approximately 112 million metric tons in 2020, and approximately 171 million metric tons
in 2050 (Ladha et al., 2012). Half of the world's food production comes from fertilizer. Stewart et al.
(2005) reported that, at the global scale, crop yields have increased by at least 30-50% as a result of
fertilization. Furthermore, the use of adequate amount of N fertilizers in cropping systems is part of
the solution to world food security. Philips and Norton (2012) reported that global wheat production
has increased over two and a half times since 1960 to 2010, as a result of better farming practices,
improved cultivars, and balanced nutrition. At the same time, fertilizer use in all agriculture has risen
4.3 times to keep up with the growing food demand. It is estimated that growers use around 15% of
the fertilizer consumed to produce the current 650 million metric ton of wheat (Philips and Norton,
2012). These authors further reported that increase in fertilizer use mirrors the gains in productivity,
although to maintain production it will require continual review of nutrient inputs. The challenge will
be to ensure that future growth in food production is met by careful and targeted use of fertilizers.
Looking into the importance of fertilizers and N in particular in crop production, the objective of this
introductory chapter is to provide an overview of the functions of N in crop plants.
1.2 FUNCTIONS
Nitrogen plays an important role in the growth and development of plants. Its functions in the plants
are extensive. N is mainly responsible for the growth and development of morphological traits in
the cereals and legumes. In addition, N is also responsible for many physiological and biochemical
functions in plants. The important biochemical functions of N include an essential constituent of
enzymes, chlorophyll, nucleic acids, storage proteons, cell walls, and a vast array of other cellular
components (Harper, 1994). Most of these reactions are responsible for improving the growth and
development of crop plants. N is essential in the structure and function of amino acids, amides,
nucleotides, nucleic acids, pigments, and some hormones (Hull and Liu, 2005; Bauer et al., 2012).
The
NO
3
−
ion undergoes transformation after it is absorbed and is reduced to the amine form (
NH
2
−
). It is then utilized to form amino acids. Twenty amino acids are precursors of polypeptide chains
comprising all protein (Bennett, 1993). Two other amino acids, glycine and glutamate, are precur-
sors of N bases. Amino acids are essential to protein formation and are considered its building
