Functions of Nitrogen in Crop Plants
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 signicant share of yield growth. Using a measure of agricultural area standardized for land quality, 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 dramatic 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 lagging productivity growth in that region (Fischer et al., 2009).