Fertilizer Effects

Increased fertilization of crops was a major force behind agricultural intensification and the Green Revolution. It was only after the synthesis of ammonia from atmospheric nitrogen was discovered in 1909 by Fritz Haber, and scaled up to industrial production in 1913 by Carl Bosch that nitrogen could be produced as urea and various nitrate forms of fertilizer.40 This did not happen at a global level until after two world wars and many years of agronomic research had determined what rates of fertilizer to apply for optimal yields. Nitrogenous fertilizer has been analogized to fossil fuel where food is to nitrogen as energy is to carbon.41 Plant growth rates had always been limited by nitrogen availability as natural sources of nitrogen, biofixation by bacteria symbiotic with legumes, atmospheric deposition and recycling of plant residues and manure, could only supply half of the global demand for



1961 1965 1969 1973 1977 1981 1985 1989 1993 1997 2001 Year

Figure 9.2 World consumption of total and nitrogenous fertilizers in agriculture.

Values adjacent to each trace indicate the fold increases in usage between 1961 and 2002.43

nitrogen.42 The ready availability of nitrogen to crops through supplied fertilizer beginning in the late 1940s and 1950s is what drove the tremendous increases in crop productivity thereafter. The demand also increased for the other two plant macronutrients, phosphorous and potassium, and was met by the mining of rock deposits containing phosphates and potassium. However, the rate of consumption of nitrogenous fertilizers has increased 7.3-fold between 1961 and 2002, compared to a 4.5-fold increase for total fertilizers (see Figure 9.2).43

Enhanced nitrogen content of plants has been shown to positively influence growth and development of immature insects.11'44 Higher nitrogen content in plant tissue often improves assimilation efficiency leading to greater accumulation of body tissue in insects.45 Although faster growth rates do not always occur with enhanced nitrogen content in plant tissue,46 in most cases laboratory performance measures do improve while population growth rates in the field also are accelerated.47-50 Some of the benefit to insects that feed on nitrogen-fertilized plants may result more from accumulation of foliar biomass in a vigorous plant than plant nitrogen concentration.50 Phloem-feeding insects often respond favorably to higher nitrogen content, in part because they are not exposed to secondary plant compounds, such as alkaloids and tannins, to the same degree as plant-chewing insects and can therefore more readily utilize nitrogen-containing compounds.44 Various studies have shown positive effects of nitrogen fertilizer added to host plants of B. tabaci.51-54 In the Sudan Gezira, substantially higher densities of B. tabaci occurred consistently over three field seasons in nitrogenous fertilized cotton compared to unfertilized cotton.55 Collectively, these studies all point to the potential benefit to B. tabaci populations that nitrogenous fertilization provided to the Sudan Gezira and subsequently to many other locations worldwide where outbreaks of B. tabaci have occurred.

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