One of the greatest changes in agriculture has been the use of inorganic fertilizers, which expanded dramatically after World War II in response to the demand for increased agricultural output. In the developed countries, large processing plants were built to manufacture nitrogenous fertilizers and convert imported rock phosphate into a variety of water-soluble and partially water-soluble phosphorus fertilizer products. Basic slag, a by-product from the steel industry, also became widely used in the manufacturing of phosphorus fertilizer. In the United States, the use of inorganic phosphorus fertilizer rose rapidly in the 1940s and 1950s, but has been relatively constant since 1960. The rate of use of inorganic nitrogen fertilizer, on the other hand, continued to rise rapidly until the early 1980s (Figure 5-4). This relative gain in nitrogen use over phosphorus use resulted primarily from favorable crop yield responses, especially corn, to nitrogen fertilizers.

Over the last 30 years, agricultural production systems in the United States have become more specialized and concentrated. During this time, overall agricultural production has more than doubled (Evans et al. 1996), and is occurring on less agricultural land and on fewer but larger farms (Evans et al. 1996). Since 1950, U.S. farmland has decreased from 1,200 to 970 million acres (20 percent) and the number of farms has dropped from 5.6 to 2.1 million (63 percent), while average farm size has increased from 213 to 469 acres (120 percent).

In many states, animal feeding operations (AFOs) are now a major source of agricultural income. The rapid growth of the animal industry in certain areas of the United States has been coupled with an intensification of operations. For example, current census information shows an 18 percent increase in the numbers of hogs in the United States over the last 10 years, at the same time as a 72 percent decrease in numbers of hog farms. Over the same 10 years, the number of dairy farms decreased 40 percent, but herd size increased 50 percent. A similar intensification of the poultry and beef industries has also occurred, with 97 percent of poultry production in the United States coming from operations with more than 100,000 birds and over a third of beef production coming from just under 2 percent of the feedlots (Gardner 1998). Driving this intensification is an increased demand for animal products and improved profitability because of advances in transportation, processing, and marketing. But animal feeding operations pose significant challenges with the management of wastes produced.

Prior to World War II, farming communities tended to be self-sufficient, in that enough feed was produced locally to meet animal requirements

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