126-131 Smil.p65

Von Liebig noted in his most famous book that agriculture’s principal objective is the production of digestible N (1). This task was particularly challenging in all traditional (pre-industrial) agricultures. They had 3 ways in which to provide N for crops: i) recycling of organic wastes (mainly crop residues and animal and human wastes); ii) crop rotations including N-fixing leguminous species; ii) and planting of leguminous cover crops (alfalfa, vetches, clovers) that were plowed under as green manures. Even the most intensive use of these practices in areas whose climate allowed year-round cropping could not supply more than 120–150 kg N ha yr (2). Nationwide means were, naturally, much lower. Buck’s surveys indicate that in early 20-century China, N applications from organic recycling and leguminous crops averaged only about 50 kg N ha, and the resulting yields could support about only 5.5 people ha on an overwhelmingly vegetarian diet (3). In contrast, applications of inorganic N fertilizers in today’s China average nearly 200 kg N ha. In the most intensively cultivated provinces, where rice is double-cropped, they surpass 400 kg N ha, and the country now supports more than 10 people ha on a diet whose total food energy content is almost the same as in Japan and that contains more than twice as much animal protein than it did just 20 years ago (2, 4). Borlaug summed up the role that N fertilizer played in this grand agricultural transformation by using a memorable kinetic analogy: “If the highyielding dwarf wheat and rice varieties are the catalysts that have ignited the Green Revolution, then chemical fertilizer is the fuel that has powered its forward thrust...” (5).