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It is also clear that some parts of the world use far too much fertilizer, and could reduce this without any drop in productivity. In China many growers use double or more the necessary fertilizer on their rice, cotton, or vegetable crops, causing chronic nitrogen pollution problems as a result. Studies show that parts of China are so overloaded with fertilizer that inputs could be cut in half with no reduction in crop yields.28 It is also likely that farmers in both Europe and the U.S. could use less fertilizer, though to a lesser extent. On the other hand, growers in Africa face a serious shortage of fertilizer. If African farm productivity is to be raised, and poverty and malnutrition reduced, then substantially more reactive nitrogen needs to be supplied to replenish soils degraded by decades of overuse. Africa missed out on the Green Revolution of the 1970s and 1980s, when yields tripled in the developing world as a whole thanks to high-yielding crop varieties and large quantities of additional fertilizer.29 Soil health will not be restored by nitrate chemicals on their own; animal manures and crop residues also need to be dug back into the soil rather than being removed for cooking fuel. Farmers also need access to markets and to credit—bringing the Green Revolution to Africa will involve a whole host of intertwined antipoverty strategies. But more fertilizer is an essential part of the mix, so globally the potential to reduce the amount of nitrogen used in agriculture by humanity is limited.

I realize that this runs counter to the accepted Green wisdom that organic farming—which eschews artificial fertilizers—is always best for the environment. Certainly organic agriculture, if practiced globally, would dramatically reduce nitrogen pollution from currently cultivated areas. But it would also leave millions starving, thanks to a major reduction in the food produced per unit of land. This conclusion emerges logically from looking at nutrient flows: with less nitrogen added, crops produce less protein in their seeds and less biomass overall, so yields go down. African agricultural productivity is very low precisely because it is stuck in organic subsistence farming and lacks additional nitrogen. Certainly this nutrient deficiency can be reduced by assiduous recycling, from both human and animal wastes. Until recently, China had a thriving “night soil” industry, which collected sewage and spread it straight back onto the fields. Sewage sludge is also spread on farmers’ fields today as a matter of course. But none of this adds nitrogen overall—it simply recycles it more efficiently.

A look back at history confirms this picture. In 1900 world agriculture was entirely organic and fertilizer-free, and could sustain 1.6 billion people on about 850 million hectares of cultivated land. According to the Canadian scholar Vaclav Smil, the same agronomic practices extended to today’s 1.5 billion hectares of plowed farmland would feed only 2.9 billion people. The numbers could be improved somewhat if people in rich countries agreed to surrender some of their overconsumption to poorer countries, and everyone ate less meat and dairy. (Feeding grain to livestock and then consuming meat and milk is less efficient than just eating the grain directly.) But according to Smil the basic conclusion is stark: “Only half as many people as are alive today could be supplied by prefertilizer agriculture with very basic, overwhelmingly vegetarian, diets.”30 That is not an appealing prospect.

This is not to say that organic farming is bad, wrong, or necessarily backward-looking. In better-off countries with no immediate scarcity of land, it may be an important way to meet ecological objectives, and to provide choice for consumers who prefer not to have any chemicals used in growing their food. But the key reason why free-choice organic agriculture (as opposed to organic agriculture practiced unwillingly by subsistence farmers who cannot afford chemical inputs) will always be a marginal activity globally is because it uses much more