How the World Really Works: The Science Behind How We Got Here and Where We're Going

by Vaclav Smil

The only way for them to benefit from the nitrogen-fixing abilities of legumes is to rotate them with alfalfa, clovers, or vetches, grow these nitrogen fixers for a few months, and then plow them under so the soils are replenished with reactive nitrogen to be picked up by the succeeding wheat, rice, or potatoes.[18] In traditional agricultures, the only other option to enrich soil nitrogen stores was to collect and apply human and animal wastes.

Between 1900 and the year 2000, the global population increased less than fourfold (3.7 times to be exact) while farmland grew by about 40 percent, but my calculations show that anthropogenic energy subsidies in agriculture increased 90-fold, led by energy embedded in agrochemicals and in fuels directly consumed by machinery.[46]

Between 1800 and 2020, we reduced the labor needed to produce a kilogram of grain by more than 98 percent—and we reduced the share of the country’s population engaged in agriculture by the same large margin.[50]

The nitrogen content of cereal straws (the most abundant crop residue) is always low, usually 0.3–0.6 percent; manure mixed with animal bedding (usually straw) contains only 0.4–0.6 percent; fermented human waste (China’s so-called night soil) has just 1–3 percent; and manures applied to fields rarely contain more than 4 percent.

In contrast, urea, now the world’s dominant solid nitrogenous fertilizer, contains 46 percent nitrogen, ammonium nitrate has 33 percent, and commonly used liquid solutions contain 28–32 percent, at least an order of magnitude more nitrogen-dense than recyclable wastes.[52]

Global inventory of reactive nitrogen shows that six major flows bring the element to the world’s croplands: atmospheric deposition, irrigation water, plowing-under of crop residues, spreading of animal manures, nitrogen left in soil by leguminous crops, and application of synthetic fertilizers.[53]

Between 1961 and 1980 there was a substantial decline in the share of applied nitrogen actually incorporated by crops (from 68 percent to 45 percent), then came a levelling off at around 47 percent.[77]

A nearly perfect solution would be to develop grain or oil crops with the capabilities common to leguminous plants—that is, with their roots hosting bacteria able to convert inert atmospheric nitrogen to nitrates. Plant scientists have been dreaming about this for decades, but no releases of commercial nitrogen-fixing varieties of wheat or rice are coming anytime soon.[80]

our food is partly made not just of oil, but also of coal that was used to produce the coke required for smelting the iron needed for field, transportation, and food processing machinery; of natural gas that serves as both feedstock and fuel for the synthesis of nitrogenous fertilizers; and of the electricity generated by the combustion of fossil fuels that is indispensable for crop processing, taking care of animals, and food and feed storage and preparation.