Using energy: food, materials, mobility and buildings: Food: nitrogen, ammonia and fossil-fuelled farms

2.1 Crops are short of nitrogen even though air is mostly nitrogen

Every protein, every strand of DNA and every chlorophyll molecule contains nitrogen. Crops need more of it than any other nutrient they take from the soil, and in most fields it is the first thing to run out. The puzzle is that 78% of the air is nitrogen. The catch is chemical. Air nitrogen exists as N2, two atoms held together by a triple bond, one of the strongest bonds in nature. Plants cannot break it. They can only use nitrogen that has already been 'fixed' into ammonium or nitrate.

Nature fixes nitrogen in small amounts. Lightning makes a little. Most comes from bacteria, especially those living in the root nodules of legumes such as beans, peas, lentils, clover and alfalfa. Traditional farmers worked with this limit for thousands of years. They spread manure and human waste, ploughed in crop residues and rotated grain with legumes. Smil shows that these methods recycle nitrogen well but cannot supply enough of it to support high yields for large and growing populations.

By the late nineteenth century, Europe was searching hard for new sources. It mined guano, the dried droppings of seabirds, from Peruvian islands. It imported sodium nitrate from Chile's Atacama desert. It recovered some ammonia from coke ovens. It tried energy-hungry electric arcs and calcium cyanamide. All were limited, expensive or both. In 1898 the chemist William Crookes warned that the wheat-eating world would face shortages within a generation unless chemists learned to fix nitrogen from the air.

The lesson for energy is that food production was capped not by sunlight or land alone but by a single chemical bottleneck. Breaking it would take a lot of energy, delivered at high pressure and temperature.

• Smil, Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production (2001) · via Smil Mastery Course, M10 'Nitrogen: the nutrient that limits harvests'

2.2 Haber-Bosch turns natural gas and air into ammonia at a cost of about 28 to 41 GJ a tonne

In 1909 Fritz Haber showed in his laboratory that nitrogen and hydrogen could be combined into ammonia (NH₃) over a catalyst at about 200 atmospheres and 500 degrees Celsius, with unreacted gas recycled round a loop. Carl Bosch at BASF then did the harder part. He built steel reactors that could survive hot hydrogen at high pressure, and his colleague Alwin Mittasch found a cheap iron catalyst after thousands of trials. The first plant, at Oppau, opened in 1913. During the First World War its output went into explosives. Smil calls the synthesis of ammonia the most important technical invention of the twentieth century, because world population could not have grown from 1.6 billion in 1900 to over 6 billion by 2000 without it.

The energy logic is simple. Nitrogen comes free from the air. Hydrogen must be made, and today most of it comes from natural gas through steam reforming, or from coal in China. Compressors then squeeze the gases to high pressure. Most of the energy that goes into ammonia is in the hydrogen.

After 1950, natural gas feedstock, large single-train plants with centrifugal compressors, and better catalysts more than halved energy use per tonne. The IEA's Ammonia Technology Roadmap puts the best gas-fed plants at about 28 GJ per tonne of ammonia, not far above the theoretical minimum, and the best coal-based plants at about 36 GJ. The world average, dragged up by older and coal-based plants, is about 41 GJ. In 2020 the world made about 185 million tonnes of ammonia, and making it took around 2% of global final energy, about 8.6 EJ. Roughly 70% of the ammonia goes into fertilisers.

Most ammonia becomes fertiliser. In India it is mainly converted to urea, a white solid containing 46% nitrogen that is easy to store, ship and spread. The process also produces pure CO₂ as a by-product, which is used to make the urea itself.

• Smil, Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production (2001) · via Smil Mastery Course, M10 'Haber and Bosch: the most important invention'
• Smil, How the World Really Works (2022) · via Smil Mastery Course, M10 'Haber and Bosch: the most important invention'
• IEA, Ammonia Technology Roadmap: Towards More Sustainable Nitrogen Fertiliser Production, International Energy Agency, 2021 · web

2.3 A modern meal embeds fossil fuel, measured best in spoonfuls of diesel

Smil likes to translate the energy inside food into everyday volumes of diesel fuel. Farming uses energy directly as diesel for tractors and combines, electricity for irrigation pumps and fuel for drying grain. It uses even more indirectly, in making fertilisers (above all ammonia), pesticides, machinery and greenhouses, and then in processing, refrigeration and transport.

The numbers are surprising. Growing wheat on the American Great Plains takes about 100 millilitres of diesel equivalent per kilogram of grain. Growing, milling and baking a one-kilogram sourdough loaf needs at least 250 millilitres, about a cup. A kilogram of roasted chicken needs at least 300 to 350 millilitres, almost half a wine bottle, because the bird must eat about three kilograms of feed for each kilogram of edible meat. Tomatoes grown in heated greenhouses and shipped across Europe can embody around 650 millilitres per kilogram. Wild seafood averages about 700 millilitres per kilogram, and wild shrimp or lobster can exceed 10 litres.

There is a trade-off. Traditional farming returned 10 to 20 times the energy invested, but all that investment was human and animal muscle plus sunlight, and it fed few people per hectare. Modern farming often puts in more fossil energy than the food energy it delivers, yet it frees most of the workforce from the fields and feeds far more people per hectare.

The dependence is deep. Smil estimates that without synthetic nitrogen, 40 to 50% of today's people could not be fed. And it is leaky. Crops take up only about half of the nitrogen applied. The rest washes into water as nitrate, escapes as ammonia, or leaves as nitrous oxide, a potent greenhouse gas. Better timing, precision application and less food waste can cut both the energy and the losses.

• Smil, How the World Really Works (2022) · via Smil Mastery Course, M10 'Eating fossil fuels: energy in modern farming'
• Smil, Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production (2001) · via Smil Mastery Course, M10 'Haber and Bosch: the most important invention'

See it in the table

Check your understanding

Why can plants not use the nitrogen in air directly?

In a gas-based ammonia plant, where does most of the energy go?

Why does a kilogram of chicken embed more fossil energy than a kilogram of bread?