In April 1935 a dust storm from the Great Plains reached Washington, DC, while a US Senate committee was hearing evidence about soil erosion. Hugh Bennett, who led the new Soil Conservation Service, is said to have pointed out of the window at the darkening sky. The Dust Bowl had stripped topsoil from millions of hectares of ploughed prairie in a decade of drought, and within weeks Congress passed the Soil Conservation Act. Soil that took centuries to build had been lost in seasons.
Now run the film forward. Since the 1960s world cereal yields have roughly tripled, and the number of people has gone from about three billion to eight (the United Nations marked the eight-billionth person in November 2022). Much of that came from fertiliser made with the Haber–Bosch process, which takes nitrogen out of the air. Farming also covers about half of the world's habitable land and produces about a quarter of its greenhouse gas emissions. This is a course about a system with huge successes and huge costs, and a graduate's job is to tell the two apart.
Soil is rock turned into a living material. Vasily Dokuchaev in Russia and, later, Hans Jenny in California described it as the product of five things: climate, organisms, relief (the slope), parent material and time. Jenny's 1941 equation puts it in one line. The point of the five factors is that they act slowly. Under most conditions a centimetre of topsoil takes decades to centuries to form.
Losing it takes far less. Raindrops knock particles loose, and running water carries them away; wind does the same on dry ground. The standard tool for water erosion is the Universal Soil Loss Equation (Wischmeier and Smith, in use since the 1960s and 1970s):
The last two factors are the ones a farmer controls. Keep the ground covered (C) and break up the slope with contour rows or terraces (P), and loss falls by factors of ten. David Montgomery's 2007 review of more than 1,600 measurements found that erosion from conventionally ploughed fields averaged one to two orders of magnitude faster than soil is produced, while no-till fields lost soil at rates close to formation. He estimated the net loss from ploughed fields at around a millimetre a year, enough to strip a typical hillslope soil over spans like the lifetime of a civilisation, and the book that followed, Dirt, argued that this is why some societies declined. Archaeologists disagree about how much of any one collapse soil explains. FAO's 2015 report on the world's soil resources estimated about a third of soils as moderately to highly degraded by erosion, salt, compaction, acidity and pollution.
USLE with K fixed at 0.03 t·ha·h per ha·MJ·mm. The cover factors (bare 1, ploughed maize 0.3, no-till 0.08, grass 0.01) and practice factors (contour 0.6, terraces 0.15) are round illustrative values; real ones depend on crop stage, residue and region. 1 mm of soil over 1 ha is about 13 tonnes at 1.3 t per cubic metre. The model cannot see gullies or tillage erosion.
At the defaults, ploughed maize on an 8% slope loses about 29 tonnes per hectare a year, 2.2 mm, so 20 cm of topsoil would last about 90 years. No-till with contour rows loses about 4.6 tonnes (0.35 mm), enough for about 570 years, and grass loses under 1 tonne. Natural soil formation is usually estimated at under about 0.1 mm a year, or roughly a tonne per hectare, so only the grass case sits near it. Raise the slope to 20% and every number climbs, which is why steep land is terraced in the Andes, in the Philippines, in Yemen and in southern China.
Not all soils are the same story. The Amazon's terra preta, dark earths rich in charcoal and pottery, were built up by Indigenous farmers over centuries and stay fertile today, evidence that people can make soil as well as lose it. Soil organic matter holds carbon, water and nutrients, which is why cover crops, mulch and manure matter.
Plants need nitrogen more than any other nutrient except what the air and water provide, and nitrogen is everywhere: 78% of the atmosphere. But the N2 molecule has a triple bond that almost no organism can break. Bacteria can, including those in the root nodules of beans, peas and clover, and lightning breaks a little. For most of history this set a ceiling on food: farmers rotated legumes, spread manure, mined guano and Chilean nitrate, and populations stayed under the limit.
Fritz Haber showed in the summer of 1909 that ammonia could be made from nitrogen and hydrogen, and Carl Bosch scaled it up at BASF's Oppau plant, which began production in 1913. The reaction runs at high pressure and temperature over an iron catalyst, and the hydrogen comes mostly from natural gas, so ammonia production uses roughly 1–2% of the world's energy. Haber won the Chemistry Nobel in 1918, Bosch in 1931. Haber's other legacy was leading Germany's chemical-weapons programme in the First World War, the first use of chlorine gas at Ypres in 1915. The same chemistry fed and killed.
Erisman and colleagues estimated in 2008 that synthetic nitrogen fertiliser feeds nearly half the world's people, about 48%. The rest of the story is that much of it never reaches a plant. Global assessments put the share of applied nitrogen that ends up in the harvested crop at roughly half; the rest is lost as nitrate in water, ammonia and nitrous oxide in air. Nitrous oxide is a greenhouse gas about 273 times as strong as carbon dioxide per tonne over 100 years (IPCC AR6), and nitrate in rivers feeds the dead zones seen in Ecology (35/48).
Quadratic-plateau response, a standard agronomic form, with invented round parameters: 5 t/ha with no fertiliser (soil supply), plateau 11 t/ha at 200 kg N/ha. Grain is taken as 1.5% nitrogen (15 kg per tonne). Real responses differ by crop, soil, weather and previous crop.
At 100 kg the field yields 9.5 tonnes and each extra kilogram of nitrogen still buys 30 kg of grain. At 150 kg it buys 15. At 200 kg it buys none, and every kilogram above that is surplus: at 250 kg, 85 kg per hectare is left over. With nitrogen costing the equivalent of 5 kg of grain a kilogram, the profit-maximising dose is about 183 kg, a little under the plateau. Farmers who add a safety margin, because a bad year costs more than the extra bag, aren't foolish. This is why policies on fertiliser pricing, timing, precision application and legume rotations work on the slope of this curve. Worldwide the picture is lopsided. Parts of China, India and Europe have applied far more than crops can use, while much of sub-Saharan Africa applies far too little and has the largest yield gaps.
Higher yields should spare land: if you grow twice as much per hectare you need half the land for the same food, leaving the rest for forest. This was Norman Borlaug's argument, and in crude terms it has held (see Plant Biology, 35/53). But economists have long noted a rebound: a more profitable crop can lead farmers to clear more land, which researchers have argued happened with soy and cattle in South America and oil palm in Southeast Asia. Yield gains spare land only if something else (law, price, enforcement, the cost of clearing) holds the frontier.
A second argument is about what kind of farming to put on the land. In 2005 Rhys Green and colleagues set out the choice. Land sharing farms the whole landscape in a wildlife-friendly, lower-yield way. Land sparing farms a smaller area intensively and keeps the rest wild. Which wins depends on how fast a species' numbers fall as yield rises. Make a simple model: the density of a species is 1 on wild land and 1 − yp at relative yield y, and the landscape must produce a fraction P of the maximum. Farming a fraction P/y of the land at yield y leaves total species density:
A teaching model from the Green et al. (2005) set-up, with a made-up density curve. Yield is limited to at least P (all land farmed). Real species differ: some thrive on farms, others vanish at the first plough.
With half the food needed and p = 0.5 (species fall quickly at first), the best landscape farms half the land at full yield and spares the other half: species density 0.50, against 0.29 when the whole landscape is farmed gently. Change p to 2 and the answer reverses: sharing gives 0.75 and sparing 0.50. Field studies point both ways. Phalan, Balmford and colleagues (2011) found that most of the species they counted in Ghana and India did better under sparing, but other places, and other groups such as birds on traditional farms, favour sharing, and the sparing argument needs the spared land actually to be protected.
The biggest lever on food's footprint is what is on the plate. The most-cited data set is Joseph Poore and Thomas Nemecek's 2018 analysis in Science, which pulled together data from about 38,700 commercial farms in 119 countries and around 40 products. They estimated that food produces about 26% of global greenhouse gas emissions, with livestock and fisheries contributing about 31% of food emissions. Animal products, they found, use about 83% of farmland while providing about 18% of calories and 37% of protein. About 80% of agricultural land, counting grazing and feed crops, serves meat and dairy; only about half of the world's cereal goes directly to people, while about 41% goes to animal feed.
| Food (average) | kg CO2e per kg | m² of land per kg |
|---|---|---|
| Beef (beef herd) | 99.5 | 326 |
| Lamb and mutton | 39.7 | 370 |
| Cheese | 23.9 | 88 |
| Pork | 12.3 | 17 |
| Poultry | 9.9 | 12 |
| Eggs | 4.7 | 6.3 |
| Rice | 4.5 | 2.8 |
| Tofu | 3.2 | 3.5 |
| Peas | 1.0 | 7.5 |
| Wheat and rye | 1.6 | 3.9 |
| Potatoes | 0.5 | 0.9 |
Poore and Nemecek (2018), via Our World in Data, reference year 2010. Global averages: the same food varies widely between producers, and the paper stresses that how a food is produced can matter as much as which food it is.
Three honest complications. First, the unit changes the ranking: per kilogram, per calorie and per gram of protein give different orders for foods like nuts, rice and pulses. Second, the average hides the system: beef from deforested Amazon pasture and beef from dairy herds on Irish grassland are very different products, and Poore and Nemecek's table lists a separate figure for beef from dairy herds (33 kg CO2e per kg). Third, grazing land is often unsuitable for crops, so a defender of pasture-based livestock can fairly argue that removing the animals does not turn that land into farmland, and defenders of herding economies in the Sahel, the Andes and Mongolia point out that livestock is a livelihood there, not a luxury. The reply is that most of the land in the data is spent on a small fraction of food. Methane is also treated differently by different accounting methods, and the 100-year metric used here is itself debated by scientists. Poore and Nemecek's headline was that a global shift away from animal products could cut food's land use by about three-quarters and its emissions by about half; whether and how much people should change is a values question that this piece does not decide.
The diets are invented round examples, not surveys; the other foods follow the preset. Emissions and land use per kg are the Poore and Nemecek averages in the table above (and for other foods in the same data set); processing, transport and waste are inside those figures only partly, and the figures are 2010 global means.
With the "mixed" preset, the year's food comes to about 1.9 tonnes of CO2e and about 4,800 m² of land. The "meat at most meals" diet is about 3.5 tonnes and 10,000 m², and the vegan one about 0.5 tonnes and 960 m². Move only the beef slider and you will find that it dominates. The list can mislead, so remember the scale: these are model averages, and what a real person emits depends on where and how their food is grown. Cutting waste helps too: the UN Environment Programme's Food Waste Index 2024 put waste at around a billion tonnes in 2022, roughly a fifth of food available to consumers, and Poore and Nemecek estimate waste as about a quarter of food emissions.
"Organic" is a set of production rules (no synthetic fertilisers or most synthetic pesticides) and not a measure of health or environmental impact. The evidence is mixed because the claims are about different things.
The case for. Organic fields hold more soil carbon and support more species on the farm. A 2012 meta-analysis (Tuomisto and colleagues) found about 30% higher species richness on organic farms. Organic systems that rely on legumes and manure avoid the energy cost of Haber–Bosch and lower the risk of pesticide pollution in water.
The case against. Yields are lower. Seufert, Ramankutty and Foley (2012) found organic yields about 25% below conventional on average, with the gap anywhere from 5% to 34% depending on crop and conditions. A 2015 re-analysis by Ponisio and colleagues put it at about 19%, shrinking to about 8–9% with rotations and multi-cropping. Lower yields mean more land for the same food, and because land conversion releases carbon, the per-kilogram emissions advantage shrinks or reverses for some crops. Without synthetic nitrogen, supplying enough nutrients is the limit.
The middle. Modelling by Muller and colleagues (2017) suggested that a fully organic world could feed 9 billion people in 2050, but only if food waste fell by half and animals ate far less crop feed, which is a diet change by another name. Many agronomists favour options that borrow from both camps: legume rotations, integrated pest management, precision fertiliser and cover crops.
Arithmetic: land needed = 1 + share × (1 ÷ (1 − gap) − 1), for the same total output and no change in diet or waste. The range 5–34% is from Seufert et al. (2012); 19% is the Ponisio et al. (2015) average.
At a 19% gap, converting all cropland to organic needs about 24% more land for the same output; converting 30% needs about 7% more. Whether that is a good trade depends on what the extra land would have been, which is a judgement about biodiversity, carbon and food prices, not a number from a model.
Genetic modification inserts one or a few genes into a crop. The two big commercial traits are insect resistance (the Bt gene from a soil bacterium) and herbicide tolerance. The science and the politics have moved apart.
What the evidence says about safety. The US National Academies' 2016 report on genetically engineered crops found no substantiated evidence that approved GE foods are less safe to eat than conventional ones, and major scientific bodies worldwide have said much the same, while stressing that each new product should be assessed on its own traits, not on the technique. The public is far less convinced than scientists.
What the evidence says about farms. A 2014 meta-analysis (Klümper and Qaim) found that GM adoption cut chemical pesticide use by about 37%, raised yields by about 22% and farmer profits by about 68% on average, with the largest gains in developing countries; yield gains were bigger for insect-resistant crops (about 25%) than for herbicide-tolerant ones (about 9%). Critics point to results that vary by place and year, to the use of some non-peer-reviewed studies, to the evolution of resistance (glyphosate-resistant weeds, and pests that adapt to Bt), and to seed prices and corporate concentration. India's Bt cotton raised yields and cut insecticide sprays in the years after its 2002 approval, though how much and for whom has been disputed.
Where regulation differs. The European Union authorises GM crops case by case and grows almost none, while the United States, Brazil, Argentina, Canada and India grow large areas (India only cotton). Nigeria approved a pest-resistant Bt cowpea in 2019. Golden Rice, engineered to supply beta-carotene against vitamin A deficiency, was approved for cultivation in the Philippines in 2021, but in April 2024 a Philippine court of appeals halted its commercial propagation and that of a Bt eggplant on safety and legal grounds, and the case has gone on appeal. Newer gene-editing techniques such as CRISPR raise the same questions in new legal forms.
Most of the textbook examples come from the temperate North. Farmers have developed other answers, adapted to local water, soil and labour.
| Place | System | What it shows |
|---|---|---|
| Africa | Niger and Burkina Faso: farmer-managed natural regeneration and zaï planting pits | Farmers have restored degraded drylands by protecting tree stumps that resprout, and by digging pits that catch rain and manure; reported to have re-greened millions of hectares in Niger. |
| Asia | Hani rice terraces (Yunnan, China, UNESCO 2013); paddy rice-fish culture | Water, fish, ducks and rice in one managed landscape; terraces turn steep ground into level fields. |
| South America | Andean terraces and potato diversity; Amazonian terra preta | Altitude zones, thousands of potato varieties and soils built by people. |
| North America | The Haudenosaunee "three sisters" (maize, beans, squash); the mechanised Great Plains | Companion planting where beans fix nitrogen for maize, and the opposite scale: huge farms, big machines, ploughing lessons from the Dust Bowl. |
| Central America | Milpa; the chinampas of the Valley of Mexico | Mixed maize fields, and raised garden beds on lake shallows still farmed at Xochimilco. |
| Europe | Iberian dehesa and montado; Dutch greenhouses | Oak-pasture agroforestry for pigs, cattle and cork; and the opposite: high-tech, high-yield protected horticulture. |
| Oceania | Kuk Swamp (Papua New Guinea, UNESCO 2008); Aboriginal fire management | Drained garden plots dating back thousands of years, among the world's earliest agriculture sites; land managed by fire, not the plough. |
| Antarctica | No farming; a test greenhouse (EDEN-ISS) at the German Neumayer III station | Food grown with no soil, in a closed system, to practise for places where soil does not exist. |
The common thread is that every system is a bundle of choices about water, nutrients and labour, tuned over generations. The modern argument is less "tradition against technology" than which parts of each bundle fit which landscape and market.
Food is soil, nitrogen and land, in that order of hidden importance. Soil is a slow stock; nitrogen is a fast and leaky flow; land is where the two meet wildlife. Most debates in the field are about which trade-off to accept, so check which claim a speaker is making and what it is measured per. This describes how the system works, not what anyone should eat or farm.
Try this: pick one food you ate today and trace it back: where it grew, what fed it (fertiliser, manure, feed), and how far it travelled. Then look up its number in the table and ask what the average hides.
Sources. Soil-forming factors (Jenny, 1941); the USLE (Wischmeier and Smith); Montgomery, "Soil erosion and agricultural sustainability", PNAS 2007; FAO Status of the World's Soil Resources (2015); Haber and Bosch dates and Nobel years; Erisman and colleagues, Nature Geoscience 2008; IPCC AR6 for the nitrous oxide warming potential; Green and colleagues, Science 2005, and Phalan and colleagues, Science 2011; Poore and Nemecek, Science 2018, with per-kilogram figures from Our World in Data (reference year 2010); Seufert and colleagues, Nature 2012; Ponisio and colleagues, 2015; Tuomisto and colleagues, 2012; Muller and colleagues, 2017; National Academies 2016; Klümper and Qaim, PLoS ONE 2014; Philippine Court of Appeals rulings on Golden Rice (April and August 2024); UNEP Food Waste Index 2024. Regional farming-system examples and some dates are given in outline and should be checked against UNESCO and FAO pages before being quoted. The Dust Bowl anecdote is the usual telling and details vary between accounts. All interactive numbers were computed in node; the models are illustrative.