Long-Fallow Swidden Agriculture

Early agriculture interspersed a few years of crop production with long intervening fallow periods. Abundant land and low population densities allowed communities to clear forest for a few years of cropping, until soil fertility and yields declined; then they cleared new land and planted crops elsewhere. Farmers returned only after 20 years or more, when they could once again unlock nutrients built up in forest biomass by burning (Pyne 1997). Because swidden farmers relied on fire to clear land, anthropologists called it “slash and burn” farming, a derogatory term for what they considered primitive and wasteful land use (Stewart 1956). The agroecosystems of farm communities practicing swidden agriculture centered on moving Cropland and Horticulture through the landscape. Nutrient transfers between intensive and extensive land-use compartments using livestock, for example, were absent. The removal of nutrients with crops and nutrient recycling by burning biomass, which enriched soils with potassium and phosphorus, was the extent of human-mediated nutrient flows. Fires instantaneously unlocked nutrients in woody vegetation; then cultivation and harvest depleted nutrients from the soil in short order. As long as ample land was available, swidden produced abundant, if short-term, harvests with minimal labor.

Swidden clearance of arable land by burning is typically associated with Indigenous and tropical agriculture, but examples existed in European farming as well. When Russians pushed southward in the eighteenth century into Ukraine’s great steppe grasslands, for example, they practiced a form of swidden with 20- to 30-year rest intervals (Moon Reference Moon2013a: 19). Likewise, in 1750s Galicia, Spanish farmers supplemented permanent cropland by occasionally slashing, burning, and cultivating rough brushland on a 30- to 50-year cycle (Corbacho and Padró Reference Corbacho González and Roc Padró2021). But long-fallow swidden was a thing of the past in most of Europe by the Middle Ages.

Short-Fallow European Agriculture

The alternative to moving cropland through an uncultivated landscape was to create permanent cropland whose annual rotations moved fallow through arable land. The most widespread European systems were biannual and triannual rotations, with fallow every second or third year (Gingrich et al. Reference Gingrich, Haidvogl, Krausmann, Preis and García-Ruiz2015; Hoffmann Reference Hoffmann2014). Another alternative to this approach was several years of natural fallow followed by a few years of cropland, such as the “Egart” system in alpine Austria. The classic three-field rotation included two different annual crops plus a year of natural fallow that revolved through cropland (Cunfer and Krausmann 2009). The short fallowing period served various purposes, including pest control and food diversification, as well as fertility restoration (Sieferle and Müller-Herold Reference Sieferle and Müller-Herold1996). In traditional open-field systems, such as those that persisted in Austria until the late eighteenth century, farmers used natural fallow as communal livestock grazing.

In contrast to swidden, short-fallow systems required farmers to direct a flow of nutrients toward Cropland and Horticulture, as fallow alone could not restore all the nutrients depleted by harvests. The trade-off was permanent cultivation, with stable but lower yields. Interspersing a fallow year between one or two crop years did not replenish all the nutrients lost in harvests. Small annual surpluses of nitrogen and phosphorus accumulated during the fallow period, mainly from natural processes such as atmospheric deposition (Güldner Reference Güldner2021). Tillage of fallow land built fertility for subsequent harvests as microorganisms decomposed organic matter and slowly released nutrients. The replenishment of soil nutrients facilitated by fallow was not sustainable in the long run. Long-term agriculture thus hinged on additional nutrients transferred from extensive land-use compartments using manure, decomposition, or burning of collected biomass.

Livestock propelled the transfer of nutrients throughout the agricultural landscape, subsidizing Cropland and Horticulture with nutrients originating in outlying parts of the agroecosystem. In some places, livestock ranged widely day and night, eating grasses, forbs, and brush, and deposited their manure throughout the landscape, where it was unavailable for Cropland and Horticulture. In eighteenth-century Austria, for example, peasants whose cattle roamed freely barely produced enough manure to support cropland needs and struggled to feed their livestock (ibid.). A similar manure shortage occurred in eighteenth-century Galicia, Spain. Extensive grazing in hilly brushland supported large numbers of livestock, but it was impossible to collect their manure for cropland fertilization (Corbacho and Padró Reference Clifford2021).

Alternatively, herding livestock into confined shelters at night (“stabling”) enabled manure collection for field application, effectively transferring nutrients from Rough Grazing to Cropland and Horticulture. In the nineteenth century, Galician farmers continued to pasture livestock during the day, but began to stable them at night in the village, where they collected manure to fertilize nearby cropland. Yields, labor costs, and population all rose, but over time Rough Grazing land suffered a decline in soil nutrients (ibid.).

A yet more labor-intensive form of transferring nutrients, evident in dry Mediterranean areas that could not support high livestock densities, depended on composting and burning. Farmers transferred vegetation from Woodland and Brushland to Cropland and Horticulture for composting or burning to release nutrients (Olarieta et al. Reference Olarieta, Padrò, Masip, Rodríguez-Ochoa and Tello2011; Tello et al. Reference Tello, Garrabou, Cussó, Olarieta and Galán2012). In Catalonia, Spain, farmers dug hormigueros, where they hauled brush into vineyards, buried it, and burned it underground, unlocking potassium and phosphorus (Galán Reference Galán del Castillo2021).

The configuration of European landscapes into nutrient sources and sinks was not arbitrary. Communities reserved the richest land for Cropland and Horticulture. Where sufficient rainfall allowed, meadows and pastures were an adaptation to characteristics of the local environment, including topography, soil structure, climate, and water availability. Their relative extent reflected the land cost for a rural society to sustain its livestock for draft power and manure production. Soil nutrient sustainability rested on extensively used Rough Grazing, Hay Meadows, and Woodland and Brushland that replenished nutrient shortfalls on intensively used Cropland and Horticulture (Güldner and Krausmann 2017). With short fallow, a ratio emerged between cropland on fertile soils and outlying marginal land unsuitable for cropping. While yields were stable, these systems faced hard productivity limits, and a nitrogen bottleneck prohibited further intensification (Allen 2008).

The ratio between nutrient inputs and outputs in various land-use categories became unbalanced when demographic pressure or market opportunities encouraged higher crop production. Then land-use types serving as nutrient sources experienced significant depletion that compromised the long-term sustainability of the entire system. Furthermore, farmers’ techniques to manage recycling and boost transfer pathways were complex and often inefficient. Nitrogen, for example, is highly volatile and prone to gaseous losses during manure storage and application. The availability of land and its stockpiled nitrogen were the most limiting constraints on agricultural intensification and the greatest sustainability challenge for agricultural systems reliant on short-fallowing (García-Ruiz et al. Reference García-Ruiz, Gonzalez de Molina, Guzman, Soto and Infante-Amate2012; Tello et al. Reference Tello, Garrabou, Cussó, Olarieta and Galán2012).

Intensification: Advanced Organic Agriculture

German agronomist Nepomuk von Schwerz (Reference Von Schwerz1836) compared preindustrial agriculture to a machine, “where one gearwheel continuously interlocks with the other, but the main driver is livestock and, therefore, the production of forage crops.” His notion of agriculture as an “organic machine” fits an analysis of soil nutrient flows across coupled landscape compartments and invokes the spirit of industrialization, which presented farmers with new challenges and opportunities. Eighteenth- and nineteenth-century agronomists were aware of the shortcomings of nutrient transfer and recycling pathways, complaining about manure shortages and nitrogen scarcity in short-fallow farm systems. These worries grew from a concern about food shortages for growing populations. Their solution was simple: increase fertility by funneling forage crops on fallow through livestock stabled year-round to collect their manure. Under the rational banner of the Enlightenment, liberal governments and universities generated innovations that, alongside liberal land-tenure reforms, saw farm communities across Europe transition to an “advanced organic economy” that intensified production (Overton Reference Overton1996; Wrigley 2006).

Von Schwerz was at the cusp of an agricultural revolution that turned remaining European feudal states into advanced organic economies. Allen (Reference Allen1992) has proposed that in England there were two consecutive transitions, the first led by the yeomen and the second by landlords who took over the agricultural innovations developed by the former. The literature presents the English system as a forerunner of advanced organic farming that subsequently spread across the continent. In central and northern Europe, however, farmers were already intensifying fallow with cover crops in response to population pressure and market opportunities (Slicher van Bath Reference Slicher van Bath, Rich and Wilson1978). Besides, England’s agricultural revolution coincided with the influx of Flemish and Dutch immigrants in the late sixteenth and early seventeenth centuries who brought such techniques with them. Advanced organic systems coexisted in continental Europe from the sixteenth until the nineteenth century. In Galicia and Catalonia, Spain, annual farming spread in the seventeenth century, followed by Austria in the eighteenth. Differences in advanced organic agriculture in these places arose from biophysical resource endowments that varied widely. Raising forage for livestock required adequate water availability, so in drier places farmers rarely found long-term success. Instead, they adopted other strategies better suited for semiarid climates (Galán Reference Galán del Castillo2021).

Europe’s agricultural revolution into advanced organic economies intensified using only biological techniques, without external energy subsidies or synthetic fertilizers. Agroecosystem diversification with new varieties of forage cover crops, green manure, and an intensification of livestock production were the main gearwheels to increase nutrient recycling and transfer flows. These strategies provided more manure and organic amendments, improving nitrogen supply to cropland soils (Allen Reference Allen2000, Reference Allen2008; Gingrich et al. Reference Gingrich, Marco, Aguilera, Padro, Cattaneo, Cunfer, Guzman, MacFadyen and Watson2018). Increasing the efficiency and intensity of nutrient recycling and transfers depended on increased labor in the context of growing rural populations. At the same time, crop diversification and livestock intensification presented unforeseen challenges and created new sustainability problems.

Farmers in abundantly watered England adopted “high farming” that replaced the old natural fallow with forage crops like clover or turnips that served as either livestock feed or green manure. No longer was a third or half of arable land unproductive each year. The English Norfolk four-course rotation is an often-cited example of “seeded fallow,” comprising one or two “break crops” interspersed between summer and winter cereals (Overton Reference Overton1996). Seeded fallow in four- to six-year rotation cycles prevailed across Europe in the early twentieth century, and current organic farming systems still use these sequences.

Cover crops were legumes (clover, alfalfa, and pulses like peas or vetch), root crops, (turnips, beets, or potatoes) or corn. One part of this new land-use strategy resulted from the “Columbian Exchange” that introduced new crops from the Americas (Crosby Reference Crosby1972). Most important were corn and potatoes, which in Europe provided livestock forage long before they became a staple in human diets. Legumes, however, were traditional Old World crops now enjoying a renaissance. Farmers long understood that growing legumes improved yields. Roman farms, for example, rotated legumes such as field beans and vetches with grain to improve yields (Brevik Reference Brevik and Verheye2005; White Reference White1970). Livestock may have been the physically largest gearwheel driving nutrient flows in the organic machine, but equally important were microscopic lifeforms. Long unknown to science, fungi, bacteria, and a myriad of other tiny creatures enriched the earth by powering decomposition (Winiwarter Reference Winiwarter and Unger2013). In fact, the mass of microbiota populating the soil weighs more than twice as much as the aboveground livestock grazing on any given patch of grass (Dash and Dash Reference Dash and Dash2009). These tiny creatures were crucial for the decomposition of organic residues, such as roots and stubble, turning them into soil nutrients available for next year’s crops.

Some bacteria naturally fix atmospheric nitrogen. One variety, called rhizobia, formed a symbiosis with legumes. These microscopic, organic power plants in root nodules made legumes invaluable. They improved cropland nitrogen replenishment and created a positive feedback loop in advanced organic agriculture. A small portion of nitrogen derived from the atmosphere went directly into the soil through “rhizodeposition,” but most of it remained in the root nodules and bodies of the plants until livestock consumed them or they decomposed (García-Ruiz et al. Reference García-Ruiz, Gonzalez de Molina, Guzman, Soto and Infante-Amate2012). Because the largest part of the newly fixed atmospheric nitrogen resided in legumes’ above-ground plant matter, famers utilized these crops as forage to increase livestock production and, subsequently, the manure supply. Farmers fed nitrogen-rich legumes, such as clover, alfalfa, or field beans, to livestock who converted it to manure. Thus, nitrogen originating in the atmosphere moved through legumes before livestock recycled it as manure, which then fertilized Cropland and Horticulture. Root crops also found their way into the new rotations. Through a similar soil-to-plant-to-livestock recycling pathway, deep-rooted crops such as turnips and potatoes appropriated nutrients from deep soil layers that cereals rarely reached. Nutrients in root crops returned through livestock via manure and onto cropland, steadily increasing the soil’s content of plant-available nitrogen and phosphorus for later cereal crops (ibid.).

This legume and root-crop intensification strategy came without the additional “land costs” of nutrient transfers from outlying Rough Grazing, Hay Meadows, or Woodland and Brushland. Long before the twentieth-century revolution in synthetic fertilizers, this groundbreaking agricultural innovation lifted the limitations on farm systems that had been constrained by finite land availability for nutrient transfers and recycling. The ability of legumes to fix nitrogen from the atmosphere and of root crops to draw nutrients upward from deep underground broke through agriculture’s rigid land-use constraints. Legume and root crop cultivation on a small fraction of cropland mobilized considerably more nitrogen than the old nutrient transfers from outfields could manage, even with large labor inputs. Legumes effectively multiplied the land area of a farm. In Austria, for example, legumes contributed 30 kg of new nitrogen per hectare to cropland soils. Legumes grown on one hectare of cropland provided as much nitrogen as manure-based transfers from 13.5 hectares of Rough Grazing, or from 4 hectares of Hay Meadows (Güldner Reference Güldner2021). Legumes created a remarkable boost in available soil fertility with no need for additional land or labor.

In adopting new crops, rotations, and livestock densities, farmers restructured their traditional landscape. Europe’s advanced organic economies expanded and intensified Hay Meadow cultivation and reduced the extent of Rough Grazing. Advanced organic farmers decreased the number of extensively grazed livestock and relocated them to stables near cropland for intensive feeding and efficient manure collection. In some cases, intensive forage production and livestock stabling increased manure supply even as the number of livestock declined (Corbacho and Padró Reference Corbacho González and Roc Padró2021; Güldner Reference Güldner2021). Liberal land reforms reduced common land through enclosure, fallow area declined, and communities converted open fields to private management. The ability to access soil nutrients from the atmosphere and from deep underground transformed agricultural landscapes and restructured farm communities from top to bottom.

Figures 2 and 3 present variations of advanced organic agriculture in Europe. Figure 2 models rural Austria in the early nineteenth century (Güldner Reference Güldner2021). It highlights the nutrient transfers from Woodland and Brushland, Hay Meadows, and Rough Grazing to Cropland and Horticulture. Abundant rainfall in Central Europe allowed a diversity of techniques for accessing and transferring nutrients, including internal recycling within Cropland and Horticulture. Figure 3, in contrast, presents the advanced organic farm system in Catalonia, Spain around 1920 (Galán Reference Galán del Castillo2021). Legume cultivation, both as a source of nitrogen and as livestock forage, played an important role here, contributing a substantial amount of nitrogen inputs into cropland. However, Catalonia’s dry climate meant that Hay Meadows were virtually nonexistent, closing off one nutrient pathway that was important in Austria. Similarly, burning as a means to quickly recycle nutrients was crucial to Catalan nutrient management, but rarely employed in Austria.

Figure 2. A model of agroecosystem soil nutrient flows for Austria’s advanced organic farm system in the early nineteenth century.

Figure 3. A model of agroecosystem soil nutrient flows for Catalonia’s advanced organic farm system in the early twentieth century.

By unlocking new nutrients, farmers created a positive synergy of livestock-keeping and forage production, tightening the gears of the organic machine. Soon, however, the revved up organic machine faced different sustainability challenges. Once the nitrogen constraint on crop growth eased, other nutrients became limiting. Higher crop yields meant farmers mined phosphorus and potassium at faster rates, exporting some of them to external markets. On Austria’s manorial estates, more legumes increased nitrogen availability but also caused phosphorus depletion (Güldner and Krausmann 2017). Unlike with nitrogen, farmers could not replenish phosphorus from the atmosphere. Natural mineralization of the land’s bedrock deep underground was the only source of additional phosphorus, and it could not keep up with rising demand (Frossard et al. Reference Frossard, Bünemann, Jansa, Oberson and Feller2009; Güldner and Krausmann 2017). Livestock consumed phosphorus in forage crops and recycled it via manure into the soil, but some phosphorous departed the system each year through exported grain, and total soil phosphorus declined. Similar processes were evident in Spain’s Basque region (Olarieta et al. Reference Olarieta, Besga and Aizpurua2019). After decades of advanced organic agriculture had surpassed the old nitrogen constraints, many European agroecosystems encountered phosphorus limitations for the first time.

Extensification: New World Land Colonization

Another important strategy for accessing scarce soil nutrients was the colonization of new land. Practiced since time immemorial, converting forest, grassland, or swamp into arable land provided access to additional soil fertility for growing, expanding, or colonizing populations. The scale of land colonization might be small—a few extra furrows plowed into adjacent pasture—or large—sodbusting 45 million hectares of the North American Great Plains—but the fundamental objective was the same: to tap into stockpiled soil nutrients in new land (Cunfer Reference Cunfer2005).

Land colonization varied depending on local ecosystems. In many places it initially entailed deforestation: back-breaking labor to kill, cut, remove, or burn trees, grub out their roots, then plow cleared land (Cronon Reference Cronon1983). The job could take decades, as slash slowly decomposed and subterranean roots snarled plowshares (Donahue Reference Donahue2004). Fire might speed the processes and deposit a quick dose of ash fertilizer, but many pioneers planted their first crops among the standing or felled trunks of large trees resistant to surface fires (Pyne Reference Pyne1982, 1997). Years might pass before fields were clear and easily cultivated.

In other places colonization of new arable land first required drainage, damming, canal-digging, and diking to remove water from wetlands and then to keep it out (Stewart Reference Stewart1996). Drained wetlands could be immensely rich in nutrients, but making them dry enough to cultivate required considerable labor and constant maintenance to prevent flooding (Amato et al. Reference Amato, Janet and Amato2001; Stunden-Bower Reference Stunden-Bower2011). In the Netherlands, for example, the Dutch expanded cropland into the ocean from the thirteenth century, diking, draining, and eventually planting some 700,000 hectares of land formerly inundated with sea water. Across the English Channel, farm communities drained and “reclaimed” the fens on England’s eastern coast from the seventeenth century.

Between the 1780s and 1920s, new land colonization focused on grasslands (Cunfer et al. 2018; Moon Reference Moon2013b). Grassland sod is difficult to plow, but it covers some of the world’s richest soils. In general, grassland soils are deeper and more fertile than forest soils, and their global extent is enormous. Connected especially with European colonization, agricultural settlers undertook a massive plow-up of grasslands in the long nineteenth century. Displacing Indigenous people, European farmers brought livestock draught power (oxen, horses, and mules), iron and steel technology (moldboard plows), and a rapidly increasing population to the project. In just more than a century, they opened millions of hectares of new cropland in the Russian and Ukrainian steppes (Moon Reference Moon2013a), the Argentine pampas (Adelman Reference Adelman1994; Scobie Reference Scobie1964), the North American Great Plains (Cunfer Reference Cunfer2005), and in Australia and New Zealand (Brooking and Pawson Reference Brooking and Eric2011). It was the largest and fastest conquest of new arable land in human history. The crops grown on those rich soils fed the world’s growing population (up 60 percent during the century) with grain and meat, and delivered fiber, hides, and lubricating oils to industrializing cities (Clifford Reference Clifford2021).

Boserup (Reference Boserup1965) argued that increasing population pressured communities to find new resources, but emerging market opportunities also drove expansion into new lands. A few examples illustrate the varied character of this ubiquitous soil appropriation strategy. Medieval Europe’s “Great Clearances” provide a starting point. Following the collapse of the Roman Empire, Western Europe’s population first stagnated, then began long-term growth in the ninth century that peaked in the fourteenth. Between 850 and 1350 Europeans cleared forest for new arable land to feed ever more mouths (Hoffmann Reference Hoffmann2014; Le Roy Ladurie Reference Le Roy Ladurie1974). In the modern era, Enlightenment-inspired liberal land reforms broke up feudal estates and freed land for small-holder expansion (Carmo and Domingos, 2021; Infante-Amate et al. Reference Infante-Amate, Villa, Jimenez, Martinez Martin, Lopez, Cunfer and Gonzalez de Molina2016). In the Americas, democratic ideology drove state land distribution policies begun by Thomas Jefferson’s 1795 Northwest Ordinances, the pinnacle of which were the 1862 Homestead Act in the United States and the 1872 Dominion Lands Act in Canada (Robbins Reference Robbins1942). A century later Brazil reprised those settlement schemes, opening the Amazon for rainforest clearance and agricultural settlement by hundreds of thousands of impoverished citizens (Pereira Reference Pereira2003).

Key to new land colonization was the richness of previously uncultivated soils. Land never farmed, or unfarmed for centuries, contained soil nutrient stockpiles built up over time. Plant litter—leaves, stems, branches, dried grass, and all manner of wetland sedges, reeds, and forbs—dropped annually onto the ground. There they decomposed, sometimes burned, or sank into shallow mud, depositing nutrients into soils year after year, decade upon decade. When farmers cleared such land, plowed the soil, and planted seeds on fresh ground, they gained access to centuries of accumulated soil fertility, now diverted from native plants toward human-selected crops. Yields on freshly plowed ground were phenomenal—quadruple or more those on old cropland. Bumper crops in the early years were key incentives for land clearance. In some places—tropical rainforests are notorious—productivity lasted only a few years. In other places stockpiled soil nutrients sustained high crop production with limited fertilization for 50 years or more. Farmers colonizing colonial New England in the seventeenth century (Donahue Reference Donahue2004) and 200 years later in the Great Plains (Cunfer Reference Cunfer2004, Reference Cunfer2021) benefited from the natural supply of nitrogen for decades before yields declined and labor-intensive nutrient management became necessary.

While yields may have been spectacular on newly colonized land, they could not persist. Every frontier began with impressive production that fell away. Simply cultivating soils released nitrogen into the atmosphere. Rainfall leached nutrients below the plowline and erosion channeled them into rivers and lakes. Harvests took away nutrients annually—after all, that was the goal. Year by year, yields declined. Either farmers switched to the laborious and endless process of restoring soil nutrients, or they moved on, searching for fresh land just over the horizon or perhaps across the ocean.

Whether driven by hunger, greed, ideology, or opportunity, people incrementally added to cultivated land area over millennia, sometimes in long-term ebb-and-flow cycles, other times in great bursts of expansion. Whether they cleared forests, plowed grasslands, or drained wetlands, the objective was the same: farmers converted natural landscapes into managed agroecosystems. They tapped soil nutrients that had previously sustained natural vegetation. Now those nutrients flowed into human-managed pathways, through selected crops, domesticated animals, people, and eventually into human economies. New land colonization was one of the most important ways people acquired access to the essential soil nutrients at the base of human livelihood and wealth.

The Advent of Industrial Agriculture

By the late nineteenth century in Catalonia and Galicia, Spain, as well as on Austria’s manorial estates, phosphorous shortages emerged on Cropland and Horticulture. Rough Grazing and Hay Meadows developed shortages of both phosphorus and potassium. It was a tipping point for agriculture, requiring the introduction of soil nutrients derived from outside farm systems at ever-increasing energy costs. With artificial fertilizers, farmers could relieve phosphorus and potassium bottlenecks and further boost soil nitrogen. Early artificial fertilizers were a consequence of the Industrial Revolution, political liberalization, and mechanized sea and land transport. Factories supplied phosphorus fertilizer as industrial byproducts, such as bone meal from slaughterhouses or Thomas Slag and superphosphate from steel mills (Mazoyer and Roudart Reference Mazoyer and Roudart2006). Guano, the accumulated excrement dropped by Pacific Ocean birds on the coast of Peru, was an organic source of phosphorus and nitrogen that arrived in Europe and North America through global trade routes (Cushman Reference Cushman2013). Once farmers solved phosphorus and potassium deficiencies, nitrogen again became the limiting nutrient. Rich nitrate deposits in Chile’s Atacama Desert also flowed across the ocean. To increase yields to satisfy markets in industrializing cities, farmers tapped into global nitrogen, phosphorus, and potassium reserves through the international fertilizer industry.

Early in the twentieth century, German chemist Fritz Haber and BASF industrial engineer Carl Bosch brought agriculture fully into the industrial age when they developed a process to synthesize ammonia (Smil 2001). BASF opened its first plant in 1913, and by the mid-twentieth century the Haber–Bosch process flooded farms with inexpensive, abundant nitrogen fertilizer. Ammonia synthesis depends on fossil fuels, primarily natural gas, to fix atmospheric nitrogen. Haber–Bosch synthesis operates under very high temperatures (2,000 degrees C) and pressures (100 atmospheres), conditions that require large energy inputs. Natural gas not only provides the energy (heat and pressure) but it also provides the hydrogen for ammonia. By the end of the twentieth century, nitrogen fertilizer production had doubled the amount of plant-available nitrogen in the world, utterly transforming the global nitrogen cycle and feeding increased cropland productivity worldwide (Galloway et al. Reference Galloway, Dentener, Capone, Boyer, Howarth, Seitzinger, Asner, Cleveland, Green, Holland, Karl, Michaels, Porter, Townsend and Vöosmarty2004). For thousands of years agriculture had been an energy supplier to society; now it became an energy consumer through its reliance on synthetic nitrogen (Gingrich et al. Reference Gingrich, Marco, Aguilera, Padro, Cattaneo, Cunfer, Guzman, MacFadyen and Watson2018).

North American Great Plains farmers broadly adopted synthetic nitrogen fertilizers in the 1950s, with stunning results (Cunfer Reference Cunfer2004). In Nemaha County, Kansas, for example, corn, hay, and oat yields had fluctuated between 1.5 and 3 tons per hectare from 1880 to 1950 (Cunfer 2021). Thereafter they quadrupled, from 2 tons per hectare in 1950 to 8 tons in 1997. New hybrid seed varieties were capable of exploiting higher levels of soil nutrients, and crop productivity accelerated across the plains (Anderson Reference Anderson2009: 53–54). In Rooks County, Kansas, wheat yields rose from 1.5 to 5 tons per hectare in the second half of the twentieth century. With a boost from irrigation, farmers in Pawnee County, Kansas did even better, raising wheat and sorghum yields from under 1 ton per hectare in 1940 to nearly 6 by the end of the century. To the north in Canada, wheat farmers in Wise Creek, Saskatchewan raised yields from 0.5 to 2.5 tons per hectare by the 1980s (Larsen Reference Larsen2021). In the more diversified Livingston, Saskatchewan, farmers planting wheat, canola, and hay saw yields rise from 1.5 tons per hectare in 1941 to 4 in 2001. Across North America’s breadbasket, farm communities delivered energy-rich synthetic fertilizers to their soils, doubling crop production and then doubling it again.

Figure 4 presents a nutrient model of management practices across the North American Great Plains in the late twentieth century (Cunfer 2021; Larsen Reference Larsen2021). This industrial, export-oriented agricultural system relied on high inputs of synthetic fertilizer with very few nutrient transfers between landscapes. Livestock transferred some nutrients across the landscape, but in the twentieth century Great Plains, most of the nutrients were recycled within cropland or came from imported fertilizer, with comparatively few transfers between land-use components. Burning, although rare, happened occasionally when crop residues were especially heavy. Green manure is one way that North American farmers utilized legumes, by plowing the entire plant into the soil, a method often found in farm systems with no livestock and thus no manure (García-Ruiz et al. Reference García-Ruiz, Gonzalez de Molina, Guzman, Soto and Infante-Amate2012). Legume green manure crops fixed atmospheric nitrogen during their growing period, then “plow-down” utilized decomposition to unlock that nitrogen in the soil for future crops (Cherr et al. Reference Cherr, Scholberg and McSorley2006; Mazoyer and Roudart Reference Mazoyer and Roudart2006).

Figure 4. A model of agroecosystem soil nutrient flows for the North American Great Plains industrial farm system in the late twentieth century.

Similar processes transformed European farms in the second half of the twentieth century, once again restructuring soil nutrient management, landscape organization, and agricultural productivity. Most importantly, livestock lost their central role in soil nutrient management. With synthetic fertilizers, it was no longer necessary to integrate livestock with cropland, hay production, forage crops, and extensive grazing. Each of those agricultural functions continued, but they no longer remained in close proximity. Farms could specialize exclusively in grain production, dairy, or livestock feeding (Krausmann Reference Krausmann2004). At regional and global scales, some places focused on crop production, others on meat, dairy, fiber, or oils. Fossil fuel-powered soil nutrient supplies, coupled with fossil fuel-powered transportation networks, created a disaggregated agricultural landscape where supply chains and markets spanned continents and crossed oceans. The application of fossil fuel energy to farming in the mid-twentieth century was a turning point in the industrial transformation of agriculture. Prior to fossil fuel’s widespread adoption, biomass stored society’s available energy and fertility, and land area limited available biomass. Deploying petroleum and, especially, natural gas for agricultural applications lifted the energetic boundaries of a society that had been limited by its land-based energy and soil nutrient resources (Cunfer et al. 2018; Grigg Reference Grigg1992). Fossil fuels decoupled nutrient flows from land and labor resources. The “Boserupian link between decreasing labour productivity and increasing population density [was] overridden by the industrial link between increasing use of fossil fuels and industrial technology increasing labour productivity” (Fischer-Kowalski et al. Reference Fischer-Kowalski, Singh, Lauk, Remesch, Ringhofer and Grünbühel2011: 155).