In an often-cited example, the expansion of capitalist agriculture in Europe and North America led to a soil fertility crisis during the 19th century. A mad dash for new sources of fertility ensued, notably for South American guano and saltpeter, and a nascent synthetic fertilizer production industry developed. The scramble to locate new sources of fertility drove imperialist expansionism which ultimately displaced the metabolic rift elsewhere . As Engels explained in the late 19th century, each technological triumph over nature leads to other crises: “For each such victory takes its revenge on us. Each victory, it is true, in the first place brings about the results we expected, but in the second and third places it has quite different, unforeseen effects which only too often cancel the first” . These short-term technological fixes inevitably generate new metabolic rifts, amounting to “a shell game with the environmental problems [capitalism] generates, moving them around rather than addressing the root causes” . However, this shell game is not just a matter of space, but also a matter of scale. While a rift in a particular metabolic process occurs at a particular scale, social metabolism of nature continues at new spatial and temporal scales as production is relocated or becomes dependent on new inputs. Capitalist rationalization of agriculture arose from the pursuit of new markets and from the need to avert crises of production, such as falling rates of profit due to competition, a decline in availability of raw materials, flower buckets wholesale or environmental pollution and declining worker health resulting from production practices . These shifts in production severed particular metabolic interactions.

The separation of animal and crop production in industrial farming systems, for example, ruptured cycling of nutrients at the farm scale, leading to an increased reliance on off-farm inputs, such as fertilizers and feed shipped in from other regions. This rift in nutrient cycling therefore resulted in a rescaling of social metabolism; put simply, the inputs necessary to sustain human life under this new production system came from farther and farther away. Sustaining social metabolism under a food production system that depletes rather than regenerates the resource base depends not only on such spatial rescaling, but also on temporal rescaling. Rescaling requires what ecologists refer to as spatial and temporal “subsidies” to the food web , inputs that are produced on a different geographic and/or time scales. Since a subsidy is cross-scalar , its incorporation into a metabolic system inherently creates a new ecological rift as it is depleted; it is impossible to close the loop between the source and sink of such a cross-scalar subsidy. During the aforementioned crisis in soil fertility, for example, guano and nitrates were mined from decades- and centuries-old deposits from Peru and Chile, then transported across oceans to Europe and America . Replenishing these stocks would have been impossible within the span of a single cropping season, much less within the span of a human life. Once guano stocks were exhausted, agribusiness interests turned to synthetic fertilizers. The natural gas and petroleum needed to produce synthetic fertilizer and power tractors is millions of years-old, drawn from gas fields and oil wells around the globe and shipped to factories and refineries before being used thousands of miles from the point of extraction.

It becomes easy to see how ecological rift scales up, making social metabolism a global affair, dependent on millions-year-old subsidies from tens of thousands of miles away. If, as Huber argues, fossil fuel use is “an internal and necessary basis to the capitalist mode of production,” ecological rift and the resulting spatiotemporal rescaling of social metabolism is internal and integral to the contemporary agri-food system. Relocalizing these nutrient cycles and reducing dependence on petroleum-based food production lie at the heart of urban agriculture’s potential to mitigate metabolic rift. British agronomist Sir Albert Howard , concerned that organic wastes were rarely cycled back to their point of origin in large-scale agriculture, plaintively pondered, “Can anything be done at this late hour by way of reform? Can Mother Nature secure even a partial restitution of her manurial rights?” . While unclear if he was aware of Marx’s views on social metabolism , Howard echoed the concerns of Liebig, Marx, and Engels. Noting that “the Chinese have maintained soil fertility on small holdings for forty centuries” and inspired by the traditional farming practices he witnessed around him in the colonies, Howard championed compost use over chemical fertilizers and pondered a possible transformation of the industrial model where waste would be cycled back to farmland. In this same tradition, mending ecological rift via the recycling of organic waste is central to urban agriculture across the globe. This concept of returning nutrients to agricultural soils in the form of urban waste is vital to overcoming the “antithesis between town and country” and is fundamental to a “restitutive” agriculture. While few urban planners and mainstream development practitioners likely look towards Marx and Engels for inspiration, these obscure passages describing metabolic rift are particularly prescient, relevant not only to the development of sustainable agriculture, but also to urban waste management and the impending environmental crises of mega-urbanization . For millennia, farmers worldwide have maintained soil fertility on small plots through the application of organic waste; urban farmers are no exception. Adapting to the rising cost of chemical fertilizers and stagnant market prices for their produce, urban farmers in many parts of the South rely on intensive applications of manure from urban and peri-urban livestock production, ash, and composted garbage as a free or low-cost fertilizer and soil conditioner.

Periurban livestock producers, in addition to tapping rising urban demand for meat, dairy, and eggs, sell manure to urban market gardeners and to large-scale vegetable farms in the urban outskirts. To profit from compost’s fertilizing potential, farmers frequently cultivate the peripheries of garbage dumps or establish illicit contracts with garbage truck or cart drivers to obtain compost for their fields, paying them to simply dump a load of garbage in their fields while en route to central collection facilities. Advocates argue that redirecting the organic fraction of waste streams to agricultural production in urban areas and their hinterlands will help to boost soil fertility, as well as reduce soil and water pollution arising from heavy agrochemical use and large concentrations of waste deposited in landfills, dumps, and waterways . Yet to truly close the nutrient cycle and diminish the impacts of this ecological rift, human waste from urban consumers would need to be returned to the crops’ fields of origin. Every day, on average, every human produces 1 to 1 ” kg of nutrient-rich feces. Human waste, or “night soil”, is a common source of organic fertilizer in urban and peri-urban agriculture, though less commonly promoted due to cultural biases and to the higher public health risks associated with its application. Despite the social stigma, foul odor, and contamination risk of its use, there is stiff competition among farmers for access to night soil. Inone study, two-thirds of farmers surveyed in two peri-urban zones in northern Ghana used human waste in their fields . In China, in particular, flower harvest buckets application of human waste to farmland has been central to both urban waste management and agricultural production, but has been diminishing as rapid industrialization and urbanization transform agricultural production at the urban edge . While such forms of restitutive soil fertility management In the Global South generally arise from creative exploitation of limited resources and adaptation to limited access to land, fertilizer, and credit, they have been celebrated by urban farming advocates worldwide as fundamentally sustainable practices. In North America and Europe, where the discourse of ecological sustainability generally informs urban agriculture practice, the age-old nutrient cycling practices used in the Global South are a cornerstone of urban agriculture advocacy. Practices such as compost application, planting of nitrogen-fixing cover crops, and incorporation of crop residues are presented as a sustainable way to close the nutrient cycle and reduce urban ecological footprints. Indeed, application of compost to urban soils can also provide other environmental services, such as reducing erosion, improving drainage and water holding capacity, controlling pathogens, and immobilizing heavy metals. For commercial growers in peri-urban areas, a growing consumer demand for local and organic food often drives the transition to more ecologically-sound farming practices. A growing number of municipalities collect green waste for composting. Much of the compost is sold at low cost or provided for free to local farmers, landscapers, and gardeners.

Infrastructure for the collection, composting, and distribution of compost seems to be the greatest hurdle preventing urban agriculture’s ability to minimize ecological rift in nutrient cycling. Nevertheless, development workers and planners are optimistic about its role and argue that with improved waste management technology, access to land, and policies favoring agricultural production in urban areas, urban agriculture can contribute significantly to feeding the world’s cities and mending ecological rift by restoring “Nature’s manurial rights”, rescaling production to a more local level, and relying less on petroleum-based inputs and other crossscalar subsidies.Understanding this social rift is not only essential to explaining urbanization, but to elucidating the linkages between urbanization and the agri-food system. The rise of large- and industrial-scale farming has entailed the consolidation of land and expansion of mechanization and other new farming technologies, both of which reduce the demand for agricultural labor. This was evident in Europe at the dawn of the capitalist era, in the US during the latter half of the 20th century , and more recently in China where as many as 70 million farmers were dispossessed by expanding land markets in the last decade of the 20th century . In the Global South, a host of pressures—structural adjustment programs, land consolidation, drought, war, expansion of natural resource extraction and biofuels plantations—has dispossessed rural populations over the last several decades and fueled the growth of megacities and their slums across the globe . Indeed, as Marx predicted, “Part of the agricultural population is therefore constantly on the point of passing over into an urban or manufacturing proletariat” . Social rift is a central driver of urban agriculture in the Global South, where production of food is often a subsistence activity. Between 70 and 75 percent of farmers in a survey of urban agriculture in Nairobi, for example, produced for household consumption, citing hunger and the need for food as their principal motivation . Similar rates have been found in other parts of Africa, with lower rates in Asia, and Latin America . A recent FAO study revealed that over 30 percent of households in 11 of the 15 countries studied engage in some form of urban agriculture. The results also showed the urban poor are more likely to practice urban agriculture than wealthier city dwellers . Many must therefore improvise new means of survival, particularly in those cities where social services were gutted under structural adjustment during the 1980s and ‘90s. Many embark on small-scale agriculture on marginal plots of land tucked in between housing, industry, and infrastructure, within the city itself or in its immediate hinterlands, in order to buffer themselves from the socio-economic upheaval of dispossession from their land and from the lack of formal employment opportunities in the city and its peripheral slums. The slashing of government jobs under structural adjustment in many parts of the Global South also drove members of the urban professional class to embark on urban agriculture projects to augment their diets, and for those selling on informal local markets, to supplement their income. According to Guyer subsistence and small-scale urban food production, along with the informal food economy to which it contributes, often undermine the expansion of more formal markets. At the same time, however, self-provisioning effectively subsidizes the cost of social reproduction within the larger capitalist economy ; in short, wages can stay lower if workers are feeding themselves, ultimately facilitating the accumulation of capital.29 Urban agriculture therefore exists in tension with capital, arising as a strategic response to social rift on one level by exploiting underutilized land and buttressing against the expansion of commercial agri-food markets in poor areas, while subsidizing ongoing accumulation on a more macro-level.