Soil decline is not an unavoidable outcome of agriculture — under specific conditions, human land use has increased fertility and kept it in place for centuries
By Craig Hartsough
Historically, human land use has more often depleted soils than improved them. As agriculture intensified, soils eroded, compacted, salinized, or lost organic matter — a pattern visible from ancient river valleys to upland farming regions worldwide.
Archaeology, geology, and agronomy tell the same story: productive land was pushed hard, then gradually worn down. Fertile land is often seen as something you’re simply lucky to have because of where you live — like the deep soils of the American Midwest or the rich, black soils of Ukraine.
But it may not be true that long-term fertility is something you only inherit from the land. Perhaps it’s something we can build — from soil that would otherwise be unsuitable for agriculture. Indeed, the historic and anthropological record is more complicated. There are world-wide historical and archaeological cases where rich agricultural soils were created by people in otherwise infertile areas. These important soils were built slowly through everyday land use, intentional waste handling, controlled burning, and settlement practices that concentrated organic matter and nutrients over long periods. These fertile soils made by people emerged from repeated, every-day decisions about waste, animal husbandry, and other sustainable stewardship decisions and were created based on where people lived and worked.
Africa: Soils Shaped by Settlement
In West Africa, particularly in Guinea and southern Mali, dark soils are found beneath Iron Age and pre–Iron Age settlements as discrete patches within otherwise low-fertility Ferralsols and Acrisols. Archaeological excavations consistently recover ceramic sherds, charcoal, bone, and ash mixed through surface horizons rather than confined to single layers, pointing to repeated inputs over long periods. Elevated phosphorus levels are common and widely interpreted as signals of accumulated food waste, manure, bone, and organic refuse.
Ethnoarchaeological work in Guinea also shows how these soils formed through routine domestic activity. Household waste, food residues, plant material, animal remains, and ash from cooking fires accumulated gradually within stable village footprints. Repeated low-intensity burning produced fine charcoal and ash that became incorporated into the soil, increasing carbon persistence, nutrient retention, and cation exchange capacity. These soils were not deliberately engineered but emerged from the material flows of sedentary life.
The key feature is persistence. Astonishingly, hundreds and thousands of years later, these former settlement soils remain preferred planting sites — recognized by farmers for darker color, better moisture retention, and higher yields without added fertilizer or other inputs. This longevity reflects durable changes to soil structure and function rather than short-term enrichment from ionic inputs.
Similar soils occur elsewhere in Africa. In the Lake Chad Basin, long-occupied earthen mounds contain dark, nutrient-rich soils with abundant cultural material and higher organic matter than surrounding sandy or lateritic soils. Geological explanations such as flooding have been largely ruled out due to the tight confinement of these soils to settlement footprints. In Central Africa, including Cameroon and the Republic of Congo, dark soils beneath precolonial villages contrast sharply with surrounding highly leached forest soils, showing improved aggregation, higher base saturation, and greater nutrient retention that persist under secondary forest cover. Comparable “village soils” are also documented in the Ethiopian highlands and Malawi, where abandoned settlement sites consistently outperform surrounding fields decades to centuries after use.
In sub-Saharan Africa, archaeologists and soil scientists have documented localized patches of dark, fertile soils directly associated with long-term human settlement. These soils usually occur beneath or adjacent to abandoned villages, household compounds, and settlement mounds and are sharply differentiated from surrounding highly weathered soils, indicating clear human origin.
Africa offers many examples of long-lasting fertile soils, created by people, but they all share some core traits: confinement to habitation zones, abundant cultural materials mixed through surface horizons, elevated nutrient status, and long-term persistence after abandonment. Mechanistically, fertility accumulated through repeated domestic inputs and low-intensity combustion that retained organic matter and nutrients in place, rather than through specialized technologies or external inputs. Where people remained in place long enough to concentrate waste, organic residues, and burnt products, soil function improved rather than declined. African dark soils therefore stand as rare counterexamples to the dominant historical pattern of soil degradation — and as clear evidence that long-term human land use can build, rather than exhaust, soil fertility.
Asia: Hydrology and Redox as Drivers of Fertility
Irrigated rice systems in East Asia represent one of the largest and longest-running examples of human-guided soil formation. Across China, Korea, and Japan, paddy soils developed through sustained management rather than inherited soil quality. Unlike settlement-centered anthropogenic soils, these systems are field-centered, formed through integrated water control rather than domestic waste accumulation.
Paddy soils accumulated fertility through continuous organic inputs, including rice straw and roots, animal manures, composted plant materials, and human wastes. Irrigation water delivered fine sediments and dissolved nutrients, gradually modifying soil texture and chemistry. Over centuries, these inputs produced thick, organic-rich surface horizons despite continuous cultivation. Flooding is the central driver of fertility in these systems. Inundation rapidly depletes oxygen, shifting soils to reduced conditions dominated by iron and manganese reduction. Under these conditions, iron-bound phosphorus is released into soil solution, increasing plant availability. Seasonal drainage reverses this process, re-oxidizing iron and re-immobilizing phosphorus, which limits leaching losses. Repeated redox cycling functions as a nutrient buffering mechanism that sustains fertility under historically low external inputs.
Carbon persistence in paddy soils results from suppressed aerobic decomposition rather than consistent inputs. Anaerobic conditions slow organic matter mineralization, allowing carbon to accumulate through kinetic limitation. Charcoal inputs are generally minor and incidental, not a primary driver of fertility.
This long-term cultivation also altered soil structure. Repeated puddling during transplanting of rice disrupts aggregates and forms dense plow pans that restrict percolation, retain nutrients in the rooting zone, and reinforce hydrological control. Fine sediment deposition via irrigation further improves water holding capacity and nutrient retention.
These mechanisms are well documented in long-managed paddy soils of the Yangtze basin and across East Asia, under diverse climates and parent materials. Other examples of human-made fertile soils on the continent include reclaimed tidal soils in Korea, formed through seawall construction and freshwater irrigation, and Japanese kuroboku soils, where long-term cultivation on volcanic parent materials stabilized organic carbon through strong mineral-organic associations rather than flooding.
In all these systems, microbial activity is structured by redox conditions — the balance between oxygenated and reduced microsites in soil or sediment. Flooding drives soils into reduced states, suppressing nitrification and nitrate mobility while favoring ammonium retention and anaerobic microbial pathways. When water levels recede, brief re-oxidation phases restore oxygen to pore spaces, allowing controlled nitrogen transformations to occur. When these redox oscillations are short and well managed, nitrogen cycling is enhanced without large gaseous or leaching losses, maintaining fertility while limiting export.
East Asian paddy soils demonstrate a distinct pathway to anthropogenic fertility based on hydrological control, redox-mediated nutrient cycling, and continuous organic inputs. Their persistence depends on ongoing management rather than inherent soil properties; when flooding regimes or organic inputs are disrupted, stored carbon and fertility decline rapidly. These systems show that long-term soil improvement can be achieved without modern inputs when management aligns with underlying biogeochemical constraints.
European Anthropogenic Soils: Limited Urban Dark Earths and Plaggic Systems
In Europe, most soils described as “dark earths” formed in Roman and medieval towns rather than in agricultural fields. These urban dark earths developed within settlement footprints through long-term accumulation of household waste, ash, charcoal, food residues, animal bone, and construction debris. They are well documented in places like London, Cologne, York, and Paris, often formed during periods when waste removal declined and refuse accumulated in place.
While these soils show elevated organic carbon and phosphorus, their formation was tied to urban material flows, not to field management. As a result, they remained spatially confined, were frequently disturbed by later development, and had little lasting relevance for agriculture beyond the settlement itself. For farmers, they are primarily an archaeological record of habitation rather than a durable soil-building system.
Europe’s most important anthropogenic agricultural soils are instead the Plaggic Anthrosol of northern Europe, particularly in the Netherlands, northwestern Germany, and nearby regions. These soils formed through centuries of deliberate nutrient transfer. Turf and topsoil were cut from surrounding commons and used as livestock bedding; the resulting manure-rich material was then repeatedly applied to arable fields. Over time, this practice built thick, dark, organic-rich topsoils capable of sustaining long-term production.
Plaggic soils are persistent and productive, but they relied on continuous external inputs drawn from surrounding landscapes. Fertility was concentrated in fields by degrading donor ecosystems, rather than generated through closed, in-place nutrient cycling. When sod-cutting practices ended, new plaggic soil formation largely ceased.
What These Soils Actually Tell Us
Some of the world’s most productive soils — Chernozems in Ukraine and southern Russia, and Mollisols across the North American Great Plains — are sometimes grouped into these discussions because they are dark, organic-rich, and highly productive. But they are fundamentally different.
Chernozems and Mollisols are natural grassland soils. Their deep, dark, surface horizons formed over thousands of years under perennial prairie and steppe vegetation through root turnover, biological cycling, and slow decomposition. Fertility in these soils emerged from ecology, not from long-term settlement, waste accumulation, or domestic activity. While small amounts of charcoal may be present from natural fires, there is no archaeological or stratigraphic evidence linking their formation to sustained human inputs. In many cases, these soils were later degraded rather than improved by agriculture.
The existence of the dark soils described above shows that people can create durable fertility where it would not naturally exist. Grassland soils show the opposite: exceptional fertility can arise without human intervention, but it can be rapidly lost when extraction exceeds regeneration. Chernozems and Mollisols are fertile by nature, not by culture. Keeping that distinction clear is essential to understanding what humans have actually done to soil.
Amazonian Dark Earth — terra preta — is the most studied and widest-spread example of a soil made fertile through long-term human activity. Its significance lies not only in productivity, but in the fact that it formed in landscapes where fertility would not otherwise persist. The same is true of comparable anthropogenic soils around the world: fertility was built and retained through human land use, not inherited from geology. The phenomenon is worldwide and repeated via several different methods.
It’s inspiring to realize there are so many examples of human activity improving and stabilizing soil, not degrading it. These outcomes were not accidents, nor the result of modern inputs or specialized technologies. They emerged where people stayed in place long enough for organic matter, nutrients, and combustion residues to accumulate rather than be exported, and where everyday practices reinforced retention instead of loss.
Just as important is what failed to produce durable fertility. In Europe, dense settlement generated localized enrichment that rarely translated into lasting agricultural soils. In grassland regions, fertility existed without human intervention but proved vulnerable once extraction exceeded regeneration. Across most of history, soils were either inherited and mined, or briefly enriched and then reset.
The common thread in persistent anthropogenic soils is not a single input or technique, but structure: permanence, repeated return of organic residues, low-intensity combustion, and minimal net nutrient export. Where those conditions aligned, soils improved. Where they did not, degradation followed. These soils are not recipes to copy, nor arguments for permanence everywhere. They are evidence that soil decline is not an unavoidable outcome of agriculture. Under specific conditions, human land use has increased fertility and kept it in place for centuries.
At a time when soil loss is widespread and restoration is often framed as technically complex or economically unrealistic, these examples are important. They show that long-term soil improvement has occurred before — without industrial inputs — and as a byproduct of how societies organized waste, settlement, and land use. Understanding those conditions can inform our decisions as land stewards today.
