Читать книгу Dirt - David R. Montgomery - Страница 12
ОглавлениеFOUR
Graveyard of Empires
To Protect Your Rivers, Protect Your Mountains
EMPEROR YU (CHINA)
IN THE EARLY 1840s NEW YORK LAWYER, adventurer, and amateur archaeologist John Lloyd Stephens found the ruins of more than forty ancient cities in dense Central American jungle. After excavating at Copán in Guatemala, traveling north to Mexico's ruined city of Pelenque, and returning to the Yucatán, Stephens realized that the jungle hid a lost civilization. His revelation shocked the American public. Native American civilizations rivaling those of the Middle East didn't fit into the American vision of civilizing a primeval continent.
A century and a half after Stephens's discovery, I stood atop the Great Pyramid at Tikal and relived his realization that the surrounding hills were ancient buildings. The topography itself outlined a lost city, reclaimed by huge trees, roots locked around piles of hieroglyphic-covered rubble. Temple-top islands rising above the forest canopy were the only sign of an ancient tropical empire.
With different characters and contexts, Tikal's story has been repeated many times around the world—in the Middle East, Europe, and Asia. The capital of many a dead civilization lives off tourism. Did soil degradation destroy these early civilizations? Not directly. But time and again it left societies increasingly vulnerable to hostile neighbors, internal sociopolitical disruption, and harsh winters or droughts.
Although societies dating back to ancient Mesopotamia damaged their environments, dreams of returning to a lost ethic of land stewardship still underpin modern environmental rhetoric. Indeed, the idea that ancient peoples lived in harmony with the environment remains deeply rooted in the mythology of Western civilizations, enshrined in the biblical imagery of the garden of Eden and notions of a golden age of ancient Greece. Yet few societies managed to conserve their soil—whether deliberately or through traditions that defined how people treated their land while farms filled in the landscape and villages coalesced into towns and cities. With allowances for different geographical and historical circumstances, the story of many civilizations follows a pattern of slow, steady population growth followed by comparatively abrupt societal decline.
Ancient Greece provides a classic example of too much faith in stories of lost utopias. Hesiod, a contemporary of Homer, wrote the earliest surviving description of Greek agriculture about eight centuries before the time of Christ. Even the largest Greek estates produced little more than needed to feed the master, his slaves, and their respective families. Like Ulysses' father, Laertes, the early leaders of ancient Greece worked in their own fields.
Later, in the fourth century BC, Xenophon wrote a more extensive description of Greek agriculture. By then wealthy landowners employed superintendents to oversee laborers. Still, Xenophon advised proprietors to observe what their land could bear. “Before we commence the cultivation of the soil, we should notice what crops flourish best upon it; and we may even learn from the weeds it produces what it will best support.”1 Xenophon advised farmers to enrich their soil both with manure and with burned crop stubble plowed back into the fields.
Ancient Greeks knew about the fertilizing properties of manure and compost, but it is not clear how widely such practices were followed. Even so, for centuries after the revival of classical ideals during the European Renaissance, historians glorified the ancient Greeks as careful stewards of their land. But the dirt of modern Greece tells a different story—a tale of destructive episodes of soil erosion.
With thin rocky soils covering much of its uplands, only about a fifth of Greece could ever support agriculture. The adverse effects of soil erosion on society were known in classical times; the Greeks replenished soil nutrients and terraced hillside fields to retard erosion. Nonetheless, the hills around Athens were stripped bare by 590 BC, motivating concern over how to feed the city. Soil loss was so severe that Solon, the famed reformer of the constitution, proposed a ban on plowing steep slopes. By the time of the Peloponnesian War (431—404 BC), Egypt and Sicily grew between a third and three-quarters of the food for Greek cities.
Plato (427-347 BC) attributed the rocky slopes of his native Attica to pre-Hellenistic soil erosion following deforestation. He also commented on soil's key role in shaping Athenian society, maintaining that the soils of earlier times were far more fertile. Plato held that the soil around Athens was but a shadow of its former self, citing evidence that bare slopes were once forested. “The rich, soft soil has all run away leaving the land nothing but skin and bone. But in those days the damage had not taken place, the hills had high crests, the rocky plain of Phelleus was covered with rich soil, and the mountains were covered by thick woods, of which there are some traces today.”2 Seeing how harvesting the natural fertility of the surrounding land allowed Athens to blossom into a regional power, Plato held that the root of his city's wealth lay in its soil.
Aristotle (384-322 BC) shared Plato's conviction that Bronze Age land use degraded soil productivity. His student Theophrastus (371—286 BC) recognized six distinct types of soil composed of different layers, including a humus-rich layer above subsoil that supplied nutrients to plants. Theophrastus made a point of distinguishing fertile topsoil from the underlying earth.
Both Plato and Aristotle recognized signs that Bronze Age land use had degraded their region's soil. Several thousand years and several civilizations later, archaeologists, geologists, and paleoecologists vindicated Aristotle's estimate of the timing: farmers arrived about 5000 BC and dozens of agricultural settlements were scattered throughout the region by 3000 BC; cultivation intensified about the time Aristotle posited the first serious effects of soil erosion there. Such knowledge, however, did not prevent classical Greece from repeating the pattern.
Over the past several decades, studies of soils throughout Greece—from the Argive Plain and the southern Argolid in the Peloponnese to Thessaly and eastern Macedonia—showed that even the dramatic climate change at the end of the last glaciation did not increase erosion. Instead, thick forest soils developed in the warming climate as oak forest replaced grassland across the Greek countryside. Over thousands of years the soil grew half a foot to several feet thick depending on local conditions. Soil erosion began to exceed soil production only after introduction of the plow.
The first Greek settlements were located in valleys with good soils near reliable water supplies. As the landscape filled with people, farmers began advancing onto steeper, less productive slopes. Extensive tilling and grazing stripped soil from hillsides and piled thick deposits of reworked dirt in valleys. Ancient agricultural artifacts can still be found on the rocky slopes of areas that lack enough soil to grow much vegetation.
Figure 6. Map of ancient Greece.
Sediments trapped in valley bottoms, and remnant pockets of soil on the slopes themselves, record cycles of erosion and soil formation throughout Greece. The deepest layers of valley-filling sediments date from glacial to interglacial climate changes during the past quarter million years. Higher layers in the stack of dirt tell of more recent episodes of hillslope erosion as well as intervening periods when soils developed. The first postglacial deposits of reworked hillslope soils in the valleys generally date from the Bronze Age arrival of agriculture. Erosional episodes similar in outline, but different in detail, occurred across ancient Greece where farming spread out of the valleys and onto hillslopes.
Figure 7. Parthenon. Albumen print by William James Stillman, 1869 (courtesy of Research Library, the Getty Research Institute, Los Angeles, California [92.R.84]).
Soils of the southern Argolid, for example, record four periods of postglacial erosion during times of intensive land use. The first, from roughly 4500 to 3500 BC, was a time when thick woodland soils were widely settled by early farmers. Introduction of the plow and the spread of farming into steeper terrain led to widespread erosion around 2300 to 1600 BC. Hillslope soils gradually rebuilt during the dark age before the rise of classical Greek civilization. The area was again densely settled in late Roman times and another period of depopulation followed in the seventh century AD. About fifteen inches of soil are estimated to have been lost from Argolid uplands since the start of Bronze Age agriculture. As many as three feet of soil may have been stripped from some lowland slopes.
Valley bottom sediments of the Argive Plain in the northeastern Peloponnese also testify to four periods of extensive soil erosion in the past five thousand years. Today, thick red and brown soils are found only in hollows and at the foot of slopes protected from streams. Remnants of hillslope soils and archaeological evidence show that since the Bronze Age there have been centuries-long periods with high settlement density, intensive farming, and accelerated soil erosion separated by millennia-long periods of low population density and soil formation.
Alexander the Great's homeland of Macedonia in eastern Greece underwent similar episodes of soil erosion accompanied by stream filling, and followed by landscape stability. The pace of soil erosion doubled in the late Bronze Age, and then doubled again from the third century BC to the seventh century AD. Another round started after the fifteenth century—defining a cycle with a roughly thousand-year periodicity, just as in other parts of Greece.
Regional climate changes cannot explain the boom-and-bust pattern of human occupation in ancient Greece because the timing of land settlement and soil erosion differed around the region. Instead, modern geoarchaeological surveys show that soil erosion episodically disrupted local cultures, forced settlements to relocate, led to changes in agricultural practices, and caused periodic abandonment of entire areas.
An ancient geopolitical curiosity provides further evidence that people destroyed Greek soils. The northern slopes of Mount Parness define the border between Boeotia and Attica. Oddly, the region belonged to Attica but was accessible only from Boeotia. So the region remained forested because Athenians could not get to it and Boeotians could not use it. While both city-states suffered severe soil erosion in their cultivated heartland, the no-man's-land on the border still retains a thick forest soil.
Extensive Bronze Age soil erosion coincides with changing agricultural practices that allowed a major increase in human population. The transition from highly localized, spring-fed agriculture using digging sticks to rain-fed agriculture based on clearing and plowing whole landscapes fueled an expansion of settlements. Initially, very low hillslope erosion rates increased slowly as agriculture spread until eventually erosion increased tenfold during the Bronze Age. Subsequently, erosion rates dropped back to close to the natural rate before once again increasing tenfold during the classical and Roman eras.
Almost the entire landscape was cultivated by classical times. Massive piles of dirt deposited in valley bottoms document extensive erosion of forest soils from hillsides disturbed by initial agricultural colonization. In places, later episodes of soil erosion were not as severe because continued farming and grazing prevented rebuilding thick soils. Even so, ancient erosion control measures like terraced hillsides and check dams built to slow the growth of gullies provide direct evidence of a fight to save soil.
The variety of crops excavated from Neolithic sites in Greece indicate that pre-Bronze Age agriculture was highly diversified. Sheep, goats, cows, and pigs were kept on small, intensively worked mixed-crop farms. Evidence of plow-based agriculture on estates worked by oxen records a progressive shift from diversified small-scale farms to large plantations. By the late Bronze Age, large areas controlled by palaces specialized in growing cereals. Olives and grapes became increasingly important as small farms spread into progressively more marginal areas prone to soil erosion. This was no coincidence—they grew well in thin, rocky soils.
Hesiod, Homer, and Xenophon all described two-field systems with alternate fallow years. It was normal to plow both fallow and planted fields three times a year, once in the spring, once in summer, and again in the autumn right before sowing. All this plowing gradually pushed soil downhill and left fields bare and vulnerable to erosion. Whereas Hesiod recommended using an experienced plowman who could plow a straight line regardless of the lay of the land, by later classical times terraces were built to try and retain soil and extend the productive life of hillside fields.
Modern examples show just how rapidly Greek soils can erode. On some overgrazed slopes, thickets of fifty-year-old oak standing on one-and-a-half-foot-high soil pedestals testify to modern erosion rates of just over a quarter of an inch per year. Live trees with roots exposed up to two and a half feet above the present ground surface record decades of soil erosion at around half an inch per year. When exposed to the direct effects of rainfall, land can fall apart at a rate apparent to even casual observers.
Little more than six centuries after the first Olympics were held in 776 BC, the Romans captured and destroyed Corinth, assimilating Greece into the Roman Empire in 146 BC. By then, after the second round of widespread soil erosion, Greece was no longer a major power. Some remarkable geologic detective work has shown how, like the ancient Greeks, the Romans also accelerated soil erosion enough to impact their society.
In the mid-1960s Cambridge University graduate student Claudio Vita-Finzi picked Roman potsherds from the banks of a Libyan wadi out of deposits previously thought to date from glacial times. Puzzled by the large amount of sediment so recently deposited by the stream, he poked around ancient dams, cisterns, and ruined cities and found evidence for substantial historical soil erosion and floodplain deposition. Intrigued, he set about trying to determine whether these geologic changes in historical times told of climate change or land abuse.
Traveling from Morocco north to Spain, and then back east across North Africa to Jordan, Vita-Finzi found evidence for two periods of extensive hillslope erosion and valley bottom sedimentation in river valleys around the Mediterranean. Deposits he called the Older Fill recorded erosion during late glacial times. Convinced that what he at first thought to be a Libyan curiosity was instead part of a broader pattern, Vita-Finzi attributed the younger valley fill to lower stream discharge caused by increasing aridity at the beginning of the late Roman era.
As often happens with new theories, people trying to fit additional observations into a simple framework found a more complicated story. The timing of soil erosion and valley filling differed around the region. How could Vita-Finzi's proposed regional drying affect neighboring areas at different times, let alone cause repeated episodes of erosion in some places? Just as in Greece, evidence now shows that people accelerated soil erosion in the Roman heartland as well as Roman provinces in North Africa and the Middle East. Even so, a simple choice of causes between climate and people is misleading. Droughts and intense storms accelerated erosion periodically on land where agricultural practices left soils bare and vulnerable.
As in other Paleolithic hunting cultures in southern Europe, an almost exclusive reliance on hunting large animals in central Italy gave way to more mixed hunting, fishing, and gathering as forests returned after the glaciers retreated. Thousands of years later, sometime between 5000 and 4000 BC, immigrants from the east introduced agriculture to the Italian Peninsula. Sheep, goat, and pig bones found along with wheat, barley seeds, and grinding stones reveal that these first farmers relied on mixed cereal cultivation and animal husbandry. Occupying ridges mantled with easily worked, well-drained soils these farmers relied on an integrated system of cereal cultivation and grazing similar to traditional peasant agriculture described by Roman agronomists thousands of years later. Between 3000 and 1000 BC, agricultural settlements spread across the Italian landscape.
From the early Neolithic to the end of the Bronze Age, Italian agriculture expanded from a core of prime farmlands into progressively more marginal land. The basic system of small-scale farms practicing mixed animal husbandry and growing a diversity of crops remained remarkably stable during this period of agricultural expansion—Bronze Age farmers still followed the practices of their Neolithic ancestors. Between about 4000 and 1000 BC, agriculture spread from the best sites used by the first farmers to steeper slopes and hard-to-work valley bottom clays.
Figure 8. Map of Roman Italy.
Iron came into widespread use about 500 BC. Before then only the wealthy and the military had access to metal tools. More abundant and cheaper than bronze, iron was hard, durable, and readily formed to fit over wood. Farmers began fitting plows and spades with iron blades to carve through topsoil and down into denser subsoil. Most of Italy remained forested around 300 BC, but new metal tools allowed extensive deforestation over the next several centuries.
When Romulus founded Rome in about 750 BC, he divided up the new state into two-acre parcels, a size his followers could cultivate themselves. The soils of central Italy were famously productive when the Roman Republic was founded in 508 BC. The average farm still consisted of roughly one to five acres (half a hectare to two hectares) of land, just enough land to feed a family. Many prominent Roman family names were derived from vegetables their ancestors excelled at growing. Calling a man a good farmer was high praise in the republic. Cincinnatus was plowing his fields when summoned to become dictator in 458 BC.
Early Roman farms were intensively worked operations where diversified fields were hoed and weeded manually and carefully manured. The earliest Roman farmers planted a multistory canopy of olives, grapes, cereals, and fodder crops referred to as cultura promiscua. Interplanting of understory and overstory crops smothered weeds, saved labor, and prevented erosion by shielding the ground all year. Roots of each crop reached to different depths and did not compete with each other. Instead, the multicrop system raised soil temperatures and extended the growing season. In the early republic, a Roman family could feed itself working the typical plot of land by hand. (And such labor-intensive farming is best practiced on a small scale.) Using an ox and plow saved labor but required twice as much land to feed a family. As plowing became standard practice, the demand for land increased faster than the population.
So did erosion. Extensive deforestation and plowing of the Campagna increased hillslope erosion to the point that antierosion channels were built to stabilize hillside farms. Despite such efforts, sediment-choked rivers turned valley bottoms into waterlogged marshes as plows advanced up the surrounding slopes. Malaria became a serious concern about 200 BC when silt eroded from cultivated uplands clogged the Tiber River and the agricultural valley that centuries before supported more than a dozen towns became the infamous Pontine Marshes. Large areas of worn-out hills and newly marshy valleys meant that formerly cultivated regions were becoming pastures of little use beyond grazing. Once-flourishing towns emptied as pastures supported fewer farmers than did the former fields.
Romans recognized that their wealth came from the earth; after all they coined the name Mother Earth (mater terra). As did the Greeks before them, Roman philosophers recognized the fundamental problems of soil erosion and loss of soil fertility. But unlike Aristotle and Plato, who simply described evidence for past erosion, Roman philosophers exuded confidence that human ingenuity would solve any problems. Cicero crisply summarized the goal of Roman agriculture as to create “a second world within the world of nature.” Yet even as Roman farmers used deeper plows and adapted their choice of crops to their denuded slopes, keeping soil on the Roman heartland became increasingly problematic. As Rome grew, Roman agriculture kept up by expanding into new territory.
Central Italy has four main types of soil: clay-rich soils prone to erosion when cultivated; limestone soils including ancient, deeply weathered Terra Rossa; fertile, well-drained volcanic soils; and valley bottom alluvial soil. Agricultural practices induced severe erosion on both clay-rich and limestone soils that mantled upland areas. The original forest soils have been so eroded in places that farmers now plow barely weathered rock. In many upland areas, limestone soils have been reduced to small residual pockets. Across much of central Italy, centuries of farming and grazing left a legacy of thin soils on bare slopes.
Roman farmers distinguished soils based on their texture (sand or clay content), structure (whether the particles grouped together as crumbs or clods), and capacity to absorb moisture. They assessed a soil's quality according to the natural vegetation that grew on it, or its color, taste, and smell. Different soils were rich or poor, free or stiff, and wet or dry. The best soil was a rich blackish color, absorbed water readily, and crumbled when dry. Good dirt did not rust plows or attract crows after plowing; if left fallow, healthy turf rapidly covered it. Like Xenophon, Roman agriculturalists understood that different things grew best in different soils; grapevines liked sandy soil, olive trees grew well on rocky ground.
Marcus Porcius Cato (234—149 BC) wrote De agri cultura, the oldest surviving Roman work on agriculture. Cato focused on grape, olive, and fruit growing and distinguished nine types of arable soils, subdivided into twenty-one minor classes based mainly on what grew best in them. He called farmers the ideal citizens and considered the agricultural might of its North African rival, Carthage, a direct threat to Roman interests. Carthage was an agricultural powerhouse capable of becoming a military rival. In perhaps the earliest known political stunt, Cato brought plump figs grown in Carthage onto the Senate floor to emphasize his view that “Carthage must be destroyed.” Ending all his speeches, no matter what the subject, with this slogan, Cato's agitating helped trigger the Third Punic War (149—146 BC) in which Carthage was torched, her inhabitants slaughtered, and her fields put to work feeding Rome.
Cato's businesslike approach to farming appears tailored to help Rome's rising class of plantation owners maximize wine and olive oil harvests while keeping costs to a minimum. Low-tech versions of the plantation agriculture of colonial and modern times, the agrarian enterprises he described became specialized operations with a high degree of capital investment. Falling slave and grain prices began driving tenant farmers off the land and encouraged raising cash crops on large estates using slave labor.
The next surviving Roman agricultural text dates from about a century later. Born on a farm in the heart of rural Italy, Marcus Terentius Varro (116—27 BC) wrote De re rustica at a time when these large estates dominated the Roman heartland. Varro himself owned an estate on the slopes of Vesuvius. Recognizing almost one hundred types of soil, he advocated adapting farming practices and equipment to the land. “It is also a science, which explains what crops are to be sown and what cultivations are to be carried out in each kind of soil, in order that the land may always render the highest yields.”3 Like most Roman agricultural writers, Varro emphasized obtaining the highest possible yields through intensive agriculture.
Although cereals grew best in the alluvial plains, Italy's lowland forest was already cleared and cultivated by Varro's time. Increasing population had pushed cereal cultivation into the uplands as well. Varro noted that Roman farmers grew cereals all over Italy, in the valleys, plains, hills, and mountains. “You have all traveled through many lands; have you seen any country more fully cultivated than Italy?”4 Varro also commented that the widespread conversion of cultivated fields to pasture increased the need for imported food.
Writing in the first century AD, Lucius Junius Moderatus Columella thought the best soil required minimal labor to produce the greatest yields. In his view, fertile topsoil well suited for grain should be at least two feet thick. Cereals grew best on valley bottom soils, but grapes and olives could flourish on thinner hillslope soils. Rich, easily worked soils made grains the major cash crop along Italy's river valleys. Focused like his predecessors on maximizing production, Columella chastised large landowners who left fields fallow for extended periods.
Columella described two simple tests of soil quality. The easy way was to take a small piece of earth, sprinkle it with a little water, and roll it around. Good soil would stick to your fingers when handled and did not crumble when thrown to the ground. A more labor-intensive test involved analyzing the dirt excavated from a hole. Soil that would not settle back down into the hole was rich in silt and clay good for growing grains; sandy soil that would not refill the hole was better suited for vineyards or pasture. Although little is known about Columella himself, I learned a version of his first test in graduate school at UC Berkeley.
Roman agriculturists recognized the importance of crop rotation—even the best soils could not grow the same crops forever. Farmers would periodically let a piece of ground lie fallow, grow a crop of legumes, or raise a cover crop well suited for the local dirt. Generally, they left fields fallow every other year between cereal crops. As for plant nutrition, Romans understood that crops absorbed nutrients from the soil and recognized the value of manure to achieve the greatest yields from the soil and prevent its exhaustion. In line with Cato's advice to keep “a large dunghill,” Roman farmers collected and stored manure from oxen, horses, sheep, goats, pigs, and even pigeons for spreading on their fields. They applied marl—crushed limestone—as well as ashes to enrich their fields. Varro recommended applying cattle dung in piles but thought bird droppings should be scattered. Cato recommended using human excrement if manure was unavailable. Columella even cautioned that hillside fields required more manure because runoff across bare, plowed fields would wash the stuff downslope. He also advised plowing manure under to keep it from drying out in the sun.
Above all else, Roman agronomists stressed the importance of plowing. Repeated annual plowing provided a well-aerated bed free of weeds. Varro recommended three plowings; Columella advised four. Stiff soils were plowed many times to break up the ground before planting. By the peak of the empire, Roman farmers used light wooden plows for thin easily worked soils, and heavy iron plows for dense soils. Most still plowed in straight lines with equal-size furrows. Just as in Greece, all that plowing slowly pushed soil downhill and promoted erosion, as runoff from each storm took its toll—slow enough to ignore in one farmer's lifetime, but fast enough to add up over the centuries.
Roman farmers plowed under fields of lupines and beans to restore humus and maintain soil texture. Columella wrote that a rotation of heavily manured beans following a crop of cereal could keep land under continuous production. He specifically warned against the damage that slave labor did to the land. “It is better for every kind of land to be under free farmers than under slave overseers, but this is particularly true of grain land. To such land a tenant farmer can do no great harm…while slaves do it tremendous damage.”5 Columella thought that poor agricultural practices on large plantations threatened the foundation of Roman agriculture.
Caius Plinius Secundus, better known as Pliny the Elder (AD 23—79), attributed the decline of Roman agriculture to city-dwelling landlords leaving large tracts of farmland in the hands of overseers running slave labor. Pliny also decried the general practice of growing cash crops for the highest profit to the exclusion of good husbandry. He maintained that such practices would ruin the empire.
Some contemporary accounts support the view that the Romans' land use greatly accelerated erosion despite their extensive knowledge of practical husbandry. Pliny described how forest clearing on hillslopes produced devastating torrents when the rain no longer sank into the soil. Later, in the second century, Pausanias compared two Greek river basins: the Maeander, actively plowed agricultural land, and the Achelous, vacant land whose inhabitants had been removed by the Romans. The populated, actively cultivated watershed produced far more sediment, its rapidly advancing delta turning islands into peninsulas. But by how much did Roman agriculture increase erosion rates in Roman Italy?
In the 1960s Princeton University geologist Sheldon Judson studied ancient erosion in the area around Rome. He saw how the foundation of a cistern built to hold water for a Roman villa around AD 150 stood exposed by between twenty and fifty-one inches of erosion since the structure was built, an average rate of more than an inch per century. He noted a similar rate for the Via Prenestina, a major road leading west from Rome. Originally placed flush with the surface of the ridge along which it runs, by the 1960s its basalt paving stones protruded several feet above easily eroded volcanic soil of the surrounding cultivated slopes. Other sites around Rome recorded an average of three-quarters of an inch to four inches of erosion each century since the city's founding.
Sediments accumulated in volcanic crater lakes in the countryside confirmed the account of dramatic erosion. Cores pulled from Lago di Monterosi, a small lake twenty-five miles north of Rome, record that land shedding sediment into the lake eroded about an inch every thousand years before the Via Cassia was built through the area in the second century BC. After the road was built, erosion increased to almost an inch per century as farms and estates began working the land to produce marketable crops. Sediments from a lake in the Baccano basin, less than twenty miles north of Rome along the Via Cassia, also recorded an average erosion rate on the surrounding lands of a little more than an inch every thousand years for more than five thousand years before the Romans drained the lake in the second century BC. Thick deposits of material stripped from hillslopes and deposited in valley bottoms along streams north of Rome further indicate intense erosion near the end of the empire.
These diverse lines of evidence, together with Vita-Finzi's findings, point to a dramatic increase in soil erosion owing to Roman agriculture. Considered annually, the net increase was small, just a fraction of an inch per year—hardly enough to notice. If the original topsoil was six inches to a foot thick, it probably took at least a few centuries but no more than about a thousand years to strip topsoil off the Roman heartland. Once landowners no longer worked their own fields, it is doubtful that more than a handful noticed what was happening to their dirt.
It was easier to see evidence of soil erosion downstream along the major rivers, where ports became inland towns as sediments derived from soil stripped off hillsides pushed the land seaward. Swamped by sediment from the Tiber River, Rome's ancient seaport of Ostia today stands miles from the coast. Other towns, like Ravenna, lost their access to the sea and declined in influence. At the southern end of Italy, the town of Sybaris vanished beneath dirt deposited by the Crathis River.
Historians still debate the reasons behind the collapse of the Roman Empire, placing different emphases on imperial politics, external pressures, and environmental degradation. But Rome did not so much collapse as consume itself. While it would be simplistic to blame the fall of Rome on soil erosion alone, the stress of feeding a growing population from deteriorating lands helped unravel the empire. Moreover, the relation worked both ways. As soil erosion influenced Roman society, political and economic forces in turn shaped how Romans treated their soil.
When Hannibal razed the Italian countryside in the Second Punic War (218 to 201 BC), thousands of Roman farmers flooded into the cities as their fields and houses were destroyed. After Hannibal's defeat, vacant farmland was an attractive investment for those with money. The Roman government also paid off war loans from wealthy citizens with land abandoned during the war. The estimated quarter of a million slaves brought back to Italy provided a ready labor supply. After the war, all three of the primary sources of agricultural production—land, labor, and capital—were cheap and available.
The growth of large cash crop—oriented estates (latifundia) harnessed these resources to maximize production of wine and olive oil. By the middle of the second century BC, such large slave-worked plantations dominated Roman agriculture. The landowning citizen-farmer became an antiquated ideal but a convenient emblem for the Gracchi brothers' popular cause in 131 BC. They promoted laws giving a few acres of state-owned land to individual farmers, yet many of those who received land under the Gracchi laws could not make a living, sold their land off to larger landowners, and went back on the dole in Rome. Less than two centuries after the Gracchi brothers were assassinated, huge estates accounted for nearly all the arable land within two days' travel from Rome. Forbidden to engage directly in commerce, many wealthy senators circumvented the law by operating their estates as commercial farms. The total area under Roman cultivation continued to expand as the transformation from subsistence farming to agricultural plantations reshaped the Italian countryside.
The land fared poorly under these vast farming operations. In the first decade AD the historian Titus Livius wondered how the fields of central Italy could have supported the vast armies that centuries before had fought against Roman expansion—given the state of the land, accounts of Rome's ancient foes no longer seemed credible. Two centuries later Pertinax offered central Italy's abandoned farmland to anyone willing to work it for two years. Few took advantage of his offer. Another century later Diocletian bound free farmers and slaves to the land they cultivated. A generation after that, Constantine made it a crime for the son of a farmer to leave the farm where he was raised. By then central Italy's farmers could barely feed themselves, let alone the urban population. By AD 395 the abandoned fields of Campagna were estimated to cover enough land to have held more than 75,000 farms in the early republic.
The countryside around Rome had fed the growing metropolis until late in the third century BC. By the time of Christ, grain from the surrounding land could no longer feed the city. Two hundred thousand tons of grain a year were shipped from Egypt and North Africa to feed the million people in Rome. Emperor Tiberius complained to the Senate that “the very existence of the people of Rome is daily at the mercy of uncertain waves and storms.”6 Rome came to rely on food imported from the provinces to feed the capital's unruly mobs. Grain was shipped to Ostia, the closest port to Rome. Anyone delaying or disrupting deliveries could be summarily executed.
North African provinces faced constant pressure to produce as much grain as possible because political considerations compelled the empire to provide free grain to Rome's population. The Libyan coast produced copious harvests until soil erosion so degraded the land that the desert began encroaching from the south. The Roman destruction of Carthage in 146 BC, and its salting of the surrounding earth to prevent its resurrection are well known. Less widely appreciated are the longer-term effects of soil degradation when the growing Roman demand for grain reintensified cultivation in North Africa.
The Roman Senate paid to translate the twenty-eight volumes of Mago's handbook of Carthaginian agriculture salvaged from the ruined city. Once the salt leached away, land-hungry Romans turned the North African coast into densely planted olive farms—for a while. Major farming operations centered around great olive presses developed in the first century AD. Proconsuls charged with producing food for Rome commanded legions of up to two hundred thousand men to protect the harvest from marauding nomads. The barbarians were kept at bay for centuries, but the threat of soil erosion was harder to stop as political stability under the pax romana encouraged continuous cultivation aimed at maximizing each year's harvest. By the time the Vandals crossed from Spain into Africa and took Carthage in AD 439, the Roman presence was so feeble that fewer than fifteen thousand men conquered all of North Africa. After the Roman capitulation, overgrazing by herds of nomadic sheep prevented rebuilding the soil.
