Learn · AP® Human Geography

Unit 5.6 — Sustainability Challenges in Agriculture

Desertification, vanishing aquifers, and the GMO debate — how the unit's farming systems collide with environmental limits, and what it all adds up to for the exam.

17 minUnit 5AP® Human Geography
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This closing lesson of Unit 5 gathers the environmental pressure points that farming, in every form covered so far — from swidden plots to von Thünen's rings to Green Revolution grain belts to agribusiness supply chains — ultimately runs up against. Three challenges dominate current geographic and policy discussion: desertification, freshwater depletion, and the contested role of genetically modified crops. A short synthesis at the end ties the whole unit together.

Desertification

Desertification is the process by which productive dryland — land that is naturally semi-arid but historically capable of supporting crops or grazing — degrades into desert-like conditions with little vegetation and minimal agricultural value, typically through a combination of human land-use pressure and climatic stress rather than either cause alone. The most closely watched example is the Sahel, the semi-arid belt running east-west across Africa immediately south of the Sahara, spanning parts of Senegal, Mali, Niger, Chad, and Sudan among other countries. Contributing pressures there include overgrazing by livestock herds beyond what the land's vegetation can regenerate, deforestation for fuelwood and farmland expansion in a region already short on tree cover to begin with, and rainfall variability that recent decades of climate change have made both less predictable and, in many years, sparser than the historical average. The consequences compound: as vegetation cover thins, exposed soil loses moisture faster and erodes more easily in wind and the region's occasional intense rains, which further reduces the land's ability to support the crops and grazing that communities depend on, in turn pushing herders and farmers to place even more pressure on whatever land remains productive. International recognition of the problem produced the United Nations Convention to Combat Desertification, opened for signature in 1994, one of three major UN environmental conventions alongside the climate change and biodiversity conventions from the same era — a sign of how seriously the international community had come to treat dryland degradation as a distinct global crisis rather than a purely local farming problem.

Water use and aquifer depletion

Agriculture is by a wide margin the largest human use of fresh water worldwide, accounting for roughly seventy percent of global freshwater withdrawals, driven above all by irrigation. Irrigation expanded enormously over the twentieth century, both as a direct requirement of the Green Revolution's water-demanding high-yield crop varieties (Unit 5.3) and as commercial farming generally intensified. A large share of that irrigation draws not from rivers or rainfall but from groundwater aquifers, underground layers of water-bearing rock or sediment that accumulated over thousands or millions of years and refill, if at all, far more slowly than farmers are pumping from them.

The Ogallala Aquifer, which underlies parts of eight U.S. states from South Dakota down through the Texas Panhandle, is the clearest domestic case study and a frequent exam reference point. It supplies roughly a third of all irrigation groundwater used in the United States, and decades of center-pivot irrigation (the same circular fields visible within the Public Land Survey System's grid, covered in Unit 5.5) have drawn its water table down measurably across large portions of its extent, in some areas by tens of feet since large-scale pumping began in the mid-twentieth century. Because natural recharge in this semi-arid region is extremely slow, hydrologists generally describe Ogallala withdrawal as functionally mining a mostly non-renewable resource rather than sustainably using a renewable one — a distinction that matters directly for the long-term viability of the Great Plains grain and ranching economy discussed earlier in this unit. Similar aquifer stress appears globally, including in India's Punjab, where the same Green Revolution intensification that tripled wheat output also accelerated groundwater withdrawal well beyond natural replenishment rates.

Genetically modified organisms (GMOs)

Genetically modified crops, introduced commercially beginning in the 1990s as an extension of the Green Revolution's logic through direct genetic engineering rather than selective breeding, remain one of agriculture's more geographically and politically contested technologies. Proponents point to concrete gains: Bt corn and cotton, engineered to produce their own insect-resistant protein, have measurably reduced insecticide applications in the regions that adopted them; herbicide-tolerant soybean varieties simplified weed control and supported reduced-tillage farming practices that in turn cut soil erosion; and "golden rice," engineered to produce beta-carotene (a vitamin A precursor), was developed specifically to address vitamin A deficiency, a leading cause of preventable childhood blindness in parts of the developing world, though its actual adoption has been slowed for years by regulatory and public-acceptance hurdles in several target countries.

Critics raise a different set of concerns, several with a distinctly geographic dimension. Patent control over GMO seed varieties, concentrated among a small number of agribusiness firms discussed in Unit 5.4, has generated real legal conflict over farmers' traditional right to save and replant seed from their own harvest — the Canadian case of farmer Percy Schmeiser, sued in the late 1990s after patented canola appeared on his land without his purchase, remains a widely cited example of the tension between seed-patent enforcement and older farming customs. Regulatory acceptance of GMOs also varies sharply by region: the United States and most of the Americas permit GMO cultivation broadly, while the European Union has maintained far more restrictive approval and labeling rules, a divergence that itself illustrates how a single agricultural technology can diffuse unevenly across otherwise similar economies for essentially political and cultural, rather than technical, reasons.

Toward sustainable practice

A number of practices, some old and some newly refined, aim to soften these pressures without abandoning the yield gains agriculture has built up since the First Agricultural Revolution. Crop rotation and cover cropping restore soil nitrogen and structure much as Charles Townshend's four-field system did two and a half centuries ago. No-till farming leaves crop residue on the field and avoids turning over the soil between plantings, cutting erosion and preserving soil moisture. Precision agriculture uses GPS-guided equipment and field-by-field soil and moisture data to apply water, fertilizer, and pesticide only where and when a field actually needs them, reducing both cost and runoff compared to older uniform-application methods. Agroecology and organic farming approaches try to substitute biological pest and fertility management — beneficial insects, composting, crop diversity — for some of the Green Revolution's chemical inputs, generally trading some yield for reduced input cost and environmental impact.

Unit 5 synthesis: why this matters for the exam

Unit 5 traces one continuous argument: agriculture began independently in several world regions roughly ten thousand years ago (5.1), was reorganized around distance and transportation cost once market cities existed (von Thünen, 5.2), was radically intensified twice more — first by Britain's eighteenth-century mechanization and later by the Green Revolution's high-yield seed package (5.1 and 5.3) — was reorganized again around corporate consolidation rather than distance (5.4), left its own permanent fingerprint on the physical shape of rural land (5.5), and now runs up against the environmental limits of the water, soil, and biodiversity it depends on (this lesson). A free-response question spanning the unit might ask you to trace exactly this arc, or to argue whether a specific technology — the Green Revolution, GMOs, or precision agriculture — represents a genuine long-term solution or only a short-term fix that defers the same underlying sustainability problem. A model synthesis sentence for that kind of prompt: "While each wave of agricultural intensification, from Norfolk's four-field rotation to the Green Revolution's dwarf grains to today's genetically modified crops, has succeeded in raising yield per acre, each has also deepened agriculture's dependence on inputs — fossil-fuel fertilizer, groundwater, and patented seed technology — that are themselves geographically uneven and, in the case of water, not renewable on any human timescale."

Practice: Free-Response Questions

Real AP-format prompts for this unit, each with a full model answer and the exact points a College Board reader would award. Click a question to see the answer — not AI-graded, just scored the way the real exam is scored.

LEQ

Respond to the following prompt.

Respond to the following prompt. Your response must address all seven parts

(a) through (g).

Agricultural systems around the world developed from a common set of origins and continue to diffuse, evolve, and shape the rural landscape.

(a) Define subsistence agriculture.

(b) Identify one hearth (region of origin) of the First Agricultural Revolution and name one plant or animal domesticated in that hearth.

(c) Describe one characteristic of shifting cultivation (swidden agriculture) as a form of subsistence agriculture.

(d) Describe one characteristic of pastoral nomadism as a form of subsistence agriculture.

(e) Explain one way relocation diffusion differs from expansion diffusion in the spread of an agricultural innovation, providing one example of each type of diffusion.

(f) Explain one way the Second Agricultural Revolution increased agricultural productivity in Europe.

(g) Explain one way that rural settlement patterns (for example, dispersed versus clustered settlement) reflect the agricultural system practiced in a region.

Model answer

(a) Subsistence agriculture is farming carried out primarily to feed the farmer's own household or local community, with little or no surplus produced for sale in a market.

(b) One accepted hearth of the First Agricultural Revolution is the Fertile Crescent in Southwest Asia (the region including parts of modern Iraq, Syria, and Turkey), where wheat and barley were domesticated (other accepted hearths: Mesoamerica — maize/beans/squash; the Andes — potatoes/quinoa; East Asia — rice/millet; Sub-Saharan Africa — sorghum/yams).

(c) Shifting cultivation involves clearing a plot of land, often by cutting and burning vegetation, farming it for a few growing seasons until soil fertility declines, then abandoning the plot to let it lie fallow and regenerate while the farmer clears a new plot elsewhere. It is typically practiced in tropical rainforest environments and supports only low population densities.

(d) Pastoral nomadism involves herding domesticated livestock (such as cattle, sheep, goats, or camels) over seasonal, non-fixed routes in search of pasture and water, rather than cultivating crops on fixed plots of land. It is typically practiced in arid, semi-arid, or high-altitude environments unsuitable for crop farming.

(e) Relocation diffusion occurs when people who carry a practice physically migrate from one location to another, taking the practice with them and establishing it in the new location while it may cease to spread from the origin — for example, European colonists carrying wheat cultivation techniques to the Americas as they settled there. Expansion diffusion occurs when a practice spreads outward from its origin through contact between people, without the originators relocating, and the practice keeps expanding within the source region as well — for example, contour plowing techniques spreading farmer-to-farmer across a agricultural region as neighboring farmers observe and adopt the method.

(f) The Second Agricultural Revolution, associated with the Industrial Revolution, increased productivity through innovations such as mechanized farm equipment (e.g., the seed drill and, later, mechanized reapers and tractors), improved crop rotation systems, and selective breeding of livestock and crops, which increased yields per unit of land and reduced the labor required per farm worker.

(g) Rural settlement patterns reflect the agricultural system practiced: for example, a dispersed (scattered) settlement pattern is common in areas of extensive commercial agriculture (such as U.S. Midwest grain farming) where large individual landholdings require farmhouses to be spaced far apart, while a clustered settlement pattern is common in areas of intensive subsistence rice farming (such as parts of Southeast or South Asia) where farmers live together in a central village and walk out daily to small, nearby fields, allowing shared labor and efficient use of limited arable land.

Scoring · 7 points
1. Correctly defines subsistence agriculture as farming to feed the household/local community with little or no surplus for sale.
2. Identifies a valid hearth of the First Agricultural Revolution and names a plant or animal domesticated there.
3. Describes a valid characteristic of shifting cultivation (e.g., clear/burn/fallow cycle, tropical environment, low population density supported).
4. Describes a valid characteristic of pastoral nomadism (e.g., seasonal herd movement, arid/semi-arid environment, no fixed crop land).
5. Explains the distinction between relocation and expansion diffusion with one valid example of each.
6. Explains a valid mechanism by which the Second Agricultural Revolution raised productivity (mechanization, crop rotation, or selective breeding).
7. Explains how a specific rural settlement pattern (dispersed or clustered) connects logically to a specific agricultural system, with a supporting example or reasoning.
LEQ

Use the data table below to respond to the following prompt.

Use the data table below to respond to the following prompt. Your response must address all seven parts

(a) through (g).

Source: Hypothetical land-use and rent data for farmland surrounding the market town of Meadowbrook

Distance from Meadowbrook town center | Average land rent per hectare | Dominant land use 0–5 km | $420 | Dairying and market gardening (fruits and vegetables) 5–15 km | $310 | Forest for fuelwood and timber 15–40 km | $150 | Grain farming (wheat) 40–100 km | $60 | Ranching and livestock grazing

(a) Identify the agricultural geographer who developed the model of rural land use that best explains the pattern shown in the table.

(b) Define the concept of "bid-rent" as it is used in this model.

(c) Describe why land located closer to Meadowbrook commands a higher rent per hectare than land located farther away.

(d) Describe why dairying and market gardening are located in the ring nearest the town center rather than in the outermost ring.

(e) Explain why, in the original version of this model, forest for fuelwood and timber was located closer to the town center than grain farming, with reference to transportation costs.

(f) Explain one limitation of applying this model to agricultural land use in the present-day United States.

(g) Using the data table, explain how the widespread introduction of refrigerated trucking would likely alter the relationship between distance from the town center and land rent for dairy products.

Model answer

(a) The model shown is von Thünen's model of agricultural land use, developed by Johann Heinrich von Thünen.

(b) Bid-rent refers to the rent a farmer is willing and able to pay for a parcel of land, which reflects the value of the land for a given agricultural use; bid-rent decreases as distance from the central market increases because transportation costs to move goods to market rise with distance, reducing the profit available to pay as rent.

(c) Land closer to Meadowbrook commands higher rent because transportation costs to move goods to the central market are lower for nearby land, leaving farmers more profit to bid for that land; competition among farmers who want to minimize transport costs and time (especially for perishable goods) also drives up rent for the closest parcels.

(d) Dairying and market gardening are located nearest the town because they produce perishable goods (milk, fresh produce) that must reach market quickly before spoiling and are also costly to transport per unit of value, so farmers of these goods can afford, and need, to outbid other land uses for the most accessible land.

(e) In von Thünen's original model, wood was a bulky, heavy commodity that was expensive and difficult to transport over land before mechanized transportation existed, and it was in constant demand in town for fuel and building material, so it needed to be located close enough to the market to keep transport costs manageable — closer than lower-value, less bulky grain, which could be transported profitably over greater distances.

(f) One valid limitation: von Thünen's model assumes an isotropic plain (uniform, featureless terrain, transportation cost equal in all directions, a single central market, and no external trade), conditions that do not hold in the present-day United States, which has variable terrain, multiple transportation modes and networks (highways, rail) that create transport corridors rather than uniform rings, multiple competing markets, and global trade that allows perishable or bulky goods to be shipped long distances profitably using modern refrigeration and rapid transport.

(g) Refrigerated trucking would reduce the spoilage risk and effective transportation cost of moving dairy products long distances, weakening the locational advantage that land near Meadowbrook currently holds for dairying; as a result, dairy production could shift farther from the town center where land rent is lower, and the steep rent gradient favoring the innermost ring for dairying and market gardening would likely flatten, since farmers producing perishables would no longer need to be located immediately adjacent to the market to remain profitable.

Scoring · 7 points
1. Correctly identifies von Thünen as the geographer associated with this land-use model.
2. Correctly defines bid-rent as the rent a land use can support, which reflects value net of transportation cost to market.
3. Describes the inverse relationship between distance from the market and rent, tied to rising transportation costs.
4. Describes why perishable/high-transport-cost goods (dairying, market gardening) locate nearest the market.
5. Explains the historical transportation-cost reasoning for placing forest/timber closer to town than grain in the original model.
6. Explains a valid limitation of applying the model to the modern U.S. (e.g., non-isotropic terrain, transport networks/corridors, multiple markets, modern refrigerated/global trade).
7. Explains, using the data, how refrigerated trucking would plausibly flatten or shift the rent-distance gradient for dairy products.
LEQ

Use the two sources below to respond to the following prompt.

Use the two sources below to respond to the following prompt. Your response must address all seven parts

(a) through (g).

Source 1: Hypothetical average wheat and rice yields, South Asia, 1965–1985

Year | Average wheat yield (kg per hectare) | Average rice yield (kg per hectare) 1965 | 850 | 1,100 1975 | 1,600 | 1,700 1985 | 2,300 | 2,450

Source 2: Statement from a smallholder farmers' cooperative, 2020 "Since the large agribusiness firm began contracting local growers here, most farmers plant only the single hybrid seed variety supplied under contract, sell their entire harvest exclusively to the company's processing plant, and no longer save seed from their own harvest to replant the following year."

(a) Define the Green Revolution.

(b) Using Source 1, describe the overall trend in wheat and rice yields between 1965 and 1985.

(c) Identify two agricultural inputs associated with the Green Revolution that help explain the yield trend shown in Source 1.

(d) Using Source 2, describe one characteristic of agribusiness illustrated by the cooperative's statement.

(e) Explain one economic risk faced by the smallholder farmers described in Source 2 as a result of contract farming with a single agribusiness firm.

(f) Explain one environmental sustainability concern associated with the Green Revolution practices that produced the yield increases shown in Source 1.

(g) Explain one way that vertical integration by agribusiness firms, as described in Source 2, has both benefited and threatened rural farming communities.

Model answer

(a) The Green Revolution (also called the Third Agricultural Revolution) refers to the period beginning in the mid-twentieth century in which the development and spread of high-yield hybrid seed varieties, synthetic fertilizers, irrigation, and mechanization dramatically increased agricultural output, especially of grain crops such as wheat and rice, in many developing countries.

(b) Source 1 shows that both wheat and rice yields per hectare increased substantially and consistently from 1965 to 1985 — wheat yields roughly tripled (from 850 to 2,300 kg/hectare) and rice yields more than doubled (from 1,100 to 2,450 kg/hectare) over the twenty-year period.

(c) Two valid inputs: high-yield variety (HYV) hybrid seeds (developed through selective breeding to produce more grain per plant) and synthetic chemical fertilizers (which supply nutrients that allow the HYV seeds to reach their higher yield potential). Other acceptable inputs: expanded irrigation and mechanized equipment (tractors, mechanized harvesting).

(d) Source 2 illustrates vertical integration / contract farming, a characteristic of agribusiness in which a single large firm controls or coordinates multiple stages of the agricultural supply chain — in this case supplying the seed input, and also purchasing and processing the entire output — so that independent farmers effectively grow a single standardized crop for one buyer rather than making independent planting and marketing decisions.

(e) One valid risk: because the farmers sell exclusively to one firm and plant only the variety it supplies, they have no alternative buyer and little bargaining power over the price they receive, leaving them economically vulnerable if the firm lowers the price it pays, changes contract terms, or ends the contract; because they no longer save their own seed, they must also purchase new seed from the company each season, increasing their costs and dependency.

(f) One valid concern: intensive Green Revolution farming relying on heavy irrigation and synthetic fertilizer/pesticide application can degrade soil quality over time, deplete or contaminate groundwater and surface water supplies through runoff, and encourage monocropping of a narrow set of HYV seeds, which reduces agricultural biodiversity and can increase vulnerability to pests or disease that affect a single genetically similar crop.

(g) Vertical integration by agribusiness firms has benefited rural farming communities by providing farmers with guaranteed access to improved seed inputs, a reliable buyer for their harvest, and often technical support, which can raise and stabilize household income compared to selling on an open, price-volatile market. At the same time, it has threatened these communities by concentrating economic power and decision-making in a single outside firm, eroding farmers' independence (loss of seed-saving and choice of what to grow), and making the community's livelihood dependent on the continued presence and pricing decisions of one company, with little recourse if the firm withdraws or changes terms.

Scoring · 7 points
1. Correctly defines the Green Revolution as the mid-twentieth-century expansion of high-yield seeds, fertilizer, irrigation, and mechanization that raised agricultural output.
2. Describes the upward yield trend in Source 1 for both wheat and rice from 1965 to 1985 (with reference to the data).
3. Identifies two valid Green Revolution inputs (e.g., HYV seeds, synthetic fertilizer, irrigation, mechanization) linked to the yield trend.
4. Describes vertical integration/contract farming as the agribusiness characteristic shown in Source 2.
5. Explains a valid economic risk to smallholder farmers from single-buyer contract dependency (e.g., loss of bargaining power, seed-purchase cost, no alternative buyer).
6. Explains a valid environmental sustainability concern tied to Green-Revolution-style intensive farming (soil degradation, water depletion/contamination, or reduced biodiversity from monocropping).
7. Explains both a benefit and a threat that vertical integration poses to rural farming communities, connected to Source 2.
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