Growing food means bending an ecosystem to human ends — and every method of doing so has an environmental price tag. Plow the soil and you loosen it for planting but expose it to erosion. Irrigate a dry field and you grow crops in a desert but risk poisoning the soil with salt. Spray a pesticide and you kill the bugs eating your crop — until the survivors breed a resistant swarm. This lesson surveys how humans farm: the ways we prepare land and deliver water, and the chemical and genetic tools we use to fight pests. The recurring theme is the trade-off — and the search for methods that produce food with less damage.
Tillage is plowing/turning soil to prepare for planting and control weeds. - Conventional (intensive) tillage breaks up soil thoroughly but leaves it bare and vulnerable to erosion and moisture loss. - No-till / conservation tillage leaves crop residue on the surface and disturbs soil minimally, reducing erosion, retaining moisture, and building organic matter — a key sustainable practice.
Slash-and-burn agriculture (shifting cultivation): forest is cut and burned, releasing nutrients into the soil for a few years of farming; then the plot is abandoned as fertility declines and farmers move on. Sustainable at low population density, but at scale it drives deforestation, carbon release, and soil degradation.
Agriculture uses the majority of human freshwater. Methods vary in efficiency:
| Method | Description | Efficiency / issue |
|---|---|---|
| Flood (furrow) irrigation | Flooding fields/channels | Cheap but wasteful; much water lost to evaporation and runoff |
| Spray (center-pivot) irrigation | Sprinklers spray water over crops | More efficient than flooding, but evaporation losses |
| Drip irrigation | Tubes deliver water directly to roots | Most efficient; minimal evaporation; best for water conservation but higher cost |
Aquifers and groundwater: Much irrigation draws on aquifers (underground water in porous rock). Pumping faster than natural recharge causes aquifer depletion — a falling water table, land subsidence (sinking ground), and, near coasts, saltwater intrusion. The Ogallala Aquifer (US Great Plains) is a classic over-drawn example. Diverting rivers for irrigation can shrink lakes — the Aral Sea famously collapsed after its feeder rivers were diverted for cotton.
Both degrade farmland; solutions include efficient irrigation (drip), proper drainage, and salt-tolerant crops.
[DIAGRAM: Salinization process — irrigation water applied to a field in a dry climate; water evaporates from the surface, leaving salt crystals that accumulate over time; crop yield declines as soil salinity rises. Contrast with a drip-irrigated field showing minimal evaporation and salt buildup.]
Pesticides (insecticides, herbicides, fungicides) increase yields by killing pests, but bring problems: - The pesticide treadmill: spraying kills susceptible pests, but resistant individuals survive and reproduce, so the pest population becomes resistant, requiring more or stronger pesticides — an escalating cycle. - Killing non-target species (beneficial insects, pollinators, natural predators of pests). - Bioaccumulation and biomagnification (Lesson 27): persistent pesticides like DDT concentrate up the food chain, harming top predators (e.g., raptor eggshell thinning). - Runoff contaminating water.
Alternatives: Integrated Pest Management (IPM) combines biological controls (natural predators), crop rotation, targeted limited pesticide use, and monitoring to minimize chemical use (detailed in Lesson 16).
Genetically modified organisms (GMOs) have DNA altered to add traits such as pest resistance (Bt crops), herbicide tolerance, drought tolerance, or higher nutrition. - Benefits: higher yields, reduced pesticide use in some cases, drought/pest resistance, potential nutritional gains. - Concerns: reduced genetic diversity, potential gene flow to wild relatives, pest resistance evolution, corporate seed dependence, and debated ecological/health effects.
Irrigation efficiency, salinization, and aquifer depletion are frequent MC and FRQ topics with clear cause-and-effect chains. The pesticide treadmill and IPM contrast recurs. These methods set up the sustainable-agriculture solutions in Lesson 16.
Rank flood, drip, and spray irrigation from most to least water-efficient, and give the reason.
Solution: Drip > spray > flood. Drip delivers water directly to roots with minimal evaporation; spray loses some to evaporation; flood loses the most to evaporation and runoff.
Interpretation: Efficiency rises as water is delivered more directly to roots.
A farm in a hot, dry region has irrigated for 20 years using flood irrigation. Crop yields are falling and white crusts appear on the soil. Explain.
Solution: Salinization. Irrigation water carries dissolved salts; in the hot, dry climate water evaporates from the soil surface, leaving salts behind. Over 20 years, salts have accumulated to levels toxic to crops, forming visible white crusts and reducing yields.
Interpretation: Evaporation concentrates salts; poor drainage and dry climate make it worse.
Explain the pesticide treadmill and one way IPM breaks the cycle.
Solution: Spraying kills susceptible pests, but resistant individuals survive and reproduce, so over generations the pest population becomes resistant, requiring more or stronger pesticide — an escalating, self-defeating cycle. IPM breaks it by relying on biological controls, crop rotation, and monitoring, using pesticides only as a targeted last resort, which slows resistance evolution.
Interpretation: Overuse → resistance → more use. IPM reduces the selection pressure.
An aquifer is pumped at 120 million liters/year but recharges at only 90 million liters/year. (a) Calculate the annual net loss. (b) State one consequence of continuing.
Strategy: net change = recharge − withdrawal.
Solution:
Net = 90 − 120 = −30 million liters/year (a net loss of 30 million L/yr)
(b) Continued over-pumping lowers the water table, can cause land subsidence, dry wells, and (near coasts) saltwater intrusion.
Interpretation: Withdrawal > recharge = depletion, an unsustainable draw on a slowly-renewing resource.
(B) Biological controls, rotation, monitoring, minimal targeted pesticides.
Flood irrigation applies large volumes of water containing dissolved salts; in a hot, arid climate the water evaporates from the soil surface, leaving salts behind to accumulate until the soil is too saline for crops. Prevention: switch to drip irrigation (less water, less evaporation) and install drainage to flush salts / use salt-tolerant crops.
(a) 200 − 260 = −60 million m³/yr (net loss of 60). (b) Not sustainable (withdrawal exceeds recharge); consequence: falling water table, subsidence, dry wells, or saltwater intrusion.
FRQ rubric (10 pts):
- (a) 1 pt setup 150 − 210; 1 pt = −60 million m³/yr. (2)
- (b) 1 pt setup 1,800 ÷ 60; 1 pt = 30 years. (2)
- (c) 1 pt irrigation water contains salts and much evaporates in the dry heat; 1 pt salts accumulate in soil, reducing fertility. (2)
- (d) For each of two practices: 1 pt name + 1 pt justification. Acceptable: drip irrigation (less withdrawal + less evaporation/salt buildup); improved drainage (flushes salts); water pricing/quotas (reduce pumping to ≤ recharge); salt-tolerant crops; lining canals/reducing losses; recharge enhancement. (4)
A large farming region relies on flood irrigation from an aquifer that recharges at 150 million m³/yr but is pumped at 210 million m³/yr. Soils are also showing salt buildup.
(a) Calculate the annual net change in aquifer volume. Show work. (2 pts) (b) At this rate, if the usable aquifer holds 1,800 million m³ above the depletion threshold, how many years until it reaches that threshold? Show work. (2 pts) (c) Explain why flood irrigation in this dry region also causes salinization. (2 pts) (d) Propose two practices to address both the water depletion and salinization, and justify each. (4 pts)
MC: 1. (C) Drip. 2. (B) Salt accumulation as water evaporates. 3. (B) Aquifer depletion and subsidence. 4. (B) Escalating pesticide use as resistance evolves. 5. (B) Soil erosion. 6. (B) Reduced genetic diversity/gene flow. 7. (B) Large scale/high population density. 8. (B) Rivers diverted for irrigation. 9. (B) Saturating soil, depriving roots of oxygen. 10. (B) Biological controls, rotation, monitoring, minimal targeted pesticides.
Flood irrigation applies large volumes of water containing dissolved salts; in a hot, arid climate the water evaporates from the soil surface, leaving salts behind to accumulate until the soil is too saline for crops. Prevention: switch to drip irrigation (less water, less evaporation) and install drainage to flush salts / use salt-tolerant crops.
(a) 200 − 260 = −60 million m³/yr (net loss of 60). (b) Not sustainable (withdrawal exceeds recharge); consequence: falling water table, subsidence, dry wells, or saltwater intrusion.
FRQ rubric (10 pts):
- (a) 1 pt setup 150 − 210; 1 pt = −60 million m³/yr. (2)
- (b) 1 pt setup 1,800 ÷ 60; 1 pt = 30 years. (2)
- (c) 1 pt irrigation water contains salts and much evaporates in the dry heat; 1 pt salts accumulate in soil, reducing fertility. (2)
- (d) For each of two practices: 1 pt name + 1 pt justification. Acceptable: drip irrigation (less withdrawal + less evaporation/salt buildup); improved drainage (flushes salts); water pricing/quotas (reduce pumping to ≤ recharge); salt-tolerant crops; lining canals/reducing losses; recharge enhancement. (4)
⭐ Exam strategy: For any groundwater problem, compare withdrawal vs. recharge — if withdrawal > recharge, it's unsustainable, full stop. For irrigation efficiency, remember drip beats spray beats flood, and in dry climates evaporation is the villain behind both water waste and salinization.
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