Agriculture, the Green Revolution and Pest Management

The Green Revolution

Between about 1950 and 1970, scientists such as Norman Borlaug developed high-yield varieties (HYVs) of wheat, rice, and corn. These crops put more energy into grain, and they could produce two or three times more food per acre. Farmers in Mexico, India, and the United States adopted them quickly.

HYVs are a package deal. They produce big yields only when they get plenty of water and nutrients, and when pests are held back. That is why the Green Revolution depended on irrigation, synthetic fertilizer, and pesticides all together.

Why do HYVs raise the demand for water and fertilizer? A plant that makes more grain must take in more water and more nitrogen and phosphorus to build that grain. Growing the same variety over huge fields (a monoculture) also gives pests an enormous, uniform food supply, so pest outbreaks spread fast.

Soil Conservation: Tillage and Crop Rotation

Bare, loose soil is easy for wind and rain to carry away. The Dust Bowl of the 1930s, when plowed Great Plains soil blew away in giant storms, is the classic example. Modern farmers use several practices to protect soil.

Tillage means turning over the soil before planting. In conventional tillage the whole field is plowed. In no-till farming seeds are planted directly into the leftover residue of the last crop.

Other soil-saving practices:

• Crop rotation: growing different crops in a repeating sequence (for example corn, then soybeans, then oats). Soybeans host nitrogen-fixing bacteria that add nitrogen to the soil, so the next crop needs less fertilizer. Rotation also breaks pest and disease cycles and keeps nutrients from being drained in only one way.

• Contour plowing: plowing across a slope, along its curves, so furrows slow runoff.

• Terracing: building flat steps on a steep hillside so water doesn't rush downhill.

• Cover crops: planting clover or rye between harvests so the soil is never bare.

Irrigation Methods

Irrigation supplies water that rain doesn't. Agriculture is the largest user of freshwater worldwide, so how the water is delivered matters a lot.

Flood irrigation loses the most water to evaporation, because the water spreads over a large open surface. Drip irrigation puts the water where the plant needs it.

The problem of salinization

All irrigation water carries small amounts of dissolved salts. In a dry climate, the water evaporates and the salts stay behind and build up in the soil. This is salinization. Salty soil pulls water out of roots by osmosis, so plants wilt even in wet soil, and yields drop. Poor drainage makes it worse. Waterlogging, too much water in the soil, can also starve roots of oxygen.

Other irrigation problems include aquifer depletion, such as the declining Ogallala Aquifer under the Great Plains, and the diversion of rivers like the Colorado.

Pests and Integrated Pest Management

A pest is any organism that damages crops: insects, weeds, fungi, and rodents. Pesticides are chemicals that kill them. A broad-spectrum pesticide kills many species, including helpful ones such as bees and the predators that eat pests.

Pesticides cause several problems:

• Non-target effects: pollinators, birds, and natural pest enemies die too.

• Pesticide resistance: in any pest population, a few individuals survive by chance. They reproduce, and their offspring inherit the resistance. After several rounds of spraying, most pests are resistant (natural selection). Farmers then spray more or switch chemicals, a cycle called the pesticide treadmill.

• Runoff and contamination of streams and groundwater, and buildup in food chains (see the persistent pollutants page).

Integrated pest management (IPM) combines several methods and uses chemicals only when needed.

Components of an IPM program:

1. Monitor fields and set an action threshold: spray only when pests reach a damaging number, not on a fixed calendar.

2. Biological control: introduce or protect natural enemies, such as ladybugs that eat aphids, or use pheromone traps.

3. Cultural practices: crop rotation, planting pest-resistant varieties, and keeping fields clean.

4. Targeted, limited pesticide use: a narrow-spectrum chemical, applied only where needed.

IPM slows resistance because fewer pesticide sprays mean less selection pressure, and rotating different control methods means pests rarely face the same threat repeatedly. Unsprayed, non-resistant pests also survive nearby and mate with resistant ones, which dilutes resistance.

Weighing Tradeoffs: Conventional and Sustainable Farming

Real decisions in farming involve tradeoffs: more yield now versus less damage later. AP® questions often give a table and ask you to calculate and then evaluate.

Percent reduction = $\dfrac{\text{old} - \text{new}}{\text{old}} \times 100$.

When you evaluate a tradeoff, use numbers from the data, name who benefits and who pays, and think about the long term. Here a small yield loss may be worth it: lost topsoil takes hundreds of years to rebuild, and less fertilizer means less runoff, so less eutrophication downstream.

Two approaches to feeding a growing population

For a region with limited freshwater, a low-input approach with efficient irrigation (drip) and drought-tolerant crops is usually the stronger recommendation. HYVs depend on a lot of water and fertilizer, and the water may not be there in the long run. A good argument also admits the cost, lower peak yields, and suggests a fix such as combining efficient irrigation with improved varieties.

Back to the start: Iowa farms are so productive because of Green Revolution crops, fertilizer, irrigation, and pesticides. That productivity came at a cost in soil, water, and wildlife, and practices such as rotation, no-till, drip irrigation, and IPM help farmers keep high yields while limiting the damage.

Summary