AP Environmental Science

## Learning Outcomes

- Define the **Green Revolution** and identify its core agricultural practices
- Explain how **high-yield varieties**, irrigation, fertilizers, pesticides, and mechanization worked together to raise crop yields
- Distinguish the original Green Revolution from later genetic engineering and GMO developments
- Describe the main environmental costs of Green Revolution practices, including salinization, eutrophication, and biodiversity loss
- Compare Green Revolution agriculture with traditional subsistence agriculture
- Apply these ideas in AP Environmental Science-style questions about benefits, drawbacks, and sustainability

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand Green Revolution practices, with a focus on the following syllabus points:

- Know that the **Green Revolution** was a mid-20th-century shift toward input-intensive agriculture designed to increase food production
- Identify the main practices: **high-yield crop varieties (HYVs)**, irrigation, synthetic fertilizers, pesticides/herbicides, and mechanization
- Explain how these practices interact rather than treating them as separate inventions
- Understand that HYVs were mainly associated with **selective breeding** in the original Green Revolution, not genetic engineering
- Explain the environmental effects of irrigation, including **waterlogging**, **salinization**, and groundwater depletion
- Describe how fertilizer runoff can cause nutrient pollution and **eutrophication**
- Explain how pesticide use can harm non-target species and select for pesticide resistance
- Understand that Green Revolution agriculture often reduced **genetic diversity** and encouraged monoculture

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What are the main agricultural practices associated with the Green Revolution?
2. Why do high-yield crop varieties usually need irrigation and fertilizer to reach their highest yields?
3. How can irrigation both increase crop production and damage soil?
4. What is the difference between the original Green Revolution and later GMO-based technologies?
5. How can synthetic fertilizers and pesticides increase food production but also create environmental problems?

## Introduction

The Green Revolution is one of the most tested agricultural ideas in AP Environmental Science because it shows a classic environmental trade-off. Humans increased food production for a growing population, but did so by relying on more water, more chemicals, and more simplified farming systems.

For the exam, do not think of the Green Revolution as just “better seeds.” It was a package of linked practices: improved crop varieties, irrigation, synthetic fertilizers, pesticides, and mechanization. The gains in yield were real, especially for staple crops such as wheat and rice, but the ecological costs were also real.

> **Key Term: Green Revolution**
> A mid-20th-century transformation in agriculture characterized by the use of high-yield crop varieties together with irrigation, synthetic fertilizers, pesticides, and mechanization to increase food production.

### The core package: how Green Revolution practices worked together

The First Green Revolution began in the **1940s** and spread most strongly through the **1950s to 1970s**. Its main purpose was simple: produce more food per unit land. That mattered because population growth increased demand for staple crops.

The major practices were:

- planting **high-yield crop varieties**
- expanding irrigation
- using synthetic fertilizers
- using pesticides and herbicides
- increasing mechanization

These practices worked as a system. That is the key idea students often miss.

A high-yield variety can produce more grain than a traditional variety, but only if enough resources are available. If water is unreliable, nutrients are low, and pests are severe, the improved seed cannot perform at its best. This is why APES questions often link HYVs with fertilizer and irrigation rather than treating them separately.

> **Key Term: high-yield crop varieties (HYVs)**
> Crop strains bred to produce more grain per unit area, especially when supplied with enough water, nutrients, and pest control.

HYVs were especially important for major cereal crops such as wheat and rice. In the original Green Revolution, these varieties were mainly produced through **selective breeding**, not modern genetic engineering. Students often confuse the Green Revolution with GMOs because later biotechnology built on some of the same food-production goals.

A simple cause-and-effect chain looks like this:

- selective breeding produces HYVs
- HYVs can convert more resources into grain
- irrigation supplies reliable water
- fertilizers supply key nutrients, especially nitrogen and phosphorus
- pesticides reduce crop loss to insects, fungi, and weeds
- mechanization allows large-scale planting and harvesting
- total crop yield rises

That is why Green Revolution gains came from a **package** of practices.

> **Test Tip:** If an APES question asks what most increased yields during the Green Revolution, avoid choosing just one factor unless the question is very narrow. The best answer is usually the combined system of HYVs, irrigation, fertilizers, and pesticides.

Mechanization also matters. Tractors, pumps, harvesters, and other machinery make it possible to farm larger areas with less human labor. This increases efficiency, but also increases fossil fuel use and often makes farming more dependent on purchased inputs.

### Irrigation, fertilizers, and pesticides: why yield gains created new problems

Irrigation was one of the main reasons Green Revolution agriculture succeeded in places where rainfall alone was not enough. Reliable water stabilizes crop growth and can allow multiple cropping in one year. A farmer relying only on rainfall faces uncertainty; a farmer with irrigation can better control growing conditions.

But irrigation creates two major APES-linked soil problems: **waterlogging** and **salinization**.

> **Key Term: waterlogging**
> Saturation of soil with excess water, which reduces oxygen available to roots and can damage or kill crops.

When too much irrigation water remains in the soil, root zones lose oxygen. Plants need water, but roots also need oxygen. So more water is not always better. This is a frequent exam trap.

> **Key Term: salinization**
> The buildup of dissolved salts in soil, often caused by irrigation water evaporating and leaving salts behind.

Salinization is especially likely in dry climates. Irrigation water always contains some dissolved salts. If the water evaporates from the soil surface, the salts remain. Over time, this salt buildup reduces soil fertility and can eventually make land difficult or impossible to farm.

The Green Revolution also increased pressure on groundwater. Large-scale irrigation often pumps water from aquifers faster than they recharge. This can lower water tables and threaten long-term water availability.

Synthetic fertilizers were another major input. They supplied nutrients quickly, especially **nitrogen** and **phosphorus**, which often limit plant growth. This is why fertilizer use can sharply increase productivity.

However, fertilizers do not stay neatly in fields. Rain and irrigation can wash them into streams, ponds, wetlands, and lakes.

The environmental chain is important:

- synthetic fertilizer added to fields
- nitrogen and phosphorus move in runoff
- water receives excess nutrients
- algal growth increases
- decomposers break down dead algae
- dissolved oxygen falls
- aquatic organisms may die
  > **Key Term: eutrophication**
  > Nutrient enrichment of water, usually by nitrogen and phosphorus, causing algal blooms and oxygen depletion.

That is one of the most common APES cause-effect sequences.

Pesticides and herbicides also played a major role by reducing crop loss. In theory, if fewer insects, fungi, and weeds damage crops, more of the crop can be harvested. But pesticide use has three big environmental costs students should know.

First, pesticides can harm **non-target species**, including pollinators, natural predators, and other beneficial organisms.

Second, repeated use creates strong selection pressure. The most resistant pests survive and reproduce.

> **Key Term: pesticide resistance**
> The evolution of pest populations so that a pesticide becomes less effective over time.

Third, pesticide residues can move through soil and water, affecting wider the environments.

One subtle point matters here. Green Revolution farming often did not reduce pest problems in the long term as much as people expected. Why? Because monocultures create large, uniform food supplies for pests, and because resistance evolves. So the same practice that raises yield in the short term can make pest control harder later.

> **Test Tip:** When asked for one benefit and one drawback of a Green Revolution practice, be specific. Example: “Irrigation increases yield reliability, but it can cause salinization.” Specific pairings score better than vague statements like “it helps farming but harms the environment.”

### High-yield varieties, monoculture, and the loss of diversity

One of the biggest long-term ecological concerns was the reduction of biodiversity. This happened at two levels.

First, farms often shifted toward **monoculture**, the planting of one crop across a large area. That simplifies planting and harvesting, especially with machinery, but it reduces habitat variety and makes it easier for pests and diseases to spread.

> **Key Term: monoculture**
> The repeated large-scale planting of a single crop species over a wide area.

Second, when farmers replace many local crop varieties with a smaller number of HYVs, the **genetic diversity** of crops decreases. This matters because genetic diversity gives populations a wider range of traits. A more varied crop population is more likely to include resistance to new diseases, new pests, or climatic stress.

If one genetically similar crop dominates a region, a disease outbreak can do more damage because many plants share the same vulnerabilities. This is why reduced diversity is not just an abstract problem; it affects food security itself.

Another distinction students should know is the difference between the original Green Revolution and the later GMO era. The Second Green Revolution, beginning in the **mid-1980s**, included genetic engineering and global policy changes affecting food systems. Examples include **Golden Rice** and **Bt corn**. These are later technologies, not the main defining feature of the original Green Revolution.

Golden Rice was designed to produce more beta-carotene, addressing vitamin A deficiency. Bt corn contains a gene linked to insect resistance. These examples show that later biotechnology often targeted the same problem—food quantity and quality—but through different methods.

> **Key Term: genetically modified organism (GMO)**
> An organism whose genetic material has been altered in a laboratory, often by adding genes that do not occur naturally in that combination.

For APES, the safest distinction is this:

- **Original Green Revolution** = selective breeding + irrigation + fertilizers + pesticides + mechanization
- **Later biotech developments** = GMOs and genetic engineering

Do not collapse these into one idea.

The benefits of the Green Revolution were real. It raised crop yields, improved food supply in many places, and may have reduced pressure to clear even more forests and grasslands by increasing production per acre. But the benefits were uneven. Farmers with irrigation, money, and access to inputs gained more than poorer rainfed farmers. That matters because APES often asks you to evaluate both environmental and human outcomes.

### Evaluating Green Revolution practices in APES terms

AP Environmental Science likes balanced evaluation. The Green Revolution is not simply “good” or “bad.” It is a trade-off between higher production and greater environmental strain.

A strong APES evaluation usually works like this:

Benefit to humans

- increased grain yields
- better food supply
- reduced famine risk in some regions
- more output from existing farmland

Environmental or long-term cost

- groundwater depletion
- waterlogging and salinization
- fertilizer runoff and eutrophication
- pesticide resistance and non-target harm
- lower biodiversity and genetic diversity
- higher fossil fuel use in mechanized farming

Students also need to be careful about uneven impacts. Rainfed farmers often could not benefit as much because HYVs perform best under controlled, high-input conditions. This is a useful explanation if an FRQ asks why Green Revolution gains varied by place.

A compact APES-style judgment would be:

The Green Revolution increased food production by combining HYVs with irrigation, fertilizers, pesticides, and machinery. However, it also increased dependence on water, chemicals, and simplified farming systems, creating soil degradation, pollution, and biodiversity loss.

That is exactly the kind of balanced line that works well in free response.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- The Green Revolution was a mid-20th-century shift toward input-intensive agriculture
- Its main goal was to increase food production for a growing human population
- Core Green Revolution practices included HYVs, irrigation, synthetic fertilizers, pesticides/herbicides, and mechanization
- HYVs in the original Green Revolution were mainly produced through selective breeding, not genetic engineering
- Green Revolution yield gains depended on practices working together as a package
- Irrigation can increase and stabilize yields, but may cause waterlogging, salinization, and groundwater depletion
- Synthetic fertilizers increase crop growth, especially through nitrogen and phosphorus inputs
- Fertilizer runoff can cause eutrophication in aquatic systems
- Pesticides reduce crop losses but can harm non-target species and select for pesticide resistance
- Monoculture supports mechanized farming but reduces biodiversity and can increase pest vulnerability
- Replacing traditional varieties with HYVs can reduce genetic diversity
- Later GMO developments such as Golden Rice and Bt corn are more associated with the later phase of agricultural biotechnology than the original Green Revolution

## Key Terms and Concepts

- Green Revolution
- high-yield crop varieties (HYVs)
- waterlogging
- salinization
- eutrophication
- pesticide resistance
- monoculture
- genetically modified organism (GMO)


The Green Revolution - Green Revolution practices