AP Environmental Science

## Learning Outcomes

- Explain how damaging agricultural practices degrade soil and water systems
- Distinguish between **runoff** and **leaching**, and between **eutrophication** and **hypoxia**
- Identify how tillage, monocropping, overgrazing, irrigation, and CAFO waste create environmental harm
- Use cause-and-effect chains to explain agricultural pollution on AP-style questions
- Apply historical and modern examples such as the **Dust Bowl** and nutrient pollution in major watersheds
- Propose sustainable alternatives such as cover crops, conservation tillage, rotational grazing, and improved irrigation

## AP Environmental Science Syllabus

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

- Know that soil erosion can be accelerated by agriculture, especially through tillage, bare fields, overgrazing, and farming on slopes
- Identify the difference between **salinization** and **waterlogging** as irrigation-related soil problems
- Explain how excess fertilizer or manure causes **nutrient runoff** to surface water and **leaching** to groundwater
- Understand how nutrient pollution leads to **eutrophication**, algal blooms, and **hypoxia**
- Know that agricultural pollution is often **nonpoint source pollution**
- Recognise how monocropping reduces biodiversity and can increase fertilizer and pesticide dependence
- Understand how CAFO waste contributes to nutrient pollution, pathogens, and water contamination
- Use examples such as the **Dust Bowl** to show the effects of poor soil management

## 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. How does conventional tillage increase soil erosion?
2. What is the difference between salinization and waterlogging?
3. How can excess nitrogen and phosphorus from farms lead to hypoxia in water?
4. Why is agricultural pollution often described as nonpoint source pollution?
5. Name two ways overgrazing damages soil.

## Introduction

Modern agriculture can produce very high yields, but it often does so by putting heavy pressure on soil, water, and nearby habitatss. On the AP Environmental Science exam, this topic is not just about listing bad farming methods. You need to explain the mechanism: what farmers do, how that changes the land or water, and what environmental result follows.

A useful way to revise this subtopic is to group damaging practices into two big categories. First, there are **soil-focused harms** such as erosion, compaction, and nutrient depletion. Second, there are **water-focused harms** such as runoff, leaching, eutrophication, and groundwater contamination. Many AP questions ask you to connect these categories, because soil damage often leads directly to water pollution.

> **Key Term: soil erosion**
> The removal and transport of soil by water, wind, or human activity; it is a natural process but can be greatly accelerated by poor land use.

### Soil degradation from tillage, monocropping, and overgrazing

A major agricultural problem is the loss of topsoil. Topsoil contains much of the soil's organic matter, nutrients, and biological activity. When it is removed, fertility drops sharply. Since soil forms very slowly, this is a serious long-term problem rather than a temporary inconvenience.

**Conventional tillage** increases erosion because it breaks up soil structure and leaves the ground exposed. Crop residue that would normally cushion rainfall and reduce wind exposure is removed or buried. Rain then hits loose soil directly, dislodging particles. Water runs across the surface, carrying sediment away. This can become a feedback loop: erosion reduces infiltration, which increases runoff, which causes more erosion.

> **Key Term: conventional tillage**
> Intensive plowing that disturbs soil structure and leaves soil more exposed to wind and water erosion.

Tillage also harms soil in ways students often miss. It can break apart soil aggregates, reduce water-holding capacity, and create compact layers such as hardpan or plow pan. It may also speed the decomposition of soil organic matter, reducing long-term fertility. That means the problem is not just that soil moves away; the soil left behind is often less productive too.

**Monocropping** or **monoculture** creates a different kind of soil problem. Repeatedly planting the same crop draws down the same nutrient profile year after year. A corn field, for example, keeps demanding the same major nutrients. Over time, farmers often respond by applying more synthetic fertilizer. This can maintain yields in the short term, but it ties the farm to higher chemical inputs and raises the risk of runoff and leaching.

> **Key Term: monocropping**
> Repeated planting of the same crop on the same land, which can reduce biodiversity, deplete specific nutrients, and increase pest vulnerability.

Monocropping also lowers biodiversity. A more diverse system often contains a wider range of root structures, soil organisms, and natural pest controls. A uniform field is easier for pests and diseases to spread through, so monocropping often pushes farmers toward greater pesticide use as well.

**Overgrazing** is another major cause of soil degradation. When livestock remove vegetation faster than it can regrow, the protective ground cover disappears. Plant roots that once held soil in place shrink or die back. At the same time, animal trampling compacts soil, reducing pore space and infiltration. Then rainfall is more likely to run off the surface instead of soaking in.

This is why overgrazing can lead to **desertification**, especially in dry regions. The chain is important:

overgrazing -> vegetation loss -> less root binding and infiltration -> more erosion and compaction -> poorer regrowth -> even less vegetation

That chain is classic APES reasoning.

> **Test Tip:** If a free-response question asks how farming harms soil, do not stop at “erosion increases.” Add the mechanism: less vegetation cover, lower infiltration, more runoff, topsoil loss, then reduced fertility.

### Irrigation damage: salinization, waterlogging, and groundwater stress

Irrigation is not automatically harmful. It is essential in many dry regions. The damage comes from overirrigation, poor drainage, and repeated use without managing salt and water balance.

**Salinization** happens when irrigation water evaporates and leaves dissolved salts behind in the soil. This is especially common in arid and semiarid climates, where evaporation is high and rainfall is too low to flush salts away naturally. Over time, salts accumulate in the root zone and interfere with plant growth.

> **Key Term: salinization**
> The buildup of soluble salts in soil to levels that reduce soil function and crop productivity, often due to irrigation and high evaporation.

Salts make it harder for plant roots to take up water, even when water is present. So a field can be irrigated but plants still suffer water stress because of the osmotic effect of excess salt. This is why salinized land may show stunted growth and falling yields before it becomes nearly unusable.

**Waterlogging** is different. Waterlogging means the soil becomes saturated with excess water, often because of overirrigation and poor drainage. Water fills soil pores and reduces the oxygen available to roots. Plants need oxygen for root respiration, so prolonged saturation damages or kills them.

> **Key Term: waterlogging**
> Saturation of soil with excess water, reducing oxygen available to roots and harming plant growth.

Students often confuse these two. Salinization is a **salt buildup** problem. Waterlogging is a **too much water in the soil** problem. They often occur together because both are linked to poor drainage and rising water tables, but they are not the same process.

Large-scale irrigation can also deplete aquifers. Agriculture uses most of the freshwater withdrawn by humans, and when groundwater is pumped faster than it recharges, water tables fall. This raises pumping costs and threatens long-term water availability. In some regions, this can eventually make current farming systems unsustainable.

A comparison of irrigation methods helps here. Flood and furrow irrigation are cheap and common but waste large amounts of water through evaporation, runoff, and uneven infiltration. Drip irrigation is far more efficient because it delivers water directly to roots, reducing both waste and the risk of salinization and waterlogging.

> **Test Tip:** If an APES question asks for an irrigation-related harm, be precise. Say “salinization due to evaporation leaving salts behind” or “waterlogging due to saturated soil reducing root oxygen,” not just “irrigation damages soil.”

### Water pollution from fertilizer, manure, and CAFO waste

Many agricultural water problems begin with nitrogen and phosphorus. These nutrients are necessary for crop growth, but when more is applied than crops can absorb, they become pollutants.

There are two main pathways. **Runoff** moves pollutants across the land surface into streams, ponds, lakes, and coastal waters. **Leaching** moves dissolved substances downward through soil into groundwater.

> **Key Term: runoff**
> The movement of water and pollutants across the land surface into nearby water bodies.
>
> **Key Term: leaching**
> The downward movement of dissolved substances through soil, often carrying nitrates, salts, or pesticides into groundwater.

This distinction matters on the exam. Surface water problems usually come from runoff. Groundwater contamination is often linked to leaching, especially of nitrate because nitrate is highly soluble.

Once nitrogen and phosphorus enter surface water, they can trigger **eutrophication**. Nutrients fuel rapid algal or plant growth. When the algae die, decomposers break them down and consume dissolved oxygen. If oxygen falls to very low levels, the result is **hypoxia**. Fish and many other aquatic organisms may die or leave the area.

> **Key Term: eutrophication**
> Nutrient enrichment of water, especially by nitrogen and phosphorus, causing excessive algal or plant growth.
>
> **Key Term: hypoxia**
> Low dissolved oxygen in water, often caused when decomposers use oxygen while breaking down algal blooms.

The sequence is one of the most tested in APES:

excess fertilizer or manure -> runoff into water -> eutrophication -> algal bloom -> decomposition -> oxygen depletion -> hypoxia or dead zone

Do not skip steps in this chain. AP scorers reward the full process.

Agricultural nutrient pollution is usually **nonpoint source pollution** because it comes from many fields, not one obvious pipe. That makes it harder to regulate and monitor than a single discharge source.

**CAFOs** intensify this issue. In concentrated animal feeding operations, large numbers of animals are kept in small spaces, producing huge amounts of manure and wastewater. If manure lagoons leak, overflow in storms, or manure is spread improperly on fields, nutrients, pathogens, antibiotics, and other contaminants can enter surface water or groundwater.

> **Key Term: CAFO**
> Concentrated animal feeding operation; a facility where large numbers of livestock are confined, producing concentrated waste that can pollute air and water.

CAFO pollution is especially serious because the waste is concentrated. Instead of manure being spread naturally over wide grazing land, it accumulates in one place. This raises the chance of spills and nutrient loading. CAFO waste is also linked to methane emissions and ammonia release, so it connects Unit 5 to climate and air pollution topics.

A strong real-world example is nutrient delivery through major farm watersheds into coastal waters. Agricultural nitrogen and phosphorus carried by river systems can contribute to seasonal hypoxic zones in places like the Gulf of Mexico. Even if a question does not name that example, it is a good model for thinking.

### Historical example and sustainable alternatives students should propose

The **Dust Bowl** of the **1930s** in the U.S. Great Plains is a classic example of severe wind erosion made worse by poor farming practices and drought. Extensive plowing removed native grasses that had anchored the soil. When drought came, exposed topsoil dried out and strong winds carried it away in massive dust storms.

This example matters because it shows that environmental damage usually has multiple causes. It was not just drought. It was drought plus poor land management. AP questions often reward that kind of multi-causal explanation.

When asked to propose solutions, connect each damaging practice to a matching sustainable alternative:

- erosion from tillage -> **no-till farming**, **cover crops**, **contour plowing**, **terracing**
- nutrient depletion from monocropping -> **crop rotation**, especially including legumes
- overgrazing -> **rotational grazing**
- runoff into streams -> **vegetative buffer zones**
- irrigation waste -> **drip irrigation** and improved drainage
- pesticide dependence -> **integrated pest management**

These work because they target the mechanism of harm. Cover crops keep soil covered. Buffers slow runoff and trap sediment. Crop rotation breaks pest cycles and restores nutrient balance. Rotational grazing allows vegetation to recover.

A useful APES comparison is that some methods solve more than one problem at once. For example, no-till farming reduces erosion, keeps more organic matter in soil, and can reduce carbon loss from plowing. Buffers reduce both sediment pollution and nutrient pollution.

> **Test Tip:** For “propose a solution” prompts, do more than name a practice. Explain how it reduces the specific harm in the question. “Use cover crops” is weak. “Use cover crops to keep soil covered between harvests, reducing erosion and nutrient runoff” is stronger.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Damaging agricultural practices often harm both soil systems and water systems at the same time
- Conventional tillage increases erosion by disturbing soil and leaving it exposed to wind and water
- Monocropping depletes specific nutrients, lowers biodiversity, and can increase dependence on fertilizers and pesticides
- Overgrazing removes vegetation, increases compaction, reduces infiltration, and can contribute to desertification
- Salinization is salt buildup in soil, while waterlogging is excess water saturating soil and reducing root oxygen
- Runoff moves pollutants across land into surface water, while leaching moves dissolved pollutants downward into groundwater
- Excess nitrogen and phosphorus can cause eutrophication, algal blooms, and hypoxia
- Agricultural nutrient pollution is often nonpoint source pollution
- CAFO waste can contribute nutrients, pathogens, and other contaminants to water supplies
- The Dust Bowl is a major historical example of severe erosion linked to drought and poor farming practices
- Sustainable responses include no-till farming, cover crops, crop rotation, buffers, drip irrigation, and rotational grazing

## Key Terms and Concepts

- soil erosion
- conventional tillage
- monocropping
- salinization
- waterlogging
- runoff
- leaching
- eutrophication
- hypoxia
- CAFO


Impact of Agricultural Practices - Damaging agricultural practices