AP Environmental Science

## Learning Outcomes

- Understand how **pedogenesis** turns parent material into soil over time
- Identify the five major soil-forming factors in **CLORPT**
- Explain the difference between **weathering** and **erosion**
- Describe the four main soil-forming processes: additions, losses, transformations, and translocations
- Interpret a basic **soil profile** and the main horizons: O, A, E, B, C, and R
- Apply soil formation ideas to examples such as slopes, forests, grasslands, and primary succession

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand the soil formation process, with a focus on the following syllabus points:

- Know that soil forms from **weathered parent material** and decomposed organic matter over long periods of time
- Identify the five soil-forming factors in **CLORPT**: climate, organisms, relief, parent material, and time
- Explain how **physical, chemical, and biological weathering** contribute to soil development
- Understand the difference between **soil-forming factors** and **soil-forming processes**
- Identify the four soil-forming processes: **additions, losses, transformations, and translocations**
- Interpret a **soil profile** and the common horizons: O, A, E, B, C, and R
- Explain how climate, slope, and parent material affect soil depth, texture, drainage, and horizon development
- Understand that **weathering breaks down rock in place**, while **erosion removes and transports material**

## 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 does **CLORPT** stand for in soil science?
2. How is **weathering** different from **erosion**?
3. What are the four main processes of soil development?
4. Why do steep slopes usually have thinner soils than flat land?
5. Which soil horizon is usually called **topsoil**?

## Introduction

## Introduction

Soil may look simple, but for AP Environmental Science it is one of the most important links between the lithosphere, hydrosphere, atmosphere, and biosphere. Soil stores water, holds nutrients, supports plant growth, filters water, and affects how biotic systemss develop. Because of that, understanding how soil forms is more than just geology. It helps explain agriculture, succession, water quality, and why human disturbance can cause long-term environmental damage.

On the exam, soil formation questions often test cause-and-effect thinking. You may be asked why one region has deeper soil than another, how climate changes weathering rates, or why a certain soil profile forms in a forest rather than a grassland. That means you need to know both the sequence of soil formation and the factors that control it.

> **Key Term: pedogenesis**
> The process of soil formation and development from weathered parent material and organic matter over time.

### How soil begins: from rock to regolith to true soil

Soil does not appear all at once. It starts with rock. The first step is **weathering**, which breaks down rock into smaller particles or changes its minerals chemically. The loose material produced above bedrock is often called regolith. Regolith is not automatically fertile soil. It becomes soil only when organic matter, water movement, air, and biological activity begin to change it.

> **Key Term: weathering**
> The breakdown or alteration of rock in place by physical, chemical, or biological processes.

A common exam error is confusing weathering with erosion. Weathering happens **in place**. Erosion happens when particles are then moved away by water, wind, ice, or gravity.

> **Key Term: erosion**
> The removal and transport of rock or soil material from one place to another.

This distinction matters because soil formation depends on weathering, but too much erosion can remove soil faster than it forms. So one process builds the raw material; the other can strip it away.

There are three main kinds of weathering involved in soil formation.

**Physical weathering** breaks rock into smaller pieces without changing its chemical composition. Examples include freeze-thaw cracking, abrasion by wind, and grinding by moving water. This matters because smaller particles expose more surface area, which speeds later chemical reactions.

**Chemical weathering** changes the minerals themselves. Water, oxygen, and acids react with minerals to form new substances. For example, some minerals break down into clays. Chemical weathering is generally faster in warm, wet conditions because heat and moisture speed reactions.

**Biological weathering** is caused by living organisms. Plant roots pry apart cracks in rock. Lichens and roots release acids. Soil organisms mix and disturb material. This is often where students miss the link between biology and geology: living things do not just use soil, they help create it.

A useful sequence to remember is:

1. Rock is weathered
2. Regolith forms
3. Organisms colonize the surface
4. Organic matter accumulates
5. Water moves materials through the profile
6. Horizons become distinct

This sequence is especially clear in **primary succession**. On bare rock, pioneer species such as lichens begin weathering the rock and adding organic matter. Thin soil forms, then grasses and larger plants can establish. Soil formation is therefore one reason succession can progress from bare substrate to more complex communities.

> **Test Tip:** If an FRQ asks how soil begins in a newly exposed surface, start with weathering of parent rock, then add pioneer species, organic matter, and horizon development.

### CLORPT: why soils differ from place to place

A standard way to explain soil formation is **CLORPT**. This is the set of five major soil-forming factors: climate, organisms, relief, parent material, and time. These factors explain **why** one soil differs from another.

> **Key Term: CLORPT**
> The five major soil-forming factors: climate, organisms, relief, parent material, and time.

#### Climate

Climate is often the strongest control because it affects both weathering and biological activity. Warm, wet climates usually produce faster chemical weathering and stronger leaching. Cold or dry climates usually slow these processes.

Why? Warm temperatures speed chemical reactions. Water allows minerals to dissolve and move. More rainfall usually means more downward movement of dissolved materials, which can produce clearer soil horizons.

But more rainfall does not always mean more fertile soil. In tropical climates, intense rainfall can leach nutrients from the upper horizons, leaving soils that look deep and strongly developed but are nutrient-poor once the vegetation is removed. Students often assume thick soil means fertile soil. That is not always true.

In dry regions, by contrast, less leaching means certain minerals may accumulate rather than wash away. So climate controls not just the **rate** of soil formation, but also the **type** of soil profile that develops.

#### Organisms

Organisms include plants, animals, fungi, microbes, and humans. They affect soil in several ways.

Plants add leaf litter and roots, which become humus after decomposition. Roots also break rock and alter soil chemistry. Burrowing animals mix soil layers. Microbes break down dead material and recycle nutrients. Earthworms improve mixing and structure.

The A horizon is often strongly shaped by organisms because this is where most biological activity occurs. In grasslands, deep root systems add large amounts of organic material to topsoil, often making the A horizon thick and dark. In forests, litter tends to build at the surface, so the O horizon may be more obvious.

Students often treat “organisms” as meaning only organic matter. That is too narrow. Organisms shape soil mechanically, chemically, and biologically.

#### Relief

Relief means topography: slope, elevation, aspect, and drainage conditions.

> **Key Term: relief**
> The topographic features of a location that affect drainage, erosion, sunlight exposure, and soil depth.

Steep slopes usually have thinner soils because runoff removes material before deep horizons can develop. Flatter land often has thicker soils because water infiltrates more and material accumulates. Aspect matters too. A slope facing stronger sunlight may be warmer and drier than one facing less sunlight, which changes evaporation and biological activity.

Think in terms of water movement. On a steep hill, water runs downslope quickly, carrying particles with it. On flatter land, water moves more slowly, so there is more time for infiltration, weathering, and horizon formation.

This is a frequent APES application point: same climate, different topography, different soils.

#### Parent material

Parent material is the original mineral or organic substance from which soil develops.

> **Key Term: parent material**
> The original rock or sediment from which a soil forms.

Parent material affects soil texture, mineral content, drainage, and initial fertility. For example, material rich in quartz often weathers into sandy soil. Shale tends to produce finer particles, including more silt and clay.

That matters because sandy soils usually drain quickly and lose nutrients more easily, while clay-rich soils retain more water and nutrients but may have poor permeability. So parent material helps set the starting conditions for soil development.

However, parent material is not the whole story. Given enough time and strong climate effects, soils from different starting materials can become more altered. On the exam, avoid saying parent material alone determines soil character. It influences the soil strongly, but climate and time may modify that influence.

#### Time

Time allows the other factors to act. Young soils tend to have weak or poorly developed horizons. Older soils usually show more weathering and stronger differentiation between layers.

But “older” does not simply mean “better.” An older soil may be deeper and more developed, yet also more leached or less fertile. Time increases change; it does not guarantee ideal agricultural quality.

A simple exam rule is: more time usually means more horizon development, assuming the soil is not constantly eroded or disturbed.

> **Test Tip:** When asked why one soil is more developed than another, use a chain such as: longer time + stable surface + less erosion = more distinct horizons.

### How soils change: additions, losses, transformations, and translocations

CLORPT explains why soils differ. The four main soil-forming processes explain **how** soils develop.

> **Key Term: translocation**
> The movement of soil materials from one horizon to another, often by water.

The four processes are:

- **Additions**
- **Losses**
- **Transformations**
- **Translocations**

#### Additions

These are materials added to the soil. Examples include leaf litter, dead organisms, windblown dust, alluvium, and rainwater inputs. Additions help build the upper soil and increase organic matter.

If a grassland keeps adding root material year after year, the topsoil becomes darker and richer. If wind deposits fine sediment, the texture of the surface may change.

#### Losses

These are materials removed from the soil. Losses happen through erosion, leaching, plant uptake, and decomposition products leaving the system.

Losses can reduce fertility, thin horizons, and weaken structure. A steep slope with heavy rainfall may lose topsoil quickly. A sandy soil may lose dissolved nutrients through leaching.

#### Transformations

These are changes to materials already in the soil. Rock minerals weather into smaller particles or new minerals such as clays. Organic matter decomposes into humus.

Transformations are crucial because they turn raw material into soil material with different chemical and physical properties. Without transformations, there would be loose broken rock, but not developed soil.

#### Translocations

These are movements of material within the profile, often caused by water. Clay, iron, aluminum, and dissolved organic matter may move downward from upper horizons and accumulate lower down.

This helps create the characteristic pattern of horizons. Upper layers may lose materials, while lower layers gain them.

A common extension of this is **eluviation** and **illuviation**. Eluviation means washing out from an upper horizon, usually the E horizon. Illuviation means washing in and accumulating in a lower horizon, usually the B horizon. These terms may appear in some materials, but the central APES idea is simpler: materials are moved from upper layers to lower layers.

### Soil horizons: reading the soil profile

A **soil profile** is a vertical section showing horizons from the surface downward.

> **Key Term: soil profile**
> A vertical cross-section of soil showing its horizons or layers.

The standard horizon sequence is:

- **O horizon**: organic litter and humus
- **A horizon**: topsoil, minerals mixed with organic matter
- **E horizon**: leached layer, often lighter in color, not always present
- **B horizon**: subsoil, zone of accumulation
- **C horizon**: partially weathered parent material
- **R horizon**: bedrock
  > **Key Term: horizon**
  > A distinct soil layer with characteristic composition and properties.

The **O horizon** is rich in decomposing leaves and organic debris. The **A horizon** is usually called topsoil and is often the most biologically active. The **E horizon** is a leached zone where materials have been removed. The **B horizon** is where those materials often accumulate. The **C horizon** is weakly altered parent material, and **R** is hard bedrock below.

Students often assume every soil has every horizon. It does not. The E horizon, in particular, is not always present.

The profile reflects both the four processes and CLORPT factors. For example:

- Heavy rainfall can make E and B horizons more distinct through leaching and accumulation
- Grassland organisms can make the A horizon thicker
- Steep slopes may leave only shallow A and C horizons because erosion removes upper material

That is why the profile is not just a diagram to memorize. It is evidence of the processes that formed the soil.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- **Pedogenesis** is the process of soil formation and development
- Soil forms from **weathered parent material** plus organic matter over long periods of time
- **Weathering** breaks down rock in place, while **erosion** removes and transports material
- The three main forms of weathering are physical, chemical, and biological
- The five main soil-forming factors are **climate, organisms, relief, parent material, and time**
- **CLORPT** explains why soils differ from place to place
- The four main soil-forming processes are **additions, losses, transformations, and translocations**
- Warm, wet climates usually increase chemical weathering and leaching
- Steep slopes often have thinner soils because runoff and erosion remove material
- Parent material affects soil texture, mineral content, and drainage
- A soil profile commonly includes **O, A, E, B, C, and R horizons**
- The **A horizon** is usually topsoil, and the **E horizon** is not always present

## Key Terms and Concepts

- pedogenesis
- weathering
- erosion
- CLORPT
- relief
- parent material
- translocation
- soil profile
- horizon


Soil Formation and Erosion - Soil formation process