Teacher's Guide - How do Soils Form? and CLORPT for Short

How do Soils Form? and CLORPT for Short

Introduction
Soils can be deep, shallow, new, old, bright orange, black, or even blue (and anywhere in-between)!  What soil looks like depends on the location that it forms in.  It can take over 500 years to form an inch of topsoil on the surface!

Soil may look simple at first glance, but it is actually a complex and dynamic system that tells a story about Earth’s past and present. When students dig into the ground, they uncover layers called soil horizons, which together form a soil profile. Each layer represents changes that have taken place over hundreds or even thousands of years. Soil begins with parent material, such as weathered rock or materials moved by wind, water, or ice. Over time, natural forces—including climate, living organisms, and the shape of the land—transform this material into the soil we see today.

Soil formation is an ongoing process. Materials are constantly added, removed, moved, and changed. For younger students, this can be understood as soil being “built up and broken down” over time. Older students can explore these processes more deeply as additions, losses, translocations, and transformations. The characteristics of soil are influenced by five key factors: climate, organisms, relief (landscape), parent material, and time—often remembered by the acronym CLORPT.

This section of the book supports students in observing, describing, and explaining how soils form and change. Elementary learners can focus on identifying soil layers and recognizing that soil changes over time, while middle school students can analyze the processes and factors that drive soil formation. Understanding soil helps students make connections between Earth systems, ecosystems, and human impacts, and highlights the importance of soil in supporting life on Earth.

Learning Objectives: 

• Identify and describe soil horizons and recognize that soils are made of layers that form over time.
• Explain how soil forms through processes such as weathering, movement of materials, and decomposition.
• Analyze how the five soil-forming factors (CLORPT) influence soil characteristics in different environments.

NGSS Connections: 

• ESS2.A: Earth Materials and Systems
• ESS3.A: Natural Resources
• PS1.A: Structure and Properties of Matter

Glossary:

• Bedrock
• CLORPT
• Horizon
• Humus
• Parent material
• Slope
• Soil profile
• Weathering

Click here for a Powerpoint  (Note: The revised slide deck will be linked here.)

Discussion Questions: 

1. What is a soil profile? What can it tell us?

• A soil profile shows layers of soil. It tells us what the soil is made of.
• A soil profile shows the arrangement of soil horizons and helps explain how the soil formed over time.
• A soil profile has layers called horizons.  Horizons are designated with the capital letters O, A, E, B, C - from the surface down.  All horizons are not always present.  The most typical profile is A-B-C in fields or O-A-B-C in forests.  The R horizon (consisting of the bedrock) may be very shallow or very deep.

2. How are soil layers (horizons) different from each other?

• Some layers have more plants and roots, and others have more rocks.
• Horizons differ in composition, such as organic matter, mineral content, and evidence of movement like leaching or accumulation.

3. How does soil change over time?

• Soil changes when things are added, washed away, or broken down.
• Soil additions: rain adds water, dust adds minerals, as plants die and animals poop organic matter is added, and humans also add fertilizer.  
• Soil losses: loss of water by evaporation, nutrient up take, and leaching (removing) of nutrients and elements.  
• Soil translocations: gravity pull water and dissolved materials down, organic matter can move in many directions due to critters, clay movement, and an eluviated (leached) horizon develops.  
• Soil transformations: decomposition, weathering, iron rusting (reddening) or dissolving (graying), clay formation. Through these processes, horizons can form and can get thicker, horizons can change colors (may get redder if well drained or get grayer if wet), and the E horizon becomes more evident.

4. What factors affect how soil forms?

• Weather, plants and animals, and land shape affect soil.
• Soil formation is influenced by climate, organisms, relief, parent material, and time (CLORPT).

5. Why are soils different in different places?

• Different places have different weather and plants.
• Variations in soil-forming factors lead to differences in soil properties across regions.

Suggested Activities and Curriculum Connections 

General Soils:

• Lesson Plans - Soil Science Society of America  (All Grades): Explore comprehensive background materials with helpful links to various soil science topics.
• Educational Links - USDA NRCS  (Grades K-6): This collection provides fundamental information and resources from the USDA-NRCS suitable for elementary learners.

Soil Profile Photos and Descriptions:

• State Soils Booklets (All Grades): Access detailed booklets and information regarding specific state soils, including a related student activity.
• Soil Horizons (Grades 4+): Offers an introductory overview of the different layers within a soil profile.
• New England Soil Horizons (Grades 4-12): A presentation providing a thorough overview of soil profiles; while focused on New England, the concepts are broadly applicable.

Soil Formation:

• Soil Formation and Classification  (Grades 8+): A USDA-NRCS deep dive into soil-forming factors (CLORPT). Note: taxonomic sections may be advanced for K-8 students.
• New England Soil Genesis (Grades 4-12): Provides teacher-level background and regional examples of soil development that can be adapted for upper elementary lessons.
• Soil Mini Monoliths (Grades 4+): Instructions for creating soil profile cards. For best results, use dry, pulverized materials rather than coarse sand.

Other Suggested Activities:

• Local Soil Investigation
  • Collect soil from backyards or the school yard. Observe and describe soil using senses.
  • Collect soil from backyards or the school yard. Analyze soil characteristics and infer formation processes.
• Soil Layer Model (Grades 3–5 adaptation / Grades 6–8 extension)
  • Students build a simple layered model using different materials to represent soil horizons.
  • Students label horizons and explain how each formed, including processes like leaching and accumulation.
• Soil in a Jar Investigation
  • Observe how soil separates into layers after shaking with water.
  • Measure layer proportions and connect to soil composition and particle size. 
• CLORPT Exploration Activity: Soil Formation & Edible Horizons
  • Introduce factors with visuals (layering of materials).
  • Comparing soils formed from different factors (Food represents the soil forming factors) 
• Weathering and Erosion Demonstration
  • Observe how water moves soil.
  • Connect observations to real-world processes like runoff, leaching, and sediment transport.
• “Soil Story” Writing Activity (ELA Connection)
  • Write a short story about how soil forms.
  • Write a scientific explanation describing soil formation processes using vocabulary.