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Earth Science
Grade 4-10
Soil Layers & Composition Cheat Sheet
A printable reference covering soil horizons, soil composition, particle sizes, permeability, porosity, humus, and weathering for grades 4-10.
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Soil is a mixture of minerals, organic matter, water, air, and living organisms that forms in layers over time. This cheat sheet explains how soil horizons make a vertical profile from the surface down to bedrock. Students need these ideas to understand plant growth, erosion, water movement, and how landscapes change. It is useful for reviewing diagrams, comparing soil types, and connecting soil to ecosystems.
Key Facts
- The O horizon is the surface organic layer made mostly of leaf litter, decomposing plants, and humus.
- The A horizon is topsoil, and it usually contains minerals, humus, roots, water, air, and many organisms.
- The B horizon is subsoil, where clay, iron, and other minerals often collect after being washed down from above.
- The C horizon is weathered parent material, made of partly broken rock with little organic matter.
- Soil composition is often described as about 45% minerals, 25% water, 25% air, and 5% organic matter in healthy loam soil.
- Particle size controls texture: sand is the largest, silt is medium-sized, and clay is the smallest.
- Porosity is the amount of pore space in soil, and it can be estimated with porosity = pore volume / total volume x 100%.
- Permeability is how easily water moves through soil, and sandy soil usually has higher permeability than clay soil.
Vocabulary
- Soil Horizon
- A soil horizon is a distinct layer of soil that has different color, texture, composition, or organic content from nearby layers.
- Topsoil
- Topsoil is the upper soil layer where most plant roots, humus, minerals, and soil organisms are found.
- Humus
- Humus is dark, decomposed organic matter that adds nutrients and helps soil hold water.
- Parent Material
- Parent material is the rock or sediment from which soil forms through weathering.
- Porosity
- Porosity is the percentage of a soil's total volume that is made of pore spaces filled with air or water.
- Permeability
- Permeability is the ability of soil to let water pass through its connected pore spaces.
Common Mistakes to Avoid
- Confusing topsoil with bedrock is wrong because topsoil is rich in roots and organic matter, while bedrock is solid rock beneath the soil profile.
- Thinking all soil layers have the same composition is wrong because each horizon has different amounts of minerals, humus, water, air, and weathered rock.
- Saying clay has the largest particles is wrong because clay has the smallest particles, while sand has the largest particles.
- Assuming high porosity always means high permeability is wrong because pores must be connected for water to flow through them easily.
- Forgetting that soil forms slowly is wrong because weathering, decomposition, and horizon development can take hundreds to thousands of years.
Practice Questions
- 1 A soil sample has 30 mL of pore space in a total volume of 100 mL. What is its porosity percentage?
- 2 A healthy loam soil sample has a total volume of 200 cm3. Using the common 45% mineral, 25% water, 25% air, and 5% organic matter model, how many cm3 are minerals?
- 3 Put these soil horizons in order from the surface downward: C horizon, A horizon, O horizon, B horizon.
- 4 A farmer wants soil that holds nutrients but also drains well. Explain why a balanced loam is usually better than pure sand or pure clay.