Soil Science
Grade 6 · Science · Study Guide
Dig into what soil is made of, how it forms, and why it's critical for life on Earth.
Soil might look like just dirt, but it is one of Earth's most complex and precious resources. It takes about 500 years to form just 2.5 cm of topsoil naturally. Soil is a mixture of weathered rock particles, mineral nutrients, water, air, and organic matter from decomposed living things.
Soil forms through weathering — the breaking down of rocks into smaller particles. Physical weathering (like ice expanding in cracks) and chemical weathering (like rainwater dissolving minerals) both break rock into particles over thousands of years. Decomposers like earthworms, fungi, and bacteria break down dead organic matter into humus — the dark, nutrient-rich part of soil that gives topsoil its fertility.
Soil has layers called horizons. The O horizon (if present) is organic matter like leaf litter on the surface. The A horizon, or topsoil, is dark and fertile — most plants grow their roots here. The B horizon (subsoil) has less organic matter and more minerals that have washed down from above. The C horizon is partly weathered rock, and bedrock lies below.
Soil texture — how much sand, silt, and clay it contains — affects how it holds water and nutrients. Sandy soil drains quickly but doesn't hold nutrients well. Clay soil holds water but can become waterlogged. The ideal soil for growing crops is loam, a balanced mixture of sand, silt, and clay. Healthy soil also contains billions of microorganisms in every teaspoon — bacteria, fungi, and tiny animals that cycle nutrients and keep the ecosystem going.
| Horizon | Name | Description |
|---|---|---|
| O | Organic layer | Leaf litter and partially decomposed organic material |
| A | Topsoil | Dark, fertile; rich in humus and minerals; most plant roots here |
| B | Subsoil | Less organic matter; minerals leached down from A horizon |
| C | Parent material | Partially weathered rock fragments |
| R | Bedrock | Solid unweathered rock |
Key terms
- Soil
- A mixture of weathered rock particles, organic matter, water, air, and living organisms.
- Weathering
- The breakdown of rocks into smaller particles by physical and chemical processes.
- Humus
- Dark organic matter in soil formed from the decomposition of plants and animals; increases fertility.
- Soil horizon
- A distinct layer of soil, each with different characteristics (O, A, B, C horizons).
- Topsoil
- The uppermost layer of soil (A horizon); most fertile and important for plant growth.
- Loam
- Ideal garden soil with a balanced mix of sand, silt, and clay.
- Erosion
- The wearing away and transport of soil by wind, water, or ice.
- Decomposition
- The breakdown of dead organic matter by decomposers like bacteria and fungi.
Flashcards
- Soil
- A mix of weathered rock, organic matter, water, air, and living organisms.
- Humus
- Dark, organic-rich material in soil formed by decomposition; improves fertility.
- Topsoil (A horizon)
- The most fertile upper soil layer; where most plant roots grow.
- Weathering
- Physical and chemical breakdown of rock into smaller soil particles.
- Erosion
- Transport and removal of soil by wind, water, or ice.
- Loam
- Ideal soil type — balanced mix of sand, silt, and clay.
Practice questions
Which soil layer is the most fertile and best for plant growth?
- O horizon
- A horizon (topsoil)
- B horizon (subsoil)
- C horizon
Show answer
Answer: A horizon (topsoil)
Topsoil (the A horizon) contains the most humus and mineral nutrients, making it the most productive layer for plant growth.
Humus forms from the decomposition of plants and animals.
Show answer
Answer: True
Decomposers break down dead plants and animals into humus — the dark organic material that gives fertile soil its rich color and nutrient content.
The wearing away of soil by wind or water is called ___.
Show answer
Answer: erosion
Erosion moves topsoil away from fields and hillsides. It's a major threat to agriculture and can be reduced by planting cover crops or terracing slopes.
Which soil type is considered ideal for growing most crops?
- Sandy soil
- Clay soil
- Loam
- Gravel
Show answer
Answer: Loam
Loam is about 40% sand, 40% silt, and 20% clay. This balance allows good drainage while retaining moisture and nutrients for plants.
Soil is formed quickly — it only takes a few years to make new topsoil.
Show answer
Answer: False
Forming just 2.5 cm of topsoil takes about 500 years. This is why soil is considered a non-renewable resource on human timescales.
Fun facts
- One teaspoon of healthy soil contains more living organisms than there are people on Earth.
- Earthworms are soil engineers — one earthworm can eat its own body weight in soil every day, improving drainage and nutrient mixing.
- About one-third of the world's soil is degraded due to erosion, overuse, and pollution, threatening global food supplies.
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