Describe the soil horizons and the factors of soil formation, and classify soil texture using sand, silt, and clay proportions.
Soil forms extremely slowly by human standards -- it can take 500 years or more of weathering to form just one inch (about 2.5 cm) of new topsoil, which is exactly why soil is often treated as a renewable resource only in theory and a fragile one in practice. A vertical slice through soil reveals horizons (layers): O (organic litter on the surface), A (topsoil, rich in decomposed organic matter called humus), B (subsoil, where minerals leached from above accumulate), C (weathered parent material), and R (unweathered bedrock).
Soil formation depends on five interacting factors: climate, living organisms, relief/topography (slope and drainage), parent material (the underlying rock or sediment it forms from), and time. Separately, soil TEXTURE depends on the relative proportions of sand, silt, and clay particle sizes present: sandy soil is gritty and drains quickly but holds water and nutrients poorly; clay soil is smooth and sticky when wet, drains slowly, and holds nutrients well but can compact and suffocate roots; loam, a balanced mixture of all three, is generally the most prized soil for agriculture because it drains reasonably well while still holding onto water and nutrients.
Mastery looks like: They can list the soil horizons in order, correctly classify a described sample's texture, and explain why topsoil formation is so slow.
Common stumbles: Treating soil as a simple, inert material ("dirt") rather than a complex, slowly formed system with distinct layers and living components.