How do soils develop from weathered rock and what are typical soil horizons?

How do soils develop from weathered rock and what are typical soil horizons?

• A. Soils form from weathering plus organic matter; horizons commonly include O, A, E, B, C, and R.
• B. Soils form only from organic matter; horizons include X and Y.
• C. Soils cannot develop without water; horizons include O and B only.
• D. Soils form from bedrock only and have no horizons.

Answer: (A)

Soils form from the combination of weathered rock and input of organic matter, and over time this builds a layered profile with distinct horizons. The uppermost layer is the O horizon, rich in organic material from plant and animal debris. Just below is the A horizon, a mineral-rich surface layer with some mixing of organic matter. In many soils, an E horizon appears beneath A, formed by leaching that removes clay and minerals, leaving lighter material behind. Underneath, the B horizon accumulates materials leached from above, such as clay, oxides, and organic compounds. Deeper still is the C horizon, consisting of partially weathered parent material, and the bottom layer is R, the unweathered or consolidated bedrock. This O–A–E–B–C–R sequence reflects how soils develop and how their properties change with depth. The answer matches the idea that soils arise from weathering of rock plus organic matter and that a typical soil profile includes these standard horizons. Other choices either imagine soils forming from organic matter alone with nonstandard horizons, or rely on water as the only requirement with a limited set of horizons, or claim there is no horizon at all.

Soils form from the combination of weathered rock and input of organic matter, and over time this builds a layered profile with distinct horizons. The uppermost layer is the O horizon, rich in organic material from plant and animal debris. Just below is the A horizon, a mineral-rich surface layer with some mixing of organic matter. In many soils, an E horizon appears beneath A, formed by leaching that removes clay and minerals, leaving lighter material behind. Underneath, the B horizon accumulates materials leached from above, such as clay, oxides, and organic compounds. Deeper still is the C horizon, consisting of partially weathered parent material, and the bottom layer is R, the unweathered or consolidated bedrock. This O–A–E–B–C–R sequence reflects how soils develop and how their properties change with depth.

The answer matches the idea that soils arise from weathering of rock plus organic matter and that a typical soil profile includes these standard horizons. Other choices either imagine soils forming from organic matter alone with nonstandard horizons, or rely on water as the only requirement with a limited set of horizons, or claim there is no horizon at all.