Which horizon is defined as the topsoil?

• A. A Horizon
• B. O Horizon
• C. B Horizon
• D. C Horizon (bedrock)

Answer: (A)

Soil horizons are distinct layers with different makeup. The topsoil is the upper mineral layer that sits beneath any surface organic litter and consists of mineral material mixed with organic matter from decaying plants. This layer, called the A horizon, is usually darker due to humus and is where most plant roots grow and nutrients are most available. Above it is the organic surface layer, the O horizon, while below it lies the B horizon where minerals and clays accumulate, followed by the C horizon of weathered parent material and the R horizon (bedrock). Therefore, the horizon defined as topsoil is the A horizon.