Final answer:
Soil horizons are characterized by physical, chemical, and biological properties, not just physical features. The A horizon is known as topsoil, and the B horizon is called subsoil, each with different compositions and roles in the soil profile.
Step-by-step explanation:
Soil horizons are defined by a variety of characteristics including physical features, which indeed contribute to their distinction, but they are also characterized by their chemical and biological composition.
Therefore, the statement 'Soil horizons are defined by physical features' is not entirely accurate. Soil profiles consist of several layers, namely the O horizon, A horizon, B horizon (subsoil), and C horizon (soil base), each with distinct properties.
The O horizon contains organic matter such as humus, while the A horizon, also known as topsoil, has a mix of organic and inorganic materials, is crucial for plant growth, and is dark in color due to the organic content. The B horizon, or subsoil, is where fine material accumulates, and the C horizon is composed of parent material and the materials that have broken down to form soil.