What component is present in topsoil that classifies it as such?

Topsoil is characterized primarily by its rich organic material content. This organic matter, which includes decomposed plant and animal material, significantly enhances the soil's fertility and its ability to retain moisture. It provides essential nutrients for plant growth and helps create a soil structure that promotes aeration and drainage.

While components like sand, clay, and gravel can be present in topsoil, they do not define it. Sand and gravel contribute to the soil's texture and drainage properties, whereas clay can affect compaction and water retention. However, without a significant amount of organic material, the soil would not be classified as topsoil. Therefore, the presence of organic material is what distinctly classifies topsoil and greatly influences its role in supporting healthy ecosystems.