Soil is the topmost part of Earth’s crust. It is a mixture of organic and inorganic materials, some moisture, and air. The remains of dead plants and animals make up the organic materials while clay, sand, silt, and small particles of rocks comprise the inorganic materials. All these compose the soil’s three layers: topsoil, subsoil, and bedrock.

Topsoil

The topsoil is the uppermost layer of the soil. It contains waste matter from animals and plants and is rich in nutrients that plants need for growth. It also contains water and air and may vary in thickness from 2 to 60 cm.

This layer is darker in color than the other layers and is more fertile because it also contains humus, or the dark brown organic material that is produced from decaying plants and animals. Thick layers of topsoil characterize the soil in farmlands, particularly those located in valleys.