The loss of the fertile topsoil is known as soil erosion. The layer below the topsoil is called subsoil layer, and it is not suitable for growing plants. The major reasons for soil erosion are heavy rainfall and strong winds.
In the case of ​deforestation—that is, cutting of large number of plants and trees over a wide range of area—the fertile layer of the topsoil is exposed and carried away by the water and the wind easily.
Afforestation—that is, planting more number of trees—solves the problem of soil erosion by holding the fertile layer of soil by the roots of plants and trees tightly and thus cannot be washed away by strong winds or heavy rainfall.