Due to a lack of which of the following substances the lower layers beneath the topsoil are rocky and hard in nature?

Humus is a component of topsoil that is formed from the decomposition of organic matter, such as dead plant and animal material. It is a dark, crumbly, and nutrient-rich substance that helps to provide the necessary nutrients for plant growth. Deforestation can lead to soil degradation, which is a decline in soil quality and fertility.