The photograph shows soil layers. what is the dark layer of soil at the top of the photograph

"The top layer of the soil represents the dead organism and plants, top soil,  the decomposed materials are used to retained the water."

Explanation:

The topsoil represents the humus, which represents the top most soil layer.The layer is contains the decomposed materials in it which enhances the soil ability to retain water inside it. Along with that it also increases the ability of the plant to grow more plants in it.