Which layer of the earth contains humus?

The top layer cause when leaves and branches drop they pile up

Greetings! 

Most of the Humus found in soil is in the Top Layer of the soil or Topsoil. 

Here is the Formation of Soil for Reference:
From the Top Layer to Bottom.

1) Topsoil

2) Subsoil

3) Parent Material

4) Bedrock 

Hope this helps.
-Benjamin

Google it that might work