What is humus layer, the hotspot of soil biological activities?

In: Milsil, 2015, 8(5), pp.12-14 (in Japanese, translated from English by Keiko Mori). What can we find under our feet when walking in a forest? As an example, have a look on the humus layer of a pine forest. The humus layer can be treated as a book, flipping its pages allowing us to decipher the story of organic matter. Humus forms can be used as a diagnostic tool for the assessment of the good health of forests and other terrestrial ecosystems.