Humus is the dark, spongy organic matter composed mainly of large carbon-based molecules that forms when dead plant and animal matter has been fully decomposed through the action of insects, fungi and microorganisms. Unlike the decomposing organic matter in soil, which appears rough and still has visible remains of the original plant and animal matter, humus no longer has such easily visible remains, although tiny plant, animal and microbial remains may still be seen when viewed under a microscope.