The traditional definition of humus is a dark, organic material that consists of partially decomposed plant and animal matter. It is uniformly black or a dark brown in colour, and has a spongy texture.
To add to much confusion around humus, it is also often used to refer to compost.
It has been argued that humus may take years or even millennia to decay, as it resists breakdown by microorganisms. Yet other sources argue that it is a source of valuable nutrients which feed the same microorganisms.
However, humus has recently come across a problem.
Humus is supposed to be formed of long, recalcitrant carbon molecules – i.e. molecules that don’t break down.
Yet when scientists study soil or compost, they can’t see those molecules. This has led some scientists to announce “the death of humus”.
Unfortunately, education and literature hasn’t caught up with the controversy, and many composters will wax lyrical about its benefits.
As a composter, I think it’s worth ignoring talk of humus, at least until the current controversy is settled.
Garden Myths: Humus Does not Exist – Says New Study
National Geographic: Humus
Lehmen and Kleber, 2015, Nature: The contentious nature of soil organic matter
Gabriel Popkin, Quanta Magazine, 2021: A Soil-Science Revolution Upends Plans to Fight Climate Change