Humus is organic matter that is decomposed to the point where it resists further decomposition and is stable and accumulating in the soil. It is mostly extremely stable carbon compounds with no phosphorus or nitrogen. The stable form makes it difficult to break down by microorganisms.
Why do we care?
Nitrates (NO3-) stay in solution between the soil particles and are readily leached from the soil. Humus has a positive charge and aids in holding the negatively charged nitrates in the soil profile, making them available for plants to take up.
Humus has extremely high absorption abilities. It can hold and release water and nutrients as needed. It also improves the physical structure of soil so that it is crumbly and aerated.
‘Humus’ is often incorrectly used instead of the term ‘compost’. Compost is able to be broken down in the soil releasing nutrients for plants, unlike humus.