It is the dark brown, almost black substance that results from the decay of organic matter in the soil or in the compost pile.
It also gives soil a certain sponginess and helps it retain water. Humus is also full of microorganisms that help break down various chemicals in the soil so they can be absorbed by plants. Make sure your soil has a healthy humus content by adding organic matter in the form of compost, shredded leaves, hay, straw or grass clippings to your flower and vegetable gardens on a regular basis, ideally once or twice a year.