Humus is often described as the 'life-force' of the soil.
Yet it is difficult to define humus in precise terms; it is a highly complex substance, the full nature of which is still not fully understood.
In the earth sciences, "humus" means any organic matter which has reached a point of stability, where it will break down no further and might, if conditions do not change, remain essentially as it is for centuries, or millennia.
Physically, the difference between humus and organic matter is that organic matter is rough looking material, with coarse plant remains still visible, while once fully humified it becomes more uniform in appearance (a dark, spongy, jelly-like substance) and amorphous in structure. That is, it has no determinate shape, structure or character.