A complex mixture of brown or dark brown amorphous and colloidal substances modified from the original tissues or synthesised by various soil organisms is referred as

A complex mixture of brown or dark brown amorphous and colloidal substances modified from the original tissues or synthesized by various soil organisms is referred to as Humus.
Humus is a complex rather resistant mixture of brown or dark brown, amorphous and colloidal substances modified from the original tissues or synthesized by the various soil organisms.
It is a colloidal substance and increases the cation-exchange capacity of the soil, hence its ability to store nutrients by chelation. While these nutrient cations are available to plants, they are held in the soil and prevented from being leached by rain or irrigation.