Humus complex and resistant mixture of brown, dark brown or black amorphous and colloidal substances, which are modified from the original plant and animal tissues or synthesised by soil microorganisms. Humus is nearly insoluble in water, but is soluble in dilute alkali (NaOH or KOH) solutions. It contains about 30% of each nitrogen-rich protein, the slow-to-decompose lignins, and complex sugars (polyuronides). The polyuronides comprise much of the varied organic substances that cement soil aggregates together. Humus has about 50% carbon, some oxygen, 5% nitrogen, and small amounts of sulphur, phosphorus and other elements.

Humus increases with the flooding of the soil. In general, amounts of fulvic acid predominated over humic acid in forest soils. The predominance of fulvic acid over humic acid in all forest soils was due to the low amounts of forest litter entering into the soil, acid reaction of the soil, low moisture content and more ready oxidation. Humin fraction predominated over all other fractions of humus substances in all soils including some cultivated soils of Bangladesh.

The amount and composition of humus in different soils of Bangladesh is an extremely important factor that influences various chemical and biochemical processes going on in the soil, its physico-chemical properties and its use in agriculture. [Md Khalilur Rahman]