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Humus is a complex and rather resistant mixture of brown or dark brown amorphous and colloidal organic substance that results from microbial decomposition and synthesis and has chemical and physical properties of great significance to soils and plants.

Humus Formation

The humus compounds have resulted from two general types of biochemical reactions: Decomposition and Synthesis.

1. Decomposition:

a) Chemicals in the plant residues are broken down by soil microbes including lignin.
b) Other simpler organic compounds that result from the breakdown take part immediately in the second of the humus-forming processes, biochemical synthesis.
c) These simpler chemicals are metabolized into new compounds in the body tissue of soil microbes.
d) The new compounds are subject to further modification and synthesis as the microbial tissue is subsequently attacked by other soil microbes.

2. Synthesis: Involve such breakdown products of lignin as the phenols and quinones.

a) These monomers undergo polymerization by which polyphenols and polyquinones are formed.
b) These high molecular weight compounds interact with N-containing amino compounds and forms a significant component of resistant humus.
c) Colloidal clays encourage formation of these polymers.
d) Generally two groups of compounds that collectively make up humus, the humic group and the nonhumic group.

Current Category » Introduction to Soil Science