Factors of Pedogenesis - Major Soil Groups of the World - page 62

Table 2.7 Approximate half-life of some organic compounds.

Labile original organic compounds Synthesized humic substances

Sugars 3 months Fulvic acids 200-1000 years

Hemicellulose 4 months Humic acids 200-1000 years

Cellulose 6 months Humin More than 2000 years

Resistant original compounds
Lignin

1 year

Waxes 2 years

Phenols 7 years

2.5.3 Types of Humus
A fine characterization of humus involves the following elements
(RP 1995):
￿ nature of layers present,
￿ more or less rapid transitions between the layers present,
￿ colour, because it is useful for distinguishing residues with
brown colour rich in proteinaceous nitrogen and polyphenols
from residues bleached by abundant fungi, which are capable of
utilizing the brown pigments (Schwartz and Toutain 1999),
￿ bonding between organic matter and mineral matter: organic
matter alone, organic matter and minerals in juxtaposed grains,
or even intimate association of the two constituents,
￿ structure of the layers: lamellae, fibres, aggregates, individual
organic or mineral particles,
￿ acidity (saturated with calcium or not).
On these bases, three major layers have been identified, which are
as many diagnostic horizons of humus, some of the latter exhibiting the
three superposed layers:

￿ a layer of fresh litter, called, according to the classification used,
'L' or 'OL' in which needles, leaves and residues are scarcely
transformed and easily identifiable,

￿ a 'F' or 'OF' layer, called the fermenting layer, containing
aggregated, fragmented, more or less identifiable residues,

￿ a 'H' or 'OH' layer containing humic substances, where the
organic matter is greatly transformed, often dark-coloured,
containing organic residues impossible to separate. This layer
might be continuous, without visible structure (continually moist
environments) or can be extremely rich in small faecal pellets of