Agriculture Reference
In-Depth Information
It should also be noted that not all of the master horizons considered below occur
in every soil.
1.1
Predominantly organic horizons
The O or surface organic horizon is dominated by organic materials usually increasingly
decomposed with depth. The FAO-UNESCO (1990) scheme requires that O horizons
should contain more than 20 % organic carbon by weight. Under this system, O and
H horizons are differentiated on the basis that the former are saturated with water for no
more than a few days per year while H horizons undergo extended saturation.
Due to faunal mixing, mineral matter increases in concentration towards the bases of
many O horizons.
The H horizon
The H horizon comprises accumulations of organic matter that remain saturated for long
periods each year. This horizon is characteristic of peats, or the histosols of the two
classification schemes mentioned above. In Soil Taxonomy, the term H horizon is not
used and such materials are considered to be O horizons.
The O horizon
A range of classification schemes exists for O horizons and most depend on the degree
of decomposition of the accumulated organic materials. Because O horizons are best
developed under forests, some older classification systems have been narrowly based on
the layers pertaining in particular forest types. One such example is the classification
of the litter layers of temperate-climate conifer forests in which three distinct layers may
be recognised namely, the L (litter) layer comprising freshly-fallen needles, the F
(fermentation) layer comprising needles in an advanced stage of decomposition but still
recognisable as such and the H (humus) layer in which the original form of the needles
is no longer recognisable. However, grasslands may also develop substantial O horizons,
particularly where anecic earthworm populations are not sufficiently active to bury
the surface litter. These may be quite different morphologically from those described
above and sometimes comprise a diffuse layer with poor ground contact.
Other systems of classifying the O horizons divide them into layers depending
whether the original form of most of the vegetation is still recognisable to the naked eye.
One other system (Guthrie and Witty, 1982) bases its subdivisions on the reduction
of the fibre content with state of decomposition after the material has been rubbed
between the fingers.
Table II.1 presents the approximate equivalents of the divisions found in O horizons.
The dotted line represents the approximate division between materials whose origin
remains apparent to the unaided eye and those that are not.
