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￿ colour of the water obtained by squeezing a sample in the hand;
it is clear or loaded with organic compounds giving a 'coffee'
colour, indicating a certain degree of decomposition of fibres; at
the extreme limit when the fibres have totally disappeared, all
the organic matter slips through the fingers,
￿ topographic position (sloping peatlands, peatlands of bottoms
of depressions).
In the laboratory, the bulk density is found to be low (0.2 to 0.3 kg
dm -3 when the peat is fibrous), the carbon content is high (40-50%) and
so is the cation exchange capacity (150-250 cmol + kg -1 ), the reaction is
acid (pH generally from 3.5 to 4.5) and ash content is low (1-10%).
Marsh soils are not always rich enough in organic matter to be classified as
Histosols. Then they correspond to various types of Gleysols, especially
'Humic Gleysols'. Earlier authors distinguished submerged and reduced
humus ( sapropel ) or submerged humus with signs of oxidation ( gyttja ).
But these terms are not precise. The WRB of 2006 wisely defined gyttja
within the ' limnic ' diagnostic material.
Highly organic soils related to prolonged waterlogging fall in
Histosols and for once all classifications are in agreement on this.
Prefixes serve to define the condition and content of plant fibres found
in these environments. In present vocabulary, it is the same in WRB as
in Soil Taxonomy (Table 12.7).
Taxonomy
Table 12.7 Classification of Histosols based on proportion of fibres.
Proportion of fibres
WRB
Soil Taxonomy
2/3 or more
Fibric Histosol
Fibr-ist
2/3 to 1/6
Hemic Histosol
Hem-ist
Less than 1/6
Sapric Histosol
Sapr-ist
The pH value can be used for separating the peatlands (acid) from
marshes (neutral or alkaline). The WRB proposes to distinguish Histosol
Alcalic (pH > 8.5), Histosol Eutric (base saturation 50% or more) and
Histosol Dystric (base saturation < 50%).
In addition, shallow peats are the surface horizon of various soils,
among which are giant tropical Podzols, very typical of equatorial
environments (Dubroeucq and Volkoff 1998). To give an example: Histic
Podzol (WRB) or Histic Trop-aqu-od (Soil Taxonomy).
 
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