49
Blanket mires are independent of drainage
hydrology. They are the most widespread among
the peat lands. They cover more than a million
square kilometres of the northern hemisphere, i.e.
>1 % of the land surface. These blanket systems
rely on solutes in precipitation and blown dust for
their nutrition. Consequently, they are the world’s
most oligotrophic terrestrial surfaces. Hence,
they are described as ombrotrophic (Gk. ombros:
a rainstorm).
The primary and secondary peats of valley
mires are less nutrient deficient, remaining in capillary contact with flowing groundwater. These are
the rheotropic peats (Gk. rheos: stream) and their
peat growth is a function of wetness and anoxia
alone without the further effects of nutrient deficiency and acidity. Alkaline fen peats do, in fact,
oxidise very fast when drained, hence the great
wastage or land surface level during the past two
centuries in the English fenlands (Godwin 1978).
The early stages of ombrotrophic peat formation at high latitudes depend on the ability of the
mire to maintain its own perched water table even
when it overlies a permeable substratum. Sphagnum
is a high latitude genus but does occur in tropical
montane mires and is recorded from the transition
between reed swamp and the wet soil palm,
Phoenix reclinata, in Ugandan swamplands.
Soil is the natural medium for growth of biota
in both land and water and is a heterogeneous
mixture. The physical parameters of soil in any
area determine the different types of biota it can
sustain. The methodology recommended by
Jackson (1973) is largely followed.
The air-dry (in shade) soil is normally passed
through 2-mm sieve before chemical analysis.
Before sieving, the soil clods be lightly crushed
in wooden mortar and pestle. Plant residues,
gravel and other foreign matters retained on the
sieve be discarded.
4.3
Mineral Soils: Gleys
(Mineral Wetland Soil)
Mineral soils develop certain characteristics
when flooded for extended periods. These help in
their identification. These characteristics are
collectively called ‘redoximorphic features’.
These may be defined as the features formed by
the reduction, translocation and/or oxidation of
iron and manganese oxides (Vedpraskas 1995).
Another characteristic of some mineral wetland soils is the presence of an ‘oxidised rhizosphere’. Further, mineral soils which are
seasonally flooded, particularly by alternate
wetting and drying, develop spots of highly
oxidised materials called ‘mottles’ or ‘redox
concentrations’.
Soils are categorised according to zonal types
which broadly correspond with geographical and
climatic conditions. However, poor drainage coupled with a high precipitation–evaporation (P/E)
ratio, sometimes, creates a range of intrazonal
wet soils. The latter have a number of common
characteristics related to oxygen deficiency and
low redox potential which typify them as gley
soils. Irrespective of climate, gley soils sustain
different types of wetland vegetation such as
marsh, swamp, fen, fen with swamp forests and
cultivated rice paddy in the tropics.
Soils are illustrated by reference to their profiles, which are the sequence of layers (horizons)
exposed in the wall of a soil pit. Horizons formed
during soil development are visibly different from
each other and have contrasting physical and
chemical characteristics. The horizons of a gley
soil usually include an upper organically rich or
peaty layer overlying a paler coloured gley horizon in which the background colour of grey or
grey-green is frequently mottled with brown or
ochreous patches of iron oxide enrichment. Gley
soils have their pore spaces partly or completely
flooded for at least a portion of the year.
4.4
Physical Characteristics
Temperature, texture, colour, moisture content,
bulk density, infiltration rate
4.4.1 Soil Temperature
Main materials required: soil thermometer,
notebook
4.4 Physical Characteristics
Blanket mires are independent of drainage
hydrology. They are the most widespread among
the peat lands. They cover more than a million
square kilometres of the northern hemisphere, i.e.
>1 % of the land surface. These blanket systems
rely on solutes in precipitation and blown dust for
their nutrition. Consequently, they are the world’s
most oligotrophic terrestrial surfaces. Hence,
they are described as ombrotrophic (Gk. ombros:
a rainstorm).
The primary and secondary peats of valley
mires are less nutrient deficient, remaining in capillary contact with flowing groundwater. These are
the rheotropic peats (Gk. rheos: stream) and their
peat growth is a function of wetness and anoxia
alone without the further effects of nutrient deficiency and acidity. Alkaline fen peats do, in fact,
oxidise very fast when drained, hence the great
wastage or land surface level during the past two
centuries in the English fenlands (Godwin 1978).
The early stages of ombrotrophic peat formation at high latitudes depend on the ability of the
mire to maintain its own perched water table even
when it overlies a permeable substratum. Sphagnum
is a high latitude genus but does occur in tropical
montane mires and is recorded from the transition
between reed swamp and the wet soil palm,
Phoenix reclinata, in Ugandan swamplands.
Soil is the natural medium for growth of biota
in both land and water and is a heterogeneous
mixture. The physical parameters of soil in any
area determine the different types of biota it can
sustain. The methodology recommended by
Jackson (1973) is largely followed.
The air-dry (in shade) soil is normally passed
through 2-mm sieve before chemical analysis.
Before sieving, the soil clods be lightly crushed
in wooden mortar and pestle. Plant residues,
gravel and other foreign matters retained on the
sieve be discarded.
4.3
Mineral Soils: Gleys
(Mineral Wetland Soil)
Mineral soils develop certain characteristics
when flooded for extended periods. These help in
their identification. These characteristics are
collectively called ‘redoximorphic features’.
These may be defined as the features formed by
the reduction, translocation and/or oxidation of
iron and manganese oxides (Vedpraskas 1995).
Another characteristic of some mineral wetland soils is the presence of an ‘oxidised rhizosphere’. Further, mineral soils which are
seasonally flooded, particularly by alternate
wetting and drying, develop spots of highly
oxidised materials called ‘mottles’ or ‘redox
concentrations’.
Soils are categorised according to zonal types
which broadly correspond with geographical and
climatic conditions. However, poor drainage coupled with a high precipitation–evaporation (P/E)
ratio, sometimes, creates a range of intrazonal
wet soils. The latter have a number of common
characteristics related to oxygen deficiency and
low redox potential which typify them as gley
soils. Irrespective of climate, gley soils sustain
different types of wetland vegetation such as
marsh, swamp, fen, fen with swamp forests and
cultivated rice paddy in the tropics.
Soils are illustrated by reference to their profiles, which are the sequence of layers (horizons)
exposed in the wall of a soil pit. Horizons formed
during soil development are visibly different from
each other and have contrasting physical and
chemical characteristics. The horizons of a gley
soil usually include an upper organically rich or
peaty layer overlying a paler coloured gley horizon in which the background colour of grey or
grey-green is frequently mottled with brown or
ochreous patches of iron oxide enrichment. Gley
soils have their pore spaces partly or completely
flooded for at least a portion of the year.
4.4
Physical Characteristics
Temperature, texture, colour, moisture content,
bulk density, infiltration rate
4.4.1 Soil Temperature
Main materials required: soil thermometer,
notebook
4.4 Physical Characteristics