Geoscience Reference
In-Depth Information
Peat has a high moisture content and water holding capacity. The high water
content results in high buoyancy and high pore volume, leading to low bulk density
and low bearing capacity of the peat. The bulk density of the peat varies according
to its degree of decomposition. Highly decomposed peat generally has a higher bulk
density. The degree of decomposition and the bulk density are intrinsically related.
The high water content results in high buoyancy and high pore volume, leading to
low bulk density and low bearing capacity. Excessive drainage of the peat will cause a
transformation of its colloids, resulting in irreversible drying.
In tropical peat there is a vacant zone, which is more common and extensive under
the root mat of the Alan forest compared with that found in mixed peat swamp forest.
The vacant zone is found within the top 0.5m of the surface horizon below the root
mat. It makes the surrounding of the forest floor dry. In fact, the surface horizon of the
peat is actually floating on the inter-winding root mats. The thickness of the vacant
zone ranges from 0.25-0.40m. The surface of tropical peat is often very raw and
fibrous. The very fibric and porous nature of the peat results in its low bulk density,
thus decreasing the growth media and availability of nutrients.
Peats are very dynamic. This is because they undergo subsidence and oxidation
upon drainage. Initially, it involves principally the loss of buoyancy and compaction
of the organic column under its own weight. Compaction results in the changes in
the hydropedological parameters like the hydraulic conductivity, bulk density, pore
volume and moisture content. The subsequent dominant process, which may last for
decades, is oxidation and shrinkage. The rate of subsidence varies strongly depending
on the peat's profile morphology, composition and depth; the depth of drainage; and
land use. Due to the differential rate of subsidence upon drainage, the micro relief of
peat surfaces is hummocky (Ambak and Melling, 1999).
Surface topography and peat thickness, however, do not influence the vegetation
directly. They operate through changes that are brought about in other characteristics
of the peat land, especially hydrology, chemistry and organic matter dynamics (the
balance between peat accumulation and peat degradation).
The peat thickness, the nature of the subsoil, the humification level and the peat
surface altitudes and gradients influence the chemical composition of the peat. The
chemical properties of a typical tropical peat are shown in Table 8.4.
Tropical peat, like temperate bog peat, is very acidic in nature, with a pH of 3 to
4. Aluminium (Al) toxicity, however, is not a problem for deep peat, which has a high
CEC. The value is very misleading because it is due to the dissociated carboxyl groups,
which determine the high acidity. The amount of exchangeable bases is actually very
small. Thus the base saturation is very low and strongly buffered.
The high CEC is not due to the presence of Na, K, Mg or Ca but because of the
amount of exchangeable H
+
. The N content of peat is rather high, but its availability
for plant uptake is rather low. The high C:N ratio, coupled with the low pH, results in
lowmaterialization in peat. The ash content can be less than 10%, showing a very high
organic matter content. This is indicated by a loss of ignition value exceeding 90%
(Ambak
et al
., 1991; Ambak and Tadano, 1991). Peat is highly deficient in micronu-
trients such as Cu and B. From the viewpoint of nutrient dynamics, the potential use
of reclaimed peat land was rather limited, especially under low input management
(Yonebayashi, 2003).
