Geoscience Reference
In-Depth Information
soils; thus subsidence may further modify soil moisture conditions beyond the effects
of lower water table levels due to drainage alone. The total porosity is not sharply
reduced with increasing bulk density. However, bulk density is very closely related to
peat pore size distribution (which regulates soil water retention), and saturated and
unsaturated water transport dynamics.
Greater soil water contents are expected after drainage, which are actually asso-
ciated with greater peat bulk density. Peat bulk density, water retention and capillary
moisture transport from the water table, and saturated hydraulic conductivity are thus
related to the degree of peat decomposition.
The woody nature of tropical peat, for instance, is a big hindrance to land prepa-
ration. The very low bulk density and bearing capacity presents many problems in the
use of heavy machinery. In an undrained condition, the machine tends to sink in the
peat due to the low bearing capacity of peat soil. To support mechanization, the water
table has to be sufficiently lowered to attain the required bearing capacity. Machinery
has to be modified by having lighter weight and wider tracks to reduce ground pressure
so that it will not sink into the peat. Construction of farm roads on peat will require
fill materials to improve the bearing capacity of the porous peat soil to withstand the
load of a vehicle. A 3.5 ton vehicle creates a pressure of 8.75 tonm
−
2
or 88.85 kNm
−
2
.
The water table has to be lowered by about 1.8m at mid-field in order to achieve this
bearing capacity on peat (Salmah
et al.
, 1991)
It can be seen that there is a conflict of function between the need to drain for
traffic access and the moisture supply required for crop growth. The mechanization
problem becomes particularly critical during harvesting. The regular harvesting of
oil palm, particularly if heavy machinery is used, will then lead to differences in the
height of planting rows and harvesting rows. This will affect the management of the
required water table and will also make heavy demands on the surface structure and
consistency of the soil. The rate of surface compaction is also accentuated. Mecha-
nization is also more difficult during the high rainfall period because the ground is
soggier.
The high acidity and low nutrient levels in tropical peat demand the use of lime
and fertilizer. Liming and fertilization will encourage microbial activity and rate of
decomposition by increasing the pH and decreasing the C:N ratio. Therefore the rate
of subsidence will increase. Newly developed tropical peats, such as those in Sarawak,
Malaysia, are still very porous. This, together with the extremely high rainfall and
fluctuating water table, gives rise to excessive leaching of applied fertilizers in deep
peat. Peat soil, being an organic material, will continuously mineralize and thus cause
a dynamic change in its nutrient content, especially the available nitrogen. This will
tend to cause an imbalance between nitrogen and potassium.
All of these problems will have an adverse effect on the sustainable utilization
of the peat for agricultural development, for which a balance solution has yet to be
found. Tie (1990) suggested that agriculture development, where necessary, should be
confined to the fringes of the peat dome, using PC1 and PC2. PC3 may be considered
for silviculture with proper management, while PC4 and PC5 are best kept under
natural conditions.
Figure 8.6 illustrates the sequence of impact to peat land due to logging.
Even when drainage is not actually involved, there is still an associated loss in
biodiversity.
