Page 75 - Soil Degradation, Conservation and Remediation

2.6 Desertifi cation 61

the reclamation and development of peat lands in many parts of the world. Draining
brings the surface soil layers above the submergence level, which enhances aeration.
So, reclamation of peat soils will always accompany oxidation of these layers.
Experiences in the Netherlands show that reclamation of peat areas in the Middle
Ages have gradually subsided from 0.5 m above mean sea level to 1–2 m below.
Land subsided about 2 m over a period of 8–10 centuries in spite of a continuously
shallow drainage (Nieuwenhuis and Schokking 1997 ). Some 85 % of this subsid-
ence can be ascribed to the oxidation of organic matter, which will continue at a rate
−1
of 5 cm year (Schothorst 1982 ). Land subsidence is the most commonly observed
response of Histosols to drainage for agricultural purposes. Worldwide subsidence
−1
−1
rates in drained peaty areas vary from less than 1 cm year to more than 10 cm year .
−1
The arable organic soils experienced an average subsidence rate of 2.5 cm year
between 1924 and 1978 (Ingebritsen et al. 1999 ) in Florida. Histosols in California
−1
settled at a rate of up to 8 cm year between 1922 and 1950 (Rojstaczer and Deverel
1995 ). Records of land subsidence in Malaysia reveal that the subsidence rates
−1
−1
decreased from 12 cm year over the period 1960–1974 to 6.4 cm year in the
−1
following 14 years and to 2 cm year thereafter (Wosten et al. 1997 ). Stephens et al.

( 1984 ) recognized at least five sources of organic soil subsidence following drain-
age: shrinkage due to desiccation, consolidation, wind and water erosion, burning,
and biochemical oxidation. The latter has been found to be the dominant cause of
land subsidence in temperate and tropical peat soils (Andriesse 1988 ; Deverel and
Rojstaczer 1996 ). In areas with low-bearing capacities, buildings and structures
have to be built on pile foundations. Subsidence will change the relative elevation of
piled buildings and structures with respect to the surrounding area. The areas
surrounding these buildings and structures will have to be raised from time to time
by the addition of earth or other fi ll material. Special measures have to be taken in
connecting utilities (power lines, water mains, etc.). On soils with soft clay or peat
layers, the design height of embankments has to be corrected to take the future
subsidence into account; otherwise, the safety requirements may not be met. Segeren
and Smits ( 1980 ) suggested that clay content, depth of peat layer, loss of water due
to drainage, water table, and load of machineries affect the rate and extent of subsid-
ence. Clayey sediments lose more water than sandy sediments. As a consequence,
clay soils will subside more. The subsidence caused by the shrinkage of different
soil layers at a given clay content decreases with depth. The greater the thickness of
the compressible layers, the greater will be the subsidence. Cultivation with heavy
machineries compacts the peat soils considerably.

2.6 Desertifi cation

Desertification is a process of land degradation which occurs mainly in the arid and

semiarid regions due to human actions on ecosystems combined with adverse
climatic conditions. Dregne ( 1977 ) defi ned, “Desertifi cation is the impoverishment
of terrestrial ecosystems under the impact of man. It is a process of deterioration in