Agriculture Reference
In-Depth Information
relic riverbeds, and other factors: Entisols and Inceptisols in young deposits
and Alfisols and Ultisols in older. More humic soils, Mollisols, occur locally in
depressions in richer floodplains and Vertisols (swelling clay soils that are 'self-
mulching' as a result of seasonal shrinking and cracking) in basins receiving
base-rich water from adjacent higher areas.
Alluvial Terraces
Terraces vary in age, parent material, height above the base drainage and
topography. Within a given river system, the lower terraces are the youngest and
the main soils are Inceptisols with Alfisols in poorly drained areas. Higher, older,
Pleistocene terraces have soils reflecting their age and degree of pedogenesis.
The relatively low Pleistocene terraces accompanying many rivers in Southeast
Asia are important rice areas and the soils are Ultisols and to a lesser extent
Alfisols. The highest, oldest Pleistocene terraces are often so dissected and highly
weathered that they are not suitable for rice.
Coastal Plains
Rapidly aggrading coastal plains are all wet and young and many of the soils are
Entisols with no distinct profile development. Upon ripening, whether through
drainage or further accretion of sediment, they may develop into Inceptisols.
Further inland, soils are increasingly older and profile development more con-
spicuous. The groundwater is shallow and the soils are grayish with mottling
reflecting seasonal fluctuations in the water table. Similar soils occur in slowly
aggrading or stationary coastal plains, but the greater extent and duration of man-
grove vegetation and more intense tidal influence often cause accumulation of
pyrite leading to potential 'acid sulfate' soils. Upon drainage and aeration these
become extremely acid and are notoriously difficult to manage (Section 7.2).
Where the land is protected from direct intrusion by the sea by beach ridges, and
sedimentation is minimal, Histosols may develop. Typically these areas have a
domed relief with raised central portions where the peat-forming forest vegetation
grew, grading into Entisols close to the rivers and sea.
