Agriculture Reference
In-Depth Information
but has similar texture. Over time the surface soil often becomes more coarse-
textured, possibly because of weathering of clay under alternate flooding and
drainage (Brinkman, 1970; Moormann and van Breemen, 1978). Clay may also
be lost from the surface during puddling by movement downslope with surface
water. But equally clay may be added from upslope. Freely drained soils that
are repeatedly flooded and puddled for rice may show downward movement of
reduced Fe and Mn and their subsequent accumulation in oxidized forms at the
boundary with oxic subsoil. However this is rarely seen in naturally hydromor-
phic, finer textured soils.
Because of rice's origins as a wetland plant, it is more sensitive to water
deficiency than most other crops. But provided sufficient water is supplied to
periodically inundate the land and the soil is able to retain the water, rice will
thrive on almost any type of soil. The productivity of rice land therefore often
depends more on position in the landscape and soil physical properties than
on the finer attributes of the soil. Nonetheless, subtle differences in properties
distinguish productive and 'problem' soils and affect the behaviour of the soil in
the environment.
1.3.3 RELATION BETWEEN SOILS AND LANDFORM
Most of the landforms in which wetlands form can be seen in tracing a river from
its source in hilly or mountainous areas to its outflow in coastal floodplains and
the sea. The main landforms are inland valleys, alluvial fans or fan complexes,
meander or lacustrine floodplains, and alluvial terraces (Figure 1.4), and each
of these is associated with particular soils as illustrated for ricelands in Asia
in Table 1.6. This section gives a brief description of these associations. More
complete descriptions are given in Moormann and van Breemen (1978), Driessen
and Moormann (1985) and Richardson and Vepraskas (2001). Following these
authors I use the USDA (1999) soil classification.
Inland Valleys
Wetlands occur on the valley floors and the lower slopes. The soils vary widely
with parent materials and other factors, but there are some general patterns. On the
valley floors, slopes decrease from the top to the bottom and the age and texture
of the deposits vary accordingly. Where deposition is most active, the soils are
young and have little profile development. These are Entisols. But most soils in
the valley bottoms show at least some profile development and are Inceptisols or
Alfisols where there is a pronounced dry season.
Where the valley slopes have been terraced for rice and the soil has remained
in situ
for a long time-hundreds of years-there may be inherited clay illuviation
leading to man-made Alfisols and Ultisols. Artificial Entisols may occur where
