Agriculture Reference
In-Depth Information
water necessarily accumulates. Even good quality water contains 200 mg L
−
1
of
soluble salts, so, for example, a ricefield receiving 1000mm of irrigation water
will accumulate 2 t ha
−
1
of salt per year (Greenland, 1997). Also, salts in subsoils
and groundwater in waterlogged land may be brought into the surface by mass
flow and diffusion.
In land that is flooded for part of the year but drains naturally after the flood-
water recedes, accumulated salt is removed with the draining water and there is
a natural renewal of the land. Percolation and lateral drainage at the start of the
following rainy season but before the land is re-flooded also wash out accumu-
lated salt. However this natural recovery is prevented if the water table remains
above or close to the surface. This may happen in depressions, but also where
large reservoirs have been established at a higher elevation than the flood-prone
land, or where an unlined canal that carries a large volume of water has been
built on permeable soil (Greenland, 1997). Such problems are more common in
arid or semi-arid areas where there is both less leaching of the soil and ground-
and irrigation-waters are more likely to be saline.
A further problem associated with waterlogging and salinity is sodicity. When
the quantity of Na
+
in the soil exceeds about 15% of the CEC, the soil may
become dispersed, i.e. aggregation is lost, and it will then dry to large tough clods.
Salts in groundwater are often high in Na
+
, so sodicity may be a problem even
though salinity is not. Sodic soils are not necessarily problematic for wetland rice
cultivation, though rates of percolation may be sub-optimal but they are difficult
to cultivate for following dryland crops.
About 5Mha of tidal wetlands are cultivated with rice (IRRI, 2002). Although
this is only a small part of the 147Mha of land currently cropped with rice
globally, there is some scope for expanding the area in response to increasing
demand for rice and loss of more favourable land to non-rice uses (Greenland,
1997). There is also a need to improve the productivity of existing tidal rice
areas to relieve pressure to clear marginal lands. The principal soil chemical
stress in tidal wetlands is high salinity. Rice is only moderately tolerant of salt
and is more sensitive to it than some cereals (Flowers and Yeo, 1981; Yoshida
et al
., 1983). The sensitivity varies over the growth cycle being most acute in the
seedling stage and again during flowering. Although there are large differences
in tolerance between cultivars, none are tolerant of salt throughout the growing
season (Flowers
et al
., 2000; Gregorio
et al
., 2002). The suitability of rice for
coastal saline areas therefore arises from its tolerance of soil submergence rather
than exceptional salt tolerance: because it can tolerate soil submergence, and
because flooding with less saline water dilutes the salt in the soil, rice will grow
on land not able to support dryland crops. In addition, various other soil chemical
stresses are prevalent. These include alkalinity, acidity, Fe toxicity, and deficien-
cies of P, Zn and other nutrients, typically in combination. Quijano-Guerta and
Kirk (2002) discuss the tolerance of rice germplasm to the multiple soil chemical
stresses in tidal wetlands.
