Geoscience Reference
In-Depth Information
the cropping season is very limited: it corresponds to the period of
drying of the soil. The authors mention '
a window for cultivation
'. I n
Senegal, one says '
recessional cultivation
'. The short favourable period is
best used by farmers who manually transplant rice to the water with
the triple advantage (i) of not waiting for complete drying that results
in acidification, (ii) of shortening the time for establishment of the crop
by using an already-germinated plant, and (iii) of acting when the very
low bearing capacity will not allow passage of machines. One can see
that the Asiatic rice-culture tradition has strong scientific justification
underlying it.
Sometime, the mangrove are completely drained and free of their
sulphates and chlorides. The soils can then support plantation crops
such as coconut, oil palm, sugar cane, etc. Conversely, in some parts of
the world, the mangrove swamps drained earlier by polderization have
become unsuitable for all crops after a few years. They have been
abandoned and returned to their wild state, which satisfies some
people.
12.7 RICE PADDIES
Much of the information given earlier is applicable to rice paddies that
are soils flooded artificially. But let us look into specific points here.
In developing countries, especially some in Asia, the problem of nitrogen
nutrition of rice is crucial, in the absence of significant use of fertilizers.
This has prompted several investigations that can be summarized as
follows (Fig. 12.15) (Buresh
et al
. 1980; Savant and De Datta 1982):
In a flooded (a few cm or a dm or two) rice paddy, atmospheric
nitrogen is fixed in three ways:
Nitrogen
fixation by blue-green algae that live in water and draw their
energy from photosynthesis; this source of nitrogen seems to
be dominant even if the algae are temporally and spatially
distributed unevenly in the field;
nonsymbiotic fixation, in anoxic conditions, by heterotrophic
anaerobic bacteria that draw energy from decomposition of
organic matter; but this pathway seems to be less important
than the preceding;
fixation by nonsymbiotic bacteria (e.g.
Azotobacter
spp.) living in
the rhizosphere where small quantities of oxygen transported by
the plant are still available.
