the effect of organic manures on rice yields is
mainly due to their nutrient input, including the
secondary nutrients and micronutrients that
may not be contained in mineral NPK fertilizers
used. Improvements in soil fertility through the
use of organic manures (e.g., increased soil
OM content and cation exchange capacity
[CEC], and improved soil physical properties)
are more important in rice-nonrice and rainfed
lowland and upland rice systems.
Straw management
Straw is the only major organic material
available to most rice farmers. About 40% of
the N, 80-85% of the K, 30-35%0 of the P, and
40-50% of the S taken up by rice remains in
vegetative plant parts at crop maturity.
Straw is removed from the field, burned in situ,
piled or spread in the field, incorporated in the
soil, or used as mulch for the following crop.
Each of these measures has a different effect
on the overall nutrient balance and soil fertility
in the long term. Where S-free mineral
fertilizers are used, straw may be an important
source of S. Thus, straw burning should not
be practiced. In contrast, burning effectively
transforms straw into a mineral K nutrient
source and only a small amount of K is lost in
the process. The effect of straw removal on
long-term soil fertility is much greater for K than
for P (Table 5). Spreading and incorporation
of straw, however, are labor-intensive tasks,
and farmers consider burning to be more
expedient. Straw is also an important source
of micronutrients (Zn) and the most important
influence on the cumulative Si balance in rice
(Section 3.6).
Removal of straw from the field is widespread
in India, Bangladesh, and Nepal. This explains
the depletion of soil K and Si reserves
observed at many sites. Straw can be used
as fuel for cooking, ruminant fodder, and stable
bedding or as a raw material in industrial