EFFECT OF THE SYSTEM OF RICE INTENSIFICATION (SRI) ON ...

The effect of the leafy manures is though very much depending on the time of incorporation and
growth period (De Datta, 1987). Young GM materials should be incorporated into the soil before
rice transplanting app. after eight weeks of growth or if older then incorporated several months
before the transplanting in order to increase decomposition (De Datta, 1987).
Sesbania can in average produce 15 t ha -1 of green matter with an N content of 2.67% of the dry
weight (Singh, 1984). This would roughly result in 400 kg N ha -1 which is a rather large input.
However not all N will be plant available as mentioned earlier. Given the percentages Kundu &
Ladha (1999) referred to above the Sesbania crop would result in 80-120 kg available N ha -1 . As
Sesbania can tolerate anaerobic conditions resulting from flooding (De Datta, 1987), farmers could
grow a Sesbania crop before transplanting in early wet season app. eight weeks before
transplanting. This would provide the farmers with a considerable input of N. The cultivation of a
catch crop or a GM crop before the rice transplanting would also reduce the labour of transporting
and producing composts on the homestead land. Instead organic biomass would more easily be
present to incorporate in the soils, even in fields located far from the homestead lands.
Given the traditional rice yield of 2.19 t ha -1 (grain), this would result in a straw dry mass of 3.3 t
ha -1 with a harvest index of 0.5. In theory farmers have sustained their domestic animal production
with an approximate rice straw production of 3.3 t ha -1 or less as opposed to the estimated scenario
in table 8 where all land would be converted to SRI, resulting in a straw production of 5.25 t ha -1
which is a difference of 1.95 t ha -1 . This signifies that farmers would be able to return this amount
to the field. With uptake rates estimated by De Datta (1987) this would result in a new net balance:
N (14.3 kg ha -1 ), P (14.6 kg ha -1 ) and K (- 0.7 kg ha -1 ). The balance of especially K would be quite
sustainable if farmers could afford to return the additional production of straw or some of it. This is
of course depending on their amount of cows and buffaloes per farmer. One could argue that with
additional straw they would be able to sustain more domestic animals thus a trade-off situation is
arising.
64