EFFECTS OF SESBANIA-ROSTRATA AND AZOLLA-MICROPHYLLA INCORPORATION ON TRANSFORMATION OF APPLIED ZINC AND COPPER IN LATERITIC RICE SOILS WITHDIFFERENT FLOODING REGIMES

Fixation of Zn and Cu applied to tropical rice-growing lateritic soils rich in Fe-oxides may be reduced if the soils are kept flooded for a few days before their application. There may be a further reduction if such flooding is combined with incorporation of green manures. To inv estigate this effect, a laboratory experiment was conducted to study t he effect of different periods (0 and 15 days) of preflooding combined with (0 and 0.50% of soil weight) Sesbania rostrata and Azolla microp hylla incorporation as green manures on the transformation of applied Zn and Cu in two lateritic rice-growing soils. Recovery of added Zn/Cu in DTPA (diethylene triamine pentaacetate)-extractable form was alway s found to be higher when they were applied after the soils were maint ained in a flooded state for 15 days than when applied immediately aft er flooding; this effect was more prominent in respect of Cu. Contrary to expectations, green manure incorporation along with preflooding ca used a significant decrease in recovery of Zn/Cu; the effect, however, showed a decreasing trend as incubation progressed. The effect was mo re marked with A. microphylla than with S. rostrata, particularly with Cu. Possible causes of such changes and their implications on the Zn/ Cu nutrition of rice are discussed.