Diagnosis and amelioration of iron deficiency under aerobic rice

Iron (Fe) deficiency is one of the serious nutritional disorders in aerobically grown rice on upland alkaline and calcareous soils, which leads to a decline in productivity. With a view to resolve the Fe-deficiency syndrome in aerobic rice, the influence of soil moisture regimes, farmyard manure (FYM) and applied Fe on the release of Fe was assessed under an incubation study. A field experiment was also conducted to evaluate the relative effectiveness of soil and foliar applications of Fe in alleviating Fe deficiency using four rice cultivars ('IR 36', 'IR 64', 'IR 71525-19-1-1' and 'CT 6510-24-1-2'). Results of incubation study indicated that the application of FYM marginally improved the diethylene triamine pentaacetic acid (DTPA)-Fe status of soil over control. However, application of iron sulfate (FeSO4 center dot 7H2O) at 14 mg Fe/kg with FYM released as much Fe as did the application of 27 mg Fe/kg as FeSO4 7H(2)O alone. Comparatively higher amounts of Fe were released under water saturation than that under drier soil moisture regimes and the effect of incubation period in releasing Fe was pronounced only under water saturation. Under field study, supplementation of Fe through integrated or inorganic source caused improvement in the DTPA and ammonium acetate (NH4OAc) extractable Fe similar to that recorded under incubation. The foliar application of Fe (3% FeSO4 7H(2)O solution, thrice at 40, 60, and 75 days after sowing of rice, i.e., 45 kg FeSO4 center dot 7H(2)O/ha) was most effective and economical in correcting Fe deficiency in aerobic rice, followed by soil application of 150 kg FeSO4.7H(2)O + 10 t FYM/ ha and 305 kg FeSO4 center dot 7H(2)O/ha. Among the rice cultivars, 'CT 6510-24-1-2' and 'IR 71525-19-1-1' performed better under aerobic condition compared to 'IR 36' and 'IR 64'. Differential response of rice cultivars to applied Fe was not related to Fe-nutrition; rather it was apparently related with inherent ability of cultivars to grow under water-stress condition. Ferrous iron (Fe-II) content in rice plants proved to be a better index of Fe-nutrition status compared to total plant Fe and chemically extractable soil Fe. The Fe-II concentration of >= 37 mg kg(-1) in plants (on dry weight basis) appeared to be an adequate level at 60 days after sowing for direct seeded rice grown under upland aerobic condition.