Effect of varying water regimes on soil physical properties and yield of rice in mollisols of Tarai region

Abstract

Field experiments were conducted under shallow and medium water table soils in order to identify suitable water regime(s) for wetland rice cultivation. The physical properties such as the puddling index, bulk density, porosity, moisture retention, hydraulic conductivity and infiltration rates were studied to ascertain the changes brought about by puddling under six imposed soil-water regimes; ranging from continuous submergence of 5 ± 2 cm water (Ic) to completely rainfed (Io). In intermittent irrigation regimes (Ic 1, Ic 3, Ic 5 and Ic 7) irrigation was applied respectively 1, 3, 5 and 7 days after water vanished from the surface of the soil. The depth of the naturally occurring water table fluctuated during the rice season. Shallow water table varied from 0.7 to 92.3 cm and medium water table from 12.6 to 126.3 cm. A continuous rise in water table occurred from week 26 to week 32, between weeks 32 to 38 it was around 30 cm and thereafter a continuous decrease occurred. The puddling index was 80.4% for shallow and 80.6% for medium water-table soil. Under puddling conditions, bulk density increased by 16.9% under shallow- and 14% under medium-water table conditions. This increase in bulk density enhanced microporosity by 11.6% for shallow water table and 5.1% for medium water table soils, but reduced macroporosity, hydraulic conductivity and infiltration rates respectively by 51.1, 50.7, and 54.1% of shallow water table and 53.6, 54.7 and 51.3% of medium water table soils. Microporosity decreased by 13.5% and total porosity by 8% with increasing crop growth. The reductions were more pronounced under the rainfed as compared with the continuous submergence regime. Soil moisture tension remained zero under Ic but varied from zero to 15 bar under Io. In Ic 1 and Ic 3 conditions it ranged from zero to 0.05 bar and under Ic 5 and Ic 7 from 1.7 to 3.5 bar during the entire crop growing season. For Ic, grain yield was about 6300 kg ha −1, under Ic 1, Ic 3 and Ic 5 regimes 5800 kg ha −1, and under Ic 7 and Io, 5000 kg ha −1. In shallow water table grain yield was 200 to 300 kg ha − higher than medium water table in all the water regimes tested. Thus, it is apparent that in terms of net return, continuous submergence did not appear necessary under Tarai conditions, where water table remains shallow (< 30 cm) for most of the rice growing season. Intermittent irrigation, given at 3 to 5 days after water vanished from the surface of the soil under shallow and 1 to 3 days under medium water table conditions, appears to be as effective as continuous submergence.