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Zero Flux Plane Recession under Monocropped and Intercropped Cowpea and Sorghum

Multiple Cropping Dep., The Int. Rice Res. Inst., Los Baños, Philippines
Dep. of Soil Sci., Oregon State Univ., Corvallis, OR 97331.

^* Corresponding author.

Movement of water remaining in the soil profile at harvest of rice (Oryza sativa L.) from puddled fields in the tropics has not been well documented. This water is used by crops during the dry season. To use it more effectively, however, its behavior during the postrice period must be better understood. Monocrops and intercrops of species differing in rooting habit [cowpea (Vigna unguiculata (L.) Walp.) and sorghum (Sorghum bicolor (L.) Moench)] were compared for effects on zero flux planes in an Andaqueptic Haplaquoll during three postrice seasons in the Philippines. Fallow treatments were included for comparison. Zero flux plane recession was regarded as an indicator of internal drainage. Zero hydraulic head gradients calculated from matric potentials defined the zero flux plane. In all years, the zero flux plane receded most rapidly under monocropped cowpea (to 0.9 m in 36—38 d after planting). The zero flux plane under the intercrop reached 0.9 m 7 d later. Recession under sorghum was less consistent, requiring 43 to 70 d to reach 0.9 m. Under fallow, it receded to about 0.45 m (to the Bt horizon) within 15 to 25 d and remained stationary except when a 30 mm rain 40 d after planting infiltrated and temporarily raised it to 0.3 m. Ten days later, the zero flux plane under fallow again was stationary at 0.45 m. In contrast, under cowpea the 30 mm rain infiltrated the soil under cowpea but was redistributed above the zero flux plane. Monocropped cowpea and the cowpea-sorghum intercrop were equally effective at arresting drainage of residual water.

Received for publication November 21, 1988.