| Food production in the humid tropics can be increased by expanding crop culture during the dry season, but soil moisture is usually limiting. Upland crops may also affect succeeding rice (Oryza sativa L.)--the major crop in the tropics--during the wet season. Objectives of these studies were to determine performance of upland crops during the dry season and their effects on rice during the wet season in the Philippines. Water withdrawal patterns of four upland crops--cowpea (Vigna unguiculata), mungbean (Phaseolus aureus), sorghum (Sorghum bicolor Moench), and soybean (Glycine max (L.) Merill)--were compared to water loss from bare soil. Soil hydraulic properties, profile water contents, and water flux were determined to approximate soil water dynamics. Four different methods of determining unsaturated hydraulic conductivity--IP, CGA, hot air flow, and Campbell methods--were compared. After harvesting upland crops, rice fertilized with 0, 40, 80, or 120 kg N/ha was grown. Methods for determining hydraulic conductivity agreed well for soil layers down to 60 cm, but not for the 60- to 100-cm layer. Slow drainage prevented measuring conductivity functions beyond 100 cm. Moisture uptake by crops continued below the lower limits of availability. After uniformly uptaking water for about 5 weeks, crops developed two distinctly different patterns of water withdrawal. Deep root systems facilitated greater water extraction by cowpea and sorghum during vegetative phase. Short growth duration enabled mungbean and soybean to escape severe water stress, whereas exhaustion of profile soil water during reproductive phase reduced yields of cowpea and sorghum. Mungbean, fallow, soybean, cowpea, and sorghum extracted 19.1, 20.6, 20.6, 37.5, and 39.3 cm water, respectively. All upland crops suppressed rice yield and yield components at most rates of nitrogen fertilization; rice after sorghum produced lowest yields. Nitrogen fertilization increased most rice growth parameters and grain yield. Upland crops appeared to affect rice by influencing soil N status or C:N ratio, although residual toxicity might have been involved. We concluded that upland crops might decrease performance of rice, but that higher combined economic yields of the upland crops and rice favored an upland crop--rice sequence over the fallow--rice system. |
