Optimum irrigation frequency for soybean production

Results showed that the plots that received greater number of irrigation frequency exhibited significantly taller plants than those that were irrigated the least. However, no significant variations on yield parameters e.g. pods per plant, seed per plant, seed yield per square meter, and rooting depth were detected. It is probably due to the high moisture regime of the upper 30-cm root zone brought about by high ground water level in the area throughout the growing period of the crop that no water stress was imposed by the differet irrigation schedules. The empirical evidence of delaying irrigation interval between vegetative stage (17 DAE) and early seed development (61 DAE) depletes the soil moisture from 8.90 cm to 7.67 cm, with a depletion rate of 0.024 cm/day. This finding implies that reducing water at this stage between pod-filling and early seed development would affect soybean yield. The amount of water stored by the soil two days after irrigation was very small (0.27-0.99 cm) compared to the amount of irrigation water applied (5.90-9.07 cm). This was attributed to the high soil moisture content during the imposition of irrigation water at different schedules. This resulted to the very low water application efficiency of the irrigation system (6-9%). In terms of productive efficiency, the least irrigated treatment (two times) has the highest yield/hectare per millimeter of water applied (9.0 kg/ha--mm) which was 50% higher than the treatment that received the highest irrigation frequency (five times). The adoption of two irrigations i.e. one at vegetative stage (17 DAE) and another at pod-filling stage (53 DAE) under lowland rice-based farming system with high soil moisture level would substantially conserve 73% of water and 54% of energy compared to four or more irrigations. Therefore, results suggest that under lowland rice-based farming system with high water table of 60-70 cm below the ground surface, the practice of two irrigations should be followed in order to maximize soybean yield and minimize the use of water and energy.