The objective of this research was to investigate the effect of regulated deficit irrigation on advance vegetative phase to the water productivity of Soybean. This research was conducted under plastic house on the field laboratory of Lampung University from October 2018 to January 2019. The water stress treatments in regulated deficit irrigation levels were DI1 (0 – 100 %) of total available water as a control, DI2 (0 – 80 %), DI3 (0 – 60 %), DI4 (0 – 40 %) and DI5 (0 – 20 %) of total available water (TAW) arranged in a randomized block design with four replications. The results showed that the soybean plant started to experience stress from week IV, the soybean plant started to experience stress within 0-40 % of total available water and continuing to stress until the end of growth, even the RDI treatment was stop at week VI. It means that the soybean plant which experience to tress at vegetative advance can't be recovered even the soybean plant was irrigated to bring back the water to the field capacity. The (p) value was 0.6 and the Ks value were 0.84; 0.70; 0.68; 0.80, 0.86 and 0.88 at week IV, V, VI, VII, VIII and IX, respectively. The average Ks value was 0.79. There was no significant different between DI1 DI2, and DI3 in water productivity of soybean plant. The value of water productivity were 0.65, 0.49, 0.48, 0.40 and 0.42 at DI1, DI2, DI3, DI4, DI5, respectively. The optimum water management which the high crop water productivity (WP=0.48) was reach by RDI at DI3 treatment which maintain the available water between 0-60 % or the soybean plant must be irrigate by bring back the water to the 60 % of total available water. The optimum yield of soybean (Anjasmoro veriety) was 17.9 g/pot and crop water requirement was 36746.5 ml or equal to 566.08 mm.
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