This study was carried out to investigate water-yield relationships of soybean (Glycine max L. Merril) in the southern Marmara region of Turkey under a sub-humid climate for two seasons. Field experiments were consisting of full irrigation, deficit irrigation and rainfed treatments. Deficit irrigation treatments were determined based on the phenological periods of soybean as vegetative growth, flowering, pod formation, and seed enlargement. Irrigation water was applied by drip irrigation method.

The result of study showed that effect of irrigation treatments on yield, biomass and harvest index were statistically significant. Maximum crop evapotranspiration and yield were obtained from fully irrigated treatment. On the other hand, according to linear regression analyzes of relative evapotranspiration deficiency and relative yield decrease, crop yield response factor (ky) values for vegetative, flowering, pod formation and seed enlargement stages were found as 0.37, 1.12, 1.31 and 1.89, respectively. In addition, ky for the total growing season of soybean was 1.21. As a result, under sub-humid climate conditions soybean is very sensitive to water deficit especially during pod formation and seed enlargement stages. The water stress during vegetative growing stage has not a deterministic effect on soybean yield. However, in order to obtain maximum yield, fully irrigation throughout the other development stages is necessary.