Two field experiments were carried out at El-Karada station of irrigation requirements, Kafr El-sheikh Governorate, during the two summer seasons 2008 &2009, to study the effect of irrigation regimes (irrigating soybean plants at 25, 50 and 75% of available soil moisture depletion (ASMD) and biofertilizers (inoculation soybean seeds with phosphoriene, Rizobial and their mixture, comparing with recommended NPK) on yield and its components, as well as, some water relations of soybean plants. The experimental design is split plot with three replicates, where the main plots were assigned to irrigation regimes, while the sub plots were devoted to biofertilizers.Results showed that both of irrigation treatments and biofertilizers had highly significant effect on yield and its attributes of soybean plants. Irrigated treatments after the depletion of 50% in available soil moisture, gave the highest values of plant height, No. of branches per plant, No. of pods/plant, pods weight /plant, seed weight /plant,1000 seed weight and seed yield (kg/fed) in both growing seasons. Meanwhile, the lowest values of the abovementioned parameters were recorded under irrigation at 75 % ASMD in both seasons. Data also showed that phosphorien + rhizobial inoculation produced the tallest plants; however, the remains aforementioned characters were the highest under application of recommended NPK, followed by rhizobial inoculation in both growing seasons comparing with phosphorien.Seasonal applied water (as an average of the two seasons) was 3451.4, 3006.9 and 2515.5m3 /fed for irrigation at 25, 50 and 75%ASMD treatments, respectively. Irrigating soybean plants at 25% ASMD recorded the highest seasonal water consumption as an average of the two seasons (2272.9m3 /fed), followed by irrigation treatments at 50% ASMD (2074.2m3 /fed) , while the lowest seasonal water consumption was recorded under irrigation at 75% ASMD, and found to be (1859.2m3 /fed). The highest values of crop water use efficiency (0.96&0.84 Kg/m 3 ) and water productivity (0.66&0.58 kg seed yield /m3WA) were achieved under irrigation at 50% ASMD in both seasons, respectively. Meanwhile, irrigating soybean plants at 75% ASMD recorded the lowest values of CWUE (0.56&0.39 kg/m3 ) and water productivity (0.38 &0.29 kg/m3 ) in the 1 st and 2 nd seasons, respectively.
