Effectiveness of indigenous soyabean rhizobial isolates to fix nitrogen under field conditions of Zimbabwe

Ten promiscuous soyabean isolates that proved superior in nitrogen fixation effectiveness to commercial inoculant strains under greenhouse studies were evaluated for their symbiotic potential under field conditions. The promiscuous soyabean variety, Magoye and the specific variety Solitaire were used as test hosts. Five of the isolates were obtained from the nodules of Magoye and coded M1 to M5 while the other five were isolated from the promiscuous soyabean variety Hernon 147 and coded H1 to H5. Two commercial inoculant strains, MAR 1491 and 1495 were included for comparison. Parameters tested included nodulation effectiveness, yield, nitrogen content and nitrogen fixation effectiveness. Isolate M3 gave significantly higher nitrogen contents and N2 fixation effectiveness in Magoye than the reference strains, while H1, H3 and H5 gave better nitrogen contents and N2 fixation effectiveness with Solitaire. Nodule numbers and yields obtained were highly variable and not reliable indicators of effectiveness. There were strong harvest index effects of different rhizobial isolates, which closely followed the trend for grain yield. Isolates M3, H1, H3 and H5 were concluded to be potential commercial inoculants, while the other six need to be retested over many sites and seasons before they can be recommended