Four decades of soybean [Glycine max (L.) Merr.] cultivation in South Africa has resulted in the establishment of populations of bradyrhizobia against which the recently introduced inoculant strain CB 1809 must compete. Serological and DNA fingerprinting methods were used to study the diversity of nodule isolates from soils at Bergville, Koedoeskop and Morgenzon. Dominant serogroups included Bradyrhizobium elkanii serotype 76 at Bergville (67%), Bradyrhizobium japonicum serotype 123 at Morgenzon (81%) and B. japonicum serotype 135 at Koedoeskop (100%). Their origin is unknown as they do not correspond in serotype to strains used in previous inoculants. A small percentage of isolates from Bergville (13%) and Morgenzon (16%) were serologically homologous to strain WB 1 (serotype 31/76), applied for two decades before CB 1809 (serotype 122). Nitrogen-fixing effectiveness of CB 1809 was superior to 60% of the isolates tested from Bergville and Morgenzon, but similar to 73% of the Koedoeskop isolates. Seed and liquid-in-furrow application methods increased CB 1809 nodule occupancy at least three-fold above background levels at Bergville (pH 5.16) and Morgenzon (pH 6.33). Inoculation did not, however, increase CB 1809 nodule occupancy at Koedoeskop (pH 7.76), possibly because alkaline soil conditions favoured the serotype 135 population predominant at this site.
Farm lands of resource-poor communities in South Africa are depleted of nutrients due to continuous mono-cropping, limited use of fertilisers, and sometimes leaching caused by high rainfall. Despite the well-known advantages of biological nitrogen fixation (BNF) in cropping systems, less than 10% of the grain crops planted annually in these areas are legumes. Using a participatory research and development approach, resource-poor farmers were introduced to conservation agriculture (CA) practices, including BNF, that promoted zero (or reduced) tillage, increased retention of soil cover, as well as crop diversification. Because crop rotation and intercropping of legumes with cereals are known to contribute to soil fertility while enhancing food security, resource-poor farmers from various Provinces in South Africa were trained on the benefits of legume culture for eight years. As a result, these resource-poor farmers did not only get training in inoculation techniques, but were also supplied with inoculants for use on their farms. Data collected from Farmers Demonstration Trials at Belvedere, Dumbarton and Lusikisiki, showed that the grain and fodder yield of maize planted after legumes, and maize intercropped with legumes, were comparable to those of maize receiving high N fertilizer dose (i.e. 54 kg N at planting and 54 kg N as top-dressing). The same data further showed that Rhizobium inoculation, when combined with application of low levels ofP and K, significantly increased crop yields within farmers' trial plots. BNF therefore offers a great opportunity for resource-poor farmers in South Africa to increase their crop yields and thus improve the quality of their livelihoods through the adoption of affordable and sustainable biological technologies that enhance soil fertility.
A culture of Sinorhizobium meliloti strain U 45, maintained on yeast extract-mannitol (YM) agar, produced a mixture of Congo red-absorbing (R1) and non-absorbing (W1) colonies when grown on YM medium containing Congo red. The original freeze-dried (FD) culture formed gummy (G), white (W2) and small red (R2) colony types on the above medium. All colonies were stable except G, which segregated into G and W2-like types. Immune diffusion patterns of all colony types were identical. The W1 colony type dominated R1 when a 1:1 combination was sub-cultured on YM agar. The parent cultures and their variants exhibited a range of N2-fixing effectiveness and competitiveness when inoculated onto two cultivars of Medicago sativa. Variant R2 from the FD culture was ineffective on both cultivars. Genomic DNA fingerprinting with insertion elements ISRm3 and ISRm2011-2 suggested that transposition of these elements was not a cause of variation, but a DNA band was absent in the profiles of two out of three W2-like colonies. Protein profile comparisons showed high similarity (r = 0.98) between the colony types when grown in YM broth. When grown on Tryptone-Yeast extract medium, variants from the FD and agar-maintained cultures formed separate clusters with r = 0.79. Polymerase chain reaction fingerprinting using repetitive, site-directed and arbitrary primers failed to differentiate the variants. The results emphasize the need to monitor culture variability to maintain the quality of legume inoculants.
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