Low effectiveness of native strains remains a limitation to soybean productivity in sub-Saharan Africa; while in other countries commercial inoculants are produced that provide effective strains that stimulate N fixation and growth. An experiment was set up to evaluate the response of a dual purpose promiscuous soybean variety (TGx1740-2F) and a non-promiscuous variety (Nyala) to commercial rhizobium inoculants in soils from central and coastal Kenya. Highest nodulation was observed in some of the treatments with commercial inoculants applied with nodule weights of 4.5 and 1.0 g plant?1 for TGx1740-2F and Nyala, respectively. Average biomass yields of TGx1740-2F (16 g plant?1) were twice as large as of Nyala (7.5 g plant?1) at the podding stage. Nitrogen fixation was higher in TGx1740-2F than in Nyala, and positively affected by a number of commercial inoculants with more than 50% N derived from the atmosphere. Nodule occupancy was 100% on both soybean varieties, indicating that the commercial strains were extremely infective in both of the tested soils. These results showed that commercial strains can be used to inoculate promiscuous soybean and enhance N fixation and yield. (Résumé d'auteur

A key constraint in successfully obtaining an effective inoculant is overcoming difficulties in formulating a viable and user-friendly final product and maintaining the microbial cells in a competent state. Co-cultures of rhizobia and PGPR (Plant Growth Promoting Rhizobacteria) are a logical next subject for formulation researchers as they can influence the efficacy of rhizobia. A greenhouse experiment was set to assess the formulation effect of one strain i.e. Bradyrhizobium japonicum, 532c (granules, liquid and broth) and also to determine the efficiency of co-inoculation of Bacillus with two commercial strains of B. japonicum (532c and RCR 3407) on 2 soybean (Glycine max L.) varieties. PCR-RFLP analysis was used to determine the nodule occupancy in each treatment. Most of the inoculants showed increased nodulation and biomass yields (by approximately 2-5 and 4-10 g plant(-1) respectively) as compared to the uninoculated controls. TGx1740-2F showed no significant differences in nodule fresh weights for the formulation effect while the co-inoculants increased the nodule fresh weights by up to 4 g plant(-1). The liquid and granule-based inoculants induced higher biomass yields (4-8 g plant(-1)) suggesting a possible impact of formulation on the effectiveness of the inoculants. The co-inoculants also gave higher yields but showing no significant differences to the rhizobial inoculants. Nodule occupancy was 100 % for the rhizobial inoculants as well as the co-inoculants emphasizing the infectivity and high competitiveness of 532c and RCR 3407 strains despite the high population of indigenous rhizobia.

A field study was carried out on a sixyear-old on-farm field trial during long-rains season (April-August) 2003 to investigate the effect of improved fallow systems and phosphorus application on arbuscular mycorrhiza fungi (AMF) symbiosis in maize. The trial comprised of maize rotated with a fast growing leguminous Crotalaria grahamiana fallow and a non-leguminous Tithonia diversifolia fallow for 3 years followed by continuous maize. The experiment was randomized complete block design with three cropping (continuous maize, Crotalaria fallow and Tithonia fallow) systems and two phosphorus levels (0 and 50 kg P/ha). AMF colonization in maize roots, maize yield and macro-nutrients uptake were recorded. Phosphorus applications improved (P \ 0.05) early (\8 weeks old maize) AMF colonization, nutrient uptake and maize yield in improved fallow systems. Greater differences due to phosphorus application were noted in maize in Tithonia fallow than in Crotalaria fallow. Following phosphorus application, a positive relationship existed between early AMF colonization and maize yield (r = 0.38), and phosphorus and nitrogen uptake (r = 0.40 and r = 0.43, respectively), demonstrating the importance of phosphorus fertilization in enhancing low-input technologies (improved fallows systems) in phosphorus deficient and acidic soils of western Kenya.

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Potassium (K) deficiency is emerging in intensively cropped soils of Kenya, but the magnitude of the problem has not been determined. This study was conducted to ascertain the possibilities of mapping K-deficient soils by the use of existing soil and climatic databases and validate the developed map. Mapping was done in Geographical Information Systems, using moisture data, in the Almanac Characterization Tool. Six study sites were selected: three (Itare, Keumbu, and Ndanai) in the southern geomorphic area (SGA) and three (Ebukanga, Yala, and Kabula) in the northern geomorphic area. Soil samples were analyzed for texture, clay mineralogy, exchangeable K, and other fertility attributes. Maize response to increasing levels of K (0, 25, 50, and 75 kg ha j1 , as KCl) was studied in five of the six sites. Mapped area occupied 13% of the land area in Kenya. Exchangeable K ranged from 0.2 to 1.6 cmol (p + ) kg j1 and increased significantly with increase in organic C (P = 0.015), pH (P = 0.023), Ca + Mg content (P = 0.031), and K saturation of exchange complex (P = 0.013). Maize response to KCl was not significant in all soils possibly because the three-season testing period was too short, but the probability of response and the profitability of K use in maize increased with seasons in SGA because of increased K depletion. Potassium fertilizer was recommended in soils from the SGA at 25 kg ha j1 . Application of nitrogen-phosphate-K fertilizers was not economical in the northern geomorphic area because of attack by witchweed, Striga hermonthica. (Soil Science 2006;171:610-626)