This study examined the Biological Nitrogen Fixation (BNF) potential of various legume species in Limuru, Kiambu County, Kenya, focusing on smallholder mixed crop-livestock farmers. Nitrogen is vital to plant growth, particularly for protein synthesis and photosynthesis. Biological nitrogen fixation (BNF), a process by which legumes transform atmospheric nitrogen into a usable form, increases soil fertility and decreases synthetic fertilisers need. The problem addressed was identifying the most effective legumes for improving soil fertility in this region. Using purposive sampling, community-based organisations (CBOs) participated in focus group discussions, where farmers planted experimental plots, collected soil samples, and counted nodules. Destructive sampling was employed during flowering to assess nodulation, with nodules classified based on their ability to fix nitrogen. Results showed Rose coco beans ranked highest due to their efficient nitrogen fixation and adaptability to varying soils and temperatures, followed by Albus lupin and Black and White faba beans. Desmodium also demonstrated persistent nodulation, while indigenous Kienyeji legumes, though less efficient, improved soil fertility. Garden peas, cowpeas, and green grams had lower nitrogen-fixing performance in Limuru's cool, humid climate. This study concludes that Rose coco beans and Albus lupins are highly suitable for improving soil fertility in the region. Participatory methods strengthened farmers' knowledge of sustainable agriculture, and qualitative data highlighted the practical value of BNF legumes. Future research should expand legume trials and explore the use of rhizobia inoculants to further enhance BNF efficiency, supporting long-term agricultural sustainability.
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