Underutilised Crops for Enhancing Soil Health and Water-Use Efficiency in Water-Scarce Agricultural Systems: A Systematic Review

Water scarcity and soil degradation constrain agricultural production in arid, semi-arid and rainfed systems, increasing the need for crops capable of maintaining production under limited water and input availability. This systematic review synthesised evidence on the contributions of underutilised crops to soil health, water-use efficiency, and climate-smart agricultural outcomes. Following PRISMA guidelines, peer-reviewed studies published between 2014 and 2025 were identified from Scopus, Web of Science and Google Scholar. Of 275 records identified, 28 studies met the eligibility criteria. Bambara groundnut (Vigna subterranea) was the most frequently studied crop (14 studies), followed by cowpea (Vigna unguiculata) (12), sorghum (Sorghum bicolor) (9), finger millet (Eleusine coracana) (7), pearl millet (Pennisetum glaucum) (6), amaranth (Amaranthus spp.) (4), African nightshade (Solanum scabrum) (2) and African yam bean (Sphenostylis stenocarpa) (1). Water-use efficiency was the most frequently reported water-related indicator, occurring in 23 of 28 studies (82.1%), followed by drought tolerance (11; 39.3%), rainfed performance (8; 28.6%), soil-moisture retention (5; 17.9%) and irrigation requirement (3; 10.7%). Soil-health evidence was concentrated on biological nitrogen fixation, soil nitrogen, soil organic carbon and nutrient availability, whereas soil structure, aggregation, microbial processes, and erosion control were less frequently assessed. Leguminous crops contributed primarily through biological nitrogen fixation and nutrient cycling, while drought-adapted cereals and millets were more consistently associated with water-use efficiency, rainfed performance and production under moisture stress. Wider adoption was constrained particularly by competition with major staple crops, inadequate policy support, limited agronomic knowledge, weak extension services, poor seed availability and underdeveloped markets. Overall, the evidence most strongly supports the adaptation and productivity benefits of selected underutilised crops in water-scarce systems, while long-term soil processes and climate-mitigation outcomes remain insufficiently quantified. Future research should prioritise long-term, crop-specific field studies that simultaneously measure crop productivity, soil processes and water dynamics across contrasting water-scarce environments.