Screening of Green Manure Legumes Adaptable to the Desert Southwest.

Green manure legumes can significantly improve soil fertility and provide much needed organic matter for soil health improvement of many farmlands in the southwestern United States. However, selection of adaptable species that can fit into the existing cropping systems remains challenging. Concerns raised by farmers include the adaptability of species, biomass accumulation potential, irrigation demand and nitrogen fixation capacity of the legumes. A preliminary study was conducted to screen several legume species as potential green manure candidates for the southwest. The study was conducted at the Student Centered Field Laboratory at the New Mexico State University, Las Cruces, NM. Selected species (13 species) that were tested included adzuki bean (Vigna angularis), sesbania (Sesbania exaltata), tepary bean (Phaseolus acutifolius), pigeon pea (Cajanus cajan), green bean (Phaseolus vulgaris, Linnaeus), lima bean (Phaseolus lunatus), lablab (Lablab purpureus), yellow sweet clover (Melilotus officinalis), bush bean (Phaseolus vulgaris), mung bean (Vigna radiata), guar (Cyamopsis tetragonoloba), moth bean (Vigna aconitifolia), and cowpea (Vigna unguiculata). Results showed that sesbania had significantly higher biomass (30 t/ha) than the rest of the legumes tested. This was followed by lablab (10 t/ha), cowpea (8 t/ha) and bush bean (7 t/ha). Cowpea had the highest chlorophyll content in the leaves, followed by green bean and lablab. This may be an indicator of nitrogen fixation. Canopy light penetration measurement showed that several of the species (lablab, lima bean, cowpea, green bean, bush bean and mung bean) provided good canopy cover which is an important characteristic that may help suppress weeds and reduce soil water evaporation. Pigeon pea and moth bean gave poor canopy cover. Mid-season transpiration rate was highest in lablab and lowest in cowpea, moth bean and mung bean. This study will continue to assess these species for biomass production, water use efficiency and nitrogen fixation capacity for the southwestern region.