Abstract—Soil iron (Fe) and antagonistic effects of rhizobial inoculants and organic C involved in legume root nodulation, leaf chlorophyll formation and crop yield performance are not well understood. Soil Fe, a critical micronutrient for plant growth, is highly variable in the humid, acidic environments. The objectives of the study were to assess the co-limitation of soil Fe concentrations on whole plant development, root nodulation, chlorophyll formation and marketable yield of yardlong beans (Vigna unguiculata subsp. sesquipedalis) in the humid, acidic environment. The field study was conducted in a rice-vigna rotation field in tropical Hainan Island during 2012-2013. The experimental treatments consisted of two levels of chicken compost (0 and 1.8 Mg ha-1) and vigna rhizobial inoculants at the rates of 0, 8, 16 and 24 g per kg of seeds, arranged in a split-plot design. The results showed that soil Fe concentrations were spatially variable across the field. In spite of soil Fe variability, yardlong-bean plants (cv. ‘Youfeng-3’) responded strongly to the compost and inoculant treatments. Root nodulation, whole plant biomass and marketable fresh beans yields were significantly higher in the compost and inoculant plots compared to the control (P < 0.05). Soil Fe concentrations could explain 23% of the variations in leaf chlorophyll content. Fresh bean marketable yields in early bearing stage increased strongly with soil Fe concentrations (R2 = 0.44), but there was a decline trend for late-season fresh bean yields plotted against soil Fe concentrations. It was concluded that co-limitation of excess soil Fe (> 400 mg kg-1) could prohibit legume plant chlorophyll and yield formation. Rhizobia inoculants and organic C help reduce soil Fe activity in the humid, acidic environment.