Evaluation of green manure crops for nitrogen fixation and soil improvement in tropical conditions

Abstract:

Here is a fact that surprises many agronomists outside the tropics: a well-managed stand of dhaincha (Sesbania aculeata) can fix 158 kg N ha⁻¹ in just 12 weeks more nitrogen than most cereal farmers in the US Corn Belt apply as synthetic fertiliser. This research evaluated three tropical green manure species (sunn hemp, cowpea, and dhaincha) for nitrogen fixation capacity, biomass production, and subsequent soil improvement on a Morrow-series silt loam at Starkville, Mississippi, during 2023. A randomised block design with four replications compared the three species against a bare fallow control. Dhaincha fixed the most N (158 kg ha⁻¹ by 12 weeks), followed by sunn hemp (153 kg ha⁻¹) and cowpea (127 kg ha⁻¹). Following incorporation, soil organic carbon at 0–15 cm rose by 0.14–0.21% over the fallow, and available N increased by 34–47 kg ha⁻¹ within six weeks. These results demonstrate that tropical green manure species can deliver a full season’s worth of N in a short summer window, making them practical cover-crop options for southern US cropping systems.