There are about 20 species of Sesbania in the tropics broadly classified into annual types and perennial (tree) types. The African Sesbania rostrata is an annual (matures in 180 - 270 days), aquatic legume that has the unique ability of producing nitrogen-fixing nodules on its stems and roots.
This species of Sesbania serves as a green manure and can be grown before or between rice or maize crops. It is used as a green manure especially in rice paddies because of its rapid and substantial biomass growth in a short time, its ability to fix substantial amount of nitrogen into the soil and to grow well in water-logging soils. It is estimated that S. rostrata can fix as much as 267 kg N/ha (275 lbs/acre) in 52 days of growth. When used as a green manure, 1/3 of the fixed N is transferred to the rice crop and 2/3 remains in the soil.
The seed should be scarified with boiling water by pouring the water over the seed and letting the seed soak overnight (other treatments include sand scarification and treatment with concentrated sulfuric acid.) It is recommended to plant the seed in moist, but not submerged rice paddies 6 - 7 weeks before rice is transplanted. If S. rostrata is sown for the first time in a field, the seeds need to be inoculated with a Rhizobium bacterium to ensure optimal nitrogen fixation. It is also advisable to inoculate the stems when they have a height of 25 - 30 cm (10 in -12 in). Spacing should be 20 cm (8 in) between seed in a row and 20 cm between rows (36 plants per square meter.) Sesbanias show tolerance to alkaline soils while some sesbanias grow well in acid soils.
Harvesting and Seed Production
Sesbania rostrata can grow 3 m high and set seed in about 4 months. The incorporation of Sesbania rostrata into the soil is carried out when the plants have reached a height of 1 m - 1.5 m (about 35 days old). This is done by plowing, or by any other means which seems appropriate to the farmer. Seeds have to be collected before they shatter. Make sure that you collect mature seeds, thoroughly dried, and clean. Dust the seed with insecticide and freeze it overnight to kill insects, store in air-tight containers at low temperature (5º C) and low relative humidity. Sesbania seeds can normally be stored for several years at ambient temperatures (15° - 30º C) with little loss of viability, although this varies with species and seed source. It is likely that long-term storage could occur under sub-zero temperatures.
Pests and Diseases
Nematodes infest Sesbanias, reducing their growth and inducing root galls. Blights and rots of seedlings and dieback mature plants as well as leaf spots are caused by fungal organisms. Viral and bacterial infections are less common among the Sesbanias. Insect pests include weevils, beetles, borers, thrips, chalcids, and leaf-feeding insects. INNOCULATION - The innoculum is made up of dehydrated bacteria mixed with powder of alginate. These are living organisms (but not harmful) and must be handled carefully if they are to work efficiently. Keep them in a cool place and avoid bringing them in contact with chemicals. To inoculate the seeds, put the inoculant in a plastic bag, add water, close the bag and mix well, and then leave it in a cool place overnight. After the seed has been drained from the scarification process, it can be mixed with the inoculant. To inoculate the stems, these have to be 25 cm - 30 cm in height. Nodules appear on the stem about 1 or 2 weeks after inoculation. The inoculum is prepared in the same way as at sowing, then it is sprayed on the young plants.