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Sunn Hemp (Janumu / Crotalaria juncea)

Sunn hemp (Crotalaria juncea), called janumu in Telugu, is a fast-growing tropical legume sown pre-kharif as a green manure. The crop reaches 1.2 m in 60 days and 1.8 m in 90 days, fixing biological nitrogen and adding bulk green biomass that, when incorporated, supplies free nitrogen, phosphorus and potassium to the following crop.

Principle

Sunn hemp roots form nodules with Rhizobium that fix atmospheric nitrogen during the vegetative growth phase. By the time the plant approaches flowering at 45-60 days, it has accumulated nitrogen reserves in shoots and roots that can be incorporated into the soil as a green manure. The bulky aboveground biomass also smothers weeds during its rapid growth phase and suppresses root-knot nematode populations through allelochemicals produced by the roots.

Implementation

Sunn hemp is broadcast or row-sown pre-kharif ahead of the main crop. Approximate nitrogen fixation is 50-60 kg N/ha in 60-90 days, with dry matter production exceeding 5,000 lb per acre. Standard practice is to plough the standing crop in at the vegetative-to-early-flowering stage, 45-60 days after sowing, before stems become heavily lignified and slow to decompose. The crop can also be under-sown in young orchards or grazed early as small-ruminant fodder before incorporation as green manure.

Adoption context

Sunn hemp is grown across the rainfed kharif belt of India and is the recommended pre-kharif green manure under several state organic and natural farming missions. It fits the rabi-fallow-kharif window in dryland systems where Sesbania (dhaincha) requires too much water. The crop is also a structural element in conservation-agriculture rotations and orchard inter-row management.

Limitations

Seed cost can be a barrier for cash-strained smallholders. Lignified stems if incorporation is delayed past flowering decompose slowly and tie up nitrogen short-term. Suitability declines on heavy waterlogged soils. Yield of the following crop depends on adequate incorporation depth and soil moisture for decomposition.

See also Sunn Hemp Green Manure 2, Green Manure Dhaincha, Mulching Organic Residue and In Situ Residue Decomposition.

References

  1. Crotalaria juncea. Tropical Forages database.
  2. Sunn hemp. Feedipedia.