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Sunn hemp (Crotalaria juncea) green manure

Sunn hemp (Crotalaria juncea) is a fast-growing tropical legume cultivated as a green manure, cover crop and short-duration forage. The species fixes biological nitrogen, smothers weeds, suppresses root-knot nematode populations and produces high biomass when incorporated at the vegetative-to-flowering stage.

Principle

The plant grows rapidly under warm rainy-season conditions and develops a deep, branched root system colonised by Rhizobium that fixes atmospheric nitrogen. Aboveground biomass shades out weed seedlings and intercepts rainfall, reducing surface runoff and soil erosion. The roots and shoots produce allelochemicals that disrupt root-knot nematode life cycles, providing biotic suppression of a soil-borne pest complex that affects vegetables and orchard crops.

Implementation

Approximate nitrogen fixation is 50-60 kg N/ha in 60-90 days, with growth reaching 1.2 m at 60 days and 1.8 m at 90 days. Aboveground dry matter exceeds 5,000 lb per acre at peak. The crop is incorporated at the vegetative-to-flowering stage to capture nitrogen reserves before stems lignify. Young growth is also palatable as small-ruminant fodder, allowing the standing crop to be grazed early before knockdown and incorporation. The species is widely under-sown in orchards as a living cover or rotated before kharif vegetables.

Adoption context

Sunn hemp is used across India's kharif belt as a pre-kharif green manure, in young orchard inter-rows as a cover crop, and in conservation-agriculture rotations to add nitrogen and organic matter ahead of the cash crop. State natural-farming and organic-farming missions include the species in their recommended-crop list for soil-building rotations.

Limitations

Seed cost can deter smallholder adoption. Heavy waterlogged soils slow growth. Delayed incorporation past flowering produces lignified stems that decompose slowly and immobilise nitrogen in the short term. The window between sowing and required incorporation must fit between the previous crop and the planned cash crop, which is not always feasible in tight rotations.

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

References

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