Cowpea (Vigna unguiculata L. Walp.) as a green manure to improve the productivity of a menthol mint (Mentha arvensis L.) intercropping system

A field experiment was conducted at Central Institute of Medicinal and aromatic Plants (CIMAP), Lucknow, India in a sandy loam soil (entisol) during 2004 and 2005. Cowpea (Vigna unguiculata L. Walp.) was intercropped with transplanted menthol mint (Mentha arvensis L.) for green manuring (GM) and for fodder plus green manuring (F + GM) with four levels of urea N (0, 30, 60, 90 kg N ha−1). In GM, cowpea was incorporated in the soil 30 days after sowing (DAS), while in F + GM 50% (alternate) cow pea plants were used for fodder at 30 DAS and 50% were incorporated in soil at 35 DAS. No significant differences were found between GM and F + GM with respect to herb and oil yield of menthol mint and succeeding palmarosa crop and nitrogen economy. Fresh biomass yield of menthol mint increased by 23.4% and essential oil yield by 25.2% by cowpea green manure (mean of GM and F + GM) as compared to without GM across all N levels. The contribution of green manure, as a nitrogen source, was equivalent to 30 kg N ha−1 when no fertilizer nitrogen was applied in menthol mint. The residual effect of cowpea GM was studied in a succeeding crop of fast growing essential oil yielding palmarosa (Cymbopopogon martinii (Roxb.)Wats. var motia Burk.) over two harvests (July and December). Averaged across N levels green manure resulted in an increase of 18.5% in the fresh biomass and 17.7% in essential oil yield of palmarosa over no green manuring.