Long-term effects of grain legumes on rainy-season sorghum productivity ina semi-arid tropical vertisol

In southern and central India, farmers crop Vertisols only in the post-rain y season, to avoid land management problems in the rainy season. In 1983 IC RISAT established a long-term trial seeking to intensify cropping. The tria l included intercrops, sequential crops and appropriate Vertisol management technology to allow consecutive rainy-season and post-rainy season crops t o be grown. Benefits provided by legumes to succeeding rainy-season sorghum (Sorghum bicolor) were analysed in relation to a non-legume system of sorg hum + safflower (Carthamus tinctorius). Rainy-season sorghum grain yield pr oduction was sustained at about 2.7 t ha(-1) over 12 years within a continu ous sorghum-pigeonpea (Cajanus cajan) intercrop system. With a cowpea-pigco npea intercrop system, succeeding sorghum benefitted each year by about 40 kg N ha(-1) (fertilizer nitrogen (N) equivalent). Without N fertilizer appl ication the sorghum grain yield was around 3.3 t ha(-1). Legume benefits we re less marked in the chickpea (Cicer arietinum)-based rotation than in the pigeonpea system, in which a 12-year build up of soil total N (about 125 m u g g(-1)) was observed. Although sorghum benefitted from this system, pige onpea yields declined over rime due to soil-borne fungi and nematodes. Wide r rotations of crops with pigeonpea may help to overcome these problems, wh ile sustaining sorghum production.