Long-Term Effects of Grain Legumes on Rainy-Season Sorghum Productivity in a Semi-Arid Tropical Vertisol

In southern and central India, farmers crop Vertisols only in the post-rainy season, to avoid land management problems in the rainy season. In 1983 ICRISAT established a long-term trial seeking to intensify cropping. The trial included intercrops, sequential crops and appropriate Vertisol management technology to allow consecutive rainy-season and post-rainy season crops to be grown. Benefits provided by legumes to succeeding rainy-season sorghum (Sorghum bicolor) were analysed in relation to a non-legume system of sorghum + safflower (Carthamus tinctorius). Rainy-season sorghum grain yield production was sustained at about 2.7 t ha−1 over 12 years within a continuous sorghum–pigeonpea (Cajanus cajan) intercrop system. With a cowpea–pigeonpea intercrop system, succeeding sorghum benefitted each year by about 40 kg N ha−1 (fertilizer nitrogen (N) equivalent). Without N fertilizer application the sorghum grain yield was around 3.3 t ha−1. Legume benefits were 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 μg g−1) was observed. Although sorghum benefitted from this system, pigeonpea yields declined over time due to soil-borne fungi and nematodes. Wider rotations of crops with pigeonpea may help to overcome these problems, while sustaining sorghum production.