Effects of long-term legume cover crop incorporation on soil organic carbon, microbial biomass, nutrient build-up and grain yields of sorghum/sunflower under rain-fed conditions

Low organic matter, poor fertility and erosion are common features of rain‐fed Alfisols in southern India. Build‐up of organic matter is crucial to maintain sustainable production on these soils. The possibility of on‐farm generation of legume biomass horsegram; Macrotyloma uniflorum (Lam.) Verdc.] by using off‐season rainfall was examined in two field experiments involving sorghum and sunflower from 1994 to 2003. The effects of this incorporation were assessed on crop yields and soil properties for 10 years together with fertilizer application. Horsegram biomass ranging from 3.03–4.28 t ha^?1 year^?1 (fresh weight) was produced and incorporated in situ under different levels of fertilizer application. Annual incorporation improved the soil properties and fertility status of the soil, which resulted in improved yields of test crops. With biomass incorporation, mean organic carbon content improved by 24% over fallow. Microbial biomass carbon improved by 28% at site I. Long‐term biomass incorporation and fertilizer application resulted in the build‐up of soil nutrients compared with the fallow plots. Application of N and P alone resulted in a negative balance of soil K. A time‐scale analysis of yields showed that incorporation together with fertilizer application maintained a stable yield trend over a 10‐year period in sorghum, whereas fertilizer application alone showed a declining trend. At the end of 10 years of incorporation, the increase in grain yield because of incorporation was 28 and 18%, respectively, in sorghum and sunflower over fallow when no fertilizers were applied to rainy season crops. The incorporation effect was even larger in plots receiving fertilizer. The growing and incorporation of a post‐rainy season legume crop is a low‐cost simple practice that even small and marginal farmers can adopt in semi‐arid regions of the country. Widespread adoption of this practice, at least in alternate years, can restore the productivity of degraded soils and improve crop yields.