Green manure approaches to crop production: A synthesis

A green manure (GM) is a crop used primarily as a soil amendment and a nutrient source for subsequent crops. Green manure approaches to crop production may improve economic viability, while reducing the environmental impacts of agriculture. However, such approaches are complex because they depend on interactions between the GM, the environment, and management. We suggest that the research and management techniques developed for synthetic inputs are not adequate for effective GM use. This review provides a conceptual framework to more critically evaluate GM use, and we discuss a limited number of key examples involving GM adaptation and growth, effects on soil organic matter, N release and availability for future crops, and pest control. We explore the deficiencies in our current understanding of GM approaches and argue that economic justification of GM requires provision of multiple services (such as nutrient supply, pest and weed control, and increase of soil organic matter). We propose that future research efforts make improved use of whole systems and participatory strategies to better address both the complexity of GM-based cropping systems and the obstacles preventing farmer adoption of GM approaches.