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 GMbased cropping systems and the obstacles preventing farmer adoption of GM approaches.

Legumes utilized as GM may provide on‐farm organic N. Nonetheless, data regarding GM use on sandy soils in warm‐temperate environments remains scarce. We conducted a 2‐yr field study to evaluate growth and decomposition of sunn hemp (Crotalaria juncea L.), blue lupin (Lupinus angustifolius L., winter 2001–2002), and Cahaba white vetch (Vicia sativa L., winter 2002–2003) used as GM to provide N for sweet corn (Zea mays L. var Rugosa) in a reduced tillage system in northern Florida. Intercepted solar radiation accounted for most variability in total GM dry weight and N content regardless of species or year. Sunn hemp exhibited substantial growth and N accumulation (≤12.2 Mg ha−1 and 172 kg N ha−1 in 14 wk). However, rapid N loss (45–58% within 4 wk after death) occurred after sunn hemp death. Winter GM growth (2.0–4.0 Mg ha−1 and 51–104 kg N ha−1 in 18–20 wk) appeared limited by low LAI response to growing degree days (GDD) and was not affected by previous sunn hemp GM. Sunn hemp residues and living winter legume together contained 120 to 125 kg N ha−1 at time of final sampling before sweet corn planting. Corn rotated with sunn hemp plus winter GM and supplemented with 133 kg synthetic N ha−1 produced marketable ear yields similar to monoculture corn fertilized with 200 kg synthetic N ha−1, but the practical value of this benefit is low. In northern Florida, winter and/or summer GM use may substitute only for a relatively small portion of synthetic N rates.

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Legumes used as green manure (GMN) may provide onfarm sources of N for subsequent crops. In warm-humid climates on sandy soils, however, there is a need to evaluate effects on growth and yield of subsequent crops when GMN substitutes for chemical N fertilizer. We collected time-series data for 2 years to evaluate growth and ear yield response of sweet corn (Zea mays L. var. Rugosa) in annual rotation with GMN (summer, winter or summer plus winter GMN) and supplemented with 0, 67 or 133 kg chemical N ha )1 . Control treatments receiving 0, 67, 133, 200 or 267 kg chemical N ha )1 were used for comparison. Rotation with summer plus winter GMN resulted in more growth and yield benefit for sweet corn than did the other GMN rotations. Compared with control treatments receiving the same chemical N rates, rotation with summer plus winter GMN increased sweet corn ear yield, leaf area index and total plant dry weight and N content by 15-30 %. Despite these benefits, use of high chemical N rate (267 kg chemical N ha )1 ) resulted in ear yields at least 20 % greater than sweet corn rotated with any GMN and fertilized with reduced N rates. Largest increases in sweet corn dry weight and N content occurred between 4 and 6 weeks after emergence in all treatments. After this period of rapid growth, sweet corn fertilized with 267 kg chemical N ha )1 showed higher N content than all other treatments. Prior to this period of rapid growth, multiple rainfall events in excess of 30 mm may have resulted in leaching of N from GMN residue. Considering the high rainfall levels, low water and N retention capability of sandy soils and rapid N release from GMN during decomposition, future research of GMN in this environment should evaluate options to better match GMN termination with time of greatest potential N uptake from a subsequent crop.