Does leaving green beans in the soil at the end of their life cycle have any nitrogen-fixation benefits?
Answer: Legume plants only fix nitrogen in their roots when the plant is growing. The majority of this fixation occurs prior to flowering. For example, when farmers use legumes as a cover crop to produce nitrogen, they usually terminate it during flowering to get the greatest nitrogen benefit. After flowering, the plant begins to move nitrogen into the seeds. Once in the seeds, the nitrogen is used to build protein. This protein does not decompose as easily as the nitrogen in the plant prior to flowering. When the plant dies, the nodules in the root no longer fix nitrogen. However, there is still nitrogen in the plant tissues.This is usually a small number compared to the biomass of the plant, perhaps 1 to 3 percent. But there is a nitrogen benefit to the soil if you let the bean plant decompose. As the plant material breaks down, the nutrients in the plant will release to the soil and be available for a subsequent crop. However, you should balance this nutrient benefit with disease control. Green beans are notorious for having a host of plant diseases. In order to minimize disease pressure, it is often recommended that all plant residue be cleared away at the end of the growing season. If you live in a humid area where disease pressure would be high, you might consider composting your green bean residue. Composting heats the material which helps to kill any disease pathogens and weed seeds.