Most plants need nitrogen fertilizer every year to grow their best. But legumes are different. The bacteria that nodulate legume roots can "fix" or convert nitrogen from the air into protein inside the plant. Because of this unique ability to acquire nitrogen, planting legumes and incorporating them into soil may be an attractive option for some gardeners.
When legumes are turned into the soil, microorganisms degrade the residue. Nitrogen is then released and recycled for the next crop to use. Incorporating forage legume or green manure crops like many other organic amendments can improve soil structure, increase a sandy soil's ability to hold water, improve drainage of heavier soil, and help soil resist compaction and crusting. But how much extra nitrogen should you apply when legumes are turned under? That depends on what your soil test recommends for nitrogen and the amount of nitrogen released from the legume crop.
After determining how much nitrogen will be released from the legume crop, subtract this "credit" from the nitrogen amount recommended by your soil test report. You may be surprised that sometimes no additional fertilizer nitrogen is needed. Keep in mind that time is needed for microbial breakdown. Some short-season vegetables may need extra nitrogen in addition to the legume credit to compensate for the slow release. This is especially true if temperatures are cooler than normal.
