Soil nitrogen Legumes in the rotation can be used to increase the available soil nitrogen. Symbiotic nitrogen-fixing bacteria called rhizobia form nodules on the roots of legume plants and convert or fix atmospheric nitrogen to organic nitrogen. The amount of nitrogen fixed varies with species, available soil nitrogen, and many other factors. Fixed nitrogen not removed from the land by harvest becomes available to succeeding crops as the legume tissues undergo microbial decomposition. When the legume crop is seeded, rhizobia inoculum should always be applied to the seed to ensure the most productive commercial strains are available to form nodules and that inoculating bacteria are always present.
Legumes have the capacity to fix large amounts of nitrogen.
However, only a portion of the nitrogen fixed is available to the next crop because much is removed in the harvested alfalfa. Nitrogen credits of various legume crops are listed in Table 5. Nitrogen credited to a subsequent crop of various legume crops, NDSU soil testing lab. Many farmers who rotate crops comment on the improvement in tilth or friability of soil following soybean or other row crops. Increased aggregate stability reduces the tendency of the soil to puddle or crust, increases rate of water infiltration under certain conditions, and may also reduce wind erosion.