Nodulation and Symbiotic Nitrogen Fixation

Many factors that contribute to alfalfa's excellence, but recognition must be given to the microsymbiont, the nodule bacteria that fix free nitrogen from the atmosphere. Early studies of the rhizobia:legume association revealed that there were many kinds of nodule bacteria and that various leguminous plants had their preference. Biochemical studies on N fixation in legume root nodules have been made with the soybean because it is an easy plant to culture, shows a marked response to effective rhizobia, and yields abundant nodule tissue. The principles elucidated are applicable to alfalfa and other nodulating leguminous plants. The number of rhizobia required for effective nodulation under any particular set of conditions depends greatly upon the type of inoculum employed. Inoculation is not considered necessary on the high-Ca soils of the West but is strongly recommended on the more acid soils of the Northwest USA.