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When grain legumes are grown on nitrogen-deficient soils, they may symbiotically fix enough N for good yields, even if no N fertilizer is applied. However, legumes preferentially assimilate N from the soil if this source is available to them. As a result, excess N via the soil may inhibit symbiotic N2 fixation by legumes. The physiological and morphological modifications that make symbiotic N2 fixation by legumes possible include: (I) the invasion of host roots by effective strains of Rhizobium, (2) the development of nodules to "house" the Rhizobium and, (3) the translocation of mineral nutrients and photosynthate from the host to the nodules to maintain Rhizobium activity. Nodules represent an extra sink for plant assimilates, a sink that non-nodulated legumes do not have.