Nodulation control, nitrogen-fixing

Root knot nematodes cause swollen and darkened enlargements of roots. Destroy infested plants. Control root knot nematodes by applying chitin or parasitic nematodes to the soil. Small, round, pinkish nodules attached to the roots are caused by nitrogen-fixing bacteria and are beneficial. 

Had their experimental constraints been less rigorous and any of the bean plants become contaminated with rhizobia it is beyond doubt that they would have associated the improvement in growth with nodulation and would have concluded as did Hellriegel and Wilfarth thirty years later, that nodulated legumes fix nitrogen. Anyone who has tried to keep control pots of plants free from contamination knows how difficult this can be. Earlier claims by Ville and others may have been due to sporadic contamination by rhizobia or ammonia. 

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