A Comparison of the Numbers of Two Species of Rhizobium and Ammonia-Oxidizing Organisms in Variously Treated Iowa Soils 1

T nodule bacteria of leguminous plants and the nitrifying organisms are very important in the nitrogen cycle and the maintenance of soil fertility. The results of many investigations (2)3 have shown that the nitrifying power of a soil is frequently closely related to its fertility. More recent studies (1, 3, 4, 5) have also shown that the numbers of root-nodule bacteria in soils vary greatly with fertility and especially with fertilization and cropping history.