A comparison of the numbers of two species of Rhizobium and ammonia-oxidizing organisms in variously treated Iowa soils

Thorne, D.W.; Brown, P.E.

A comparison of the numbers of two species of Rhizobium and ammonia-oxidizing organisms in variously treated Iowa soils

1937

Thorne, D.W. | Brown, P.E.

The approximate numbers of Rh. meliloti, Rh. trifolii and ammonia-oxidizing organisms in variously treated soils on the Agronomy farm of the Iowa Agricultural Experiment Station were determined at several different periods. The logarithms of the numbers of Rh. meliloti and Rh. trifolii in the soils studied had a highly significant correlation. The logarithms of numbers of Rh. meliloti and nitrite-forming organisms in the variously treated soils cropped to the 3-year rotation of corn, oats, and clover were highly significantly correlated. In similarly treated soils cropped to a 5-year rotation of corn, oats, red clover, winter wheat, and alfalfa, a highly significant correlation between these organisms was also noted. The correlation between the logarithms of numbers of these organisms in soils cropped only to corn and oats was much lower, being just significant. The correlations between the actual numbers of organisms showed a similar trend to those found between the logarithms of numbers. The presence of legumes in a crop rotation seems to promote a more stable flora of the organisms investigated. The ammonia-oxidizing organisms and the legume bacteria studied showed similar responses to fertilizer treatments. Greater fluctuations were observed, however, in the numbers of legume bacteria. The ammonia-oxidizing organisms reached their maximum numbers in the spring or early summer. The alfalfa and red clover bacteria showed no consistent seasonal fluctuations.

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