146 BACTERIA AND SOIL FERTILITY 



Crop. — Heinze called attention to the fact that the fallowing 

 of the soil increased its nitrogen-fixing power. This may be due 

 to better aeration, moisture, temperature, etc., and not to any de- 

 pressing influence exerted directly by the plant. Most experi- 

 ments which consider plant and bacterial activity may be inter- 

 preted in this light. Hiltner maintains that the non-symbiotic 

 nitrogen-iixing bacteria are stimulated in their activities by the 

 growing plant roots, There may be considerable truth in this 

 for here the higher plants are rapidly removing from the solution 

 the soluble nitrogen compounds. In this case, the nitrogen-fixing 

 organisms would be forced either to compete with the higher 

 plants for the soil nitrogen or else make use of their ability to 

 live upon the atmospheric nitrogen. It is certain that different 

 cultural methods vary sufficiently with crops to influence pro- 

 foundly a soil's nitrogen-assimilating properties, for the Azoto- 

 bacter occur more widely distributed in cultivated than in virgin 

 soil* The analyses of hundreds of samples of cultivated and 

 virgin soils in Utah have in nearly every case shown the virgin 

 soil to have a low nitrogen-iixing power as compared with the 

 cultivated soil. This was the case even where the soil was in- 

 cubated without carbohydrates and the nitrogen determined di- 

 rectly. The average results from many determinations were as 

 follows: 



Soil 



Nitrogen Fixed (rngm.) 



Virgin Soil 



Cultivated 



Wheat 



Alfalfa 



Fallow 



6.99 

 14.28 

 11.83 

 12.24 

 22.81 



It would, however, be possible to fallow or crop soils so con- 

 tinuously that extremely small quantities of plant residue would 

 be returned to the soil, under which conditions there may be a 

 decrease in nitrogen fixation. The conditions of moisture and 



