786 



PRINCIPLES OF SOIL MICROBIOLOGY 



point to a stimulating effect of fallowing upon bacterial numbers, but 

 those of Hiltner and Stormer do not. An increase in the numbers of 

 bacteria followed later by a decrease in numbers, as a result of fallow- 

 ing, has also been reported. 72 Several times as much carbon dioxide 

 was found 73 in fallowed land than in corresponding land left under 

 grass; however, in soils poor in inorganic matter carbon dioxide may 

 soon become lower in the fallowed soil. Nitrate formation 74 and 



3 



1-0 



| Apr. 1 May. | June. | July. | Aug. | Sep. | Oct. | Nov. | Ofic.~l 



Fig. 75. Influence of cropping and fallowing upon nitrate accumulation at 

 different seasons of the year in the soil (after Russell). 



nitrogen-fixation are favorably influenced by cultivation and fallowing. 

 It has been observed at an early date 75 that fallowing makes the soil 



72 Kriiger, W., and Heinze, B. Untersuchungen uber das Wesen der Brache. 

 Landw. Jahrb., 36: 383-423. 1907; A detailed study of the influence of fallow- 

 ing upon plant growth and bacterial activities has been made by Makkus, W. 

 Die Brache, ihre Physiologie, Formen, Zweck, Bedeutung und Verbreitung einst 

 and jetzt. Eine kritische Studie. Landw. Jahrb., 47: 673-718. 1914; Pfeiffer, 

 Th. Einflusz der Brache bezw. der Stallmistdiingung auf die Ernteertriige und 

 den Stickstoffhaushalt im Boden. Landw. Vers. Sta., 98: 187-222. 1921. 



"Wollny, 1897 (p. 477). 



74 Deherain, 1896 (p. 781). 



75 Boussingault, J. B. De la terre veg^tale, considered dans ses effets sur la 

 vegetation. Compt. Rend. Acad. Sci., 48: 303-318. 1859. 



