61 



Reviewing Table I, we note the failure of the various inorganic fertilizers 

 to offer any marked stimulation to nitrogen fixation. The soils all have a 

 rather high nitrogen fixing power, yet the control plat which received no 

 fertilizer has an Azotobacter flora which is excelled in only one or two 

 cases. The .maximum fixations are attained under the influence of stable 

 manure, and that with lime records the highest increase in nitrogen noted 

 in the series. The minimum fixation, that from soil No. 2 which had crim- 

 son clover turned under without lime, is not unexpected. The acid condi- 

 tion resulting from such a practice would undoubtedly account for this 

 decrease. The high fixing power of soil No. 10 is difficult to explain. It 

 does not appear probable that nitrate of soda should induce any special de- 

 velopment of Azotobacter. If so, it is significant that less nitrogen in this 

 form was more conducive to their growth than were larger quantities of 

 organic nitrogen. The action here suggests a parallel to the benefit which 

 leguminous crops are supposed to derive from light dressings of nitrate of 

 soda during the early stages of growth. 



The Azotobacter films on all of the soils were uniformly good and those 

 on soils Nos. 3, 4, and 5, were the richest ever observed in this laboratory. 



The nitrification tests are assertive in at least one point, namely, the 

 conspicuous results obtained by the use of lime. From Table II it will be seen 

 that crimson clover increased the nitrifying power of the soil 83 percent 

 while crimson clover and lime produced an increase of 526 percent. The 

 use of stable manure likewise increased nitrification 120 percent and the 

 addition of lime here produced a 407 percent increase. The combination of 

 crimson clover and lime was more efficient in promoting nitrification than 

 was stable manure and lime. The remaining soils except Nos. 10 and 11 gave 

 uniformly higher nitrifying power than the control plat, yet it is question- 

 able that any combination of fertilizers was responsible for this. Their use 

 probably induced general fertility, which stimulated nitrification somewhat, 

 yet no striking result is noted certainly none commensurate with that 

 produced by lime, for example. 



(2) Studies on Nitrogen Fixation in Various Soil Types 1 



The process of nitrogen fixation of Azotobacter has been definitely es- 

 tablished; it is probably of widespread activity; its importance is of dis- 

 puted magnitude, regarded as paramount in soil fertility by a sanguine few, 

 but held in abeyance by a significant number of others. Despite the investi- 

 gation which the problem has received, no very definite conclusions can be 

 drawn. The unexpected fixation by one soil, the near failure of another, 

 possessing the qualities usually ascribed to soils of high fixation power, are 

 obstacles to the investigator who would dogmatically assert the conditions 



a Paper 37 from the Laboratories of Plant Pathology and Bacteriology, Virginia Agr. Exp. Sta. 



