204 Bacteria in Relation to Country Life 



gain of twenty-five pounds of nitrogen per acre. Azoto- 

 bacter and other nitrogen-fixing bacteria must have 

 played a prominent part in the accumulation of this 

 nitrogen. At any rate, azotobacter has been shown 

 to be quite abundant in the Rothamsted soils. 



Gains of nitrogen have also been noted in smaller 

 quantities of soil employed in vegetation experiments. 

 A number of such gains are recorded in the literature 

 of the subject. The gains found at the New Jersey Ex- 

 periment Station occurred in a poor, sandy soil, and 

 were remarkably large in those instances when horse 

 manure was applied to the soil. It amounted, in some 

 instances, to more than one-third of the nitrogen origi- 

 nally present or supplied. The experiments in question 

 offer strong proof of the ability of these organisms to 

 add large quantities of combined nitrogen to the soil. 

 They also show the necessity of supplying to the bac- 

 teria an abundance of organic material that they may 

 use as food and fuel in manufacturing nitrogen com- 

 pounds. 



Other nitrogen- fixing forms. Aside from the two 

 large groups of nitrogen-fixing bacteria discussed here, 

 there are also a number of soil organisms that, to a 

 limited extent, may display an ability to fix atmospheric 

 nitrogen. They are not, strictly speaking, nitrogen- 

 fixing bacteria, and belong properly to the ammonifi- 

 cation or denitrification groups, or to others. Neither 

 the conditions that stimulate their power of nitrogen- 

 fixation nor their relations to the nitrogen-fixing bacteria 

 proper are well understood. It does not necessarily 

 follow that their ability to utilize atmospheric nitrogen 



