172 OUTLINES OF BACTERIOLOGY 



bacteria are active agents in making good the loss of nitrogen cannot 

 be overestimated. In this relation there are two distinct lines of 

 discovery which must be treated separately, though we are dealing 

 with only one class of organism. We have to deal with these nitrogen- 

 organisms, first, when they are free in the soil, and second, when 

 acting in conjunction with leguminous plants. 



2. THE ACTIVITIES OF THE NITROGEN-BACTERIA WHEN 

 FREE IN THE SOIL. 



Suppose a number of young dried leaves be collected from any forest 

 tree, and then exposed to the atmosphere for a twelvemonth. At the 

 end of that period it will be found that, though the total weight is less 

 because some water has evaporated, yet the leaves have gained in 

 nitrogen, in fact they will be found to contain twice as much nitrogen 

 as they did before the exposure. In the same way the soil, when left 

 untouched, will, under favourable conditions, accumulate nitrogen. The 

 results of one experiment will make this clear. Fifty kilograms of earth 

 were exposed for a certain time, and then analysed so far as its nitro- 

 genous constituents were concerned. 



-54*09 grams of nitrogen were present in earth at beginning 



of experiment. 



Total 

 54-569 



0*076 gram ,, was gained through agency of rain. 



0-053 was gained through agency of atmo- 



spheric ammonia. 

 0'35 ,, ,, was present in a few plants which 



had been inserted in the earth. 

 f56'54 grams ,, were present at close of experiment. 



Total I 0-403 gram had been carried away by water. 



59-178] 2-235 grams were present in the plants which had 



I grown in the earth. 



Hence the gain of nitrogen was 4*609 grams. 



As this abstraction of nitrogen from the atmosphere is taking place 

 wherever decomposition of this nature is taking place, it will be seen 

 that a very large gain of nitrogen must annually accrue to the earth 

 through this agency. When this process became known, attention was 

 directed to the agents which Nature employed for this purpose. It 

 was soon found that when the soil was sterilised no gain of nitrogen 

 took place, so that the explanation of the gain could not be found in 

 any chemical or physical action, but must be sought for in the 



