no. 6. department of agriculture. " 377 



7. Increase of the Nitrogen Content of Soils. 



It is a familiar fact that land left to itself in an undisturbed con- 

 dition increases in fertility from year to year. In the forests and 

 prairies of our own country we have profited by the accumulated fer- 

 tility of centuries, and much of our present National wealth we owe 

 to those biological processes of the soil we are about to enumerate. 



Hy these Ave mean the ability of the soil to accumulate nitrogen 

 through the agency of microscopic organisms, and the lower classes 

 of plants. Every observer is familiar with the growth of lichens on 

 bare rocks on which no organic food, much less nitrogen in any ap- 

 preciable quantity, is present; and yet such plants contain nitrogen 

 and contribute it to the thin mantle of soil which slowly accumulates 

 over rocky surfaces. 



Fungi are frequently found on sterile sand in which only an in- 

 appreciable quantity of nitrogen is present, and yet they may store 

 up in their tissues quantities of nitrogen greatly in excess of what 

 the soil is able to furnish. 



Mulder showed that moulds grown upon non-nitrogenous sub- 

 stances always contain protein, and his experiments are quoted by 

 Storer. 



In aqueous solutions of sugar left "for three months in stoppered 

 bottles, with a seven fold volume of air, an abundance of mould grew, 

 which, on being collected and subjected to dry distillation gave off 

 large quantities of ammonia. So, too, starch kept under water in a 

 bottle that contained air soon fermented, and the fungus which it 

 had nourished gave off ammonia on being distilled." 



It is now generally recognized that fungi, microscopic algae, and 

 perhaps other of the lower cryptogams, possess greater or less power 

 of fixing atmospheric nitrogen. 



Bertholet^"^ held, and, in fact demonstrated, that certain micro- 

 scopic organisms of the soil do appropriate the free nitrogen of the 

 air. During the growing eeason, in clayey and sandy soils, he ob- 

 «erved a slow but continual fixation of nitrogen from the air. This 

 increase, furthermore, failed to take place when the soils were steri- 

 lized by heating them to 230 degrees F. From this it was con- 

 cluded that the fixation of nitrogen was due to certain micro-organ- 

 isms which were destroyed by the heat. Bertholet, from his labora- 

 tory experiments, calculated that as much as 75 to 100 pounds of ni- 

 trogen per acre could be fixed in this way, and in two exceptional 

 cases as high as 525 and 980 pounds to the acre. 



A number of chemists, notably, Koenig, Kiesow, Armsby, Birner, 

 Kellner, Deh^rain and Avery^"^ found that when organic matter in 

 one form or another undergoes fermentative changes of a putrefac- 

 tive character, there was frequently an increase of nitrogen in the 



