THE ORIGIN OF SOIL. 6/ 



that it may be replaced in some way. This is evident from 

 the study of virgin soil. The chemical study of the water 

 draining from virgin soil shows that it is always filled with 

 nitrogen compounds, indicating that even such uncultivated 

 soil is constantly losing nitrogen. But such soil remains fer- 

 tile and has done so for ages. If we can explain how the ni- 

 trogen in the virgin soil is kept constant we can perhaps reach 

 the secret of tilling the soil without exhausting it. To be sure 

 it is quite a different problem to keep up the nitrogen supply 

 in a tilled soil and in virgin soil. In the virgin soil many 

 species of plants are commonly growing side by side, each 

 making slightly different demands upon the soil, and when dy- 

 ing each becoming soon incorporated into the earth again. 

 This is far less exhausting than on a cultivated land where a 

 large crop of one kind of plant is rapidly produced and at 

 once removed from the land. To keep up the nitrogen supply 

 in the latter case is far more difficult than in the former. 

 Nevertheless the two problems are closely related. The ni- 

 trates are drained from the virgin soil and demand to be re- 

 placed just as truly as those taken from the cultivated land, 

 even though to a less extent. The forces which restore it 

 slowly in one case may perhaps be made to restore it rapidly 

 in the other. To these fundamental problems of the carbon 

 and nitrogen supply and their transformations we next turn 

 our attention. 



