260 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



evident that a crop, such as wheat, of a high nitrogen content, 

 must diminish the supply of this element in the soil, and if 

 such a crop is continually grown on one plot, the nitrogen must 

 become exhausted unless replaced in various ways. Besides 

 the loss of nitrogen from cropping, other sources of loss occur 

 by diainage ; the nitrates pass away in the subsoil water, and 

 heavy rainfall on a porous soil accelerates this loss. A further 

 source of loss has been referred to in Chapter XIII, viz., the 

 eUmination of nitrogen from nitrates by the de-nitrifying 

 organisms. Against these sources of loss of nitrogen have 

 to be set the following sources of gain, apart from the applica- 

 tion of nitrogenous manure. A certain amount of nitrogen 

 is added to the soil in rain, though, as already explained, this 

 may wash out more than it brings. It is to the nitrogen-fixing 

 bacteria that we have largely to look for the economic mainten- 

 ance of the balance of nitrogen ; and we have here an explana- 

 tion of the advantage of growing leguminous crops at intervals. 

 If, after a succession of nitrogen-exhausting crops, such as 

 wheat, a crop of clover be grown, and the stubble afterwards 

 ploughed in, the nitrogen content of the soil is greatly in- 

 creased. This is due, as has been explained, to the action of 

 organisms, which find their habitat in the root nodules of 

 leguminous plants, such as the clover, which in some way 

 enable the plant to obtain a store of nitrogen from the air. 



Experiments on a small scale have shown that it is possible 

 greatly to iacrease the growth of such plants, when grown in 

 sand, by inoculating the sand, or the seeds of the plants, with 

 suitable cultures of nitrogen-fixing bacteria. Attempts have 

 been made to carry out this process on the large scale. The 

 best results have been obtained with species of leguminosse 

 introduced into a country for the first time, e.g., the soy bean 

 in the United States and Germany, lucerne in Scotland and 

 certain non-indigenous plants in Canada. For crops which 

 have already long been cultivated, e.g., clover in England, the 

 conditions of success do not so far seem to be fully understood. 



